Chemistry AS - Chapter 06 - Alkanes

Alkanes are saturated hydrocarbons. They contain carbon-carbon and carbon-hydrogen single bonds. They are among the least reactive organic compounds.

They are used as fuels and lubricants and as starting materials for other compounds. They are very important for industry. The main source of alkanes is crude oil.

General formula: CnH2n+2 . Hydrocarbons may be unbranched chains, branched chains or rings.

Unbranched chains: Eg. Pentane C5H12

Unbranced chains are often called "straight chains" but the C-C-C angle is 109.5. They aren't actually straight. In an unbranched alkane, each carbon atom has two hydrogen atoms except the end carbons which have three.

Branched chains: eg. methylbutane, an isomer of pentane (C5H12)

Ring alkanes: general formula is CnH2n because the end hydrogens are not required.

Alkanes are named from the root which tells us how many carbon atoms there are and the suffix, -ane, denoting an alkane.

When naming a branched chain, find the longest unbranched chain. This gives the root name. Then name the branches/side chains (methyl-, ethyl-, propyl- etc.) Then add numbers to say which carbon atoms the side chains are attached to.

Methane, ethane and propane have no isomers but after that, the number of possible isomers increase with the number of carbons in the alkane. Eg. butane has 2 isomers, pentane has 3.

Number of isomers rises rapidly with chain length. Decane has 75 and C30H62 has over 4 billion.

Alkanes are almost non-polar because the electronegativities of carbon and hydrogen are very similar. As a result, the only intermolecular forces between their molecules are weak van der Waals forces. Larger the molecule, stronger the van der Waals forces.

This increasing intermolecular force is why boiling points of alkanes increases as chain length increases. Shorter chains are gases at room temperature. Pentane is a liquid with a boiling point of 309K. At 18 carbons, alkanes are solid at room temperature.

Alkanes with branched chains have lower melting points than straight chain alkanes with the same number of carbon atoms. This is because they cannot pack together as closely as unbranched chains and so the van der Waals forces are not so effective.

Alkanes are insoluble in water as water molecules are held by hydrogen bonds which are stronger than van der Waal's forces.

Alkanes are unreactive but do burn in oxygen.

Crude oil is at present the world's main source of organic chemicals. It's a fossil fuel, formed million of years ago from the breakdown of plant and animal remains at high pressures and temperatures deep below the Earth's surface. It is not renewable as it originated millions of years ago.

Crude oil is a mixture of mostly alkanes, both unbranched and branched. Crude oil contains small amounds of other compounds dissolved in it. These come from the original plants and animals the oil was formed from. Example: sulfur. 

When sulfur is burned, sulfur dioxide is produced which is one of the causes of acid rain. Sulfur dioxide reacts with oxygen high in the atmosphere to form sulfur trioxide which reacts with water in the atmosphere to form sulfuric acid.

To convert crude oil into useful products, we need to separate the mixture. We do this by heating it and collecting the fractions that boil over different ranges of temperatures.

Each fraction is a mixture of hydrocarbons of similar chain length and therefore similar properties.

The process is called fractional distillation and is done in a fractionating tower.

• Crude oil is heated in a furnace
• Mixture of liquid and vapour passes into a tower that is cooler at the top than at the bottom.
• Vapours pass up the tower via a series of trays containing bubble caps until they arrive at a tray that is sufficiently cool (lower temp. than their boiling point). They condense into a liquid.
• Mixture of liquids is piped off.
• Shorter chain hydrocarbons condense in trays nearer to the top of the tower where it is cooler as they have lower boiling points.
• The thick residue that collects at the base of the tower is called tar or bitumen. It can be used for road surfacing but as the supply often exceeds demand, this fraction is often further processed to give more valuable products.

The naptha fraction is in huge demand for petrol and other chemicals. Longer chain fractions are not as useful and therefore of lower value economically.

Most crude oil has more of the longer chain fractions than is wanted and not enough of the naptha fraction.

Therefore, to meet the demand for the shorter chain hydrocarbons, many of the longer chain fractions are broken into shorter lengths (cracked). Produces two results:

• Shorter, more useful chains are produced, especially petrol.
• Some of the products are alkenes, more reactive than alkanes.

Alkenes are used as chemical feedstock (supply industries with the starting materials to make different materials). They are converted into a huge range of compounds including polymers, paints and drugs. Most important alkene is ethene which is the starting material for polyethene.

Alkanes are very unreactive and harsh conditions are required to break them down. There are a number of ways to carry out cracking.

Thermal Cracking: Heating alkanes to a high temperature (700K-1200K) under high pressure (700kPa).

Carbon-Carbon bonds break so that one electron from the pair in the covalent bond goes to each carbon atom. 

Initially, two shorter chains are produced each ending in a carbon atom with an unpaired electron. These fragments are called free radicals.

Free radicals are highly reactive intermediates and react in a number of ways to form a variety of shorter chain molecules.

As there are not enough hydrogen atoms to produce two alkanes, one of the new chains must have a carbon-carbon double bond, therefore an alkene is made.

Any number of C-C bonds may break and the chain does not necessarily break in the middle. Hydrogen may also be produced. Thermal decomposition usually produces a high proportion of alkenes. To avoid too much decomposition, alkanes are kept in same conditions for just one second.

Catalytic Cracking: Takes place at a lower temperature (720K) and lower pressure (more atmospheric) using a zeolite catalyst which consists of silicon dioxide and aluminium oxide.

Zeolites have a honeycomb structure with enormous surface area. They are acidic. This form of cracking is used to produce motor fuels.

Products are mostly branched alkanes, cycloalkanes and aromatic compounds.

The products obtained from cracking are separated by fractional distillation. 

To test for a carbon-carbon double bond, the product must decolourise bromine solution.

Alkanes are quite unreactive, however, they do burn and will react with halogens under suitable conditions.

Shorter chain alkanes burn completely in a plentiful supply of oxygen to give carbon dioxide and water.

These combustions give out heat. They have a large negative enthalpy of combustion and the more carbons present, the greater the heat output. For this reason, they are important as fuels.

Examples of alkane fuels include:

• methane (North Sea gas)
• propane (camping gas)
• butane (Calor gas)
• petrol (mixture of hydrocarbons of chain length C8)
• paraffin (mixture of hydrocarbons of chain lengths C10 to C18)

In a limited supply of oxygen, the poisonous gas carbon monoxide, CO, is formed. This is called incomplete combustion. With even less oxygen, soot is formed (carbon). This usually occurs with longer chain hydrocarbons as they require more oxygen to burn compared with shorter chains.

All hydrocarbon based fuels which are derived from crude oil may produce polluting products when they burn. Include:

• Carbon Monoxide - CO
• Nitrogen Oxides - NO, NO2, N2O4, produced when there is enough energy for N2 and O2 to combine in the air. This happens in petrol engines at the high temperatures present, when the sparks ignite the fuel. Oxides may react with water vapour and oxygen to form nitric acid. Contributes to acid rain and photochemical smog.
• Sulfur Dioxide - SO2 - another contributor to acid rain. Produced from the sulfur-containing impurities in crude oil. The oxide combines with water vapour and oxygen to form sulphuric acid.
• Carbon particles - AKA particulates, can cause cancer and exacerbate asthma.

• Carbon dioxide - CO2 - greenhouse gas. Always produced when hydrocarbons burn. CO2 is necessary in the atmosphere by its level is rising.
• Water vapour - also a greenhouse gas.

Power stations burn coal or natural gas to produce electricity. Some chimneys use calcium oxide or limestone to absorb sulfur dioxide. This produces gypsum which is used as a plaster. This is called flue gas desulfurisation.

The internal combustion engine produces most of the pollutants. All new cars with petrol engines are equipped with catalytic converters. These reduce the output of carbon monoxide, nitrogen oxides and unburnt hydrocarbons in the exhaust gas mixture.

The catalytic converter is a honeycomb (large surface area) made of ceramic material coated with platinum and rhodium metals. These are catalysts.

As polluting gases pass over the catalyst, they react with each other to form less harmful products.

2CO + 2NO --> N2 + CO2

hydrocarbons + nitrogen oxide ---> nitrogen + carbon dioxide + water

Reactions take place on the surface of the catalyst, on the layer of platinum and rhodium metals.

Greenhouses become very warm as visible rays from the sun pass through the glass and instead of escaping, they are being absorbed and re-radiated as infra-red energy (heat). It has a longer wavelength and cannot pass back out through the glass.

Carbon dioxide acts like glass. It traps infra-red radiation so the Earth's atmosphere heats up. This is important for life as without CO2 and other greenhouse gases, Earth would be too cold to sustain life.

Since the industrial revolution, level of these gases in the atmosphere has increased. Gradually, the Earth's temperature has been rising too. Scientists believe the rise in CO2 in the atmosphere is the cause of this.

The concentration of water vapour, the most abundant greenhouse gas tends to stay roughly the same because of equilibrium that exists between water vapour and the liquid. However, if the temperature of the atmosphere rises, there will be more vapour in the air, resulting in more greenhouse warming. Could cause more clouds to form.