Download Organic Chemistry Structures of Organic Compounds

Organic Chemistry
You will need to buy a separate book “Organic Notes” Purchase from Mrs. Marston
Why study Organic Chemistry?
Organic chemistry is the chemistry generally practiced by living things. For biological efficiency, need
readily available atoms and light atoms.
The chemistry is primarily based on C, H, N and O; also some P, Cl, Br, S (and some transition
elements in enzymes)
At the moment ~ 1 x 107 organic compounds described in the literature either natural or unnatural
Why is carbon so great? Its multiple bonding structures allow a great deal of complexity from a few
different elements.
Carbon can form stable C-X, C=X, C≡X bonds, X = C, O, and N. No other element can do this.
Thus, many different structural possibilities exist.
In the organic chemistry section, we are going to learn only 21 reactions and no new conceptual ideas.
Structures of Organic Compounds
Carbon is tetravalent - 4 covalent bonds. Carbon is usually positively charged
109.5o
sp3 hybridized
Polarity
very weak
very strong
Bond
δδC-Hδδ+
C-C
δ+
C-OδC-Nδδ+
C-Clδδ+
Bond Energy (kJ/mol)
410
350
350
300
335
polar
strongly polarized
δ+
H-ClδδN-Hδ+
430
400
Weak
O-O
I-I
140
140
Nomenclature
Naming compounds – this is less important except for communication (it’s a pain, but less difficult than
referring to “that thing with the thingy hanging off it.”
Types of Compounds
Bond Type
Class
E.g
CH, CC
Alkanes
H3C-CH3
ethane
CH, CC, CN
Amine
H3CH2CNH2
aminoethane
CH, CC, CO
Alcohol
H3CCH2OH
ethanol
CH, CC, CO
Ether
H3CCH2OCH2CH3
ethyl ether
CH, CC, CX
X=Cl, Br, I
Alkyl Halide
BrCH2CH2Br
1,2-dibromoethane
C-C, C=C, CH
Alkene
H2C=CH2
ethene
C-C, CH, C=O
Aldehyde
O
C
H 3C
H
acetaldehyde
C-C, CH, C=O
O
Ketone
C
H 3C
CH3
(2-propanone)
C-C, C≡C, CH
Alkyne
HC≡CH
acetylene, ethyne
C=C, CH,
Aromatic
benzene
C=O, C-O, C-C, CH
O
Carboxylic Acid
C
H 3C
OH
acetic acid
Alkane (CnH2n+2) Nomenclature: Structural Isomers
Number of Carbons
Root
Number of Isomers
1
2
3
Meth
Eth
Prop
1
1
1
H2
C
H 3C
4
But
CH3
C
H 3C H
5
6
7
.
10
.
40
H2
C
CH3
2
Pent
Hex
Hept
3
5
9
Dec
75
CH3
H 3C
62,491,178,805,831
H3C
-H
→
H3C-CH2-H
→
H3C-CH2-CH2 - H
Take the previous analogue, replace H with CH2H to get higher homologue.
3D Structures of Molecules
Depends on the available orbitals. For bonding, first look at the atomic orbitals:
First, establish the
Bonding no double / triple bonds
H
H
H
C
e.g. CH4
H
C
H2
CH3
geometry
for C (and all 2nd row elements)
4 atomic orbitals
1xs
3xp
2 rules
(2s)
spherically symmetric
2 px
x
axis
2 py
y
axis
2 pz
z
axis
i) Hund’s rule - leave isoenergetic electrons unpaired
ii) maximize electrostatic repulsion (i.e., separate electronic pairs as much as possible)
The carbon in CH4 has 4 bonds (one to each H). We need to use all four atomic orbitals to make the 4
molecular orbitals (4 SIGMA σ orbitals). Mix 1 x 2s + 3 x 2p and get 4 x sp3 orbitals. In a tetrahedral
compound, the 4 groups will be separated by about 109.5° - this is the normal geometry for carbon.
H
H
H
C
109.5o
H
Alkenes
Use ethene as an example; each carbon has 3 bonds (1 x C, 2 x H). Need 3 atomic orbitals to give 3
molecular orbitals. 2s + 2px + 2py → 3 x sp3 σ orbitals. (Note no z-coordinates, just 3 substituents in the x-y
plane).
How to maximize repulsion? Separate by 120°.
H
H
C
H
120 o
C
H
We have used 3 of the 4 electrons on carbon, the last electron is in a pz orbital these combine, on adjacent carbons, to make a pi bond (π bond).
These combine to give a π-bond
H
H
C
H
C
H
C
H
H
H
H
H
H
C
H
C
C
H
Alkynes
Same idea as above, but only 2 substituents for the σ orbitals. Only 2 atomic orbitals needed (1 x 2s, 1
x 2px) → 2 molecular orbitals 2 x sp2).
H
C
C
H
180o
e.g., ethyne
Note, there is still one electron in each of the py and pz orbitals (→ total of 2 σ bonds and 2 π bonds on
each carbon)
These combine to give a π-bond
py
H
C
C
H
+
H
C
C
H
pz
H
C
C
H
Other atoms participate in the same type of hybridization. See the examples below.
sp 2
sp 3
O
N
H
sp 3
sp 2
H
O
H
N
sp
More Alkane Nomenclature
Rules for naming compound
1)
take longest linear chain (this gives “root”)
2)
number the molecule from one end to get lowest substitution numbers
3)
name and number substituents
4)
if more than one substitutent, use di, tri, tetra 5)
arrange substituents in alphabetical order (excluding prefixes such as di, tri, i.e., triethyl
precedes methyl)
Nomenclature
other functional groups
The groups that are arranged in alphabetical order are: halogens (Cl chloro, Br bromo, I iodo), NH2
amino, CN cyano, NO2 nitro, and alkyl groups: methyl CH3 (Me), ethyl CH3CH2 (Et), propyl (Pr)
C
CH3
H
CH3
CH3CH2CH2, isopropyl (iPr)
Rotational isomers
Bond rotation along σ-bonds takes place readily at room temperature. However, not all “twisted”
structures are of equal energy. Generally, the most stable structures have big groups as far away from each
other as possible. The repulsion of groups are called van der Waals interactions.
Let’s look first at a simple structure – ethane.
60 o
H
H
H
H
H
H
Staggered
0o
H H
HH
H
H
Eclipsed
Your eye
H
H
H
H
H
H
H
H
H
H
H
H
When there are more groups, the situation is a little more complex
0o
CH3
H3 CCH
3
HH
H
60o
H
Eclipsed
H
CH3
H
HH
H
CH3
H CH3
120 o
H
H
H
H
CH3
180 o
H
CH3
H
Staggered
GAUCHE
Eclipsed
CH 3
CH3
Your eye
H
H
H
H
Staggered
ANTI
CH3
H
H
H
CH3
H
CH 3
H
H
H
CH3
CH 3
H
H
H
H
H
CH 3
Other functional groups take a different priority (highest priority at top, lowest at bottom and then the
“alphabetical groups” after that.
1
O
Carboxylic Acid
Always C1
1
C
H 3C
2
OH
O
Ketone
2-propan “one”
(acetone)
C
H 3C
3
CH3
O
Aldehyde
C
H 3C
ethan “oic acid”
(acetic acid)
propan “al” (1propanal)
1
H
4
Alkene
H3CHC=CH2
5
Alkyne
H3CC≡CH
1-prop “ene”
(double bond
starts at carbon 1)
1-propyne (triple
bond starts at
carbon 1)
1)
lowest # most important - find longest chain, arrange substituents in alphabetical order and
lowest substituent #
e.g.
CH3
C H CH3
H 3C H C
Cl
longest chain = 4
= butane
2)
numbers,
if same numbers, irrespective of which end you begin with, choose alphabetical and lower
i.e., 2-chloro-3-methylbutane
not
hendecagon
3-chloro-2-methylbutane
Br
Cl
C
C C
H 3C H C
H
H2 CH3 2
CH3
∴
4-bromo-2-chloro-4-methylhexane is correct
3-bromo-5-chloro-3-methylhexane is not: the lowest number is higher in this name
Order of preference for naming
Highest
R
RCOOH
carboxylic
acid
>
RCHO >
R
> R3C-OH > RNH2 > R2C=CR'2 > RC≡
≡ CR'
O
aldehyde ketone
alcohol
amine
then come other substituents: halo, methyl, nitro, etc.
* note that this order is opposite in some books
Other Functional Groups
ETHER CH3CH2OCH3
ethyl methyl ether
AMINE CH3NHCH2CH3
(CH3)3N
ethylmethylamine
trimethylamine
NITRO NO2
CH3CH2NO2
nitroethane (1-assumed)
NITRILE C≡N
CH3CN
ethanitrile
alkene
alkyne*
O
ESTER
H 3C
R
O
CH3CO2CH3 methyl ethanoate
Cyclic Alkanes (CnH2n)
Just add cyclo to name.
cyclopropane
cyclobutane
cyclopentane
cyclohexane
Substitutents on cyclic systems – geometric isomers.
H 2C
HC
H2
C
H2
CH3
C
H 2C
CH2
HC C
H
CH 3
CH2
CH
CH3 CH3
CH 3 CH3
CH3
CH3
cis-1,2-dimethylcyclopentane
trans-1,2-dimethylcyclopentane
SAME SIDE of plane defined by ring
OPPOSITE SIDES
Alkenes
ANE → ENE
Naming ends is ene
H2
C
C
H2
H
C
C
H
H2
C
ethane
H3C-CH3
→
ethene
H2C=CH2
CH3
3 - heptene
longest chain with double bond in it - gives double bond lowest possible number
i.e., 3-pentene not 4-pentene
NB
bond strength of double bond (~267 kcal/mol) less than bond strength of single bond (350 kcal/mol)
Therefore C=C is more reactive than C-C. However, for a given structural isomer, there may be two
geometric isomers that are not interconvertible @ RT (you would have to break the bond to interconvert
them).
GEOMETRIC ISOMERS
If the groups with highest atomic number are on the same side - Z-isomer (zusammen), if on opposite
sides – E-isomer (entgegen)
H
C
H
C
H 3C
CH3
H
C
CH3
C
H
H 3C
Z-isomer
E-isomer
If 2 identical atoms go to next atom in chain; next structure is Z-1-bromo-2-pentene (put starting carbon
of alkene at lowest number of chain)
Br
H2
C
H2
C CH3
C
H
C
H
Cyclic Alkenes
cyclobutene
4-methylcyclopentane
Triple bond – Alkyne
ANE → YNE
Cl
H 3C
C
C
Cl
CH
CH 3
4-chloro-2-pentyne
Alcohol
ANE
→
ANOL
yne ending
Br
Br
OH
C
H 3C H C
H2
OH
CH 3
4-bromo-2-pentanol
Ketone
ANE
→
ONE (“OWN”)
Cl
O
ONE has precedence over other groups listed above
3-chloro-2-butanone
Aldehyde
ANE
→
ANAL
4-methylpentanal
H
O
Carboxylic Acids
→
ANE
ANOIC ACID
ETHANOIC ACID (acetic acid)
3-BROMO-PROPANOIC ACID
O
O
OH
Br
Summary of Formulas & Isomers
OH
1.
Molecular Formula
C4H8
C3H8
These are clearly different
2.
Structural isomers: Same molecular formula – different arrangement of groups, Stereoisomers
have different properties
i.e., boiling point, melting point, etc.
e.g., C4H9Cl
2-chlorobutane 1-chlorobutane
Cl
Cl
Cl
Cl
2-chloro-2-methylpropane
1-chloro-2-methylpropane
3.
Stereoisomers – geometric isomers
Cyclic alkanes
Alkenes
Cl
Br
Cl
Br
cis-1-bromo-2-chlorocyclopropane
E-2-pentene
trans-1-bromo-2-chlorocyclopropane
4.
Rotational isomers
0o
3
H
60o
CH3
H3 CCH
HH
E-2-pentene
H
Eclipsed
H
CH3
H
HH
H
CH3
H CH3
120 o
H
H
H
H
CH3
180 o
H
CH3
H
Staggered
GAUCHE
Eclipsed
CH 3
CH3
Your eye
H
H
H
H
Staggered
ANTI
CH3
H
H
H
CH3
H
CH 3
H
H
H
CH3
CH 3
H
H
H
H
H
CH 3
Alkanes: Properties and Reactions
CnH2n+2
b.p.
m.p. Increasing London
Forces (going down table)
CH4
-164 -182.5
C2H6
-88
-183
C3H8
-42
-190
C5H12
36
-130
C10H22
174
-30
Polyethylene
burns
140
(C 100H202)
Tg~20
the chemistry of parent defines chemistry for the series
Homologous series each “homologue” 1 CH2 more
Natural gas = methane and some ethane, a little propane
Petroleum (black gold, texas tea)
Distillation gives
Natural gas
pet ether
Ligroin
light naptha
Gasoline
Kerosene
Loading oil
oil general paraffin nor. Asphalt
C4
C5 - C6
C7
C5-C9
C6 - C12
C12-C15
C15-C18
C16 - C20
< 20o
30-60
60-90
85-200
200-300
300-400
>400
Alkanes – other natural sources - “fart” produced in anaerobic bacterial decomposition (e.g., cow
stomach (blue angels))
also found in salt mines, coal mines
Reactions of Alkanes
As we already saw, not very reactive
1)
2)
strong bonds C-C, C-H
not polar
C - C no polarization
C - H small polarization
hard to start reactions. Reactions generally happen at “functional group (C=O, C-N, C≡C, etc.)
Alkanes - cyclo alkanes “paraffin” means unreactive
H2S O4
No reaction
KMnO4
No reaction
Na
No reaction
To decompose Na use alcohol in parafin – only the alcohol reacts
2Na + 2 CH3CH2OH
1
→
2 CH3CH2ONa+ + H2
Halogenation with Cl2, or Br2
H
H
H
Br 2
Br
+
HBr
This is a radical chain reaction – doesn’t work in the dark
3 parts
Initiation, Propagation, Termination
hν
Br 2
2 Br
Initiation
endothermic
H
Br
H
H
+
HBr
Br
+
STEP 1
Propagation
endothermic
H
H
+
Br 2
Br
STEP 2
net reaction exothermic
Termination
2 Br
H
Br 2
+
H
H
H
+
Br
Br
Let’s look at a simpler system
CH4 + Cl2 → CH3Cl + HCl
∆Hrxn = -104 kJ mol-1
1) Cl2 → 2 Cl•
∆H1 = +243 kJ mol-1
2) Cl• + CH4 → CH3• + HCl
∆H2 = +4 kJ mol-1
3) CH3• + Cl2 → CH3Cl + Cl•
∆H3 = -108 kJ mol-1
But for bromination
CH4 + Br2 → CH3Br + HBr
∆Hrxn = -34 kJ mol-1
1) Br2 → 2 Br•
∆H1 = +192 kJ mol-1
2) Br• + CH4 → CH3• + HBr
∆H2 = +66 kJ mol-1
3) CH3• + Br2 → CH3Br + Br•
∆H3 = -100 kJ mol-1
ex othermic
Note that chlorination is mildly endothermic in the first step of propagation (step 2) whereas bromination
is quite endothermic. In the second step of the propagation, both are exothermic. The overall reaction rate is
dependent upon the activation energy in the slowest step (step 2). The Ea for chlorination is much lower that
for bromination, which one might predict from the overall enthalpies of the steps.
The overall reaction with Cl2 faster than Br2
If excess halogen, e.g., Cl2, more chlorination
ie. CH4 →
→
CH3Cl
CH2Cl2
But for I2 step 1 very endothermic + 200 kJ/mol
∴ reaction very slow so not useful
For F2, steps 1 and 2 are very exothermic step a ~ -144 kJ/mol → explosive reaction
∴ use other reagents to make F- alkanes (CoF3, SF4)
If one uses unsymmetical alkanes, there are different types of CH bonds. Depending on which bond
reacts, different products are formed. The preference depends on the strength of the C-H bonds and on the
number of hydrogens of a given type. In general, effects from both factors are observed.
The bond strengths of CH bonds depend on the number of carbons connected to the central carbon.
Reactant
Products
H3CH
H3CCH2H (Bold carbon is primary)
(H3C)2CHH (Bold carbon is secondary)
(H3C)3CH (Bold carbon is tertiary)
H3C•
H3CC•H2 a primary radical
(H3C)2C•H a secondary radical
(H3C)3C• a tertiary radical
The ease of forming a carbon radical (and the order of highest stability) is
3° (tertiary) > 2° (secondary) > 1° (primary) > methyl
Recall for chlorination (and bromination)
RH
+
Cl•
→
R•
+
HCl
endo (slow)
R
+
Cl2
→
RCl
+
Cl•
exo
What happens in a molecule with both types of hydrogens – both happen
Bond Dissociation
kJ mol-1
426
405
397
376
H2
C
H 3C
H2
C
k1
CH3
CH2
H3 C
Cl
Cl2
H3C
H2
C
C
H2
Cl
a PRIMARY alkyl radical
k2
Cl
H 3C
Cl2
CH
CH3
H 3C
H2
C
CH2
Cl
a SECONDA RY alkyl radical
The rates are proportional not only to the bond strength of the CH bond being broken, but also on the
statistical number of hydrogens. The bond strength is the most important factor. (i.e., generally k2 > k1)
Rate of reaction via 1° CH ∝ k1 [CH3CH2CH2CH3][Cl•] x fn 6 H’s
Rate of reaction via 2° CH ∝ k2 [CH3CH2CH2CH3][Cl•] x fn 2 H’s
Where fn is some fractional effect of arising from the statistics.
The product ratio between these two products will be rather similar to k1 / k2
The actual ratio of products is 1° alkyl halide 45%, 2° alkyl halide 55%
2 Alkanes - Combustion
This is the fundamental reaction of the 20th century
CnH2n+2
→
(3n +1)/2 O2
H4C + 2 O2 →
CO2
→
+ 2 H2O + ∆
e.g., cigarette lighter C4H10 + 6.5 O2
Alkane
CH4
H3CCH3
C4H10
→
4CO2 + 5H2O
∆H combustion (kJ/mol)
213
373
687
656 NOTE Ring strain
(C 4H8)
C5H12
nCO2 + (n+1) H2O + heat
845
793
(C 5H10)
4.1 A
Alkyl Halides ? Haloalkanes
Chemistry controlled by bond polarity
δδδ+
C-Hδδ+
C-XδX = F high dipole moment → X = I lowest polarity
∴ alkyl halides have higher m.p. & b.p.’s than related alkanes that only have London forces
CH4
b.p.
-164 °C
H2CCl2
b.p. 35 °C
H3 C
H2
C
Lewis acids attack here:
X
C
H2
H +, Ag+, BF3
Lewis bases (nucleophiles) attack here:
-OH,
:NH 3, -SH, -CN
Preparation of Alkyl Halides
1 Haloalkane Preparations; From alkanes (Review)
Seen above
H
Cl
Cl2
hν or ∆
3 From alcohols,
OH
2-propanol
HBr
Br
a substitution reaction
4 Addition to alkene
Br
Br 2
very fast
Br
Reactions of Alkyl Halides
5 Nucleophilic Substitution
NC-
+
NC
I
+
I
Many examples of substitution reactions. (For OH- need dilute solutions, see below)
6 Elimination of HX
I
+
hot
+
conc.
HO-
I
+
H2 O
base
H
7 Reactions with Group 1 or 2 metals Li / Mg
δ+
Mg
δBr
δ- δ+
MgBr
O
ether
= Organometallic
SN2 Reactions (5)
NaI
+
Br δδ+
2-propanone
I
+
NaBr (ppt)
Kinetics process of process - d[EtBr]/dt = k[EtBr]1[I-]1
Second order – bimolecular; these kinetics are observed for MeX, 1° RX (RCH2X) and 2° RX
(RR'CHX) BUT NOT FOR 3° RX (RR'R"CX)
Called SN2 “substitution nucleophilic bimolecular”
Go from 1 isomer to a different isomer; i.e., Inverted stereochemistry at the reaction centre
Most nucleophilic substitutions take place this way.
NCBr
NC
O
trans
cis
Mechanism of the SN2 Reaction
_
HO-
H
H
HO
C
H
H
Br
C
H
H
Br
H
HO
C
H
+
Br
H
The SN1 Reaction – another substitution reaction
As we saw above, 1° and 2° alkyl halides mostly undergo SN2 reactions. For 3° alkyl halides,
however, need very polar solvents and non-basic nucleophiles to observe nucleophilic substitution. However,
the kinetics are different.
(CH3)3CBr
+
-
N3
Azide
faster
in
→
polar
solvent
Rate Law (Experimental !) Rate = k[RBr]1
(CH3)3C-N3
a first order reaction; implies the overall reaction is not an elementary step (that the rate detemining step
doesn’t need a nucleophile).
slow
Br
1
2
ionization
+
C
+
-
N3
C
+
+
Br
N3
Why doesn’t the SN2 happen with 3° alkyl halides? Two reasons – i) Steric reasons (the space the
nucleophile needs, to attack the carbon, is occupied by other groups). The alkyl groups block backside
attack. ii) the 3° cation is sufficiently stable that another reaction pathway exists.
Order of stability of carbocations (just as we found for radicals):
3° ((CH3)3C+) > 2° ((CH3)2C+H) > 1° (H3CC+H2) > H3C+
Why? The alkyl groups help stabilize the cation by donating electronic charge. Smearing out charge is
energetically favourable.
One can accelerate the rate of these reactions using Lewis acids. Silver removes the chloride in tertiary
chlorides to help form the cation. This is a classic Lewis acid: Lewis base reaction,
Cl
+
Ag
+
+
C
Ag
Cl
Ag
Cl
ppt
Nu
Nu
This is a useful chemical test for 1° 2° and 3° alkyl halides
Some nucleophilic substitutions
H2O
OH
Cl
+
HCl
water is nucleophile and solv ent
alcohol
EtOH
O
Cl
+
HCl
alcohol is nucleophile and solv ent
ether
HOOH
Cl
Cl
Cl
alcohol
NC-
CN
RNH2
N
R
+
R
nitrile
KOH
H
N
R
amine
R
Cl
ammonium salt
Summary of Nucleophilic Substitution
SN2 relative rate
Methyl
CH3X
30
1°
C-CH2X
1
2°
C2CHX
0.03
3°
C3CX
1 x 10-6
SN1
H3C+ never see
C-CH2+ almost
never see
1-10% of SN2
rate
100%
Elimination (6)
-
We saw above that OH + an alkyl halide gives an alcohol (an SN2 reaction). This is true only if COLD
DILUTE OH (KOH, NaOH) is used. If HOT CONCENTRATED (e.g., > 3M) is used, a second order
ELIMINATION occurs to give an alkene.
OHH
HOT
H2O
Br
+
CONC
KOH
In this reaction, the HO- is acting as base, not a nucleophile.
The attack of at the carbon is slower than the attack of the base at the H
-
Rate Law; Rate = k[RX][OH ]; called E2 (elimination bimolecular)
Bond Making
Br
H
H
H
R
R
H
H
H
H
R
H
H
HObond breaking
Generally the more carbon groups on a double bond, the more stable it is: Saytzeff’s rule
Cl
+
NaOH
heat
+
major
Bromo-3-methylbutane
minor
3 methyl butane
Organometallics (7)
We can completely alter the electronic distribution in a molecule by converting an alkyl halide into an
organometallic compound.
H3Cδ -Iδ + Mg → H3Cδ--Mgδ+-I methylmagnesium iodide
+
-
Br
Li
ether
+
+
Li
LiB r
These are carbanions (very strong bases and nucleophiles). This is one of the few reactions we learn in
year 1 from which C-C bonds can be formed. THIS IS AN IMPORTANT REACTION!
8 Reaction of Organometallics with water (the carbanion is a strong base)
δ- δ+
MgBr
+
H
OH
ether
H
+
HOMgB r
pKa ca. 50
pKa 15.5
9 Reaction of Organometallics with ketones or aldehydes (IMPORTANT C-C bond formation
#1)
δ-
δ+
MgBr
+R
δ+ δO
+
H3O
ether
R
R
R
OMgBr
R
R
ketone
OH
alcohol
10 Reaction with CO2 (IMPORTANT C-C bond formation #2)
+
δ-
δ+
MgBr
+O
δ+ δO
H3O
ether
O
OMgBr
pH ca. 2
O
OH
c arboxylic acid
Alkenes
Preparation of Alkenes
From Haloalkanes (6)
CH3 OK
Br
HEAT
H
1-bromo-3-methylbutane
3-methyl-1-butene
This is the E2 mechanism we described above
11
From alcohols
an alcohol + an dehydrating acid (H2SO4, H3PO4)
OH
+
∆
distill
H3PO4
catalyst
cyclohexanol
b.p. 156 °C
+
catalysts
H2O
cyclohexane
b.p. 82 °C
We shall discuss this below.
Alkene Reactivity
Dominated by π bonds
π bond ∴ energy (~ 267 kJ/mol strength of the double bond) more reactive than a σ -bond (310
kJ/mol); It is a Lewis base - attack by E+ on π-electrons, i.e. In plane of screen above or below π-bond
H
H
R
H
H
H3O
+
H
H
C
R
R
+
H
H
R
H
H
H
X
X
H
R
H
H
H
NOT
H
R
H
+
C
H
H
X
H
H
H
R
H
H
X
These are extremely reactive almost as reactive as the metal
CHECK HYPERCHEM
Fats: in the body: triglycerides
O
O
O
O
O
O
beef fat
lard
human
leving
corn
olive
C12
0
0
1
0
0
0
C14
0
1
3
5
1
0.1
C16
27
24
27
14
10
7
C18
14
9
8
3
3
2
1
Oleic
C18
49
47
48
0
50
84
2
linoleic
C18
2
10
10
0
34
5
3
limolenic
C18
0
30
0
0
Other important alkenes: squalene → cholesterol; β-carotene → retinal (vision)
β-selinene, celery oil
myrcene, bay leaf oil
α-pinene, cedar leaf oil
Alkenes may exist in two different geometric isomers (see above) (Z- and E-)
Z-2-butene
E-2-butene
This is the source of black/white vision
Opsin
H
H
O
N
BODY
hν
H
Vitamin A
BODY
OH
N
H
β-carotene
Alkene Reactions
12 Electrophilic additions
Remember that the order of cation stability is:
3° ((CH3)3C+) > 2° ((CH3)2C+H) > 1° (H3CC+H2) > H3C+
additions to alkenes usually proceed via the most stable cation.
H
H
HX
C
X
+
X
+
X = OH, need H as catalyst
X = B r, Cl, I
get both cis and trans addition
Bromination
This is a special case of electrophilic addition.
H
Br
R
H
H
R
Br2
Br
R
Br
+
Br
H
R
Br
-
Can also use ICl (I+ Cl )
Only get trans addition
OH
I
Cl
+
ICl
+
H
H2O
+
C
Cl
I
H
H
Br2
Cl
Cl
Br
H
Br
+
Br
Br
+
Br
Br
Why do Br2 and ICl add trans but HCl adds randomly: The answer: Br and I are Lewis bases
13 Reduction
In organic chemistry, addition of H’s or removal of oxygen is “reduction”. The removal of hydrogens or
addition of oxygen is “oxidation”. Conversion of C=C to HC-CH is, therefore, a reduction
D
H2
D
D
Pt catalyst
H
H
D
cis-addition of H2, the reaction happens at the solid surface of Pt
Process used commercially to “hydrogenate” fats.
Oleic acid
O
H
m.p. 4 oC
O
Pt
H2
O
H
m.p. 70 oC
O
stearic acid
Stearic acid is a saturated fat;
Not so good for you. Better for your health are polyunsaturated fats; they are more easily processed.
Alcohols & Ethers
Unlike the functional groups we have seen so far, alcohols and ethers (to a lesser extent) are polar
molecules. In the case of alcohols, there is strong H bonding and reasonably large dipole moments.
CH 3OH
O
H
O
H
H
O
H
O
pKa ca. 16
methanol
consumption
can lead to
blindness
ethanol:
in beer, wine, liquor
2-propanol (isopropanol)
rubbing alcohol
Alcohols Preparation
SN2 of alkyl halides with hydroxide (5)
I
+
HO
dilute
-
OH
+
cold
I
Hydration of alkenes (12)
OH
H2SO4
+
H 2O
Dilut e
H 2O
Addition of organometallics to aldehyde or ketone (8)
H
δ+
δ- δ+
MgBr
Mg
δBr
O
H
OMgB r
O
ether
+
H3 O
H
OH
electrophilic
C
CH3I + 2 Li
Nucleophilic
C
→
CH3Li + LiI
From electronegativity, the polarization (and reactivity) of a ZM bond, M = Na, Li, Z = first row
elements follows the reactivity
CH3 > -NH2 > -OH > -Cl
-
-
(NOTE: This is consistent with our discussion of acidity. It’s harder to make the H3C than Cl )
Any reaction that converts one of the compounds on the left to one on the right will be
thermodynamically favoured.
e.g.,
CH3Li
+
H-OH →
CH3-H + LiOH
Less stable
More stable
CH3MgBr
H-OH →
+
CH3-H +
BrMgOH
Other C-C Bond Forming Reactions
δ-
δ+
MgBr
+R
R
δ+ δO
H3O
ether
R
R
+
OMgBr
R
R
ketone
OH
alcohol
Note: CAN’T DO THE FOLLOWING REACTION.
δ-
δ+
MgBr
+
R
R
Br
ether
R
R
H
alkyl halide
Ethers are less polar than the alcohols (No OH’s for H-bonding)
They are made in a similar fashion to alcohols
Preparation of ethers (5 SN2)
+
Br
+
CH 3O- Na
O
+
NaB r
CH 3
14 Making Alkoxides (Reducing Metals)
2
OH
+
2Na
2
ONa
+
H2
Alkoxides are strong bases (stronger than hydroxide) and also good Lewis bases or nucleophiles
Recall: 2HO-H + 2K
→
2HO-K + H2 ↑
Reactions of Alcohols
Preparation of alkyl halides (3)
Alcohols & halogen acids
OH
+
e.g. HCl, HBr, HI
Br
HBr
+
strong acid
H2 O
weak acid
Note that the reaction doesn’t work under basic conditions
OH
Br
+
Br
weak base
+
HO-
strong base
+
Na
The mechanism
H
OH
+
+
HBr
O
Step 1
+
H
H
+
O
Step 2
H
+
SN2
Br
Br
Alcohol & Dehydrating Acid (11)
e.g.
80% H2SO4, or H3PO4 is required
H
O
H
H 2SO4
- H 2SO4
H
+
O
+
C
H
H
HSO4
H
+
H 2O
+
H2 O
Br
cyclopentanol
Elimination
15 Alcohols + Oxidizing Agents
One can use inorganic salts to oxidize (remove H’s or introduce oxygen) onto organic molecules.
eg.
CrO3
or
Chromium IV or
K2Cr2O7/H2SO4
Na3Cr2O7/H2SO4
≡ H2CrO 4
Chromic acid
or
KMnO 4 (MnVII)
O
Cl
+
N
Or “Organic Chromium Salts, like pyridine chlorochromate (PCC)
1o Alcohol
(a)
with PCC
OH
C - CH2OH
PCC
CH 2Cl 2
H
removes this H
(b)
with
O
Ethanol (acetaldehyde)
K2Cr2O7/H+ or KMnO 4
H
OH
H
+
KMnO 4
O
H
2o Alcohol + Any oxidant
OH
H
2-propanol
O
H
H
+
+
Na2 CrO7
H2SO 4
O
2-propanone
Cr O
OH
KMnO4
O
3o Alcohol
no reaction at normal temps, No H-COH to oxidize!
Aldehydes and Ketones
Preparation
Oxidation of 1o Alcohol (15)
OH
H
+
PCC
CH2 Cl2
O
H
2-methylpropanol
2-methylpropanal
Oxidation of 2o Alcohol (15)
H
+
KMnO4
O
OH
3-methyl-2-butanol
3-methyl-2-butanone
Reactions of Aldehydes and Ketones
Nucleophilic Addition
The carbon in C=O is electron-poor, an electrophile.
δ+
δO
O
Y
+
ionic
X
δ- δ+
X Y
OR
O
X
covalent
Y
Examples
Organometallic + Ketone (8)
aldehyde → secondary alcohol
ketone → alcohol
Ph
δ+
δBr
Mg
δ- δ+
MgBr
O
Ph
OMgB r
O
ether
+
H3 O
Ph
OH
16 Reduction
Ph
with NaBH4
NaBH 4
O
Ph
O
CH3 OH
H
17 Reduction with H2 and a Pt catalyst
NOTE: This is exactly the same as addition of H2 to an alkene
Ph
Ph
H2
O
18
Pt-catalyst
EtOH solvent
O
H
Reaction with N-Compounds
H
N
e.g., hydrazine H2NNH2; 2,4-dinitrophenylhydrazine
O 2N
NH2
NO2
; hydroxylamine H2NOH
O
+
H 2NNH2
+
H 2NNH2
N
in Et OH
O
O
O
+
N
H2
H
+
N
+
N
H2
+
NH 2
H2 O
NH2
OH
NH2
NH
NH2
N
NH2
NH2
+
H2 O
19 Oxidation - Aldehydes
Occurs very easily, while can use strong oxidants such as K2Cr2O7/H+ or KMnO 4
Can also use weak oxidizing agents
eg. Ag
O
O
H
OH
KMnO4
+
+
MnO2
Another more relevant oxidant – how to make a mirror
OH
O
HO
H
Ag
HO
HO
HO
HO
HO
HO
NOTE: Ketones + oxidants
OH
O
HO
HO
+
HO
OH
O
+
HO
HO
Ag
mirror
HO
No reaction at normal temps
Carboxylic Acids
Structures: Pure glacial acetic acid
Methanoic acid (formic acid)
O
O
OH
H
OH
Ethanoic acid
(acetic acid - 5% soln
in H2O is vinegar)
The Structure Type
O
R
Y
Is very common
R = any alkyl or aryl (aromatic)
e.g.
R
O
O
O
R
NH2
Amides
O
R
OMe
Esters
Cl
O
Acid chlorides acetic anhydride
Preparation of Carboxylic Acids
Organometallic + CO2 (10)
δ-
+
Li
δ+
O δ-
+
Li
O
ether
C δ+
O
O
+
H 3O
pH 2
OH
O
(2-Methylpropyl)lithium
Aldehyde + most oxidants (19)
H
O
2-phenylethanal
H3O
+
O
+
OH
Na2 Cr 2O7
O
2-phenylethanoic acid
(Phenyl acetic acid)
1o Alcohol + Any oxidant
H H
OH
+
OH
KMnO4
O
ethanol
+
MnO2
ethanoic acid (acetic acid)
20 Reaction of -COOH with an Alcohol → Esters
Need a strong acid catalyst
OH
+
O
H
H3O
O
+
+
H2 O
O
OH
ethanoic acid
ethyl ethanoate (ethyl acetate)
(this is related in mechanism to reaction 18)
Reaction is a Nuc. Add followed by an elimination
(2)
Acidity pKa≈ 3- 5
O
OH
+
+
H2 O
+
H3 O
O
O
ethanoic acid
(acetic acid)
-two O atoms and therefore the O-H bond is weakened
moreover, there is resonance through which the charge is dispersed.
O
O
O
O
(Boy have we already covered this)
Compare with Alcohols
O
OH
+
H2 O
Only one O to share electrons pKa ≈ 16-18
+
+
H3 O
NOTE: an alkane C-H is an unbelievably weak acid, nothing to stabilize the charge
CH4
→
ie. pKa of x 50
CH3-
+
H3O+