An alcohol is any compound with an OH group
(alcohol group) attached to single bonded
hydrocarbons (alkanes).
alcohol group alcohol general structure
The four most common alcohols are:
CH3OH
methanol
(methyl
alcohol)
CH3CH2OH
ethanol
(ethyl
alcohol)
CH3CH2CH2OH
1-propanol
(propyl
alcohol)
OH
|
CH3CHCH3
2-propanol
(isopropyl
alcohol)
Ethanol, when fermented from sugar, is the alcohol
in beverages. It can also be made from ethene by
the addition of water for nonbeverage use, like an
additive to gassoline to make "gasahol." 2-Propanol
(better known as isopropyl alcohol) is in (with
some water) rubbing alcohol. It is also used in
gasoline to prevent freezing of the gas line in
automobiles by keeping excess moisture dissolved
in the gasoline.
IUPAC Names of Alcohols
The parent chain of the alcohol must be the longest
that includes the carbon holding the OH group.
Give the -OH group the lower location number on
the chain regardless of where alkyl substituents
occur. Name the alkane attached to the OH group
and replace the -e with an -ol. For example:
H
|
H H H-C-H H
|| | |
H-C-C---C---C-H
|| | |
HO H H
|
H
Parent chain: butyl
-OH group location: 2
Substituents locations: 3-methyl
Alkane name: 3-methylbutane
Alcohol name: 3-methyl-
2-butanol
Ethers
Ethers are basically two alkyl groups joined to one
oxygen. For example:
CH3OCH3
dimethyl
ether
CH3CH2OCH2CH3
diethyl ether
CH3OCH2CH3
methyl ethyl
ether
Dimethyl ether was the "ether" once used as an
anesthetic. The simple ethers above have very low
boiling points because hydrogen bonds do not exist
between neighboring molecules. Becuase of this,
the boiling point of ethers are much lower than
comparable alcohols. For example, 1-butanol
(CH3CH2CH2CH2OH) boils at 117 °C, much higher than
its isomer, diethyl ether, which boils at 34.5 °C.
Major Reactions of Alcohols and Ethers
Ethers act very much like alkanes. Like alkanes,
they burn and are split apart when boiled in
concentrated acids. But the alcohols have much
reactivity.
Oxidation Reactions of Alcohols
To study the oxidation of alcohols, they must be
subclassified into the following:
H
|
R-C-O-H
|
H
primary
alcohol
R'
|
R-C-O-H
|
H
secondary
alcohol
R'
|
R-C-O-H
|
R''
tertiary
alcohol
Only primary and secondary alcohols are oxidized
by oxidizing agents. When an alcohol is oxidized,
an H attached to the alcohol carbon is removed as
H-, and the H atom of the OH group leaves as H+.
The two H become part of a water molecule with
an O provided by the oxiding agent.
Primary alcohols are oxidized to aldehydes. The
net ionic equation for this when a dichromate ion is
used as an oxidizing agent is:
3RCH2OH + Cr2O72+ + 8H+ ==> 3RCH=O +
2Cr3+ + 7H2O
For example, this is the oxidation of 1-propanol
into propanal:
3CH3CH2CH2OH + Cr2O72+ + 8H+ ==>
3CH3CH2CH=O + 2Cr3+ + 7H2O
Since the boiling point of propanal is much lower
than 1-propanol, it is boiled out of the solution as it
forms. If it is not permitted to leave, it will further
be oxidized into propanoic acid (CH3CH2COOH)
since its tendency to be oxidized is greater than
alcohol's.
Secondary alcohols are oxidized to ketones. For
example, the oxidation of 2-propanol gives
propanone (more commonly called acetone):
CH3
CH3
|
|
3C-OH + Cr2O72- + 8H+ ==> 3C=O +
2Cr3+ + 7H2O
|
|
CH3
CH3
Since ketones resist oxidation, they do not have to
be removed as they form.
Tertiary alcohols are not oxidized except by chainbreaking reactions, because they have no
removable H atom on the alcohol carbon.
Dehydration Reactions of Alcohols
In a strong acid and heat, alcohols can undergo
dehydration, losing a water molecule and levaing
behind a carbon-carbon double bond. For example:
CH2-CH2 ==> CH2=CH2 + H2O
| |
H OH
ethanol
ethene + water
==>
+ H2O
cyclohexanol ==> cyclohexene + water
Not only do chemists know it works, they also
want to find out and know how it works. The
examples above were elimination reactions. The
thing that makes them possible is the protonaccepting ability of the oxygen atom of the OH
group, and therefore, will react with concentrated
strong acids. The following is the mechanism to
how ethanol is changed into ethene using sulfuric
acid as a catalyst:
CH2-CH2 + H2SO4 <==> CH2-CH2+ + HSO4- <==> CH2CH2+ + H2O + HSO4- <==> CH2=CH2 + H2O + H2SO4
| |
H OH
| | <- (weak bond) |
H O
/\
H H
H
Substitution Reactions of Alcohols
In acidic conditions, the OH group of an alcohol
can be replaced by a halogen atom. For example:
CH3CH2OH + HI ==> CH3CH2I + H2O
ethanol + hydrogen iodide ==> iodoethane + water
(ethyl iodide)
+ HCl ==>
+ H 2O
cyclohexanol + hydrogen chloride ==>
chlorocyclohexane + water
The mechanism for substitution reactions like the
ones above are:
H+
/
R-OH + H+ ==> R-O
\
H
Once the OH group has been protonated (added
H+), the bond between the carbon and oxygen is
weakened, allowing a halide ion to displace the
H2O.
*By;Lama Naseer Al-Zakari