Alkanes and their Processes

Revision Slides

Alkanes Introduction

• Alkanes are an organic homologous series- a repeating unit with a general trend in their formulae- that increase by CH2 each time.

• Hence, the general formula for alkanes is CnH2n+2

• They are saturated as all bonds that aren’t C-C are C-H, so they are ‘soaked’ in hydrogen.• They are hydrocarbons- molecules made up of hydrogen and

carbon, only.

Types of FormulaeSkeletal Displaye

d

Molecular

Structural

C4H10

Unbranched Chains

• Often called straight chained alkanes, however, they aren’t actually straight as the C-C-C bond angle is 109.5˚. This causes the chain to be relatively ‘zig-zagged’. • In this structure, each carbon has two hydrogen atoms

covalently bonded to it and at least on other carbon (excluding Methane, CH4), and an extra hydrogen atom on the end, e.g.

Branched Chains• Hydrocarbons may also be branched, with some carbon

atoms having only one hydrogen attached and instead having three carbons attaching to its available spaces. • Remember, the hydrocarbon is only branched if the

branches aren’t coming off of an endpiece carbon such as in methyl butane, an isomer of pentane:

Ring Alkanes• Ring alkanes have the same general formula of an alkene-

an unsaturated hydrocarbon with at least one C=C double bond- CnH2n • The only differences between a ‘cyclic’ alkane and an

alkene are that the cyclic alkane, as the name suggests, has a ring-shape carbon chain whereas alkenes are straight. Also, alkenes are double-bonded whereas cyclic alkanes aren’t, as shown by cyclohexane:

Nomenclature of Alkanes1. Alkanes are named from their root which tells us the

number of carbon atoms, and then the suffix is –ane. 2. When naming a branched alkane, first find the longest

possible carbon chain. This gives the root name for the alkane. Then, name the side branches with the prefixes- methyl, ethyl, propyl etc. Finally add numbers to say which carbon atom they’re on, and try to keep these numbers as low as possible. Separate all letters and numbers with a dash.

3. If the atom/functional group is a halogen, use the prefixes- fluoro, chloro, iodo.

4. If the group is a hydroxyl, OH, the suffix changes to ‘ol’ e.g. pentan-2-ol (if the OH is attached to the second carbon) and so on.

Isomerism• Isomers are “one of two (or more) compounds with the

same molecular formula but with a different arrangement of atoms in space.” Shown by pictures below.• Methane, Ethane and Propane, due to their short chains,

have no isomers but after this, the number of isomers increases with chain length. E.g. butane has two isomers whereas pentane has three and decane, C10H22, has seventy five.

Pentane

Methyl Butane

Both areC5H12

H

Physical Properties- Polarity

• Alkanes, as the carbon atom , 2.5, isn’t significantly more electronegative than hydrogen, 2.1, are nearly completely non-polar. As a result, the only intermolecular forces acting between them are weak van der Waals forces, and thus the larger the molecules, the stronger the van der Waals forces acting between them.

Physical Properties- Boiling Points

• This increasing intermolecular force is why the boiling points of hydrocarbons increase as the chain length increases. The shorter chains, thus, have very low boiling points so the van der Waals forces are overcome at a low temperature and thus, methane ethane, propane and butane are gases at room temperature, in chemistry this is 25˚C. From pentane to Heptadecane (18 carbons) they are liquids and from then on they are solids with a waxy feel to them. • Branched alkanes have lower melting points than straight chains

as their branches mean that they cannot pack so closely together, so their van der Waals forces are weaker.

Physical Properties- Solubility

• As alkanes are non-polar, they are insoluble in water. Water molecules are held together by the strongest type of intermolecular force, hydrogen bonding (as well as their own van der Waals forces) and alkanes have only van der Waals forces, so they do not mix. However, they do mix with other relatively non-polar liquids.

Alkane’s Reactions

• Alkanes are relatively unreactive. They have strong C-C bonds as well as strong C-H bonds. They do not react with acids, bases, oxidising agents or reducing agents. However, they do burn when reacted with oxygen and they will, under suitable conditions, react with halogens to form halogenoalkanes.

Fractional Distillation To Separate Crude Oil

Steps of Fractional Distillation:1. The crude oil mixture is vapourised in a furnace.

Bitumen/tar has a boiling point higher than the furnace’s temperature and so is tapped off as a liquid at the bottom.

2. The mixture of gaseous hydrocarbons passes up the tower, which is cooler at the top than the bottom- it has a temperature gradient.

3. They pass through ‘bubble-caps’ until they reach a tray that is sufficiently cool, cooler than their boiling point. Here, they condense onto the tray and are tapped off.

4. Shorter-chained hydrocarbons, with their lower boiling points are tapped off at the top end of the tower whereas, with their higher boiling points, longer-chained hydrocarbons are tapped off at the bottom of the tower.

Thermal Cracking

• Thermal Cracking is the cracking of long-chain hydrocarbons to produce, mainly, alkenes. However, this is a very short reaction as if it were to be any longer, they’d decompose further into their hydrogen and carbon components. • This reaction is done industrially under pressures of up to

7000kPa (7000 atmospheres) and a temperature of 700-1200K (427-927°C).

Catalytic Cracking

• Catalytic Cracking, as its name suggests, uses a catalyst to reduce the pressure and temperature needed for the reaction. These catalysts may be silicon dioxide or aluminium dioxide (aluminosilicates) or zeolites- acidic catalysts with a honeycomb structure to give a high surface area for the reaction to take place on.• This is mainly done to produce fuels- alkanes, cycloalkanes

and aromatic compounds. • This reaction is done industrially under pressures of up to

70kPa (70 atmospheres) and a temperature of approximately 720K (447°C).

Combustion

• Alkanes may be burnt in oxygen to produce water and carbon dioxide:

• This is an example of the reaction that takes place in some power stations (those using natural gas and hydrocarbons) and in combustion engines such as those in a car.

Incomplete Combustion

• As combustion may occur in a system with poor ventilation, there may be a lack of oxygen so incomplete combustion will occur:

• As shown above, a lack of oxygen gives carbon monoxide, a colourless, odourless, toxic gas off as a product. If there were to be an even more dire lack of oxygen in the system, carbon particulates, soot, are formed, which are a cause of smog and are a danger to those with asthma and other respiratory hindrances:4CH4 + 4O2 4C + 8H2O

4CO + 8H2O4CH4 + 6O2

Flue Gas Desulphurisation

• Large numbers of power stations generate electricity by burning fossil fuels such as coal or natural gases, but these may contain sulphur compounds, also. As sulphur burns to produce sulphur dioxide, a component of acid rain (sulphuric acid), it needs to be removed from the products before their release. • To do this, calcium oxide and water are sprayed into the

flue gas (sulphur dioxide and oxygen, basically) to form calcium sulphite which can be oxidised to produce calcium sulphate, or gypsum.

Catalytic Converters• As nitrogen may also be present in the fuel before burning, in the

presence of a high temperature and a spark, nitrogen oxide may be produced. • A pollutant gas, it needs to be removed. Catalytic converters have a

ceramic, honeycomb structure plated with rhodium or palladium. These reduce nitrogen oxide into nitrogen and oxygen; its components: