CCEA ADVANCED SUBSIDIARY

CHEMISTRY

MODULE 2

2.6 Alcohols

General Formula C_nH_2n+1OH (ROH) n ≥ 1

Name	Molecular formula	Structural formula	b.pt. /^oC	Density /g cm^-3
methanol	CH4O	H ½ H-C-O-H ½ H	65	0.79
ethanol	C2H6O	H H ½ ½ H-C-C-O-H ½ ½ H H	78	0.79
propanol	C3H8O	H H H ½ ½ ½ H-C-C-C-O-H ½ ½ ½ H H H	97	0.80

Physical properties. Reference to the effect of hydrogen bonding on boiling point and miscibility with water.

Physical properties

All the alcohols are colourless liquids with a pungent odour, less dense than water. The lower members are completely miscible with water because of hydrogen bonding between alcohol and water molecules.

R

H O

d- d+

R O H d- O H

R

As the number of carbon atoms increases the influence of the O-H group becomes less important compared to the Van der Waals forces of the large hydrocarbon group, and their properties tend more towards that of alkanes i.e. they become less soluble in water.

Alcohols contain both a highly polar O-H group and a non-polar hydrocarbon group. The lower alcohols tend to be good solvents of both polar and non-polar solutes e.g. both NaOH (ionic) and hexane (covalent) dissolve in ethanol. This makes ethanol a good solvent both in the laboratory and industrially. The boiling points of alcohols are much higher than their corresponding alkanes due to association by hydrogen bonding.

The place of alcohol in society as a recreational drug which can have beneficial and harmful effects. The idea of safe limits of ‘units’ of alcohol.

Alcoholic drinks such as beer and wine are made in vast quantities. Beer is made from barley to which hops have been added to produce a distinctive flavour, while wine is made by fermenting grape juice, which contains natural glucose.

The enzymes in yeast carry out the fermentation of both wine and beer. Beer normally contains about 4% by volume of ethanol and wine about 11% by volume of ethanol – the yeast is killed off if there is much more ethanol than this. As a result, it is not possible to make stronger drinks by fermentation. Spirits such as whiskey and brandy contain higher concentrations of ethanol and are produced by distillation after the fermentation process is complete.

Drink	% of ethanol by volume
Whiskey / brandy	40
Sherry	20
Vermouth	14
Wine	11
Beer	4

Exercise 1

Use the data in the table above to answer the following.

(a) Calculate the volume of ethanol (alcohol) in 1 litre of:

(i) whiskey

(ii) sherry

(b) An advert for Campari suggested it contained 15% more alcohol by volume than sherry.

(i) What is the percentage by volume of alcohol in Campari?

(ii) What is the volume of ethanol in a 1 litre bottle of Campari?

(c) Low alcohol beer contains about 0.5% of alcohol by volume. How many litres of this beer contain the same volume of alcohol as 2 litres of normal strength beer?

Ethanol, commonly referred to as ‘alcohol’, is the most commonly used drug, legally by adults and illegally by young people. Alcoholic drinks can have a positive effect, in moderation, in a social setting as they can help people to be less inhibited and more relaxed.

As you will know, it is the ethanol in alcoholic drinks that can also make people drunk. It is the ethanol in these drinks that causes the headaches (‘hangover’), dizziness and vomiting (the body’s natural reaction to any poisonous substance which is ingested) associated with being drunk.

Ethanol can damage vital organs in your body. Excessive intake over an extended period of time can cause irreparable damage to the heart, brain, kidneys and liver(cirrhosis) and can result in death. Ethanol is a drug that acts to depress the central nervous system and is known to be addictive.

Alcohol abuse is a very serious social problem in our society. Alcoholism is physical and psychological dependence on alcohol. There are about 30 000 alcoholics in the UK and alcoholics usually have health, family and work problems. The heavy drinker tends to behave in antisocial ways and deteriorates socially and personally.

Even small amounts of alcohol in the bloodstream can reduce your judgement and skill. About a third of all road accidents are associated with alcohol. The introduction of the breathalyser in the 1960’s had a dramatic effect, leading to 40 000 fewer accidents on the UK roads in its first year. Breathalysers measure the concentration of ethanol in the breath. The legal limit of blood alcohol concentration (BAC) for driving is 80 mg per 100 cm³ of blood, which corresponds to 35 mg per 100 cm³ of breath. If you ‘fail’ the breathalyser then the BAC is confirmed at the police station by either a further breath test or a blood sample.

Different levels of blood alcohol have different effects on the average person, as shown below.

Number of drinks	BAC level	Effects
1 pint of beer	30 mg	Increased chance of accidents
3 single whiskies	50 mg	Cheerful, judgement affected, less inhibited
5 single whiskies	80 mg	Legal driving limit, 4 times more likely to have an accident

For convenience, the amount of ethanol in drinks is measured in ‘units’. The amount of different drinks that give one unit of alcohol is shown in the table below.

ONE UNIT OF ALCOHOL	½ pint of beer or cider
	1 glass of sherry
	1 glass of wine
	1 single measure of spirits

Combustion of ethanol and its production from sugar cane and use as an alternative fuel to conserve fossil fuels and to reduce air pollution from oxides of sulphur and carbon.

Combustion of ethanol

Ethanol is flammable and burns in a good supply of oxygen with an almost invisible flame, forming carbon dioxide and water and releasing energy to the surroundings.

C₂H₅OH + 3O₂ 2CO₂ + 3H₂O ΔH = -1371 kJ mol^-1

Production of ethanol

Ethanol can be produced industrially by the fermentation of sugars. The reaction is catalysed by enzymes (biological catalysts) produced by living yeasts. Slightly warm conditions are used. The reaction can be summarised as follows:

Starting materials – starch (barley), sucrose (sugar cane or sugar beet) or glucose.

Sucrose is extracted from sugar cane by soaking in hot water. It is diluted until the sugar concentration is about 15%, acidified slightly and seeded with yeast. Yeast contains an enzyme invertase which converts the sucrose into a mixture of two isomeric sugars glucose and fructose.

invertase

SUCROSE + WATER GLUCOSE + FRUCTOSE

C₁₂H₂₂O₁₁ + H₂O C₆H₁₂O₆ + C₆H₁₂O₆

Both glucose and fructose can be fermented to alcohol by the action of another enzyme zymase.

zymase

GLUCOSE ETHANOL + CARBON DIOXIDE

C₆H₁₂O₆ 2C₂H₅OH + 2CO₂

Some countries, such as Brazil, use ethanol as a fuel for cars. Up to 20% ethanol can be added to the petrol used in car engines without the need for adjustment. The Brazilian government has cut down its petrol imports by up to 60% by using this alcohol-petrol mixture. The country has little oil but grows a lot of sugar cane, which can be fermented. In addition the reduction of the amount of petrol used, reduces the amount of atmospheric pollution from sulphur impurities in the petrol ( the sulphur dioxide produced contributes to acid rain).

Oxidation of alcohols by acidified dichromate. Difference between primary, secondary and tertiary alcohols with respect to mild oxidation.

Oxidation

Oxidation can be either

1 loss of electrons

2 removal of hydrogen or

3 addition of oxygen.

The product of oxidation depends on whether it is a primary, secondary or tertiary alcohol. The oxidising agent used is either acidified potassium manganate (VII) (colour change pink to colourless) or acidified potassium dichromate (VI) (colour change orange to green).

The symbol [O] can be used for the oxidising agent.

Oxidation of primary alcohols

(i) H

½ [O]

R¾ C¾OH R¾ C = O + 2H⁺ + 2e^-

½ ½

H H

alcohol alkanal (aldehyde)

The aldehyde is readily oxidised to the acid.

(ii) [O]

R¾ C= O R¾ C = O

½ ½

H OH

alkanal (aldehyde) alkanoic acid

[O] [O]

e.g. CH₃CH₂OH CH₃CHO CH₃COOH

ethanol ethanal ethanoic acid

Overall the reaction is

RCH₂OH + 2[O] RCOOH + H₂O

When ethanol is warmed with acidified potassium dichromate and the products distilled off immediately, ethanal is collected. However if the reaction mixture is refluxed with an excess of the oxidising agent and then distilled the product is mainly ethanoic acid.

Oxidation of secondary alcohols

The product of oxidation is an alkanone (ketone)

H

½ [O]

R¾ C¾ OH R¾C = O + 2H⁺ + 2e^-

½ ½

R’ R’

alcohol alkanone (ketone)

Ketones are not readily oxidised so the reaction stops at this point.

e.g. H

½ [O]

CH₃ ¾ C¾OH CH₃ ¾ C=O

½ ½

CH₃ CH₃

propan-2-ol propanone

Oxidation of tertiary alcohols

Tertiary alcohols are not readily oxidised since they have no hydrogen atoms attached to the carbon atom carrying the OH group.

R’’

½ [O]

R¾ C¾ OH no reaction

½

R’

This is a convenient method of distinguishing between primary, secondary and tertiary alcohols.

Esterification reactions of simple alcohols with carboxylic acids and with ethanoyl chloride.

Ester formation

Alcohols react reversibly with carboxylic acids to form esters in the presence of an inorganic acid as catalyst. The reaction involves the cleavage (breaking) of the RO-H bond.

H⁺

ACID + ALCOHOL ó ESTER + WATER

reflux

H⁺

CH₃-C¾OH + H¾OCH₂-CH₃ ó CH₃C-OCH₂CH₃ + H₂O

|| reflux ||

O O

ethanoic acid ethanol ethyl ethanoate

CH₃COOH + CH₃CH₂OH ó CH₃COOCH₂CH₃ + H₂O

Alcohols react much more rapidly with alkanoyl chlorides (acid chlorides) to form esters.

R^o

CH₃C¾Cl + H¾OCH₂CH₃ CH₃C-OCH₂CH₃ + HCl

|| ||

O O

ethanoyl chloride

Reactions of primary alcohols with sodium, hydrogen bromide, phosphorus pentachloride and thionyl chloride.

Sodium

Alcohols react quietly with metallic sodium forming a solution of an alkoxide and liberating hydrogen gas.

2ROH + 2Na 2RONa + H₂

e.g. C₂H₅OH + 2Na 2C₂H₅ONa + H₂

[This is comparable to the reaction of sodium with water

2H₂O + 2Na 2NaOH + H₂ ]

Halogenation

This involves the replacement of the –OH group in an alcohol by a halide atom -X.

H⁺/reflux

ROH + HBr RBr + H₂O

The alcohol acts as a base

_{• •} H

R¾O¾H + H⁺ R¾O+

H

This makes it easier to break the C-O bond.

H

R¾O+ R⁺ + H₂O

H

Hydrogen bromide dissolves in and reacts with ethanol, in the presence of conc. sulphuric acid, forming 1-bromobutane.

The acid has two functions

(i) HBr is formed ‘in situ’ by the reaction between sulphuric acid and sodium bromide.

H₂SO₄ + NaBr HBr + NaHSO₄

(ii) Conc. sulphuric acid is a dehydrating agent and removes the water formed during the reaction.

+

CH₃CH₂ÖH + H⁺ CH₃CH₂OH₂

(from acid)

+

CH₃CH₂¾OH₂ CH₃CH₂Br + H₂O

Br ^–

Ease of substitution 3^o>2^o>1^o

Reaction with PCl₅

Phosphorus pentachloride, PCl₅, is another reagent which can be used to replace the OH group in alcohols with Cl.

R^o

R-OH + PCl₅ (s) R-Cl + HCl + POCl₃

‘steamy fumes’ phosphorus oxychloride

This reaction is used as a test for the presence of an –OH group in an organic compound.

Reaction with SOCl₂

Thionyl chloride (SOCl₂), a colourless liquid which fumes in air, is another useful halogenating agent in organic chemistry. It is often preferred to phosphorus pentachloride for this purpose as the by-products are gases and easily removed.

reflux

ROH + SOCl₂ RCl + SO₂ (g) + HCl (g)