C16 alcohols and carboxylic acids

Chapter 16
Alcohols and Carboxylic Acids

Alcohols
You should be able to:

• Write general and molecular formulae for members of
the alcohol homologous series.

• Write fully displayed structures and names of alcohols,

• Identify alcohols by their functional groups,

• Relate the properties of alcohols to their functional
groups,

• Describe the reactions of ethanol, and

• Describe the fermentation process by which ethanol is
produced from carbohydrates.

Chapter 16
Alcohols
• All alcohols belong to a family of organic compounds
which contain the –OH group of atoms.
• This group of atoms is called the hydroxyl group. It
gives the alcohol its specific chemical properties and is
the functional group of alcohols.
• Most alcohols are colourless liquids with a strong smell.
They have low boiling and melting points.
• The members of the alcohol homologous series have the
following features:
1. They have the general formula: CnH2n+1OH,
where n is an integer.
2. They have a hydroxyl functional group, –OH.
3. Their names end with ‘-ol’.
4. The formula of each member differs from the
previous one by –CH2.

Chapter 16
Alcohols
Name Molecular Relative molecular Structural
formula mass formula
Methanol CH3OH 32

Ethanol C2H5OH 46

Propanol C3H7OH 60

Butanol C4H9OH 74

Chapter 16
Properties of Ethanol
• Ethanol is the most important alcohol in the
homologous series.
• Only ethanol can be consumed.
• Ethanol is a colourless liquid with a strong
smell.
• It has a boiling point of 78 °C and is volatile.
• It mixes readily with water.
• It is neutral to litmus.

Chapter 16
Quick Check 1
1. (a) Name an alcohol with three carbon atoms.
(b) State its
(i) molecular formula.
(ii) structural formula.

2. (a) State the general formula of alcohols.
(b) State the formula of an alcohol with 20 carbon
atoms.
(c) Do you think it is a liquid, solid or gas at room
temperature and pressure?

Solution

Chapter 16
Solutions to Quick Check 1
1. (a) Propanol
(b) (i) C3H7OH (ii)

2. (a) CnH2n+1OH
(b) C20H41OH
(c) Solid, its melting point is high because it has
very large molecular mass.

Return

Chapter 16
Chemical Reactions
1. Reaction with a reactive metal like sodium
• When a small piece of sodium is dropped into some
ethanol, it reacts steadily to give off bubbles of hydrogen
gas leaving behind a colourless solution of sodium
ethoxide, CH3CH2ONa.

• On evaporating the solution to dryness, a white sodium
ethoxide solid is obtained.

• The reaction is similar to the reaction between water and
sodium as in both reactions, the –OH group is replaced by
Na+ ion.

Chapter 16
2. Combustion
• Ethanol burns in air (oxygen) to form carbon dioxide
and water. The reaction is exothermic and heat
energy is given out. Hence, ethanol can be used as
a fuel.

Ethanol + Oxygen  Carbon dioxide + Water
C2H5OH(l) + 3O2(g)  2CO2(g) + 3H2O(l)

Chapter 16

3. Oxidation to Carboxylic Acids
• (i) Ethanol is oxidised by hot acidified potassium
dichromate(VI) solution into ethanoic acid:

H+/K2Cr2O7
C2H5OH + 2[O] CH3COOH + H2O

H+/K2Cr2O7
+ 2[O] H H
+
O

During the reaction, the potassium dichromate(VI) solution is
reduced and the solution turns from orange to green in
colour.

Chapter 16
4. Oxidation to Carboxylic Acids
• (ii) The oxidation of ethanol can also be carried out by
using air and bacteria.
• When wine, beer or any liquid containing ethanol is left in
the open for a few days, it becomes sour.
• Airborne bacteria (called Acetobacter aceti) converts the
alcohol into vinegar, which is a solution of ethanoic acid.
• This method is used in the industry to make vinegar.
Ethanol + Oxygen  Ethanoic acid + Water
C2H5OH +2[O]  CH3COOH + H2O

Barrels of vinegar made by oxidation

Chapter 16
5. Formation of Esters
• Ethanol reacts with carboxylic acids in the presence of
concentrated HCl or H2SO4 to form organic compounds
called esters. For example, ethanol reacts with ethanoic
acid to form an ester called ethyl ethanoate:

C2H5OH + CH3COOH  CH3COOC2H5 + H2O
ethanol ethanoic acid ethyl ethanoate

• Esters are sweet smelling liquids which can be used for
making perfumes and flavouring agents.

Chapter 16
6. Dehydration to Alkene
• Ethanol reacts with dehydrating agents such as concentrated
sulphuric acid or anhydrous aluminium oxide to form alkenes.
This is a dehydration reaction where water is lost across two
adjacent carbon atoms in the ethanol.

H H
O

In this process, ethanol is heated with excess concentrated sulphuric
acid at 170 °C to form ethene. Alternatively, ethanol vapour can be
passed over activated alumina (Al2O3) at 450 °C.

Chapter 16
Quick Check 2
1. (a) State the products formed by the
combustion of alcohols in oxygen.
(b) Construct a balanced equation for the
combustion of propanol.

2. (a) What is formed when ethanol is
oxidised?
(b) State two different ways in which oxidation
of ethanol can be carried out.

Solution

Chapter 16

Solutions to Quick Check 2

1. (a) Carbon dioxide and water
(b) 2C3H7OH(l) + 9O2(g)  6CO2(g) + 8H2O(l)

2. (a) Ethanoic acid
(b) Ethanol can be oxidised
(i) by hot acidified potassium dichromate(VI)
solution (into ethanoic acid) or
(ii) by using air and bacteria.

Return

Chapter 16
Making Ethanol by Fermentation
of Carbohydrates
• Large quantities of ethanol are made for consumption by
fermentation.
• Depending on the raw materials used, the ethanol is
produced in different kinds of alcoholic drinks such as wine,
whisky, brandy, and beer.

Fermentation is carried out in large tanks

• Carbohydrates such as starch and sugar are mixed with
water and yeast, a unicellular fungi, and a source of
enzymes.
• The mixture is left to ferment without air for a few weeks.

Chapter 16
Making Ethanol by Fermentation
of Carbohydrates
• The temperature of the mixture should be kept around 37 oC
for the yeast to work best.
• The reaction only proceeds under anaerobic conditions, i.e.
absence of oxygen.
• During fermentation, the yeast feeds on the sugar and
changes it into glucose and then into ethanol.
• The ethanol obtained is distilled and made into wine or used
as pure ethanol. The carbon dioxide produced in the
fermentation is a by-product and can be made into dry ice.

Step 1: C12H22O11 + H2O invertase C6H12O6 + C6H12O6
sucrose glucose fructose

Step 2: C6H12O6 zymase 2C2H5OH + 2CO2
glucose ethanol

Chapter 16
Making Ethanol from Ethene
• We have learned in the previous chapter that alkenes
undergo an addition reaction with steam to form alcohols.
• This is the industrial method of making ethanol which is
much cheaper than by the fermentation method.
• Ethanol is manufactured by reacting ethene with steam
using phosphoric(V) acid as a catalyst at a temperature of
about 300 oC.
• The reaction is reversible, and the formation of the ethanol
is exothermic.

CH2=CH2(g) +H2O(g)  CH3CH2OH(g) ΔH = –45 kJ/mol

Chapter 16
Alcoholic Beverages and Their Sources
• The table below lists the types of alcoholic drinks and their
sources of starch used.

Drink Source of % volume Method of
starch of ethanol preparation
Beer Barley 3–8 Fermentation
Wine Grapes / rice 8 – 18
Whisky Barley 30 – 60 Fermentation
Brandy Grapes and
fractional
Vodka Rye / potatoes distillation

Chapter 16
Uses of Ethanol
• Ethanol is a constituent of alcoholic beverages such as
wines and whisky.

• Ethanol is used widely as a solvent for paints, varnishes,
liquid soap and other toiletries.

• Ethanol is blended with petrol and used as a fuel for
motorcars in many countries.

• Ethanol can be oxidised into ethanoic acid which is used
to manufacture plastics and drugs.

Chapter 16

Quick Check 3

1. What raw materials are required for the
production of ethanol by fermentation?
2. Construct chemical equations for the
fermentation of sugar.

Solution

Chapter 16

Solution to Quick Check 3
1. Carbohydrates
2.
Step 1:
invertase
C12H22O11 + H2O C6H12O6 + C6H12O6
sucrose glucose fructose

Step 2:
zymase
C6H12O6 2C2H5OH + 2CO2
glucose ethanol

Return

Chapter 16
Carboxylic Acids
• The general formula is CnH2n+1COOH, where n is an integer.
• They have an acid functional group:

• Their names end with ‘-oic’, derived from the corresponding
alkane by replacing the letter ‘e’ in ‘-ane’ with ‘-oic’.
E.g. methane  methanoic acid; ethane  ethanoic acid.

• Each member of the homologous series differs from the
previous one by an extra –CH2 group.

Chapter 16
Structure of Ethanoic acid
• The molecular model and structural formula of a typical
carboxylic acid, such as ethanoic acid, is as shown:

Structure of ethanoic acid

Molecular Structural
Name
formula formula
Methanoic acid HCOOH

Ethanoic acid CH3COOH

Propanoic acid C2H5COOH

Butanoic acid C3H7COOH

Chapter 16
Properties of Ethanoic Acid
• It is a colourless liquid with a strong sour smell.
• It mixes readily with water and is commonly called acetic
acid.
• It turns blue litmus red.
Chemical reactions
• Carboxylic acids are weak monobasic acids.
• Carboxylic acids show typical reactions of mineral acids.
They react less vigorously as they are weak acids and
dissociate only partially in water to form hydrogen ions.

1. Reaction with Metals
Ethanoic acid reacts with more reactive metals like sodium,
calcium and magnesium to form a salt and liberate hydrogen gas.

2Na(s) + 2CH3COOH(aq)  2CH3COONa(aq) + H2(g)

Chapter 16
2. Reaction with Metallic Oxides and Hydroxides
• Ethanoic acid reacts with the oxides and hydroxides of
metals to form salt and water. Here, the reaction is a
neutralisation reaction.
MgO(s) + 2CH3COOH(aq)  (CH3COO)2Mg(aq) + H2O(l)
NaOH(aq) + CH3COOH(aq)  CH3COONa(aq) + H2O(l)

3. Reaction with Carbonates
• Ethanoic acid reacts with carbonates to produce salt, water
and carbon dioxide.
2CH3COOH(aq) + Na2CO3(s)  2CH3COONa(aq) + H2O(l) + CO2(g)
CH3COOH(aq) + NaHCO3(s)  CH3COONa(aq) + H2O(l) + CO2(g)

Chapter 16
4. Reaction with Alcohols
• Ethanoic acid reacts with alcohols to form sweet smelling
compounds called esters.
• Concentrated sulphuric acid is added as a catalyst. The
process of forming esters is called esterification.

The formation of the esters from different carboxylic acids and
alcohols:
CH3COOH + C2H5OH  CH3COOC2H5 + H2O
ethanoic acid ethanol ethyl ethanoate

CH3COOH + CH3OH  CH3COOCH3 + H2O
ethanoic acid methanol methyl ethanoate

C2H5COOH + C2H5OH  C2H5COOC2H5 + H2O
propanoic acid ethanol ethyl propanoate

Chapter 16
Uses of Ethanoic Acid
• Ethanoic acid is a very important chemical used in
manufacturing a wide variety of things.

• Annual production of ethanoic acid in the world is
estimated to be more than 6 million tonnes.

• There is a wide range of products made from ethanoic
acid. This includes:
– making various kinds of plastics such as acetates for making
fabric, films, adhesives and paints.
– making drugs like aspirin,
– making bleaching agents in detergents; chemicals like
herbicides and dyes, and
– vinegar, which is used in cooking, as an additive to chilli and
tomato sauces and for preserving vegetables.

Chapter 16
Esters
Esters are organic compounds with characteristic fruity
smells.
• They are used in food flavouring and in making
perfumes.
• Esters can be prepared by reacting an alcohol with a
carboxylic acid using concentrated sulphuric acid as
the catalyst or dehydrating agent.

E.g. Ethyl ethanoate can be prepared by reacting ethanoic acid with
ethanol. In this case, the hydrogen in the –COOH group has been
replaced by an ethyl group. The formula of ethyl ethanoate is:

Chapter 16
Physical properties of esters
Boiling points
• Short-chain esters are more volatile than carboxylic acids
of the same number of carbons.
• They are not able to form hydrogen bonds with themselves
due to the absence of hydroxyl group.
• Thus the boiling points of esters are usually lower than that
of carboxylic acids of the same number of carbon atoms.

Solubility of esters in water
• Short-chain esters are fairly soluble in water but solubility
decreases with increasing chain length.
• Esters are soluble in water because they are able to form
hydrogen bonds with water molecules.
• The slightly positive end of a water molecule can form a
hydrogen bond with the lone pair of electrons on one of the
oxygen atoms in an ester.

Chapter 16
Chemical properties of esters
• The ester linkage can be broken by addition of water.
• The process is therefore known as hydrolysis.
• Hydrolysing esters using only water is very slow.
• The reaction is thus catalysed by using dilute acid like dilute
hydrochloric acid or dilute sulphuric acid.
• The ester is heated under reflux with a dilute acid and the
products are separated by fractional distillation.

(a) Hydrolysis of ethyl ethanoate:

(b) Hydrolysis of methyl propanoate:

Chapter 16
Alternative method
• Alkalis can also be used. When an alkali is used, the salt
of the acid, instead of the acid itself, is formed.
• Hydrolysis of esters using an alkali is sometimes known as
saponification. (used in producing soap)
• E.g. In the hydrolysis of ethyl ethanoate using sodium
hydroxide solution, sodium ethanoate instead of ethanoic
acid is formed.
CH3COOCH2CH3(aq) + NaOH(aq)  CH3COO-Na+(aq) + CH2CH3OH(aq)

Chapter 16
Quick Check 4
1. (i) What is the general formula of a carboxylic acid?
(ii) State the formula of a carboxylic acid with 10 carbon
atoms.
2. State the name and write the structural formula of a
carboxylic acid with:
(a) 2 carbon atoms, (b) 3 carbon atoms and (c) 4 carbon
atoms.
3. (a) What organic compound is formed when ethanoic acid
reacts with ethanol?
(b) What name is given to this type of reaction and what is the

catalyst required?
(c) Construct an equation to show the reaction between
ethanoic acid and ethanol.

Solution

Chapter 16
Solution to Quick Check 4
1. (i) CnH2n+1COOH
(ii) C10H21COOH

2. (a) ethanoic acid (b) propanoic acid (c) butanoic acid

3. (a) Ethyl ethanoate
(b) Esterification, concentrated sulphuric acid
conc. H2SO4
(c) CH3COOH + C2H5OH  CH3COOC2H5 + H2O
ethanoic acid ethanol ethyl ethanoate

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C16 alcohols and carboxylic acids

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