Topics 3abc – Alkanes, alkenes and ethanol

Topics 5bc – natural oil and gas and synthetic polymers

Crude oil (Petroleum)

• A mixture of hydrocarbons (chemicals which contain only hydrogen and carbon atoms)

Fractional distillation

• Separation of liquids with different boiling points

Fractional distillation

The mixture can be split into simpler fractions by fractional distillation

Welcome to a new family

The alkanes

Methane (CH4)

Ethane (C2H6)

Propane (C3H8)

Butane (C4H10)

The alkanes

• Methane CH4

• Ethane C2H6

• Propane C3H8

• Butane C4H10

• Pentane C5H12

• Hexane C6H14

• Octane C8H18

The alkanes CnH2n + 2

• Methane CH4

• Ethane C2H6

• Propane C3H8

• Butane C4H10

• Pentane C5H12

• Hexane C6H14

• Octane C8H18

General formula

Homologous group – similar chemical properties, structures and functional groups

The alkanes CnH2n + 2

• Methane CH4

• Ethane C2H6

• Propane C3H8

• Butane C4H10

• Pentane C5H12

• Hexane C6H14

• Octane C8H18

General formula

Homologous group – similar chemical properties, structures and functional groups

Generally unreactive, but burn exothermically. Longer chains burn less easily.

Structural formulae

Alkanes are saturated compounds

No double bonds

Tetrahedral bonds

In alkanes, the carbon bonds are tetrahedral

Chlorinated methane

Methane can be chlorinated. Trichloromethane is commonly known as chloroform.

Isomers

Same formula, but different structures

We have slighty different physical properties (longer chains have higher boiling

points)

Combustion

Combustion

Hydrocarbons burn producing carbon dioxide and water

CH4 + 2O2 CO2 + 2H2O

Incomplete combustion

• When there is insufficient oxygen, carbon monoxide is also formed

It reduces the blood’s oxygen carrying capability

Another family!

The alkenes CnH2n

• Ethene C2H4

• Propene C3H6

• Butene C4H8

• Pentene C5H10

• Hexene C6H12

The alkenes

• Unsaturated (contain a double bond)

• More reactive

• Can “add” atoms – addition reactions

Alkenes - equilateral

The bonds on a double bonded carbon in an alkene point towards the corners of an equilateral triangle

Test for alkenes

• Decolourises bromine water

ethene Bromine (yellow/orange)

Dibromoethane (colourless)

Alcohols - ethanol

Alcohols

• Methanol CH3OH

• Ethanol C2H5OH

• Propanol C3H7OH

• Butanol C4H9OH

• Pentanol C5H11OH

• Hexanol C6H13OH

Alcohols

• Methanol CH3OH

• Ethanol C2H5OH

• Propanol C3H7OH

• Butanol C4H9OH

• Pentanol C5H11OH

• Hexanol C6H13OH

“functional group”

Ethanol

Can be made by the addition reaction of water to ethene – when you have plenty of oil

Phosphoric acid

Ethanol - Fermentation of sugars

You can make ethanol for industry this way when you

have lots of sugar cane

Dehydration of Ethanol

Ethanol and sodium

2C2H5OH + 2Na 2C2H5ONa + H2

Hydrogen and sodium ethoxide formed

Oxidation of ethanol

Ethanol can be oxidised to form ethanoic acid (acetic acid = vinegar!)

Carboxylic acids

• Methanoic acid CHOOH

• Ethanoic acid C2H3OOH

• Propanoic acid C3H5OOH

• Butanoic acid C4H7OOH

• Pentanoic acid C5H9OOH

• Hexanoic acid C6H11OOH

Carboxylic acids

• Methanoic acid CHOOH

• Ethanoic acid C2H3OOH

• Propanoic acid C3H5OOH

• Butanoic acid C4H7OOH

• Pentanoic acid C5H9OOH

• Hexanoic acid C6H11OOH

“functional group”

Ethanol + ethanoic acid acids

Ethanol + ethanoic acid ethyl ethanoate + water

An ester (they normally smell nice!)

Ethyl ethanoate – an “ester”

Polymers

Polymers – addition polymerisation

Molecules of ethene can be joined together to form polyethene

← monomers

← polymer

Polythene

Polypropylene – Strong and resistant to chemicals

poly(chloroethene) - PVC

Condensation polymers – NylonDiamine + dicarboxylic acid

+ + + monomers

Reactive ends

polymer

Small molecules given off (HCl)

Cracking!

Too many long molecules

Crude oil contains too many of the long (and not so useful molecules)

Cracking

Luckily we can break the long molecules into shorter ones by “cracking”

The long molecules are passed over hot broken pot (ceramic) and split up into smaller molecules. A catalyst can also be used

Cracking

Cracking an alkane produces smaller alkanes and also alkenes.