Section 3: Organic Chemistry
a) Introduction
3.1 Explain the terms homologous series, hydrocarbon, saturated, unsaturated, general formula and isomerism

• Homologous series: a group of compounds that can all be represented by the same general formula 
• The members of a homologous series have similar chemical properties and have trends in physical properties (eg as the chain length increases, the boiling point increases)
•  Hydrocarbon: a molecule made up of only carbon and hydrogen atoms
• Saturated: all possible bonds that could be formed in the molecule, are already formed
• Unsaturated: more bonds can be made in the molecule
• General formula: can be used to work out the molecular formula of any compound in the homologous series
• Isomerism: when molecules have an identical molecular formula but different structural formulae

b) Alkanes

3.2 recall that alkanes have the general formula CnH2n+2

General formula of alkanes is CnH2n+2

Alkanes are saturated hydrocarbons as no more bonds can be made in the molecule

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

• Methane: CH4

• Ethane: C2H6

• Propane: C3H8

• Butane: C4H10

• Pentane: C5H12

Straight-chain isomers of:

• Butane:

The isomer pictured is called 2-methylpropane because the longest straight chain of carbon atoms is 3 (long the top), that's where the propane part comes from. The methyl group, CH3, is bonded the to 2nd carbon in the chain, that's where the 2 comes from.

• Pentane:

 

The isomer pictures is called 2-methylbutane because the longest straight chain of carbon atoms is 4 (along the bottom), that's where the butane part comes from. The methyl group, CH3, is bonded to the 2nd carbon, that's where the 2 comes from. Another isomer of pentane is 3-methylbutane (where the methyl group is on the 3rd carbon in the chain)

NOTE: if the methyl group is on the 1st or last carbon in the chain it is NOT an isomer, just the normal molecule but bent.

3.4 recall the products of complete and incomplete combustion of alkanes

Complete combustion: alkane + oxygen → carbon dioxide + water (+ energy)

CH4 + 2O2 → CO2 + 2H2O

Incomplete combustion: alkane + oxygen  → carbon + carbon monoxide + carbon dioxide + water (+ energy) 

4CH4 + 6O2 → C + 2CO + CO2 + 8H2O

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

Alkane + halogen → haloalkane + hydrogen halide

Methane + bromine →  bromomethane + hydrogen bromide

UV light is necessary for the reaction to take place

It is a substitution reaction as one of the hydrogen atoms is replace by a halogen atom

c) Alkenes

3.6 recall that alkenes have the general formula CnH2n

General formula of alkenes is CnH2n

Alkenes are unsaturated hydrocarbons as the carbon-carbon double bond can open up allowing the carbon atoms to bond with other atoms

3.7 draw displayed formulae for alkenes up to four carbon atoms in a molecule, and name the straight chain isomers

• Ethene: C2H4

• Propene: C3H6

• Butene: C4H8

But-2-ene (double bond is on 2nd carbon)

Straight-chain isomers of butene:

But-1-ene (double bond is on 1st carbon in chain)

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

Halogen + alkene → haloalkane

Bromine + ethene → dibromoethane

It is an addition reaction as the carbon-carbon double bond is split and a halogen atom is added to each carbon atom

(1,2-dibromoethane as the Br atoms are on the 1st and 2nd carbons in the chain, 'dibromo' - as 2 BR atoms are added, 'ethane' - as there are 2 carbon atoms in the molecule and no double bond and it is now saturated)

d) Ethanol (TRIPLE AWARD ONLY)

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

1. Ethene is produced from the cracking of crude oil
2. Ethene (C2H4) will react with steam to make ethanol - C2H4 + H2O → C2H5OH
3. The reaction needs: a temperature of 300C and a pressure of 60-70 atm
4. Phosphoric acid is used as a catalyst

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

1. The raw material is sugar (glucose), which is converted into ethanol and carbon dioxide using yeast (a natural catalyst)
2. A temperature of 30C is needed to make this process most efficient

3.11 evaluate the factors relevant to the choice of method used in the manufacture of ethanol

• Ethene + steam:
• Very fast reaction
• Pure product (ethanol) produced
• The raw material, ethene, is from a finite, non-renewable resource (crude oil), so it will run out at some point
• The continuous process at a high temperature and pressure needs expensive equipment
• There are low labour costs (don't need many workers) as machines take care of the process
• Fermentation:
• Very slow reaction (slower than ethene and steam)
• Very impure product produced - needs further processing
• The raw material, sugar, is a renewable resource
• A lower temperature is needed so the equipment is cheaper and more basic
• Higher labour costs as more workers involved in the process as the equipment is basic

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

This process is removing the water from ethanol to leave ethene

1. Ethanol vapour (heating ceramic wool soaked in ethanol) is passed over a hot catalyst of aluminium oxide 
2. The aluminium oxide is a powder, which provides a large surface area for the reaction
3. The ethene gas is collected over water into a gas jar or test tube
4. C2H5OH→ C2H4 + H2O