Download Alcohols General formula R-OH hydroxyl group Nomenclature

Alcohols
General formula R-OH
hydroxyl group
Nomenclature
IUPAC: name as for alkane dropping the -ane and adding -anol. Indicate the position of the hydroxyl
group with a number. Number the longest alkane chain beginning at the end nearer the hydroxyl group.
e.g.
Trivial name
IUPAC name
CH 3 OH
methanol
methyl alcohol
CH 3CH 2 OH
H
ethanol
ethyl alcohol
CH 3 C
CH3
2-propanol
isopropanol
C OH
triphenylmethanol
trityl alcohol
OH
CH 3CH 2
3-methyl-3-hexanol
H 3C C OH
CH 2CH 2CH 3
1
2
3
HOCH2CH=CH 2
CH 3CHOHCH=CHCH 3
OH
2-propen-1-ol
allyl alcohol
3-butene-2-ol
cyclopentanol
CH 2CH 2OH
2-phenylethanol
H 3C
OH
HOCH2CH 2OH
HOCH2CHCH 2OH
OH
cis-3-methylcyclopentanol
1,2-ethanediol
ethylene glycol
1,2,3-propantriol
glycerol
Properties of Alcohols
Boiling Points
B.Pt.oC
-161.5
-33
100
19
M.W.
16
17
18
20
Compound
CH 4
NH3
H 2O
HF
B.Pt.oC
-161.5
-24
64.5
-88.6
-42
-24
M.W.
16
50.5
32
30
44
46
Compound
CH 4
CH 3Cl
CH 3OH
CH 3CH 3
CH 3CH 2CH 3
CH 3OCH3
The low molecular weight alcohols, with 4 carbons or less, are water soluble. The hydrocarbons and aryl
halides are not.
There are several types of intermolecular bonding: London or dispersion forces, dipole-dipole interactions
and hydrogen bonding. Hydrogen bonding is the strongest type of intermolecular bonding. Hydrogen
bonds exist between molecules having an H attached to a strongly electronegative atom such as O, N, or F.
These forces are present in water and can build up a 3-D network.
δ + δH O
δ+H
δ+ δδ+ δH O H O
+
Hδ
δ+H
+
+
δ
δ
δ- O H δ- O H
δ- δ+
+
Hδ
δ+H
O H
Hδ+
Alcohols can also hydrogen bond in this way.
etc.
δO
δ+ δH O
δ+H
δ+ δH O
R
R
R
-
δO R δ
δ+H O R δ
+
O Hδ
δ+H
R
These hydrogen bonds have to be broken before the molecules break apart to form vapour and so they
account for the relatively high boiling points.
Synthesis of Alcohols
Grignard reactions
Generalized reaction
O
C
+
carbonyl group
RMgX
ether
Grignard reagent
OMgX
C
R
addition
complex
H3O+
Preparation of Grignard reagent
ether
R-X + Mg
RMgX
X = Br or I
Mechanism of the addition of Grignard reagents to the carbonyl group
δO
OMgX
δ+
+
δδ
MgX
+
C
C
C
C
OH
C
R
+
Mg+X-
Formation of primary, secondary and tertiary alcohols by the Grignard Reactions
Primary alcohols
O
ether
C
+ RMgX
H H
formaldehyde
e.g.
CH 3 CH CH 3 +
Mg Br
O
C
H
OMgX
H
C
OMgBr
CH 3
C CH
H
H CH 3
ether
H
H
C
H
R
primary alcohol
H
R
OH
H 3O+
OH
H3O+
CH3
C CH
H
H CH3
2-methyl-1-propanol
Secondary alcohols
O
ether
C
+ R'MgX
R H
OMgX
C
R
R'
aldehyde
CH 3CH 2MgBr + CH3
ether
CH 3
H
R
C
H
R'
secondary alcohol
H
e.g.
O
C
OH
H3O+
OMgBr
H 3O +
C H
CH2CH 3
CH 3
OH
C H
CH 2CH3
2-butanol
Tertiary alcohols
R
e.g.
O
C
R'
+
R"MgX
R
C
R"
H3O+
OH
R
C
R'
R"
tertiary alcohol
R'
OH
OMgBr
MgBr
O
+
ether
CH3 CCH3
O
MgBr
and
OMgX
ether
C
H3C C
CH3
H 3O+
H 3C C
CH3
OH
C
+
benzophenone
triphenylmethanol
Triphenylmethanol is an interesting compound. When treated with acid, it forms a very stable carbocation
which has 10 resonance structures.
Hydration of Alkenes
Acid-catalyzed addition of water to an alkene
General reaction
C
C
H 2SO4
+ H2 O
C
OH
e.g.
CH 3
CH 3 C=CH2
CH 3-CH-CH3
75%
CH 3
H 2SO4
+ H2 O
follows Markovnikov rule
get both syn and anti addition
OH
H 2SO4
CH3CH=CH2 + H2 O
H
C
CH3-C-CH 3
OH
85%
Oxymercuration-demercuration reaction
General reaction
C
C
O
+ H2 O + Hg(OCCH 3 )2
mercuric acetate
C
+ NaBH4
OH HgOCOCH 3
O
e.g. CH 3
1) Hg(OCCH 3)2
C
O
THF
+ HOCCH 3
C
acetic acid
OH HgOCOCH 3
C
+ OH H 3C
C
OH H
C
follows Markovnikov rule
get both syn and anti addition
OH
2) OH-, NaBH 4
90%
O
1) Hg(OCCH 3)2
CH3CH 2CH=CH2
2) OH-, NaBH 4
OH
CH 3CH 2CHCH3
95%
In oxymercuration-demercuration reactions, acid is not required so that carbocations are not formed and
rearrangement does not occur
CH 3
CH 3-C-CH=CH 2
CH 3
H 3O +
O
CH 3
+
CH 3C CHCH3
CH 3
1) Hg(OCCH 3)2
2) OH-, NaBH 4
CH 3
CH3-C-CH-CH3
+
CH 3
CH 3 OH
CH3-C CH-CH3
CH 3
94%
CH 3
CH3-C CH-CH3
OH CH 3
70% rearranged
product
Hydroboration - oxidation
This is a very important reaction since it gives Anti-Markovnikov addition and both the H and the OH add syn
C
C
+ BH3
δ+
C
C
H B
H δ-
Because there is a cyclic compled formed, both
B and H add to the same side of the double bond.
H
H2O2, OH-
C
OH H
C
e.g.
+
BO3 -
H2O2, OH-, 0o C
3 CH3CH=CH2
+
BH3
B-(CH2CH2CH3)3
3 CH3CH2CH2OH
tri-(1-propyl) borane
1-Propanol
Notice the different products from hydration and hydroboration of an alkene
OH
H2O, H2 SO4
H3C C CH3 Markownikov addition
H3C C CH2
CH 3
CH3
CH3
1) BH3
2) H2O 2, OH-, 0o C H C CH CH OH
3
2
CH3
OH
H 3C
anti-Markovnikov addition
H2O, H2 SO4
1) BH3
2) H2O2, OH-, 0o C
H3C
H
OH
syn addition, stereospecific reaction
very useful
Hydrolysis of alkyl halides
This reaction was studied in the context of SN 1 and SN2 reactions.
Reactions of Alcohols
Reactions with active metals
i.e., the hydrogen of the OH is slightly acidic
2RO-M+ + H2
2ROH + 2M
M = Li, Na, K fast reaction
Mg, Ca, Al very slow reaction
2CH 3CH2-Na+ +
e.g. 2CH3CH2OH + 2Na
2 H 3C
CH3
C OH
CH3
CH3
C O-K+
CH3
2 H3C
+ 2K
H2
+ H2
potassium t-butoxide
CH3
6 H3C C
H
OH
2 Al
+ 2Al
O
CH3
CH CH3
3
+ 3H2
Dehydration to Alkenes
This reaction has been studied in the synthesis of alkenes
H3O+
H2O +
C
C C
heat
H OH
Mechanism:
OH
C
C
+ H3 O+
H
C
H
+
OH2
C
slow
fast
fast
C
fast
+
OH2
C
C
+ H2 O
H
C
H
+
C
fast
slow
C
C
+ H3 O+
Note that all of the reactions are reversible. Therefore the alkene has to be removed from the solution as
rapidly as it is formed. The slow rate-determining step is the formation of the carbocation. Tertiary
alcohols react more rapidly than secondary which react more rapidly than primary.
e.g.
CH3
CH2
20% H2SO4
+ H2 O
H3C C CH3
H3C C OH
90oC
CH3
CH3
CH2
Dehydration follows the
H3C OH
Saytzeff rule - the most
H 3PO4 , 120oC
+ H2 O highly substituted alkene
+
is formed as the major
product. This is also the
OH
more stable alkene.
83%
very small amount
o
H3PO4 , 200 C
+ H2 O
Conversion to Alkyl Halides
HX
RX
R-OH
or PX3
+
H2 O or
P(OH)3
This is one of the most important methods of synthesis of alkyl halides
OH
Br
e.g.
+
100oC
HBr
+
H2 O
cyclohexanol
CH3
H3C
+
C OH
CH3
Mechanism
H3O+
CH3
H3C
C
CH3
H3C
and
CH2Cl
+
HCl
benzyl chloride
benzyl alcohol
but
CH3CH2OH
+
C Cl + H2 O
CH3
i.e. SN 1 reaction
Clnotice that another reaction
of carbocations is loss of H+
to yield an alkene. In this
example, the nucleophile Clis in large excess and so the
formation of the alkyl halide
is favored.
H3C
CH3
+
OH2
CH2OH
CH3
25oC
HCl (conc)
C+
CH3
The benzylic cation is more stable than
a tertiary carbocation.
no reaction since the chloride ion is not a strong enough
nucleophile
HCl
HCl - ZnCl2
+
CH3CH2 OH
HOZnCl + CH3 CH2Cl
ZnCl
ClPrimary alcohols and methyl alcohols react with phosphorus trihalides by an SN2 reaction
CCl4
CH3CH2CHCH3 + PBr3
CH3CH2CHCH3
80oC
OH
Br
CCl4
CH3CH2CH2CH2Cl
80oC
Using PBr3 there is no carbocation formed and so we do not get rearrangements and this is usually the
preferred reagent for converting alcohols to alkyl bromides.
CH3CH2CH2CH2OH
+
Mechanism
R-CH2-O-H + Br
P Br
Br
This is an SN 2 type of reaction
PCl3
Br+
R-CH2-O-P-Br2
H
R-CH2Br
+
HOPBr2
Oxidation of Alcohols
Primary alcohols
RCH2OH
+ CrO3
chromic
oxide
O
R C H
aldehyde
pyridine
RCH2OH + K2 Cr2O 7
H 2SO4
Secondary alcohols
H
R C R'
O
R C R'
ketone
H2SO4
+ K2 Cr2O7
OH
Tertiary alcohols
R"
R C R' + K2 Cr2O 7
N
pyridine
O
R C OH
carboxylic acid
H 2SO4
no reaction
black tar if the temperature is high enough!
oxidation would require breaking of C-C bond
OH
Examples
CH 2OH
Primary
CH3CH2CH2OH
1-propanol
CrO3/pyridine
O
CH3CH2C H
CrO3/pyridine
0oC
benzyl
allcohol
H 2SO4
Secondary
H 3C CH CH 3 + K2 Cr2O7
2-propanol
OH
+ K2 Cr2O7
H 2SO4
Cr3+
green
O
H3C C CH 3
2-propanone (acetone)
O
H 2SO4
cyclohexanone
Unsaturated
H 2C
CH CH 2 OH
CrO 3/pyridine
0oC
O
benzaldehyde
CH3CH2CH2COOH
butanoic acid
This reaction was the basis of the breathalyzer test: Cr2 O7 =
orange
OH
C
0oC
propanal
CH3CH2CH2CH2OH + K2 Cr2O 7
H
O
H2C
CH C
H
propenal
Using a stronger oxidising agent would oxidise the double bond
by ethanol oxidation