Transcript Alcohols and ethers

Alcohols and ethers
© E.V. Blackburn, 2011
Structure
R-OH alcohol
R = alkyl group (substituted or unsubstituted)
H
R C OH
H
R'
R C OH
H
1o
2o
R'
R C OH
R"
3o
© E.V. Blackburn, 2011
Sterols - the steroid ring
system
12
3
11
D 16
1
C
9
14
A
B
7
5
HO
cholesterol
© E.V. Blackburn, 2011
Physical properties of alcohols
Alcohols are noticeably less volatile; their melting points are
greater and they are more water soluble than the
corresponding hydrocarbons having similar molecular
weights.
These differences are due to the OH group which renders a
certain polarity to the molecule. The result is an important
intermolecular attraction:
- + - + O H O H O H
R
R
R
the hydrogen bond
~ 21 - 25 kJ/mol
© E.V. Blackburn, 2011
Solubility of alcohols
Low molecular weight alcohols are water soluble:
- + - + - +
O H O H O H
R
H
R
© E.V. Blackburn, 2011
Spectroscopic properties
IR:
Unassociated alcohols show a fairly sharp absorption near
3600 cm-1 due to O-H stretching. Associated alcohols
(hydrogen bonded) show a broad absorption in the 3300 3400 cm-1 range.
© E.V. Blackburn, 2011
Fermentation
Fermentation of sugar by yeast gives C2H5OH.
Methanol is added to denature it.
© E.V. Blackburn, 2011
Azeotropic mixtures
The bp of ethanol is 78.3C whereas that of water is
100C (at least on Vancouver’s waterfront!). Can we
separate a mixture by distillation?
No! An azeotropic mixture forms!
An azeotropic mixture is one whose liquid and vapor
forms have identical compositions. The mixture
cannot be separated by distillation.
eg C2H5OH (95%) and H2O (5%) - bp 78.13C
H2O (7.5%), C2H5OH (18.5%) and C6H6 (74%) - bp
64.9C
© E.V. Blackburn, 2011
Oxymercuration
© E.V. Blackburn, 2011
Hydroboration
1. (BH3)2
H3C
2. H2O2/OH-
3HC
H
syn
addition
H OH
trans-2-methylcyclopentanol
© E.V. Blackburn, 2011
Reactions of alcohols
The reactions of alcohols involve one of two processes:
• breaking of the O-H bond
• breaking of the C-O bond
© E.V. Blackburn, 2011
Reactions involving O-H bond
breaking
RO- + M+ + 1/2 H2
R-OH + M
CH3CH2OH
Na
CH3CH2O- Na +
sodium ethoxide
© E.V. Blackburn, 2011
Reactions involving C-O bond
breaking - dehydration
H
C C
OH
OH
H3PO4

+ H 2O
H3PO4/
© E.V. Blackburn, 2011
Reactions involving C-O bond
breaking - dehydration
H+
C C
H
OH
C C
H
OH2
+
B:
C C
H
OH2
+
C C
H +
C C
H +
+ H2O
C C
E1 mechanism
© E.V. Blackburn, 2011
Reaction with hydrogen
halides
ROH + HX
HX:
RX + H2O
HI > HBr > HCl
ROH: 3 o > 2o > 1o < CH3HBr or
CH3CHCH 3
OH
NaBr/H 2SO4
CH3CHCH 3
Br
© E.V. Blackburn, 2011
Experimental facts
1. The reaction is acid catalyzed
2. Rearrangements are possible
CH3 H
CH3 H
HCl
H3C C C CH3
H3C C C CH3
Cl H
H OH
3. Alcohol reactivity is 3o > 2o > 1o < CH3OH
© E.V. Blackburn, 2011
The mechanism
H+
C C
H
OH2
+
C C
H
OH
C C
H
OH2
+
C C
H +
C C
H +
+ H2O
C C
H
X
-
X
SN1
© E.V. Blackburn, 2011
Reaction of primary alcohols
with HX
1. ROH + HX
1o
+
2. ROH2 + X-
+
X
ROH 2 +
+
X R OH2
RX + H2O
SN2
HX:
HI > HBr > HCl
This reflects nucleophile strength in a protic solvent.
© E.V. Blackburn, 2011
Reactions with phosphorus
halides and with thionyl
chloride
ROH + PX3
RX + H3PO3
SN2
RCH2Cl + SO 2 + HCl
-
Cl
Creates a good leaving group from 1o and 2o alcohols.
© E.V. Blackburn, 2011
Tosylates
O
CH3CH2OH + Cl S
CH3
O
p-toluenesulfonyl
chloride
B: H O
+
O S
H3CH2C O
CH3
-
H +O
O S
H3CH2C O Cl
CH3
O
H3CH2CO S
CH3
O
ethyl p-toluenesulfonate
a tosylate
© E.V. Blackburn, 2011
Why form tosylates?
Sulfonate ions are excellent leaving groups:
Nu:
O
C O S
O
CH3
Nu C
O
O S
O
CH3
© E.V. Blackburn, 2011
Ethers
Structure:
R-O-R, Ar-O-R, or Ar-O-Ar
nomenclature
Name the two groups bonded to the oxygen and add
the word ether.
CH3CH2OCH2CH3 - diethyl ether
© E.V. Blackburn, 2011
Nomenclature of ethers
O
diphenyl ether
CH3OCH=CH2
O
CH(CH 3)2
CH3CH2CH2CHCH2CH3
|
OCH3
methyl vinyl ether
isopropyl phenyl ether
3-methoxyhexane
© E.V. Blackburn, 2011
Nomenclature of cyclic ethers
Use the prefix oxa- to indicate that an O replaces a CH2
in the ring.
oxacyclopropane
O
O
ethylene oxide
oxacyclopentane
tetrahydrofuran
O
1,4-dioxacyclohexane
O
1,4-dioxane
© E.V. Blackburn, 2011
Williamson synthesis
CH3Br + Na
+
CH3
O C CH3
CH3
CH3
H3C O C CH3
CH3
A primary halide is necessary to ensure an SN2 reaction
and not an E2 elimination.
© E.V. Blackburn, 2011
Reaction of ethers
O
CH3
HX
OH
+ CH3X
What is the mechanism?
© E.V. Blackburn, 2011
Reaction of ethers
1-Ethoxybutane reacts with HI to give a mixture of 1iodobutane, iodoéthane, ethanol and1-butanol
whereas 2-ethoxy-2-methylpropane gives a mixture of
2-methyl-2-propanol and iodoethane. Explain.
© E.V. Blackburn, 2011
Epoxides
H3C
H
O
H
R
O OH
CH3 a peroxy
acid
H3C
H
O
H3C
H
trans-2,3-dimethyl
-oxacyclopropane
(E)-2-butene
syn addition
O
Cl
OOH
m-chloroperoxybenzoic acid
© E.V. Blackburn, 2011
Mechanism
+O
H
O
R
O
O R
O +
O
H
an oxirane
© E.V. Blackburn, 2011
Reactions
O
CH3OH/H
H
+
H
CH2OH
OCH3
What is the mechanism?
© E.V. Blackburn, 2011