Alkanes

Organic Chemistry

Homologous Series

Definition

• All share same general formula

• Formula of a homologue differs from its neighbour by CH2

• Contain same functional group

• Have similar chemical properties

• Show a gradual change in physical properties as molar mass increases

• Can be prepared by similar methods

Formulae of Organic Compunds

Empirical Formula

Simplest whole number ratio of the elements in a compound.

Molecular Formula

Actual number of each element in a compound

Structural Formula

Illustrates how the atoms are arranged in a molecule

Displayed Formula

Shows how all the atoms are arranged and all the bonds between them

Common Functional Groups

Skeletal Formula

Shows Carbon skeleton only, with any functional groups, hydrogen and carbon atoms part of the main chain are not shown.

Isomerism

Chain Isomerism

Same molecular formula, but a different structure of the alkane chain (e.g branching).

They have different physical properties

Positional Isomers

Same molecular formula, same chain structure, but different position of the functional group.

Alkanes are saturated hydrocarbons. Thery contain no double C=C bonds.

Functional Group Isomerism

Same molecular formula, but a different functional group. E.g. Alkenes and cyclic alkanes

Fractional Distillation

Crude Oil is heated and vaporised and passed into a huge fractionating column

A temperature gradient exists in the fractionating column. The column is coolest at the top and warmest at the bottom.

As the gases cool, they condense at different levels. The lightest (lowest boiling point) fractions are collected at the top of the column, whilst the heaviest (highest boiling point) fractions are collected at the bottom

Cracking

Thermal Cracking

High pressure and high temperatures

Produces high % of alkenes (free radical mechanism) used to produce polymers such as poly(ethene)

FREE RADICAL = A species with an unpaired electron = reactive intermediate

Catalytic Cracking

Reactive intermediate = Carbocation -C⁺-

Also produces cyclic alkanes and branched alkanes

Catalysts: Platinum, Palladium, Rhodium

Catalysts: Spread over a inert ceramic honeycomb; increases Surface Area

Reactions of Alkanes

Complete Combustion

Produces carbon dioxide which causes global warming

Incomplete Combustion

Releases less energy reducing fuel efficiency

CO: Toxic gas that binds to the haemoglobin of RBCs, making them unable to carry oxygen

C: Global dimming and breathing problems

Unbranched hydrocarbons = particulates — global dimming

Sulphur Dioxide

Combustion of sulphur impurities in fuel.

Sulphur dioxide dissolving to form acid rain

SO₂ + CaO → CaSO₃

Global Warming

CO₂, H₂O and CH₄ all absorb IR radiation