3. Reactions of alcohols
Alcohols contain an –OH group covalently bonded
to a carbon atom.
However, this –OH group does not behave in the
same way as the hydroxide ion OH–
because NaOH
is a base and CH3OH is not.
Alcohols, when dissolved in water, do not alter the
pH of the water.
Although the hydrogen atom is connected to an
oxygen atom, alcohols do not readily donate the
proton (they are weaker acids than water).
4. Reactions of Alcohol
Like the alkanes and alkenes, alcohols
undergo complete combustion in a plentiful
supply of oxygen gas, producing only carbon
dioxide and water as products.
You may have used alcohol burners in an
investigation in chapter 6.
The complete combustion of ethanol is as
follows:
5. When balancing an equation for the
combustion of an alcohol it is important to
remember that there is an oxygen atom in the
alcohol, unlike alkanes and alkenes
6. Oxidation of ethanol
The process of oxidation was defined in chapter 10
as the loss of electrons.
In organic chemistry oxidation is easily recognized
as the gain of oxygen or the loss of hydrogen from a
compound.
The oxidation reactions of alcohols vary,
depending upon the type of alcohol involved.
Primary, secondary and tertiary alcohols all give
different reactions with strong oxidizing agents such
as acidified potassium dichromate(VI) solution or
acidified potassium manganate(VII) solution.
7.
8. Alcohol oxidizes to carboxylic acid
In the laboratory, when an aqueous solution
of a primary alcohol such as ethanol is mixed
with potassium dichromate (VI) and sulfuric
acid, and the mixture heated under reflux, the
alcohol is fully oxidized to a carboxylic acid.
9. During the process, the alcohol is initially oxidized to
an aldehyde; however, by heating under reflux the
aldehyde is further oxidized to a carboxylic acid.
When the reaction is ‘complete’, the condenser is
turned around and the reaction mixture is distilled to
collect an aqueous solution of the carboxylic acid.
If the aldehyde is the desired product during this
reaction, then the reaction can be carried out at
room temperature and the aldehyde can be distilled
off from the mixture.
Some distillation apparatus is shown at the
beginning of this chapter (p. 341).
10.
11. The oxidation reaction of the primary alcohol
(e.g. ethanol) to a carboxylic acid may be
represented simply by an equation in which
the symbol [O] represents the oxygen
supplied by the oxidizing agent:
12. Alternatively, we may write half-equations
and a complete equation to represent the
redox nature of the reaction:
13. A secondary alcohol has the hydroxyl group
on a carbon that is bonded to two other
carbons. Propan-2-ol and butan-2-ol are
examples of secondary alcohols.
14. Secondary alcohol oxidation
When secondary alcohols are oxidized,
ketones are formed.
This reaction is very similar to the one in
which aldehydes are produced, but the
placement of the hydroxyl group results in the
production of a ketone rather than an
aldehyde and, ultimately, a carboxylic acid.
15. The oxidation reaction of a secondary alcohol
such as propan-2-ol to a ketone may be
represented simply as:
16. Alternatively, we may write half-equations
and a complete equation to represent the
redox nature of the reaction:
17. When these oxidation reactions are
performed in a laboratory investigation, the
change in colour of the oxidizing agent
indicates that the reaction has proceeded.
Potassium dichromate, K2Cr2O7, changes
colour from orange (Cr2O72–) to green (Cr3+
)
during this reaction. If potassium
manganate(VII), KMnO4, is used instead, it
changes colour from purple to colourless.
18. Tertiary alcohols, those with the hydroxyl
group bonded to a carbon atom that is
bonded to three other carbon atoms, are not
easily oxidized.
An example of a tertiary alcohol is 2-
methylpropan-2-ol.
