ALCOHOL
By Puan Azduwin Khasri
6th NOVEMBER 2012
INTRODUCTION
Reaction of Alcohol
OXIDATION
SUBSTITUTION
ELIMINATION
(DEHYDRATION)
Others
ALKYL
HALIDE
SULFONATE
ESTER
KETONE
ALDEHIDE
CARBOXILIC
ACID
ALKENE
Nucleophilic substitution of Alcohol
An alcohol has a strongly base leaving group (HO-) therefore
alcohol cannot undergo a nucleophilic substitution reaction
Convert the strongly basic leaving group (OH–) into the good
leaving group, H2O (a weaker base):
Primary, secondary, and tertiary alcohols all undergo
nucleophilic substitution reactions with HI, HBr, and HCl:
5
SN1 REACTION OF ALCOHOL
Secondary and tertiary alcohols undergo SN1 reactions
with hydrogen halides:
6
Look out for rearrangement product in the SN1
reaction of the secondary or tertiary alcohol:
7
SN2 REACTION OF ALCOHOL
Primary alcohols undergo SN2 reactions with
hydrogen halides:
8
When HCl is used; SN2 reaction is slower, but the
rate can be increased using ZnCl2 as catalyst .
ZnCl2 functions as a Lewis acid that complexes
strongly with the lone-pair electrons on oxygen:
9
CLASS EXERCISE 1
Give the major product of each of the following reactions:
Other Methods for Converting Alcohols
into Alkyl Halides
Utilization of phosphorus tribromide:
PYRIDINE
Other phosphorus reagents can be used:
PBr3, phosphorus tribromide
PCl3, phosphorus trichloride
PCl5, phosphorus pentachloride
POCl3, phosphorus oxychloride
Activation by SOCl2
Pyridine is generally used as a solvent and also acts
as a base:
12
Summary: Converting of Alcohols to
Alkyl Halides
Recommended procedures:
14
Converting Alcohols into
Sulfonate Esters
15
Several sulfonyl chlorides are available to activate OH
groups:
ELIMINATION REACTION OF ALCOHOL
(DEHYDRATION)
Dehydration of alcohol requires acid catalyst and
heat
Dehydration of Secondary and Tertiary Alcohols by
an E1 Pathway
17
17
Mechanism of E1 Dehydration of an
Alcohol
The major product is the most stable alkene product:
The most stable alkene product has the most stable
transition state
19
The rate of dehydration reflects the ease with which the
carbocation is formed:
20
Look out for carbocation rearrangement:
21
Pinicol Rearrangement
Protonate alcohol:
Eliminate water:
Rearrange carbocation:
Deprotonate:
H3 C
CH3
H 3C
Resonance-stabilized
oxocarbocation
CH3
O
22
Ring Expansion and Contraction
Mechanism for this
reaction:
•Protonate the alcohol.
•Eliminate water.
•Rearrange carbocation
to afford the more
stable cyclohexane ring.
•Deprotonate.
23
Primary Alcohols Undergo Dehydration by
an E2 Pathway
24
A Milder Way to Dehydrate an Alcohol
25
Oxidation of Alcohols
Oxidation by chromic acid:
Secondary alcohols are oxidized to ketones
26
Primary alcohols are oxidized to aldehydes and
eventually carboxylic acids:
Mechanism:
The oxidation of aldehydes to acids requires the presence
of water:
In the absence of water, the oxidation stops at the
aldehyde:
PCC, a methylene chloride–
soluble reagent:
No water present
28
A tertiary alcohol cannot be oxidized and is
converted to a stable chromate ester instead:
O
O Cr O
O
No hydrogen on
this carbon
Di-tert-Butyl Chromate
29
ETHER
Nucleophilic substitution reaction of
Ether
Ethers, like alcohols, can be activated by protonation:
Ether can undergo nucleophilic substitution with HBr and
HI only (HCl cannot be used because Cl- too poor
nucleophile
Ether cleavage: an SN1 reaction:
Ether cleavage: an SN2 reaction:
Reagents such as SOCl2 and PCl3 can activate alcohols
but not ethers
Ethers are frequently used as solvents because only they
react with hydrogen halides
33
Nucleophilic Substitution
Reactions of Epoxides
Acidic condition;
HBr:
Aqueous acid:
Reaction of an epoxide in different substituent
Regioselectivity:
Mechanism:
35
Neutral or Basic condition:
When a nucleophile attacks an unprotonated epoxide,
the reaction is a pure SN2 reaction:
Therefore:
36
Epoxides Are Synthetically
Useful Reagents
Enantiomers
CLASS EXERCISE 2
Give the major product of the following reactions:
THE END