Hydrocarbons (Alkanes, Alkenes and Alkynes)

As the name suggest, hydrocarbons are compounds containing carbon and hydrogen only. They are mainly derived from coal, natural gas, petroleum and plants. They may be saturated or unsaturated, they may be aliphatic or aromatic. Saturation is the amount of substituents around carbon, in the case of alkanes each carbon has 4 substituents, the maximum allowed by valency, therefore they arer known as saturated compounds. Alkenes and alkynes have double and triple bonds respectively therefore do not have the maximum 4 valency, these types of compounds are therefore unsaturated compounds.

Hydrocarbons are the basis of many fuels: such as butane for gas cookers etc, Acetylene (ethyne) for weilding equipment, and the mixtures of hydrocarbons for motor fuel.

Naming Hydrocarbons:

1. Find the longest possible continuous chain of carbon atoms in the molecule. The molecule will be named as a derivative of this alkane.

i.e	Chain	Name
1 2 3 4 5 6 7 8 9 10		= meth- = eth- = prop- = but- = pent- = hex- = hept- = oct- = non- = dec-

2. Determine the names of the hydrocarbon groups attached to this chain. These side chains are called alkyl groups. There names are obtained by dropping the ending -ane from the corresponding alkane and replacing it with -yl i.e a side chain with 3 carbons would by propyl-.
3. Number the carbon atoms along the chain. Identify the alkyl groups by the number of the carbon atom that they are attached to.
4. If more than one alkyl group of the same type is attached to the molecule then use the prefix di- (two) tri- (three) tetra- (four) in front of the hydrocarbon name.
5. If several types of alkyl groups are attached, namer them in alphabetical order.
6. If the hydrocarbon contains unsaturation then find the number of the carbon with the double or triple bond on it and prefix this number in fron of the ending -ene (alkenes) and -yne (alkynes)

Alkanes

Alkanes are saturated hydrocarbons. The simplest hydrogen is methane (CH4) which is tetrahedral in structure.

There are also more complicated alkanes, such as branched chain alkanes and cycloalkanes. A common occuring cycloalkane is cycloheaxane:

Alkanes are very unreactive apart from cracking and burning in air with a non-luminous flame.

The only react by substituting radicals for hydrogen atoms:

Free radical substitution of Alkanes

At 20'c and in the dark Chlorine and methane do not react. If the mixture is heated above 200'c or is irradiated by ultra violet light a rapid chain reaction takes place.

This chain reaction takes place in three steps:

1. INITIATION :- the Cl2 undergoes homolytic fission and splits into two Cl' radicals.
2. PROPAGATION :- a radical combines with a stable molecule to give a different radical and molecule.

e.g CH4 + Cl' ------------> CH3' + HCl

A chain reaction is then set up when the radicals are combining with stable molecules. This continues until no more product can be formed i.e CCl₄ or the reaction is terminated.

3. TERMINATION :- the reaction can be terminated in three ways: (all involve the combination of 2 radicals)

i) Cl' + Cl' -----------> Cl₂

ii) CH₃' Cl' --------------> CH₃Cl

iii) CH₃' + CH₃' -----------------> CH₃CH₃

Alkenes

This is ethene. The smallest alkene.There is a sigma and a pi bond between the C=C. This means that the bond is shorter and stronger than a single bond.

Alkenes are unsaturated hydrocarbons containing one or more double bonds. Alkenes are prepared by the dehydration of alcohols or by the dehydrobromination of bromoalkanes.

Alkenes are named by the name of the straight chain alkane with a number representing the position of the double bond. The prefixes cis- and trans- (now Z- and E-) are also used to name the simple geometric isomers (see page on isomerizm).

Alkenes undergo electrophilic addition reactions which follow Markovnikovs rule. Alkenes react with halogens such as Br-Br or hydrogen halides such as H-Br. They also react with bromine water and sulphuric acid. All these reagents are able to react with alkenes because they have an electrophilic part on there molecule i.e H-Br has an electrophilic (electron deficient) H which is attracted to the electron rich area of a double bond. The electrophilic part attatches itself to the carbon with the most hydrogens already attached to it, and then the nucleophilic part of the molecule bonds to the carbocation intermediate.

Alkynes

This is ethyne. The smallest alkyne. The C=C triple bond is shorter and stronger than that of an alkane or an alkene.

Alkynes are unsaturated hydrocarbons with one or more triple bonds. They are formed by the hydrolysis of calcium dicarbide or by the dehydrohalogenation of 1,2-dibromoethane using hot alcoholic potassium hydroxide. Alkynes do not undergo many reactions, but are used industrially in cutting and weilding equipment.

Hydrocarbon (Alkanes, Alkenes, and Alkynes) pdf version