Transcript Chapter 7- Alcohols

Chapter 7- Alcohols
OH
O
R
S
H
R
H
Thiol
Hydroxyl group
Phenol- Aromatic alcohol
Nomenclature
e.g.
- OL ending
OH
OH
CH3-CH-CH3
(CH3)3COH
Cyclohexanol
2-propanol
tert-butyl alcohol
or
2-methyl-2-propanol
H2C
CH-CH2-OH
2-propen-1-ol
Alcohols- commercially important
Ethanol CH3CH2OH
- Produced by fermentation or hydration of ethylene
CH2=CH2 + H2O
H2SO4
CH3-CH2OH
Commercial EtOH 95%
H2O
5%
Can’t purify further by distillation
Absolute alcohol use CaO
To remove water
CaO + H2O
Ca(OH)2
Ethanol- Solvent, fuel, antiseptic, reagent
OH
H3C
CH3
2-propanol
Isopropyl alcohol (IPA)
> 1 million tons annually
OH
H2O
H+
Classification
R
R
RCH2OH
Primary 1o
R-CH-OH
Secondary 2o
R
C
OH
R
Tertiary 3o
Reactivity depends on the degree of substitution
OH
OH
Phenol
Cl
4-chlorophenol
or
p-chlorophenol
Hydrogen bonding


O
R
H
O more electronegative than H.
Bond is polarised
Molecules can interact via hydrogen bonds


R
H

H
H
H

O
O
O

O
Can form hydrogen bonded networks
R
R
H
R
O
R
H
O
R
Covalent OH bond ~ 480 kJ mol-1
Hydrogen bond ~ 20-40 kJ mol-1
Much weaker but has important effects
(i) Boiling points of alcohols are higher than would be predicted
based on molecular weight
- Extra energy required to break the intermolecular hydrogen
bonds
(ii) Lower molecular weight alcohols are soluble in water
CH3CH2OH
MW
46
B.P.
+78.5oC
CH3OCH3
46
-24oC
CH3CH2CH3
44
-42oC
Preparation of Alcohols
(i) Reduction of aldehydes, ketones, carboxylic acid derivatives
(ii) Addition of H2O to an alkene e.g. oxymercuration
OH
(1) Hg(OAc)2
(2) NaBH4
H2O
Markovnikov addition
Hydroboration-Oxidation
(1) B2H6
(2) H2O2
NaOH
OH
anti-Markovnikov addition
Intermolecular Hydrogen Bonding
OH
Ethan-1,2-diol
HO
Ethylene Gylcol




H

H

O
C
O
C
Hydrogen bond- weak, electrostatic
~ 10% of O-H bond strength
H
H
H
H


O
H
bond dipole
Alcohols are weak acids
ROH + B:
base
RO + BH
alkoxide
e.g
CH3CH2OH + NaH
CH3CH2OH
Na
(CH3)3COH + K
CH3CH2O Na + H2(g)
CH3CH2O Na + 1/2 H2(g)
(CH3)3CO K + 1/2 H2(g)
potassium tert-butoxide
pKa MeOH/EtOH ~16
Phenol
~10 (more acidic)
7.8 Dehydration of alcohols- alkenes
- Occurs by heating with a strong
acid
H
+
H2C
CH2
HC
CH
H
OH
+
H2O
o
180 C
H
OH
Example of an elimination reaction
- can be used as a route to alkenes
OH
CH3
H
H2SO 4
+ H2O
Heat
H3C
C
OH

CH3
Cyclohexanol
H2C
C
+
CH3
2-methylpropene
Cyclohexene
Fastest 3o > 2o > 1o
CH3
H+
Slowest
Rate affected by substitution
H2O
OH
H+
CH3
H3C
Heat
H
H
-H2O
CH3
or
H3C
CH3
H2C
Proton lost from either side of OH group
Usually the more substituted alkene is the major product
H3C
CH2
CH3
OH
H
H+
+
Heat
-H2O
Cyclohexanol
Trisubstituted alkene
- major product
Disubstituted alkene
Mechanism of dehydration
Protonation of hydroxyl group occurs first
This converts the OH group into a good leaving group (H2O)
H+
CH3-CH2-OH
H
CH3-CH2-O
H
H
O
H2C
H
H2O
C
H2
H
CH2=CH2 + H2O + H3O+
7.9 Reaction of Alcohols with Hydrogen Halides
Example of a substituted reaction (OH replaced by X)
Good synthetic route to alkyl halides
Rate depends on degree of substitution 3o > 2o > 1o
R-OH + H-X
R-X + H2O
alcohol
alkyl halide
Cl, Br, I
CH3
CH3
e.g.
H3C
C
OH
CH3
t-butyl alcohol
+ HCl
H3C
C
Cl
CH3
t-butyl chloride
+ H2O
Reaction of 1o alcohols with HCl can be very slow.
To increase rate add a lewis acid catalyst such as zinc chloride (ZnCl2)
CH3CH2CH2CH2OH + HCl
ZnCl2
Heat
CH3CH2CH2CH2-Cl + H2O
1-chlorobutane
There are special reagents which can also be employed e.g. thionyl chloride (SOCl2) or
phosphorus tri-bromide (PBr3)
O
CH3CH2OH + Cl-S-Cl
CH3CH2-Cl + HCl + SO2
By-products are gases- Pure alkyl chloride formed
OH
3
Br
+ PBr3
+ H3PO3
3
(or PCl3)
Usually use SOCl2 or PBr3 with 1o or 2o alcohols- reaction of 3o alcohols with H-X
is so fast these reagents are unnecessary
7.12 Alcohol Oxidation
Primary Alcohols
R-CH2OH
oxidising
O
R-C-H
aldehyde
agent
oxidising agent
O
R-C-OH
carboxylic acid
Typical oxidising agents:
(1) CrO3, H+ (H2SO4), acetone- Jones' reagent
(2) KMnO4 Potassium permanganate
With these strong oxidising agents it is not possible to isoalte the aldehyde
- only the carboxylic acid is obtained
Jones'
CH3CH2CH2OH
CH3CH2CO2H
reagent
propanoic acid
There are special reagents which allow oxidation to the aldehyde level with no further oxidation
to the carboxylic acid
e.g. Pyridinium chlorochromate (PCC)
CrO3 + HCl +
N
pyridine
N
H
CrO3 Cl
O
CH3CH2CH2OH
P CC
CH3CH2CH
propanal
Secondary Alcohols
Ketones
e.g.
H
OH
O
Jones'
reagent
cyclohexanol
cyclohexanone
OH
O
CrO3
H3C
CH3
2-propanol
H+, acetone
H3C
CH3
2-propanone
(acetone)
Tertiary Alcohols- Not oxidised under usual conditions for alcohol oxidation
CH3
Jones'
H3C
C
OH
no reaction
reagent
CH3
No H on carbon which can be removed
Thiols: S (sulfur) analogues of alcohols
e.g.
SH
CH3-S-H
methanethiol
thiophenol
Thiols are more acidic than the alcohols
RS Na + H2O
R-S-H + NaOH
pKa ~ 10
Sodium thiolate
Thiolate anions - good nucleophiles
e.g. CH3CH2-Br + CH3S Na
CH3CH2-S-CH3 + NaBr
methylethyl sulfide