Section 3: Organic Chemistry - IGCSE Chemistry

Chemistry > Section 3: Organic Chemistry

a) Introduction

3.1 explain the terms homologous series, hydrocarbon, saturated, unsaturated, general formula and isomerism.

Homologous series:

It is a family of compounds with same functional group and general formula with similar physical and chemical properties.

Characteristics of homologous series:

  • They have a general formula
  • They differs in molecular formula
  • A homologous series have similar properties
  • They have a trend
  • Each homologous series have a functional of group


Compounds that are made of only hydrogen and carbon. Hydrocarbons are of two types: Saturated and Unsaturated



Contain C-C single bond(eg: Alkanes)

Contain C=C double bond. (eg: alkenes)

Give substitution reaction

Give addition reaction

A part of saturated compound comparatively is less

A part of unsaturated is more reactive than saturated


Alkanes do not polimarise

Alkene can be polimarised

Saturated hydrocarbons cannot change the colour of

Unsaturated hydrocarbons can change the colour of

bromine water

bromine water

General formula:

General formula confirms which compound is in which homologous series.


Alkenes ==> CH2n

Alcohols==>CnH2n+1+ OH


Molecules with identical molecular formulae but with different structural formulae. Examples:

b) Alkanes

3.2 recall that alkanes have the general formula CnH2n+2

Alkanes have the general formula CnH2n+2 . n is the number of carbons.

3.3 draw displayed formulae for alkanes with up to five carbon atoms in a molecule, and name the straight-chain isomers

3.4 recall the products of the complete and incomplete combustion of alkanes

Complete combustion gives carbon dioxide and water. eg: CH4 + O2 ==> CO2 + H2O

Incomplete combustion gives carbon monoxide and water. eg: CH4 + O2 ==> CO + H2O

3.5 describe the substitution reaction of methane with bromine to form bromomethane in the presence of UV light.

In UV light bromine and methane will form bromomethane:

CH4 + Br2 ==> CH3Br + HBr

What has happened in this reaction is bromine has taken the place of hydrogen (substitution.)

c) Alkenes

3.6 recall that alkenes have the general formula CnH2n

Alkenes have the general formula CnH2n

3.7 draw displayed formulae for alkenes with up to four carbon atoms in a molecule, and name the straight-chain isomers (knowledge of cis- and trans- isomers is not required)

3.8 describe the addition reaction of alkenes with bromine, including the decolourising of bromine water as a test for alkenes.

An alkene will make its double bond into a single bond, to bond to two bromines. Bromine is added to the molecule. The product made is colourless. When alkenes are put in bromine water it turns from brown to colourless.

For example:

C2H4(g) + Br2 (aq) ==> C2H4Br2 (aq)

d) Ethanol

3.9 describe the manufacture of ethanol by passing ethene and steam over a phosphoric acid catalyst at a temperature of about 300°C and a pressure of about 60–70 atm

Ethanol is manufactured by the catalytic addition of steam to ethane. The mixture of ethane and steam is passed through phosphoric (V) acid at 300 0C and 60 atm. Phosphoric(V) acid (H3PO4) acts as a catalyst in this reaction.

3.10 describe the manufacture of ethanol by the fermentation of sugars, for example glucose, at a temperature of about 30°C

Fermentation is a chemical process in which microorganisms such as yeast act on carbohydrates to produce ethanol and carbon dioxide. Sugars and starch are examples of carbohydrates.

Yeast contains zymase enzymes(biological catalysts) which cause starch or sugar to break down to glucose. The glucose is then broken down to ethanol and carbon dioxide.

A glucose solution is mixed with yeast and the mixture are produced after a few days.

The enzymes in yeast work best at around 37oC. If the temperature is raised beyond 37o, the enzymes will die and the fermentation stops.

The fermentation of sugars produces only a dilute solution of ethanol. This is because when the alcohol content exceeds this value, the yeast dies and fermentation stops. Ethanol can be obtained from this liquid mixture by fractional distillation.m

3.11 evaluate the factors relevant to the choice of method used in the manufacture of ethanol, for example the relative availability of sugar cane and crude oil

Fermentation Hydration of ethane
Uses of resources Uses renewable resources- sugar beet or sugar cane, corn and other starchy materials. Uses finite resources –once all the oil has been used up there wont be any more
Type of process A batch process – everything is mixed together in a reaction vessel and then left for several days. That batch is then removed and a new reaction is set up - this is ineffecient A continuous flow process – a stream of reactant is constantly passed over the catalyst. This is more efficient than a batch process
Rate of reaction Slow, taking several days for each batch Rapid
Quality of product Processes very impure ethanol that needs further processing Produce much purer ethanol
Reaction conditions Uses gentle temperature and ordinary pressure Uses high temperature and pressures, needing a high input of energy.

3.12 describe the dehydration of ethanol to ethene, using aluminium oxide.

Dehydration refers to the removal of water from a compound. The dehydration of ethanol produces ethane. Ethanol vapour is passed over hot aluminium oxide, acting as a catalyst.

C2H5OH(g) ==> C2H4(g) + H2O(l)