Structure of aliphatic

hydrocarbons

Aliphatic hydrocarbons

Aliphatic hydrocarbons are hydrocarbons that consist of straight or branched chains of carbon atoms, or rings of carbon atoms other than those containing a special ring called a benzene ring

Pentane – a straight chain aliphatic hydrocarbon

A branched chain hydrocarbon

Cyclohexane – a cyclic aliphatic hydrocarbon

Homologous series

A homologous series is a family of organic compounds with the same general formula, similar chemical properties, and successive members differing by CH2

Alkanes, alkenes and alkynes are three different homologous series of aliphatic hydrocarbons

Alkanes

Alkanes are a homologous series of hydrocarbons with the general formula CnH2n+2

They are named systematically, with a prefix indicating the number of carbon atoms per molecule, and the ending “ane”

Alkanes

Methane CH4

Ethane C2H6

Propane C3H8

Butane C4H10

Pentane C5H12

Hexane C6H14

Heptane C7H16

Octane C8H18

Alkanes

Alkane molecules are tetrahedral, e.g. propane

Structural formulas

The structural formula of an alkane indicates the way atoms in a molecule of the alkane are bonded together

The following slides show the structural formulas of some of the alkanes

Methane

Ethane

Propane

Butane

Pentane

Structural isomers

Structural isomers are compounds that have the same molecular formula but different structural formulas

Butane (C4H10)is the simplest alkane which has structural isomers, called butane and 2-methylpropane respectively

The name 2-methylpropane indicates that there is a methyl (CH3) group attached instead of one of the hydrogen atoms to the second carbon in propane

Isomers of butane

An isomer of octane (C8H18)

2,2,4-trimethylpentane is so called because there are two methyl groups attached (in place of hydrogen atoms) to the second carbon atom and one (in place of another hydrogen atom) to the fourth carbon atom in a pentane molecule

Physical properties of alkanes

Physical state: The first four alkanes are gases, while pentane and higher alkanes are liquids

Insoluble in water Soluble in non-polar solvents such as

cyclohexane

Alkenes

Alkenes are a homologous series of hydrocarbons with the general formula CnH2n

They are named systematically, with a prefix indicating the number of carbon atoms per molecule, and the ending “ene”

Alkenes

Ethene C2H4

Propene C3H6

But-1-ene C4H8

But-2-ene C4H8

Ethene

Propene

But-1-ene

But-2-ene

Physical properties of alkenes

Physical state: Gases

Insoluble in water

Soluble in non-polar solvents such as cyclohexane

Alkynes

Alkynes are a homologous series of hydrocarbons with the general formula CnH2n-2

Ethyne (C2H2) is the first member of the series

Physical properties of ethyne

Physical state: Gas Insoluble in water Soluble in non-polar solvents such as

cyclohexane