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Alcohols are compounds in which one of the hydrogen atoms of an alkane has been replaced by a hydroxyl group.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 CnH2n+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 + 2CO2The 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 35oC. 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:

temperature: 300oC

pressure: 65 atmospheres

catalyst: phosphoric acid (H3PO4) adsorbed on celite

Comparison of the two methods

FermentationHydration of ethene

Rate of reactionslowfast

Type of processbatchcontinuous

Raw materialrenewable (sugars)non-renewable (ethene from oil)

Purity of productimpure (dilute solution)pure

Technologycheap, low-tech equipmentexpensive, high-tech equipment

Labourunskilled; more manpowersemi-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.

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.

Oxidation of alcohols

When alcohols are heated with acidified potassium dichromate:

1. Primary alcohols are oxidised initially to aldehydes:

CH3CH2OH+ [O] CH3CHO + H2O

ethanol

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+ + 6e- 2Cr3+ + 7H2O

orange

green

2. Secondary alcohols are oxidised to ketones, which are not oxidised further.

CH3CH(OH)CH3+ [O] CH3COCH3 + H2O

propan-2-ol

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. Tollens reagent

Tollens reagent is prepared by adding concentrated aqueous ammonia dropwise to silver nitrate solution until the initial brown precipitate of silver oxide (Ag2O) just re-dissolves to give a clear, colourless solution. The solution contains the complex ion, diamminosilver(I), [Ag(NH3)2]+.

When aldehydes are warmed with Tollens 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

2. Fehlings solution

Fehlings 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 Fehlings 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 C C H

H OH

+ H2O

C C

H H

H H

H H

OH

H C H

R

OH

R C H

R

OH

R C R

R

H

H H

primary (1o) secondary (2o) tertiary (3o)

H OH

H C C H

A biofuel is a fuel that has been made from plant matter that is recently dead.

OH

C C

H H

H H

C C

+ H2O

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).

TOPIC 12.14: ORGANIC CHEMISTRY ALCOHOLS 3