Download ALCOHOLS

ALCOHOLS
Alcohols are compounds in which one of the hydrogen atoms of an alkane has been
replaced by a hydroxyl group. They have the general formula C nH2n+1OH. The
functional group (OH) gives alcohols their characteristic properties.
The first members of the series are:
CH3OH
methanol
CH3CH2OH
ethanol
CH3CH2CH2OH
propan-1-ol
CH3CH(OH)CH3
propan-2-ol
Ethanol Production
Ethanol is produced on an industrial scale by two separate and distinct processes:
the fermentation of carbohydrates and the hydration of ethene.
1. Fermentation
In fermentation, the enzyme zymase, which is present in the cells of yeast, converts
sugars (normally glucose) into ethanol and carbon dioxide.
C6H12O6
2C2H5OH + 2CO2
The reaction is sluggish at low temperatures, but if the temperature is raised too far,
the enzyme is denatured. The normal temperature range is 25 to 35 oC. The reaction
typically takes 3 to 5 days.
Glucose is used in solution at a concentration of about 15-20% by mass; if this is
exceeded, the ethanol produced in the reaction reaches a concentration sufficient to
kill the yeast before the fermentation is complete.
2. Hydration of ethene
This is the method currently favoured in the U.K. The hydration of ethene is carried
out by the direct reaction of ethene and steam. The conditions used are:



H
H
C
H
temperature: 300oC
pressure: 65 atmospheres
catalyst: phosphoric acid (H3PO4) adsorbed on celite
+
C
H 2O
H
TOPIC 12.15: ORGANIC CHEMISTRY – ALCOHOLS 1
H
H
H
C
C
H
OH
H
Comparison of the two methods
Fermentation
Rate of reaction
Type of process
Raw material
Purity of product
Technology
Labour
Hydration of ethene
slow
batch
renewable (sugars)
impure (dilute solution)
cheap, low-tech equipment
unskilled; more manpower
fast
continuous
non-renewable (ethene from oil)
pure
expensive, high-tech equipment
semi-skilled; less manpower
Ethanol as a biofuel
Ethanol is now being used more and more a fuel. Ethanol produced by the process of
fermentation can be mixed with petrol to produce a biofuel.
A biofuel is a fuel that has been made from plant matter that is
recently dead.
One problem with this approach is that land previously used to grow sugar cane for
the human food chain is now being used to grow sugar to make biofuels. However,
ethanol produced in this way is considered to be carbon neutral since all of the CO2
released during its combustion was removed during photosynthesis by the growing
crop. It can not be considered to be 100% carbon neutral, however, as making the
fertilizer to grow the sugar cane and fuelling agricultural machinery to harvest and
transport it, will invariably involve the burning of fossil fuels.
Classification of alcohols
Alcohols are usually classified as primary, secondary or tertiary. This classification
has value since the three categories of alcohols often behave differently in chemical
reactions.
OH
H
C
H
R
primary
OH
OH
R
C
H
secondary
C
R
R
R
(1o)
R
(2o)
TOPIC 12.15: ORGANIC CHEMISTRY – ALCOHOLS 2
tertiary (3o)
Oxidation of alcohols
When alcohols are heated with acidified potassium dichromate:
1. Primary alcohols are oxidised initially to aldehydes:
CH3CH2OH + [O]
ethanol
CH3CHO + H2O
ethanal
The aldehyde can be distilled out of the reaction mixture as it is formed. (See
practical: Experiment 12.15.1). If, however, the reaction is carried out under reflux,
the aldehyde is oxidised further to a carboxylic acid:
CH3CHO + [O]
CH3COOH
ethanoic acid
The oxidising agent changes colour from orange to green.
Cr2O72- + 14H+ + 6eorange
2Cr3+ + 7H2O
green
2. Secondary alcohols are oxidised to ketones, which are not oxidised further.
CH3CH(OH)CH3
propan-2-ol
+ [O]
CH3COCH3 + H2O
propanone
The oxidising agent changes colour from orange to green.
3. Tertiary alcohols are not oxidised. The oxidising agent stays orange.
The reaction with acidified potassium dichromate(VI) distinguishes tertiary alcohols
from primary and secondary alcohols.
Distinguishing between ketones & aldehydes
It is not easy to distinguish between primary and secondary alcohols, but their
oxidation products, aldehydes and ketones, can be differentiated. Aldehydes and
ketones both contain a carbonyl group (>C=O). In aldehydes, the carbonyl carbon is
at the end of a carbon chain, in ketones it is in the middle of a chain.
1. Tollen’s reagent
Tollen’s reagent is prepared by adding concentrated aqueous ammonia dropwise to
silver nitrate solution until the initial brown precipitate of silver oxide (Ag2O) just redissolves to give a clear, colourless solution. The solution contains the complex ion,
diamminosilver(I), [Ag(NH3)2]+.
When aldehydes are warmed with Tollen’s reagent, they reduce it to silver, which is
deposited as a mirror on the walls of the test tube. There is no reaction with ketones.
RCHO + 2[Ag(NH3)2]+ + 3OH-
RCOO- + 2Ag + 4NH3 + 2H2O
TOPIC 12.15: ORGANIC CHEMISTRY – ALCOHOLS 3
2. Fehling’s solution
Fehling’s solution comprises two solutions, A & B, which are mixed in equal volume
immediately prior to use. Solution A contains copper sulphate; solution B is a strongly
alkaline solution of potassium sodium tartrate. When solutions A and B are mixed, a
deep blue solution containing a complex copper(II) ion is formed.
When aliphatic aldehydes are warmed with Fehling’s solution, they reduce it to
copper(I) oxide, Cu2O, which appears as a brick red precipitate. There is no
reaction with ketones.
RCHO + 2Cu2+ + 5OH-
RCOO- + Cu2O + 3H2O
In both these reaction, the aldehyde is oxidised to the carboxylate anion.
Elimination reaction of alcohols
If alcohols contain a hydrogen atom on the carbon atom adjacent to the one to which
the OH group is attached, i.e. the partial structure:
H
they can be dehydrated to alkenes by heating to
about 180oC with a catalyst of concentrated
sulphuric acid or concentrated phosphoric acid.
(See experiment 12.15.2).
H
H
H
C
C
H
OH
C
OH
H
H
H
C
H
C
+
C
H 2O
H
The mechanism for this process is shown over the page. NB If the alcohol
undergoing dehydration is unsymmetrical, more than one isomeric alkene can be
produced.
TOPIC 12.15: ORGANIC CHEMISTRY – ALCOHOLS 4
Mechanism:
If a H is lost from the C
atom to the left of the
alcohol group (blue arrow)
If a H is lost from the C
atom to the right of the
alcohol group (red arrow)
+ H2O + H+
but -1- ene
but -2- ene
NB don’t forget the other products of this reaction are water (this is dehydration) and
H+ (which was the catalyst).
TOPIC 12.15: ORGANIC CHEMISTRY – ALCOHOLS 5