Features of Alkanes Objectives: To look at the physical properties of the alkanes To introduce cycloalkanes To compare and contrast the physical properties of cycloalkanes and straight chain alkanes;

Long chain Alkanes

Increasing length As hydrocarbon chains increase in length The become heavier They have more possible isomers The have higher melting and boiling points They contain increasing amounts of energy per mole.

What is a cyclo-alkane They are alkanes containing a ring of three or more carbons The have the molecular formula CnH2n

Where do they come from? Petroleum or Crude oil contains approximately 150 hydrocarbons Approximately half are alkanes or cycloalkanes 1 – 4 C = 5 – 12 C = 12 – 15 C = 15 – 25 C =

Other organic sources Pentacosane (25 C) present on the waxy cuticle of most insects Hentriacontane 31 C is found in beeswax Heptacosane, nonacosane and hentriacontane have all been found on the surface coating of bees eggs.

Physical properties Boiling point of un-branched alkanes increases with length Gases = 1 – 4 C Liquids 5 – 17 C Solids 18+ C

Boiling points Attractive forces between the molecules that hold them together are called Van der Waals forces To vapourise the hydrocarbon these must be overcome by putting in energy The longer the chain – the more energy required so the higher the boiling point

Un-branched vs branched alkanes Explain what this graph shows

Branched structures The more branched the more compact The more compact the less attraction points it has with other molecules The less attraction the lower the boiling point. Branched alkanes therefore have lower BPs than their un-branched isomers.

Properties Insoluble in water Unreactive Extremely weak acids Highly combustible in oxygen Release much energy in exothermic reactions.

Heats of Combustion (-ΔH°) of several alkanes and branched alkane Compound Number of carbons per chain -ΔH° Kj/mol Hexane 6C 4 163 Heptane 7C 4 817 Octane 8C 5 471 Nonane 9C 6 125 Decane 10C 6 778 Undecane 11C 7 431 Dodecane 12C 8 086 Hexadecane 16C 10 701 2-Methylpentane 5 + 1 4 157 2-methylyhexane 6 + 1 4 812 2-methylheptane 7 + 1 5 466

What two energy patterns do you observe in the previous slide? 1 2

Heat of combustion and stability The lower the heat of combustion, the more stable the molecule Branched chains have lower heats of combustion than un-branched ones Therefore branched chains are more stable than un-branched ones Un-branched chains tend to crack! Cycloalkanes also have lower heats of combustion than un-branched chains with the same numbers of carbons

Oxidation numbers of Carbon in alkanes Oxidation number of carbon in methane is -4 as methane is CH4 and each H is worth +1 When alkanes are burnt in air CO2 is produced. In this Carbon has an oxidation number of ? This means Carbon has been? Most organic compounds contain carbon in different oxidation states (i.e. with different oxidation numbers)