Alcohols, carboxylic acids and esters

Organic chemistryLevel 3 AQA Chemistry

Alcohols 1 - Meet the familyAlcohols R OH

The first four members of the homologous series are:• Methanol CH3OH

• Ethanol C2H5OH

• Propanol C3H7OH

• Butanol C4H7OH

Think of the alkane, then take off one hydrogen

atoms, then replace it with

the –OH functional group

Alcohols 2Alcohols contain the functional group –OH This –OH group is not like the hydroxide ion in alkalis.The most important members of the homologous series are methanol and ethanol (which we call ‘alcohol’).

Ethanol 1Ethanol C2H5 OH

Think of ethane, C2H6

and then substitute one of the hydrogen atoms (C2H5) for the functional group —OHThis gives ethanol.C2H5OH

Ethanol 2Ethanol C2H5 OHEthanol’s functional group gives it different properties from ethane. These are its main properties:• it burns well with a clear blue flame• it is an excellent solvent for perfume and

scent• it is completely miscible with water in all

proportions• it is a neutral liquid

Ethanol 2 – Good and badGood points• the drinks industry is

a major employer• Scotch whisky is a

major export item• Alcohol is a social

drug• Large tax revenues

are generated from sales

• Alcohols are excellent solvents

• Alcohol gels are used to clean hands in hospitals

Bad points• drinking alcohol can

change your behaviour

• it is a depressant• it slows reactions and

makes accidents and death more likely

• Prolonged excessive drinking causes damage to the liver and brain

• Cost of policing alcohol-related crime is high

• It is a drain on the NHS

Ethanol 3 A great mixer !Water H-OH and ethanol C2H5-OH molecules are very similar because of their –OH groups. The interactions between these different molecules are very similar to those in pure water and pure ethanol.Ethanol and water are completely miscible in all proportions. The solutions are neutral, pH 7.Alcohol solutions include:• beers (containing 4 to 5% ethanol)• wines (containing 10-12% ethanol)• spirits (containing 40% ethanol)

FermentationEthanol is produced during the fermentation of sugar. This reaction is performed by enzymes found in yeast.Distilling the resulting solution changes the alcohol content from about 10% ethanol to a liquid called rectified spirit. This has 96% ethanol and 4% water.

Combustion 1Alcohols, e.g. ethanol, burn well in air in an exothermic reaction.The clear blue flame is partly due to the internal oxygen in the ethanol molecule.C2H5OH + 3O2 → 2CO2 + 3H2O

ΔH = -1370 kJ/mole

Combustion 2Ethanol makes an excellent petrol substitute. However …

Good points• made from a

renewable resource (cane sugar/corn)

• sugar and corn take in CO2 one year when they grow, and ethanol burning releases this next year, so it is carbon neutral process

Bad points• fuel crops grow on

land that could be used to feed people

• clearing the land, transporting the fuel and manufacturing the fertilisers requires energy from fuels … these are not carbon neutral processes

Reaction with sodiumAlcohols react with sodium metal. The reaction releases hydrogen gas and forms sodium ethoxide. 2C2H5OH + 2Na → 2C2H5ONa + H2

In some respects this is similar to the reaction between water (with its –OH group) and sodium. During the reaction the O-H chemical bond is broken by the sodium atom.2H2O + 2Na → 2NaOH + H2

OxidationEthanol can be oxidised by the oxygen in the air to form an acid called ethanoic acid. This is the main acid in vinegar.

ethanoic acid

ethanol

Carboxylic acids

Homologous Series Carboxylic acids R’COO

HThe first four members of the homologous series are:• Methanoic acid H COOH• Ethanoic acid CH3 COOH

• Propanoic acid C2H5 COOH

• Butanoic acid C3H7 COOH

Ethanoic acid

Carboxylic acids have characteristic smells.Vinegar gets its smell from ethanoic acid. All carboxylic acids have the -COOH functional group.

The name of the acid above comes from the fact that it has:• two carbon atoms

(therefore it will start eth-)

• the functional group –COOH (so it will end –anoic)

This is ethanoic acid.

We are family !

Methanoic acid

Ethanoic Acid

Propanoic acid

As you descend the homologous series• the boiling

points increase

• the densities decreases

Each carboxylic acid molecule has the same functional group, -COOH. However, the length of the molecule chain affects the boiling point and the density.

Acidic propertiesBecause all carboxylic acids have the same -COOH functional group they have remarkably similar chemical properties.One in every 1 000 acid molecules dissociates into ions, including hydrogen ions, H+

(aq) making these compounds acidic.

The O–H bond breaks leaving both electrons on the oxygen, releasing a hydrogen ion, H+

Typical acidic behaviourCarboxylic acids dissolve in water to make weakly acidic solutions and pH’s less than 7.They react with metal carbonates to produce carbon dioxide gas.ethanoic + sodium = sodium + water + carbonacid carbonate ethanoate dioxideThese acids also react with magnesium metal to produce hydrogen gas. The reaction is much slower than with hydrochloric acid - the acid molecules are only partially ionised.

Explaining acidic behaviourEthanoic acid molecules may split up to form

ions. One in every thousand molecules does this. This is partial ionisation.

For hydrochloric acid molecules, 100% split up to form ions. Hydrochloric acid is fully ionised.

Comparing acidic behaviourAcids like hydrochloric acid that are

fully ionised in solution are called strong acids.Carboxylic acids are weak acids. Fewer than 1% of the molecules ionise when they are dissolved in water.

For ethanoic acid, the position of equilibrium lies well over to the left.

Weak & strong acids - pHAcids have pH’s below 7. The lower the pH the more strongly acidic the solution becomes. The greater the extent of ionisation, the greater the H+

(aq) concentration becomes.

Ethanoic acid’s pH is nearly 3. Ethanoic acid:• has a higher pH than hydrochloric acid • is a weaker acid and contains a lower

concentration of hydrogen ions than HCl

Esters

Sweet smelly ester !Esters R-COO-R’Complex mixtures of different esters give fruits their characteristic smells.Ethyl ethanoate has a smell of pear drops, the sort you buy in the ‘pound’ shop. Ghastly but somehow addictive. Esters form vapours readily. We say they are volatile. Each ester has a distinctive smell.

The ester functional group -COO-

Useful estersThe different smells and tastes of esters make them useful for:• making sweet smelling perfumes,

shampoos and shower gels• flavouring foods, sweets and chocolates

Ethyl ethanoate is a useful solvent.It is cheap, non-toxic and has a pleasant smell.It is used to clean circuit boards.It is also used to ‘de-caffeinate’ coffee and tea.It’s water and raw vegetables for me from now on. Do ‘Apple’ use a unique ester packaging spray?

Making estersThe acid used is ethanoic acid, the alcohol is ethanol.The ester made from this reaction in ethyl ethanoate.The process is called esterification.

Making more esterSulphuric acid acts as a catalyst in this reaction – removing water (a product of the forward reaction) – driving the equilibrium to the right.

Name that ester in one!

The dotted red line divides the

acid (on the left) from the

alcohol (on the right)

Clue