Chemistry

Chemistry rules and formulae.

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Alkanes

Alkanes are a sub-set of hydrocarbons. They all end in -ANE and have similar chemical properties while their physical properties such as melting and boiling point show gradual changes as the molecules get bigger.

All members of the family fit the same general formula. They are said to belong to a homologous series. Larger alkanes have higher boiling points than smaller alkanes - the bigger the alkane, the higher the boiling point. The general formula is - CnH2n+2

Cycloalkanes are molecules in which one end of the alkane chain is joined to the other end of the chain. To allow this, one hydrogen atom from each end of the chain of carbon atoms has to be removed. Cycloalkanes make another homologous series of general formula cycloalkanes CnH2n

The smallest cycloalkane is cyclopropane (C3H6), and continues with cyclobutane (C4H8) etc.

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Alkenes

There is another subset of hydrocarbons called alkenes.These have a double bond between two of the carbon atoms. Alkenes have names which end in the letters -ENE

Alkenes make another homologous series of compounds and the double bond means that alkenes will have two less atoms of hydrogen than corresponding alkanes.
The general formula will be CnH2n

As both alkenes and cycloalkanes have the same general formula, isomers are possible from different homologous series.

Sidenote: Alkanes are known as saturated hydrocarbons because there are only single bonds between the carbon atoms. Alkenes are known as unsaturated hydrocarbons because there is a double bond between two carbon atoms. Alkenes decolourise bromine.

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