AS Organic Chemistry - Alcohols


Alcohols - Nomenclature

(1) Prefix :

Take the alkane (or alkene) name that corresponds to the correct number of carbon atoms (see naming alkanes in the alkanes page) and remove the final e from the name.

(2) Position number :

Count the number of carbon atoms from the nearest chain end to where the hydroxyl group, -OH, is attached. Put hyphens around this number and place it after the prefix.

(3) Suffix :

Finally the suffix ol is added to denote an alcohol.

Examples -

ethanol - propan-2-ol - 2-methylpropan-2-ol -

Alcohols - Preparation of ethanol

(1) Fermentation :

This basic and ages old procedure involves the action of zymase (yeast, an enzyme) on an aqueous solution of sucrose (a simple sugar). The by-products of the consumption of the sugar by the yeast are carbon dioxide and ethanol. The carbon dioxide simply bubbles out of the solution, leaving behind a weak aqueous solution of ethanol.

C6H12O6(aq) 2CH3CH2OH(aq) + 2CO2

This weak solution of ethanol in water can be made more concentrated by the process of (fractional) distillation,


(2) Hydration of ethene :

The fermentation of sucrose doesn't produce enough ethanol to be commercially viable, so a different process is needed to produce the volumes of ethanol needed for industry.

The process of hydration is where the elements of water (2 H atoms and 1 O atom) are added to a molecule. In this particular case water can be added to ethene to produce ethanol, with no other by-products. It is not a straight forward process and requires a high temperature and high pressure to generate the steam. The reaction also uses a catalyst of sulphuric acid to speed up the whole process.

CH2=CH2 + H2O CH3CH2OH

Alcohols - Reactions of ethanol

(1) Combustion :

Ethanol (along with all organic compounds) burns in excess oxygen to give carbon dioxide and water as the only products.

CH3CH2OH + 3O2(g) 2CO2(g) + 3H2O(g)

N.B.: Less oxygen is needed to ensure complete combustion of an alcohol, than a corresponding alkane, because it contains an oxygen atom already.

(2) With sodium metal :

Just as with water, sodium metal will react with ethanol, though not nearly as violently. The products are hydrogen gas (exactly the same as with water) and sodium ethoxide (different to the reaction with water ),

2CH3CH2OH + 2Na(s) 2CH3CH2ONa + H2(g)

The hydrogen gas produced does not ignite, as the enthalpy change of reaction is not high enough. This is because the oxygen atom in water has two hydrogen atoms attached to it and only one in ethanol.

(3) Oxidation :

Refluxing ethanol with oxidising agents such as acidified manganate(VII)(aq) and acidified dichromate(VI)(aq) ions changes it into ethanoic acid.

CH3CH2OH + [O] CH3COOH

Purple manganate(VII) ions (MnO4-) turn colourless (Mn2+) and orange dichromate(VI) ions (Cr2O72-) turn green (Cr3+).

(4) Halogenation:

Whilst it is easy to hydrolyse haloalkanes into alcohols (see the haloalkanes page), the reverse reaction requires quite extreme conditions.

A concentrated mixture of halide ions and acid are used (for example, sodium chloride in concentrated sulphuric acid) along with heat, to ensure that the -OH group leaves the alcohol (as water) and the carbon atom will accept a Cl- ion.

CH3CH2OH + Cl- + H+ CH3CH2Cl + H2O

There is an old test involving the halogenation of an alcohol to determine whether an alcohol is primary, secondary or tertiary. A solution of zinc chloride in hydrochloric acid is added to the alcohol. A primary alcohol will be the slowest to show a reaction and the tertiary will be the fastest.

(5) Dehydration :

This process is the removal of the elements of water (2 H atoms and 1 O atom) from ethanol leaving ethene. It is accomplished by refluxing ethanol with a catalyst of concentrated sulphuric or phosphoric acid, or by passing ethanol vapour over heated aluminium oxide.

CH3CH2OH CH2=CH2 + H2O
(6) Esterification :

An ester is formed by reacting an alcohol with a carboxylic acid. It is a similar type of reaction to the neutralisation of an acid with a base. A catalyst of concentrated sulphuric acid removes the water produced and helps to push the equilibrium towards the products side of the equation.

CH3CH2OH + CH3COOH CH3COOCH2CH3 + H2O
(7) Reaction summary chart :

Alcohols - Iodoform reaction

The iodoform test is a test for the existence of the CH3-CO- group in a molecule. This could be part of an alcohol (C-O single bond) or part of a carbonyl compound (C=O double bond).

The reagents use are aqueous sodium hydroxide and iodine crystals. The first stage, in the use of this test with alcohols, is the oxidation of the alcohol group to a carbonyl group,

The hydrogen atoms on the methyl group are slightly acidic and can be removed with sodium hydroxide (stage 1 below).

The carbanion formed then react with iodine molecules to give an iodide ion and an organic iodo compound (stage 2 below).

This substitution continues until a triiodo group has been formed (stage 3 below) by repeated use of sodium hydroxide and iodine.

Another hydroxide ion can then attack the carbonyl carbon atom, giving a carboxylic acid and releasing the CI3 group which abstracts a proton from a water molecule to give CHI3 (called triiodomethane or iodoform) (stage 4 below).

Triiodomethane is a straw yellow solid, insoluble in water. This test works for ethanol and all 2-substituted secondary alcohols,

ethanol :
2-substituted alcohols :
R = any organic group

Alcohols - Oxidation differences

Most of the reactions of alcohols - combustion, halogenation, dehydration and esterification - do not depend on the nature of the alcohol. The one exception is oxidation.

Oxidation can be referred to as the removal of an atom of hydrogen from a molecule. The extent of oxidation of an alcohol will therefore depend on the number of hydrogen atoms that can be removed from the carbon atom attached to the hydroxyl group,

1 alcohol -

2 hydrogen atoms are bonded to the carbon atom therefore two oxidation steps are possible i.e. alcohol aldehyde carboxylic acid.

2 alcohol -

1 hydrogen atom is bonded to the carbon atom therefore only one oxidation step is possible i.e. alcohol ketone.

3 alcohol -

0 hydrogen atoms are bonded to the carbon atom therefore no oxidation steps are possible.


Alcohols - I.R. Spectroscopy

The spectrum of light is made up of a vast range of wavelengths. Only certain wavelengths are visible to human eyes, those from about 300 nm to 900 nm (1 nm = 1x10-9 m).

Wavelengths shorter than this range have high energy, such as ultra-violet light, x-rays and g-rays. These parts of the spectrum have enough energy to break covalent bonds in molecules and can cause cancer in humans, e.g. skin cancer from strong sunlight.

Wavelengths longer than the above range have low energy, such as infra-red light, microwaves and radio waves. These parts of the spectrum are used in both short-range and long-range communications.

Additionally, infra-red light causes covalent bonds to vibrate, not enough to break the bonds, just to cause the atoms that make up the bond to shift position slightly with respect to one-another.

In infra-red spectroscopy, an organic sample is subjected to wavelengths of light from 4000 cm-1 to 400 cm-1 (a cm-1 is known as a wavenumber, or inverse-centimeter in the US and equal to 1/cm).

Covalent bonds that are formed from two different atoms will absorb particular wavelengths of that infra-red light, and the absence of these wavelengths is detected. This produces graphs such as those shown below.

The particular covalent bonds that have to be identified at AS level are shown below,

alcohol groups, O-H   an absorption at 3200 to 3600 cm-1
carbonyl groups, C=O   an absorption at 1650 to 1720 cm-1
carboxylic acid groups, -COOH   an absorption at 2500 to 3300 cm-1 (-OH group) and 1700 to 1720 cm-1 (C=O group)

an alcohol, e.g. ethanol -

a carbonyl compound, e.g. propanal -

a carboxylic acid, e.g. ethanoic acid -


written by Dr Richard Clarkson : © Saturday, 1 November 1997

Updated : Sunday, 1st May, 2011

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