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