A-level Chemistry/OCR/Chains, Energy and Resources/Basic Concepts and Hydrocarbons/Alkanes

The AlkanesEdit

Alkanes are saturated hydrocarbons. This means that they contain only carbon and hydrogen atoms and they contain no carbon-carbon double bonds.

The alkane homologous series has the general formula of  C_{n} H_{2n+2} except for the cycloalkanes in which 2 hydrogens are lost so that the carbon can form another C-C bond; they have the general formula of  C_{n} H_{2n}

Physical PropertiesEdit

The alkanes are the most complex of the organic molecules. There are a few things you will need to know about their physical properties in an exam:

Boiling/Melting PointEdit

As the relative molecular mass, or number of carbon atoms in an alkane increases, so does its boiling or melting point:


Alkane Molecular Mass Molecular Formula Boiling Point (K)
Methane 16 CH4 109
Ethane 30 C2H6 185
Propane 44 C3H8 231
Butane 58 C4H10 273
Pentane 72 C5H12 309


As you should be aware, alkanes are held together by Van der Waal's forces. The larger a molecule is, the more electrons it has. This means it can form larger dipoles and its Van der Waal's forces will be larger. It will therefore take more energy to break the bonds and so the boiling and melting point will be higher.

As you should know, however, some alkanes can be branched instead of straight chained. Boiling point decreases as a molecule becomes more branched. This is because branched molecules cannot pack so tightly together, so their Van der Waal's forces must act over larger distances (intensity of the Van der Waal's forces decreases) and thus require less energy to break. For example, hexane has five isomers:


Isomer Structural Formula Boiling Point (K)
Hexane CH3CH2CH2CH2CH2CH3 342
3-Methylpentane CH3CH2CH(CH3)CH2CH3 337
2-Methylpentane CH3CH(CH3)CH2CH2CH3 333
2,3-Dimethylbutane CH3CH(CH3)CH(CH3)CH3 331
2,2-Dimethylbutane CH3CH(CH3)(CH3)CH2CH3 323

ReactionsEdit

The alkanes are fairly unreactive, as the C-H bond is non-polar. However they undergo three reactions.

Reactions of alkanesEdit

Take the alkane C_2 H_6 .

It will under go combustion to form CO_2 + H_2O

It will under go substitution reactions (Say with Cl_2) to create C_2 H_5 Cl + HCl

It will also undergo cracking reactions to create an alkene, in this case,  C_2 H_4 + H_2

The most important of these being combustion, alkanes are very volatile & burn easily when in the presence of plentiful amounts of oxygen & thus are major fuels.

CrackingEdit

As said earlier, long chain molecules have higher boiling points and are more difficult to ignite. These long chain molecules can be broken into shorter chain (more useful) molecules through catalytic cracking. Any alkane from  C_4 to  C_{50} can be cracked. As it is catalytic cracking it requires a catalyst, previously this catalyst was either  SiO_2 or  Al_2 O_3 , but now zeolite is preferred. These reactions also need high temperatures (773K or 450ºC is usually used).

The products from cracking can vary. For example you can create several moles of one alkene and hydrogen from cracking an alkane if done right. However you can also create a mixture of alkenes & alkanes or just a mixture of alkenes.

Example:  C_8 H_{18} \rightarrow \; 4 C_2 H_4 + H_2

Under conditions of 773k and with an  Al_2 O_3 catalyst present.

ReformingEdit

Reforming reactions are reactions where a straight chain alkane is converted into a branched alkane or cycloalkane to increase the octane number in petrol. (Octane numbers represent how much pressure a fuel can be put under before it combusts & causes knocking which damages engines).

Again, reforming uses an platinum or rhodium catalyst & a high temperature, but this time it also uses a higher pressure (40 atm or so). The by-product of reforming is hydrogen molecules.

Free radical substitutionEdit

Alkanes are very unreactive due to the non-polar C-H bond, so nucleophilic & electrophilic attack reactions are not possible. However, free radical substitutions are possible due to their reactivity. Free radical substitution involves a halogen & some UV light to initiate homolytic fission.

An example with methane.

Initiation reaction:

 Cl_2 \rightarrow\; Cl^\bullet + Cl^\bullet


Propagation:

 Cl^\bullet + CH_4 \rightarrow HCl + CH_3^\bullet

This sets into motion the chain reaction, where one free radical reacts with a stable species to form another stable species & a free radical.

The next propagation steps:

 CH_3^\bullet + Cl_2 \rightarrow CH_3Cl + Cl^\bullet

Or

CH_3^\bullet + CH_3Cl \rightarrow C_2H_6 + Cl^\bullet

This goes on until 2 radicals meet & combine to form a stable species & thus once all the radicals have done this, the reaction terminates, hence why it is the termination step.

In this reaction we have 3 possible termination steps:

CH_3^\bullet + Cl^\bullet \rightarrow CH_3Cl

Cl^\bullet + Cl^\bullet \rightarrow Cl_2

CH_3^\bullet + CH_3^\bullet \rightarrow C_2H_6

Henceforth, the reaction ends.

Note that a free-radical hydrogen is never produced.

ProblemsEdit

As we know from above, alkanes burn readily in oxygen & give off carbon dioxide, which is a weak greenhouse gas and thus a contributor to global warming.

Alkanes must burn in excess oxygen for complete combustion and the production of carbon dioxide:

CH4 + 2O2 -> CO2 + 2H2O

Incomplete combustion comes from a reduced supply of oxygen and produces water and carbon monoxide:

CH4 + 1½O2 -> CO + 2H2O

Carbon monoxide is poisonous when inhaled. It binds irreversibly to haemoglobin in red blood cells and makes carboxyhaemoglobin. The red blood cells can no longer carry oxygen and this can cause suffocation if enough cells are carrying carboxyhaemoglobin.

UK regulations ensure that all gas equipment must be annually serviced and that all gas installations require adequate ventilation.

Last modified on 9 April 2014, at 14:09