Download unit 17 organic compounds containing oxygen and nitrogen atoms

UNIT 17 ORGANIC COMPOUNDS CONTAINING
OXYGENAND NITROGEN ATOMS
Structure
17.1 Introd~~ction
Ob-jective
17.2 . Aldehydes and Ketones
Structure and Reactivity
Reactions of Carbonyl Compounds
Reaction with Water
Reaction with Alcol~ol
Wittig Reaction
Aldol Condensation
Cannizzaro Reaction
Michael Addition
17.3 Carboxylic Acids
Preparation of Carboxylic Acids
From Alcohols
From Aldehydes and Ketones
From Alky l Benzene
Reaction of Carbo~iylicacids
Reaction at 0 - H Bond
Acidity and Basicity of Carboxylic acid
Reactions at the carbonyl carbon of Carboxylic Acid
becarboxy lation
IColbe's Electrolytic Method
Substitution on Carbon Chain
Hell-Volhard-Zeli~lskyReaction
17.4 Amines
Preparation ofAmines
Reaction of Anlines
Reaction of primary arnines with Carboxyl Compound
Reaction of Secondary A~nineswith Carbonyl Compounds
17.5 Summary
17.6 Terminal Questions
-
17.1 INTRODUCTION
In tliis unit w e will study the compounds containing oxygen and nitrogen atoms. As
you know nulneroils c o ~ n p o u ~ l are
d s know11 which collie under this category, like
alcoliols, plie~iols,aldehydes, ketones, acids, amides, a ~ n i n e etc.
s It is quite
impossible to take all these classes in just one unit. Therefore in tliis ~111it
w e will
clisci~ssonly some important methods o f the preparatioli and reactions of aldehydes,
ketones, carboxylic acids and amines. Here we will not discu'ss the reactiolis of
.
alcohols separately, but w e will discuss sollle o f them under different headings.
Objectives
After studying tliis nit, you should be able to :
describe tlie str~lctureof carbonyl group, and explain its polarity,
explain tlie relative reactivity C = 0 group,
discuss the different reactions of aldehydes and ketones, carboxylic acids an$
amines,
discuss the general mechanisms of different reactions of carbonyl compounds,
carboxylic acid and amines.
17.2 ALDEHYDES AND KETONES
,
Both aldehydes and ketones have a carbonyl group, )C = 0 . An aldehyde has at
least one hydrogen atom attached to carbonyl carbon atom and the other may bk
alkyl or aryl group. Only in case of methanal, two hydrogen atoms are attached to
the carbonyl group. A ketone has two alkyl or aryl or both the groups attached to
carbonyl carbon atom.
0
0
II
H-C-R
Aldehyde
II
.
R-C-R
Ketone
Aliphatic aldehydes form a homologous series with the general formula RCHO and
ketons with general formula RCOR'. Aldehydes and ketones have the same general
formula CnH,"O. Since carbonyl group is present in both aldehydes and ketones,
many of their properties are common. But in aldehyde, there is also a reactive
hydrogen atom, which confers reducing properties upon the molecule.
You have already studied in the previous units (or will study in coming sections)
several methods that can be used for the preparation of aldehyde and ketones under
different headings. Therefore here we will not discuss the preparation of aldehyde
and ketones separately.
17.2.1 Structure and Reactivity
-
A carbonyl group consists of a carbon doubly bonded to an oxygen atom. The
carbonyl double bond is similar in many respects to the carbon-carbon double bond
of an alkene. The carbonyl carbon atom is sp2hybridised and an unhybridised p
orbital is left on the carbon atom. Of the three o bonds formed by the carbonyl
carbon atom, one is with the oxygen atom. AlSo the unhybridisedg orbital of the
carbon atoms is used in forming the n-bond with oxygen atom. Thus the carbonoxygen double bond of a carbonyl compound consists of the o bond and the n bond.
Carbonyl compounds are planar and have bond angles of approximately 120" at the
carbonyl carbon atom.
'
H (1s) -C(2sp2)
0
bond formed by
overlap of 2p orbitals
of carbon and oxygen
bond
6 bond
*
Fig. 17.1 :Bonding in carbonyl compound.
Unlike carbon-carbon double bond, carbon-oxygen double bond is polar. This is
because of the higher electronegativity of oxygen atom relative to carbon atom. The
n electrons of the carbon-oxygen double bond are pulled towaids the oxygen atom
and the bond is polarised. This electron imbalance in the n bond makes the carbon
OrganicCompounds
containing Oxygen and
Nitrogen Atoms
Chemistry of Ci~rbon
Compounds
at0111electron-deficient, as a result, the carbonyl group as a whole has an electroll
withdrawing effect. Thus, the carbonyl group has t h o active centres, viz.,
The carbon atom carrying partial positiv'e charge, called electrophilic or cationic
centre; This can be attacked by nucleophilic reagents.
The oxygen atom carrying partial negative charge, called nucleopl~i 1 ic or an ionic
centre. This can be attacked by electrophilic reagents.
As you know, ni~cleopliilicadditionreaction involves addition of a nucleopliile to the
partially positively charged carbon atom of tlie carbonyl group. Tlie relative
reactivities of carbonyl group in ni~cleophilicaddition reactions may be attributed
partly to the extent of polarisatiou of tlie carbonyl carbon. The rate determining step
at the positively charged carbon atonl. Therefore,
involves tlie attack of ~iucleopl~ile
tlie reactivity of tlie carbonyl group depends up011the magnitude of the positive
cliarge on tlie carbonyl carbon atom. Thus, a greater positive charge means higher
reactivity. Iftliis partial positive charge is dispersed throughoi~tthe molecule, then
tlie carbonyl c o ~ n p o i ~is
~ iless
d reactive. Electroll withdrawing substituents at tlie
carbonyl carbon, which increase its positive character, increase its reactivity
towards nucleophilic addition reaction. Similarly, electron donating substituents
decrease its positive character and hence decrease tlie reactivity towards
nucleophilic add ition reaction.
You know that alkyl groups have electron releasing effect. Therefore, ketones,
which contain two alkyl groups, are less reactive than aldehydes. Further
cliloroetlianal, which contains tlie electron withdrawing chlorine atonl, is more
reactive tlian etlianal. Similarly, nitroetlianal, where -NO, group has stronger
electron withdrawing character than clilorine, is more reactive than cliloroethanal.
Tlius tlie order of reactivity is :
0
II
CH3CH2 t t C H
<
0
I1
CH3 t C H
0
I1
< C l t C H 2-CH <
02N+CH2
0
I1
-CH
Aromatic aldeliydes or ketones are less reactive than aliphatic aldehydes and
ketones; this can be attributed to resonance interaction between the carbonyl group
and the, aromatic ring. Tlie result of this interactio~iis a weakening of tlie positive
charge on the carbonyl carbon atom through dispersal of tlie cliarge within the ring.
+
Steric factor also plays an important role in tlie relative reactivity ofaldehydes and
ketones. A bulky group in tlie viciriity of the carbo~iylcarbon presents greater steric
hindrance than tlie smaller hydrogen atom to tlie approaching ~iucleophile.
With the above general ideas, it will be easier to study tlie reactions of aldehydes
and ketones, wliicli we will take up in the next section.
I
SAQ 1
Considering the steric factor, arrange the following compounds in the order of their
reactivity :
17.2.2 Reactions of Carbonyl Compounds
Theoretically, a carbonyl compound may be attacked either by a nucleophile or by an
electrophile.Addition of the negative nucleophilic part of the reagent to the carbon
atom or addition of the positive electrophilic part to the oxygen atom would Qve the
same product ultimately.
The addition reactions of carbonyl compounds, therefore, can theoretically proceed
by the following two mechanism.
Mechanism I : In the first mechanism, the proton adds to the carbonyl oxygen in
the first step (slow step). This further increases the electrophilic nature of the
carbonyl carbon atom. In the next step (fast step) the nucleophile attacks the
carbocation.
t
<-OH + N U
I
Fast
4.
1
- C -OH
I
Nu
Mechanism I1 : In the second type of mechanism, nucleophile attacks the polarised
carbonyl carbon in the first step and forms an anionic intermediate.
This intermediate can undergo either protonation to form an alcohol, or it might expel
oxygen as water to form a new double bond between carbon and the nucleophile.
NuH
L
.
I
An acid catalysed reaction should follow mechanism I and the base catalysed
mechanism IT.
Now let us study some important reactions of aldehydes or ketones.
,OrganicCompounds
mn&g
Oxygen and
Nltrogen Atoms
. .
.
Cheqistry of Carbon
Compounds
Reaction with Water
Aldehydes and ketones react with water to form 1, ldiols, (geminaldiols)or
hydmtes. Hydrogen atom becomes bonded to the negatively polarised carbonyl
oxygen and hydroxyl group to the positively polarised carbon. This reaction is
niversible and the hidrate formed is generally too unstable to be isolated.
OH
-C=O+H20
I
-C-OH
1
Oeminal diol
The rate of reaction depends on the nature of the carbonyl group and is influenced
by the combination of electronic and steric effects.
With increase of al kyl substitution on the carbonyl group, the reactivity of carbonyl
compounds decreases, when treated with water under similar conditions. For
example,
Methanal has no alkyl substituents to stabilise its carbonyl group and is converted
almost compktelyto the correspondingdiol(99.%%). The carbonyl group of
ethanal is stabilised by one alkyl substituent and the carbonyl of propanone by two.
Ethanal gives 50% while propanone gives only 0.14% of diols.
Reactivity of carbonyl comwunds increases when electron-withdrawing p u p s are
attached to the carbonyl carbon. For example, in contrast to the almost negligible
hydration of propanone, the hexafluoropmpanone is completely hydrated.
loo %
Now let us consider the steric e f f w on the rate of reactions. Let us examine the
abova reaction. Thepubon atom that bears two hydroxyl group is s# hybridised.
Its substituents are kre.crowded thap they are in the starting aldehyde or ketone.
-.
.b:
x
b
RCH+
Increased crowding can be better tolerated when the substituents are hydrogen than
when they are alkyl groups. Diol of methanal is least crowded and hence formed in
large amount. Diol of propanone on the other hand is more crowded, therefore,
fonned in a lesser amount. Finally, the amount of diol of ethanal is formed between
the above two limits.
As the electronic and steric effects combine hydration of aldehydes becomes mare
favourable than that of ketones.
SAQ 2
Which of the following compougds do you predict would form stable hydrates and
why?
*.
Reaction with Alcohol
Like water, an alcohol can undergo addition reactions with carbonyl group in the
presence of an acid catalyst. It is also a reversible reaction. In most cases, the
equilibrium lies to the aldehyde or ketone side. Addition of one molecule of an .
alcohol to an aldehyde or ketone give hemiacetal or hemiketal, respectively. On
the other hand, addition of two molecules of alcohol to an aldehyde or a ketone, with
the loss of water, gives acetal or ketal, respectively. Unlike hydrates, acetals and
ketals are quite stable and can be isolated.
OR
0
!I
KOHIH.
1
OR
ROHIII'
1
RCH , L R C H + H , O
I
I
OR
&I
I
Aldehyde
Hemlaceta1
(OH and OR on C)
Aceial
(hw OR'S on C)
Mechanism : The mechanism of formation of hemiacetal is analogous to that of
the acid catalysed hydration of an aldehyde.
Oxoniurn ion
In the mechanism for acetalfO&tian from the hemiacetal, protonation of
miacetal takes place followed by dehydration to give a carbocation.
OrganicCompounds
containingOxygen and
~itroientoms
Chemistry ofcarbon
Compounds
i)~,
:OH
I
-C-OR
I
-'
H+
1
-C-OR
H,O
+
-C-OR+H20
I
'-\
I
The carbocation is stabilised by electron release from its oxygen substituent to yield
oxonium ion.
Oxonium
Ion
.
Reaction of a second molecule of alcohol forms the protonated acetal which on
deprotonation gives acetal.
'
H
Acetal
An acetal can be hydrolysed back into parent aldehyde and alcohol on treatment
with aqueous mineral acid even at room temperature.
.
is just the reverse of that for the formation of the acetal.
;
Now you will see how acetal formation and hydrolysis have been applied to
synthetic organic chemistry as a means of carbonyl group protection. In some
chemical reactions one functional group may interfere with intended reaction
elsewhere in a complex molecule. We can often circumvent the problem in such
cases by first protecting the interfering functional group, carrying out the des'ired
reaction, and then removing the protecting group. For example, if we wish to oxidise
propenal to 2,3-dihydroxypropanal, there is an interference of the carbonyl group, '
since both the double bond (C=C and C=O) would be oxidised. But after converting
the carbonyl group to an acetal, we can oxidise a carbon-carbon double bond
without oxidisingthe carbonyl group.
II
CH2 =CHCH
cannot bp
done directly
I
I
II
CH2CH-CH
Since acetal formation is a reversifle reaction, it can be cleaved by hydrolysis to
regenerate the carbonyl group. Thus, 2,3-di hydroxypropanal is obtained. Similarly
take another example; conversion of ethyl 4-,oxopentanoate to 5-dydroxypentan-2-
one. We cannot reduce the ester group directly by LiAIH, as both the carbonyl
groups would be reduced simultaneously. If we first protect the ketone by forming a
ketal, however subseque~itester reduction proceed nor~nallyand acetal can be
cleaved to get back ketone. _. _
0
0
II
II
0
CH,CCH2CH2COCH2CH3
II
be >CH3CCH2CH2CH20H
done d~rectly
Ca'lnOt
I:th> l 4-o\opentanoate
'H
I
5-Hydroxypentan-2-one
HOCH2CH20H
CH2-Ci12
I
0
I
0
\/
CH2-CH2
I
0
11
CH3CCH2CH2COCH2CH3
0
LtAIH4
I
+.I
0
\/
>CH3CCH2CH2CH20H
Ether
Reaction with ammonia derivatives
Aldehydes and ketones react with a number of ammonia derivatives such as hydroxylamine, hydrazine, cer~nicarbazideetc. in weak acidic medium. In general these derivatives [nay be represented as H 2 N 4and their reaction with aldehydes and ketones can
represented as follows :
The reaction of aldehydes or ketones with ammonia derivatives is catalysed by acids.
In acidic medium, the carbonyl oxygen gets protonated. Due to the presence of positive
charge, the carbon of carbonyl group undergoes nucleophilic attack very easily.
It is necessary to adjust the reaction medium to right pH. This reaction involves nucleophilic attack by the basic nitrogen. Protonation ofcarbonyl oxygen makes it more susceptible to nucleophilic attack so, as far carbonyl group is concerned, the reaction is
favoured by high acidity. But under highly acidic conditions the ammonia derivative can
also undergo protenation to for H,N-Gion which is no longer a nucleophile. So as far
ammonia derivative is concerned, the addition is favoured by low acidity. That is why is
OrganicCompounds
containing Oxygen and
Nitrogen Atoms
Clkemistry of Carbon
Co'mpollnds
these reactions the pH is carefully coatrolled. The optimum pH is 3.5 although the
d x a ~coi~ditions
t
also depend upon the basicity of the reagent.
SAQ 3
Consider the acid-catalysed reaction of ethanal with methanol. Write structural
'formulae for,s
a) the he~niacetalintermediate
.b) the cacbbcation intermediate
c) the acetal product.
--------------------------------------.----.-----*---------------------------------------------.................................................................................................
In 1954. George Wittig
reported a ~netliodof
synthesising alkenes from
carbonyl co~npounds.
Wittig Reaction
In this reaction, carbnyl oxygen is replaced by the group = CRR' (where R and R'
are lrydrogen or alkyl group).
1 I
I
Wittig reaction
.-+c=o
+-c =c'
There are'two,mainsteps in Wigig reaction. In the first step, the nucleophilic
reqgent tripllenylphasphine reacts with only a primary or secondary alkyl halide to
give aplrosphonium salt. The alkyhalide can also contain double bond or alkenyl
group.
/
R'
Ph,P:
+ RAHX
Triphenyl
phosphine
*lky'
R'
P ~ ~ P AHRX+-
'Iaiide
Phosphonium
salt
This plwqplionium salt further reacts with a strong base, which abstracts a weakly
acidic a-hydrogen to give alkylidene triphenylphosphorane(a phosphorus ylide)
commonly known as the Wittig reagent.
R'
I
P ~ * ~ P + HR
-C X
'
+
R'
I.
C6H3Li -3 Ph3P = C R
+
C6H6
+
LiX
(ylide)
The rqsultingphosphorus ylide attacks the carbonyl carbon to form a dipolar
interniediGecal led a betahe, which often undergoes elimination spontaneouslyto
yield an alkene.
~ e c b a n i s mi~ebhanisrnof Wittig reaction.'has been'the subject of much
.discussi&, btit
is now strongly in favour of the formation of an
intermediatgbetajne.This betaine intermediate is unstable and rapidly fragments,
prdbab~yby Way of a second inteimedia~4containing a fauf-membered ring, to an
alkene qlid triphenyl-phosphineoxide.
~~~~~~~e
Organic Compounds
containingOxygenand
NitrogenAtoms
Betaine
'I'riphenyl
pliosphinc
oxlde
.
Alkene
The great value o f Wittig reaction is that pure alkenes o f known structures can be
prepared. The position at wliich the double bond is introduced is never in~dqubt.The
double bond is formed betweed the carbonyl carbon o f the aldehyde or ketone and
the negatively charged carbon oftlie ylide.
I
Aldol Condensation
In the presence o f a dilute base, such as aqueous NaOH, h o or more molecules o f
an aldehyde or a ketone, containing an a-hydrogen may combine to form a Phydroxyaldehyde or P-hydroxyketone, a compound containingalcoholic and
aldehydic or ketonic groups, respectively. This reaction is called aldol condensation.
The product results from addition o f one molecule of the carbonyl co~npoundsto a
second n~oleculein such ti way that the a-carbon'of the first is attached to the
carbo~iylcarbon o f the second. For example, consider reaction between the two
rf
ethanal molecules.
H
I
CH3C = O
H
OH
H
I
I
I
CH3C = O ---+ CH3CHCH2C = O
0
'
+
Elhi~nill
Ethanal
3-Hydroxybutand
II
di'HC1
bCH3CH = CHCH
Z-Elutenal
'The condensation product loses a water molecule to give unsaturated aldehyde or
ketone (conjugated enones). For an aldol condensation aldehyde or ketone must
contain an a-hydrogen. If the aldehyde or ketone does not contain an u-hydrogen, a
simple aldol condensation cannot take place.
AI-CHO
.. di'ute.h
*
,NO reaction
ArCOAr
ArCOCR,
C'OII~OU~U~S'
ui~ltuinil)
no tr-,iyjrDgen
Mlcbambm :Aldol condensation is s t y o step process. I n fhe.fwst step. the .dm:'
abstracts a protocl.f m
a-c&
sf-thed&hyde fa form "ehdate'ion.
cnolatc ion
In tlie next step, the enolate ion attacks the carbonyl carbon of another aldehyde
molecule to form an alkoxide ion, which abstracts a proton from water to yik~dthe
0
c?
CHICH + CH,-H!
u-
0
0
I
CHICH
I1
-
CH2CH
H0
OH
0
II
2CH,CHCH~CH
+OH
I
-
Crossed aldol condensation : As ~lientionedabove, an aldeliyde without ahydrogen does not undergo aldol condensation. However, if such an aldeliyde is
mixed with an aldehyde that does have an a-hydrogen, aldol condensation can
occul-.Aldol condensation between the two different carbonyl compounds is called
crossed aldol condensation which is oftwo types :
In type one, both the carbonyl compounds have a-hydrogen atoms. In these cases a
mixture of four possible products may be formed. Because of the formation of such
a mixture, h i s type of reaction is corn~nerciallyof no use.
In type two, one of the carbonyl compound does not have an a-hydrogen, e.g.,
13znzaldchyde
(No a-hydrogen)
IJthanol
Methyl ketones can be used successfully in crossed aldol condensation with
aldehydes that contain no a-hydrogen.
There are a large number of reactions that are closely related to aldol condensation.
At first glance each of these reactions may seem quite different from others. But a
close examination of these reactions shows; that like aldol condensation, each of
these involves an attack by a carbanion formed from one molecule on the carbonyl
group of another:
Michael Addition
Nucleophiles and carbanions generally do not add to isolated carbon-carbon double
bonds. However, when an electron withdrawing group like C=O is present in
co~ijugationwith a carbon-carbon double bond, carbanions add to the conjugated
system at the site of electron deficiency, i.e., the P-carbon atom. Such addition
reactions are known as Mkhael addition. In other words, addition of active
methylene compounds to carbon-carbon double bond of a , P-unsaturated carbonyl
co~npoundsin the presence of basic catalyst is known as Michael addition. The
following exa~nplesare illustrative :
OrganicCompounds
containing Oxygen and
Nitrogen Atoms
Mechanism : We take here the condensation of ethyl ~nalonatewith propenal as an
example. In the first step of Michael reaction, the base removes an a-hydrogen
atom from ethyl malonate to generate the corresponding carbanion (enolate anion).
6 ~ 2 ~ 5
CH2(COOC2H5)2
C H ( C O O C ~ H , )+~ C2H,0H
Enolate ion
In the next step, the carbanion attacks at the P-carbon atom of propenal to give the
more stable enolate which abstracts a proto11from the solvent to yield the final
product.
HC-CH =CH,
L/
+ CH(COOC,H,)~+HC=CH
- CH2
I
CH(COOC2H5)2
In general the colnpound from which carbanion is generated must have an acidic
hydrogen, so that the carbanion call be obtained easily. Such a compound is usually
one that contain a -CH,- or >CH- group flanked by two electron withdrawing
groups on either side.
.
Michael addition is a general reaction and is not limited to conjugated carbonyl
compounds. Conjugated esters, n itriles, amides and nitro compounds can also
undergo Michael addition. For example :
N = CCH = CH2 +CH2(COOC2H5)2-N
I
CCH2CH2CH(COOC2HS)2
SAQ 4
How can the following colnpounds be prepared using Michael reaction?
Chemistry of Carbon
Compounds
0
0
II
11
b' CH, CCH2CH2CH2CCH,
17.3 CARBOXYLIC ACIDS
*
All the organic acids contain carboxyl group as their functional group, attached to
al kyl group (RCOOH) or an aryl group (ArCOOH) except formic acid (HCOOH) :
Those which contain one carboxyl group in each molecule are called the
monocarboxylic acids. These compounds form a homologous series of general
formula C,H,,+,.COOH. The lowest members are formic acid (HCOOH) and
acetic acid (CH,COOH).
17.3.1 Preparation of Carboxylic Acid
There am number of methods for the preparation of carboxylic acids. But in this
unit we will discuss only few important methods for the preparation of carboxylic
acids.
From Alkenes : Oxidation of alkene (in which at least one carbon atom of the
carbon-carbon double bond contains a hydrogen atom) with permanganate gives an
aldehyde which is further oxidised to carboxylic acid.
Since th&carboxylic acids are formed as salt, the acidification (using mineral acids)
is needed to isolate the product as prboxylic acid.
alkene, tbe terminal (2% group is completely oxidised to
In the oxidation of te~mina-1
carbon dioxide arid water.
From Alcohols
J
An impolrtant reaction of alcohols is their oxidation to yield carbonyl compounds.
Alcohols with a-hydrogen atom (s) undergo oxidation readily. Oxidation of an
alcohol involves the loss of one or more a-hydrogens. The nature of the product
formed depends upon the number of a-hydrogens present in the alcohol, that is,
whether the atcohol is primary, secondary or tertiary. Primary alcohols first give
aldehydes by losing two hydrogens. The aldehyde formed tends to undergo further
oxidation to give a carboxylic acid. In aqueous solution, aldehydes are &ore easily
oxidised than alcohols. Therefore, oxidation usually continues until the carboxylic
acid is formed.
,
<
I
RCOH
I
I
lo', R C - 0
H
Alcohol
[01
aldehyde
,R CI = O
carboxylic acid
Oxidisi~ig
agents co~nmo~ily
used for the oxidation o f primary alcohols to carboxylic
acids
are,
chromium
trioxide
(CrO,) in aqueous sulphuric acid (Jone's reagent),
I
I.
, potassiuni permariganateor potassium dichromate. For example,
,
1
CH3(CH2) CH20H
I
I-1)ccrnol
Jone's reagent
X
,cH,(cH~), 6 =O
h n o i c acid
If tlie reaction mixture is kept in between the boiling points o f the aldehyde and the
alcohol, tlie aldehyde distils offas soon as it is formecl and further oxidation is
avoided. Yield o f aldehydes by this method is usually low.
-
From Aldehydes and Ketones
4
As~mentionedabove, aldehydes are very readily oxidised to acid, Aldehydes can be
oxidised by the same reagents that oxidise alcohols. Permanganate or dichromate
salts are the most commoli oxidising agents.
Aldehydes are so easy to oxidise that even a mild reagent like silver diammonia
complex (Tollens' reagent) can be used for oxidation. For example:
H
Tollens' reagent oxidises aldehydes in high yield without attacking carbon-carbon
double bond o i othersfunctio~;algroups.
I
RCH = C H C = O
u,P-iniseturatod
ntdtliyde
Tollen's .
.
,RCH=CHCOOH
reFgent .
a,$-unsaturated'
acid
In this reactibn, a shining coat o f silvbr metal gets deposited on the glass suetace o f
tlie reactib~ivessel. So it can be used as a test to detect the presence o f aldehydes.
'Mirrors are also prepared commercially'in this.way.
'
-
Ketones.are not easily oxidised. Oxidation o f ketones occurs only when forced by
tlie use o f strong oxidising agents and perhaps involves the cleavage o f the molecule
through tlie corresponding enol to produce an acid.
The reaction is only useful for sym~netricalketones.
Organic Compounds
containingOxygenand
NitrogenAtoms
Chemistry of Carbon
Compounds
By carbonation of Grignard Reagents
Carboxylic acids are obtained is good yield by carbonation ofGrignard reagents. In this
Grignard reagents add on to carbon dioxides which on hydrolysis give carboxylic acid
with one more carbon than those present in Grignard reagent. Grignard reagents are
obtained from corresponding alkyl halides. So this makes a good method for obtaining
carboxylic acids from alkyl halides and is used for ascending tile series in conversions.
From Alkyl Benzene :
Although benzene and alkanes are quite unreactive towards the usual oxidising
agents (KMnO,, K2Cr20,etc.), the benzene ring renders an aliphatic side chain
quite susceptibleto oxidation. The side chain, irrespective df its length, is oxidised to
a carboxylic group (XOOH). Tertiary alkyl substituted aromatic compound do not
follow this reaction. For example toluene and propylebenzene (I -methyl ethyl)
benzene are oxidised to benzoic acid in higher yields. p-Methyltoluene on oxidation
gives tetraphthalic(benzene-1 ,4-dicarboxylic) acid but tertiary butylbenzene is not
affected.
benzoic acid
'
(1 -111ethylethyI) benzene
.
C(CH,),
[OI
No reaction
tert butylbenzene
The number and the position oftlle carboxylic groups produced indicate the number
and positions of alkyl chain attached to the aromatic ring.
CH,
HOOC
J3c00
p-methyl toluene
This reaction is useful for two purposes : (1) synthesis of carboxylic acids (2)
identification ofthe position ofthe side chain
SAQ 5
~ i v the
e
ofthe following :
OrganicCompounds
containingOxygen and
NitrogenAtoms
b)
C)
CH3CH2CH0
CH3CH20H
Tollen's
reagent
Jone's
reagent
.........................................
17.3.2 Reactions of carboxylic acids
.
0
II
As you know, the carboxylic acid has a functional group - C-OH. Most of the
reactions of carboxylic acid belong to any one of the following four categories.
0
.a.
II
Reactions at 0- H bond
b.
Reaction at carbonyl carbon
c.
Decarboxylation
d.
Substitution on carbon chain
R-C-0-H
-CH,CH,-COOH
\
In this unit we will discuss some important reactions of carboxylic acids.
a) Reaction at 0-H Bond
Acidity and Basicity of Carboxylic acid
_ Carboxylic acids are very week acids. The acidity of carboxylic acid is due to polar
nature of carbony1 group. Since it is an acid, it has a tendency of forming salt on
treatment with base such as NaOH e.g.
0
II
R - C-OH
+ NaOH---+R
0
II - +
- C-ONa+ H 2 0
Salt
'This salt forming property of carboxylic acid helps us to separate water insoluble
carboxylic acid from water insoluble nonacidic substances. To do so, first convert
the acid into salt form, which is soluble in water, then separate the two and you can
get the carboxylic acid by acidification of salt with mineral acids.
Although its name indicates that it is an acid, but it also acts as a weak base.
Carbonyl oxygen contains two lone pair which gets protonated, which is
characteristic of any base. -
Chemistry of Carbon
colnpounds
This piotonation or basicity plays an important tole in the reactions of carboxylic
acids.
Hydroxyl oxygen can also accepts proton ftom mineral acids but this possibility is
very less. This type of ptotonation plays a tole in esterification when alkyl group is
bulky and, in addition, has electron donating properties, thereby fhvouring ionisation
to give an acyl carbocation. e.g.
b) Reactions at the carbonyl carbon of CarboxylicAcid
Most ofthe reactions of carboxylic acids involve addition of nucleophile to the
partially positively charged carbon atom of the carbony1 group. These reactions
generdly are catalyzed by acids because addition of H+or formation of hydrogen
bond to the carbonyl oxygen makes the carbonyl carbon more vulnerable to
nucleophile attack. e.g.
.. f i ~ +
0:
C
I1
R-C-OH +
+ H+
E?"
<1I
R-C-OH
A,
OH
I
=
b
R-C-OH
I
Nu
This is a substitution reaction. Some important examples of this type of reaction are
formation of esters, acyl chloride formation, amide formation etc. Let us take the case of esterification.
Esterification :-Reaction of alcohol with carboxylic acid in the presence of acid
give ester. Thi9 reaction is known as Fischer ester1j7cation.
Since this is a reversible reaction, the yield of ester is very poor. The yield can
increase by using one of the reactants in excess (LeChatelier's principle).
The yield can also be increased by removing water, a product responsible for
reversibilityof the reaction,
Mechanism :In the first step, tIie carboxylic acid gets protonakd.
,
The second and the key step of the reaction is the attack of alcohol at carbonyl
carbon which gives tetrahedral intermediate.
OH
H - Q - ~ e
R-L-~HR
OH
. I
R-C-OR
-
AH
AH
Tetrahedral
Intermediate
,
In the third step, the tetrahedral intermediate eliminates water and yield the required
product i.e. ester.
OH
+
+H
I
R-C-OR
I
OH
BH; .
R-C-OR
I
(;OH2
-H,O
+H@
+OH
II
R-C-OR
0
-Hi
+H
H
R-C-OR
ester
Reduction of Carboxylic Acids :
Generally carboxylic acids do not react towards most of the reducing agents. But
LiAlH, reduces the acid to primary alcohol. e.g.
R - COOH
.
2.H20,H+
b ,RCH20H
Kolbets Electrolytic Method
When a concentrated solution of sodium or potassium salt of a carboxylic acid is .
electrolysed, an alkane is formed. The method is known as Kolbe's electrolytic
method. In this method, carboxylate radicals are formed by transfer of electron h n
carboxylate ion. Decarboxylation may take place simultaneously or subsequent to,
the formation of carboxylateradicals, leading to alkyLradicals, which combines to
give alkane.
1
RCOOK+ R'COOK + H20--+RR1+2C02+
at anode
H2 + 2KOH
at cathode
The following mechanistic pathway illustratesthis method :
II
I\
CH,C-6-'PCH,C-6
acetoxy
radical
Isotope spbstitution studies
have proved that the oxygen
of the water comes from acid
and, alcohol oxygen is
incorporated into the ester.
Chemistry ofCarbon
compounds.
0
II
+ , ~ d t CH,
- - -C-- d
CO~
In case a mixture of salts of two carboxylic acids is electrolysed, a mixtureof
alkanes is formed:
-
R'R'
R'COOK + RUCOOK
i ( ' ~+"C02 + HI + KOH
+
R"R"
This reaction has limited synthetic application,because of the formation of many side
products as a result of other
of the free radicals formed.
reasons
d) Substitution on Carbon chain
Hell - volhard - zelinsky (HVZ)reactions
This reactions involves the halogenation at a-carbon of aliphatic acid. The reaction
involves the treatment of carboxylic acids with C1, or Br, is presence of red phosphorus. If the number of a-hydrogens are more then the reaction does not stop after
monohalogenation. It continues till all the a-hydrogens are replaced by chlorine or bromine. For example
CH3CH,COOH
Bb'P
)
CH, CHCOOH
8r
Br2'P
)
CH3CBr2COOH
The function of the phosphorus is to convert a little of acid into acid halide. These are
more reactive than the parent acid k d undergo a-halogenation easily. Such as in case
of propanoic acid the reaction proceeds as follows
3CH3CH,COOH + PBr,
-
0
I1
CH, C H,CBr
+ H3P03
.
1
.
The Hell-Volhard-Zelinsky reaction is an important reaction as a number of '
derivatives of carboxylic acid can be obtained from halo acids. The halogen of an ahalo acid is replaced readily by nucleopbilic reagents such as,
Organic Cornpol~nds
containing Oxygen and
Nitrogen Atonls
I
2. l1+, H 2 0
OH
2-hydroxy acid
ji)
R - C HCOOH + 2NH3 --+R
I
-CHCOOH
I
NH~
2-a mino acid
iii)
R - C HCOOH + KCN
\
X
'.
+ R -CHCOOH
I
~ M + , H ~ O
CN
2-Cyanocarboxyl~ca c ~ d
"O
,R C HCOOH
b
COOH
Dicarboxylic acid
SAQ 6
Give the mechanism Hell-Volhard-Zelinsky reaction.
17.4 AMINES
Aliphatic amines are the derivatives of ammonia formed by replacing one or more
hydrogen by alkyl group. Amines are classified as primary, secondary or tertiary,
according to the number of alkyl group attached to the nitrogen atom. It is unlike
alcohols or alkyl halides, where classification is based on the number of alkyl groups
.attached to halogen or hydroxyl group bearing carbon atom.
Pnmary amine
'
7
Sccoilllaty
'lkriinry
ami~lcs
arniocs
Aniines form a honiologous series with general formula C,H,,+IN <; Methyl amine
(CH,NH,) is the lowest member of amine. Since alkyl group is relatively inert it is
amino group which determines the reactions.
When one of the substituent attached to the nitrogen atom is an aryl group the amine
is called aryl amine.
17.4.1 Preparation of Amines
There are number of methods for the preparation of amines. Some important
methods for their preparation are given below:
~ l k ~ i a t i oofnAmmonia : Reaction of alkyl halide with ammonia it gives amine.
Ammonia is a very good nucleophile. Therefore this reaction follows S2,
mechanism. Alkylation of ammonia, primary arnines,and secondary amines gives
primary. secondary and tertiary amines respectively.
Chemistry of Carbon
Con~poutlds
f
RNH2 + R - X + K 2 N H 2 X
.
OH
-
i R2NH
Secondary amine
Primary ammne
+
-
+R-x'-----+R~NHX----+R~-N
R2NH
Secondary amine
Linlitation of this reaction is that it does not stop after single alkylation and mixture of
products are formed i.e.
R-X+NH3jR-Nl12+R2NH+R3 -N
45%
43%
Trace
Multiple alkylation can be minimized by using a large excess of anmlonia. Even then
this method is not good enough to get good yield and better methods of preparation
are needed. In order to overcome thc limitations of this method two other methods
are oftcn used i.e. azide synthesis and Gabriel synthesis.
Alkyl azidc is very explosive
so it should bc kept in
solution only.
Azide Synthesis : It is a very good method for preparing primary amine froin alkylhalide. First convert the alkyl halid to alkyl azide by the following reaction.
-
+
-
+
R-X+N=N=N----R--N=N=N
Azide ion
-
H21Pd-C
+ R - N H 2 +.N2
Alkyl azide
Since alkyl azide is a good nucleophile over alkylation can not take place. The alkyl
azide formed can be reducedto primary amine by catalytic hydrogenation over
palladium or by LiAlH,.
Sicg~~~und
Gabriel (J 851-1924)
b. Bcrlin; Ph. D-University of
Berlh~(1874).
Gobriel Synthesis : This is also a good method for preparing primary amine from
alkyl halide. Pure primary amines can be prepared conveniently if the nitrogen atom
is protected so that alkylation can take place only once, i.e., multiple alkylation can
be avoided. Such a protected nitrogen is present In phthalimide. phthalimide is quite
acidic, due to two carbonyl groups. It can be converted to potassium phthalimide on
treatment with base.
0
I'hthal~nl~dt:
ill11011
phthalimide anion is a strong nucleophile and it reacts with an alkyl hal~deby S,2
mechanism to give N-alkyl phthalimide. Basic hydrolys~sof N-alkyl phthalimide
yields prinlary amine.
L
Organic Compounds'
cotitaining Osygcn and
Nitrogen Atoms
This reaction is useful forprimary and unbranched secondary alkylhalide.
,
Reduction of Imine :
Reduction of imine gives amine The carbon nitrogen double bond of imine can be
reduced to primary or secondary amines by simple catalytic hydrogenation.
5
R2C = NH
H2/Pt
lmine
,R ,CHNH2
primary amine
H /Pt
R2C = N R ' A - 4 R2CHNHR1
Imine
secondary aminc
Hofmann Rearrangement of Amides
Primary amide react with bromine or chlorine in basic solution to give arnine. This
reaction is known as Hofmann rearrangement.
In this reaction one carbon atom of the amide is lost and alkyl group of amide gets
attached to nitrogen of the amine. An example of this reaction is :
--
Mechanism : In the first step 0 H abstract proton from amide and amidate
intermediate is formed. The electron-withdrawing nature of acyl group of the amide ,
make the amido hydrogen much more acidic than amine.
i
K-C-NH,
i
+ OH-K-C-NFI
oI1
n
-
Amidate
X-X
0
II
,R -C-NHX
N-haloainidc
In the next step halogenation take place and N-haloamide is formed, which react
- with hydroxide ion to give N-haloamidate. This anion spontaneously rearrange with
loss of bromide ion to give an isocynate. In the rearrangement the alkyl group
migrate from carbon to electron-deficient nitrogen. The isocynate formed reaction
with water to give carbamic acids which spontaniously loses CO, to yield the amine.
Cllenlistry of Carbon
Co~npounds
II
R - N = C = O + H 2 0 - - - + ~ - N H ' - C - O ~ ---+
Carbanic acid
R-NH2+C02
Curtius Rearrangement :
Curtius rearrangement is similar to Hofmann rearrangement. In this reaction acyl
halide react with sodium azide (NaN,) to give amine.
.,
R
llc-
i!
(1 -- X -- NaNi-N-a=N:
A
A
----
-.
Acyl halide
Acyl azidc
-NaC:l
.-N = C = O
-N2
There is an other similar type of rearrangement called Schmidt rearrangement. In
this case the starting material is a carboxylic acid. Reaction of carboxylic acid with
sodium azide in the presence of catalyst gives an alkhnoyl azide which after similar
rearrangement gives amine.
R - COOH
1. N ~ N ~ , H,C6H6
+
>R-NH,
2. NaOH, H 2 0
SAQ 7
Give the mechanism of Schmidt rearrangement?
17.4.2 Reaction of Amines
We have discussed some reactions of arnines earlier in this unit. Here we will
discuss some other important reactions of amines and their mechanism.
Oxidation of Amines : All types of amines are easily oxidized. Oxidation of
primary and secondary amines do not give useful product but gives some complex
mixtures. Oxidation of tertiary arninhs'm presence of hydrogen peroxide or peroxy
acids give corresponding amine oxide.
H 2 0 2 or RCOOOH
+ R3 NTertiary amine
Oxide
Tertiary arnine oxide on heating undergoes elimination to give dialkylhydroxylalnine
and alkene. This type of elimination is known as cope elimination.
I
R
- CH2CH2 -
N
I
+
-
CH3
CH3
lsOOc
I
)
RCH = CH,
+ : N - CH3
I
CH3
I
Mechanism :
CII,
-
Organic Compounds
containing Oxygen and
Nitrogen Atoms
OH
I
R--CFI=CII+N--CH,
I
CH,
Reaction with aldehydes and Ketones :
Reactions of amine with carbonyl compounds can be classified into the following
two categories.
Reaction of primary amines with carbonyl compounds
Reaction of secondary amines with carbonyl compounds
' Reaction of primary amines with Carbonyl Compounds
'
In the presence of an acid catalyst a primary amine adds to carbonyl compounds to
give an imine (compounds with C=N group). In this reaction the initial addition of
H + is followed by an attack of H,NG. Subsequent dehydration forms a carbonnitrogen double bond. The net result is substitution of oxygen by another group.
Primary amines react with aldehydes or ketones to form corresponding N-alkyl or
N-aryl substituted imines.
I
The unsubstituted imines, obtained by reaction of carbonyl compounds with NH,, are
very unstable; while substituted imines, formed form RNH,, are more stable.
Substituted imines are also called Schiff bases.
0TcH3 a
0'':
I
I
CH3NH2 d
-
CH = NCH,
-H20,
Mechanism : Formation of a Schiff base is a two step reaction. In the first- step,
the nucleophile (RNH,) adds to the partially positive carbonyl carbon. This step is
followed by the loss of a proton from nitrogen k d the gain of proton by oxygen to
give an intermediate called carbinolarnine;
'
Chemistry of Carbon
Compounds
R\
D+
C=O
R'\
-
RN112
,
L
..
0-
OF1
RCR
RCR
I
I
I
RNI I
RNI-I
I
C:nrbinoIamine
In the second step, the carbinolamine eliminates water and gives the imine,
pH2
OH
1
I
-H+
+
-H20
R2C- NHR' K
L
R 2 C- N HR' '-\
R2C =NHR
t
-
'4R2C = N R '
Both the addition and the elimination steps of the reaction are sensitive to acid
catalysis. Hence careful control of pH is essential. The rate of reaction is increased
by an increase in acidity but beyond a certain limit, the rate decreases with further
increase of acidity.
This is because, these reactions are catalysed by acids, thus, protonation of the
carbonyl compound as well as the reagent can take place.
The first step of the reaction is the addition of the amine to the carbonyl group. In
strongly acid medium, the concentration of the amine becomes very low because
we get GNH, in excess amounts. In other words, the rate of the first step decreases
with increase in acidity. As G&H, is a poor nucleophile than GNH,.
The second step involves the elimination of water. In acidic medium concentration
of the protonated carbinolamine increases with the increasing acid concentration.
(Remember
- 0 H,
is a better leaving group than -OH).
An increase in acidity causes step 2 to go faster, but step 1 to go slower, while
decreasing acidity causes step 1 to go faster but step 2 to go slower. Between these
two extremes is the optimum pH (-34), at which the rate of the over all reaction is
greatest. At this pH, some of the amine is protonated, but some are free to initiate
the nucleophilic addition. At this pH, too, enough acid is present so that elimination
of water in the second step can proceed at a reasonable rate.
Reaction of Secondary Amines with Carbonyl compounds
Aldehydes and ketones with an a-hydrogen react with secondary amines to yield
iminium ions, which undergo further reaction to give enamines (vinylamines).
L,
II
CH, C H
+ (CH,),NH
D~cthylamine
-
2
,
0
I
1
I=+
I *+
CH2 -CH = N ( C H 3 ) ~
[rnrn~n~urn
)on
CH2
=CHN(CH3)2
T,nilm~ne
Since there is no proton remaining on nitrogen of this intermediate iminium ion, the
imine formation cannot occur. Instead an enamine is formed by loss of a proton from
a carbon atom p to the nitrogen-This results in the formation of a double bond
between a and p carbon atoms.
'Like imine formation, enamine formation is reversible, and enamines can be
converted back to the corresponding carbonyl compounds.
You must have noticed from the above that the mechanism of enamine formation is
similar to the mechanism of imine formation. Reaction of aldehydes or ketones with
primary and secondary amine may appear different but they are quite similar. Both
are typical examples of nucleophilic addition reaction in which the initially formed
tetrahedral intermediate is not stable. Instead, the carbonyl oxygen is eliminated and
a new carbon-nucleophile double bond is formed.
Coupling Reaction
Aromatic diazonium salts react with highly activated aromatic compounds like phenols
to give azo compounds. Azo compounds are coloured and many of these are used as
dyes and indicators. The reaction with phenols take place in slightly alkaline medium.
The reaction is also used for the detection of aromatic primary amines and is called as
dye test.
The reaction with diazonium salts is basically electrophilic substitution reaction. The
positiyely charged nitrogen of diazonium group acts as an electrophile and the reaction
proceeds as following.
I
SAQ 8
Write the mechanism of the following reaction.
H,C
\
CHCHO
I1,C /
+ CH3NHPh --+
H,C\
/(33
I-1,C / C = C H N \ ~ h
Organic Compounds
containing Oxygen and
Nitrogen Atoms
Chemistry of Carbon
Comp0un.d~
,
r
In this unit you have studied that
The cabonylcutmn of sp2hybridized and for three o bonds. The
unhybridizedp orbital for one 7c bond with oxygen atom.
Carbonyl compounds can be classified in two groups : i.e. i) aldehyde and
ketones ii) carboxylic acid and its derivatives.
Due to higher electronegativity difference between carbon and oxygen, the
carbon-oxygen double bond is polarised, and hence it cin undergo
electrophilic attack at oxygen or nucleophilic attack at carbon.
Electron withdrawing substituents increase the reactivity while electron
donating substituents decrease the reactivity of carbonyl con~poundstowards
nucleophilic addition reactions.
The importance and mechanism of Wittig reaction aldol condensation,
Parkin condensation, Claisen condensation, Cannizzaro reaction and Michael
addition reaction.
Carboxylic acid can be prepared by oxidation of alkenes, alcohols ahd
aldehydes.
Carboxylic.acids act as an acid as well as a base.
Using appropriate reagents carboxylic acids can be converted into esters, 2halo acids, alcohols and alkanes.
Amines can be prepared by alkylation of ammonia, by reduction nf imines and
by Hofmann and curtius rearrangements of amides.