Chemical Ideas 12Organic chemistry

frameworks

12.1 Alkanes

Many carbon compounds are found in living organisms, this why their study is named organic chemistry

Carbon is unique

About 7 million compounds containing carbon and hydrogen are known to chemists

This is far more than the number of compounds from all the other elements put together

Why carbon?

Electron structure makes it the first member of Gp 4 in the centre of the periodic table – this is responsible for its special properties

Carbon atom has 4 electrons in its outer shell – too many to lose or gain – the ions would have +4 or -4 (too highly charged)

All carbon compounds are covalent rather than ionic

Methane(CH4) C shares 4 electrons with 4 H atoms

Carbon forms strong covalent bonds with itself to form rings and chains this is called catenation

Each C atom can form 4 covalent bonds- chains may be straight or branched and can have other atoms or groups substituted on to them

H

H C H H

Hydrocarbons Only contain carbon and

hydrogen General formula CXHY

Methane –CH4 is an alkane

Ethene C2H4 is an alkene

Benzene C6H6 is an arene

H l

H - C - H l H

Alkanes Saturated hydrocarbons

Contain C and H only

Contain single bonds C-C

Have 4 bonds to every carbon (C) atom

Are non polar

General formula CnH2n+2

Physical properties such as m.pt,b.pt and density change as the number of carbon atoms in the molecule increase

Alkanesshortened

Name # carbons Structural Formula

Methane 1 CH4

Ethane 2 CH3CH3

Propane 3 CH3CH2CH3

Butane 4 CH3CH2CH2CH3

Pentane 5 CH3CH2CH2CH2CH3

Names of all alkanes end in -ane

Alkanesshortened

Name # carbons Structural Formula

Hexane 6 CH3CH2CH2CH2CH2CH3

Heptane 7 CH3CH2CH2CH2CH2CH2CH3

Octane 8 CH3CH2CH2CH2CH2CH2CH2CH3

Nonane 9 CH3 CH2 CH2CH2CH2CH2CH2CH2CH3

Decane 10

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

Names of all alkanes end in -ane

A series of compounds which are related in this way are called a homologous series

Finding the formula of alkanes

Molecular formula = m x empirical formula

Where m is 1,2,3,……

For hydrocarbons , composition of mass is easily found by burning a known mass in oxygen, measuring the amounts of CO2 and H2O this is called combustion analysis

Finding the formula of alkanes

Example0.100g of hydrocarbon X on complete combustion gave 0.309g CO2 and H2O 0.142gcalculate the empirical formula of X

Answer1st calc the masses of C and H in 0.100g44g CO2 contains 12g of C mass of C in 0.100g X = (12/44) x 0.309g

= 0.0843g18g H2O contains 2g H mass of H in 0.100g X = (2/18) x 0.142g

= 0.0158g

Finding the formula of alkanes

C HRatio by mass 0.0843 : 0.0158Ratio by moles 0.00703 : 0.0158Simplest by ratio(÷ by smaller) 1 : 2.25Whole number ratio 4 : 9Empirical formula = C4H9

Finding the formula of alkanes

ExampleRelative molecular mass of X was found to be 114 by using a mass spectrometer

AnswerEmpirical formula of X is C4H9

but Mr C4H9 = 57 this is ½ of 114. So the molecular formula of X must be (C4H9)2

= C8H18

Structure of alkanes Full structural formula of

methaneShows all atoms and bonds

Propane

Shortened structural formula for propane

Further shortened to

Structure of alkanes

Branched alkanes -Isomers

Same molecular formula Same number and types of atoms Different arrangement of atoms

Alkyl GroupsBranches on carbon chains H

H C CH3 methyl H H H

H C C CH3CH2 ethyl H H

Naming Branched Alkanes

CH3 methyl branch

CH3CH2CH2CHCH2CH3

6 5 4 3 2 1 Count

3-Methylhexane

on third C CH3 six carbon chain group

Naming Summary

1. Count the C’s in the longest

chain

2. Name each attached group

3 Count the longest carbon chain

to give the first attached group

the smallest number

4. Name and locate each group

CycloalkanesCyclopropane

CH2

CH2 CH2

Cyclobutane

CH2 CH2

CH2 CH2

•The is a group of alkanes that have a cyclic structure.

•These cycloalkanes contain a carbon chain that is in a ring.

•Each cycloalkane has a formula that is 2C less than the corresponding alkane.

•For example, propane is C3H8 whereas cyclopropane ic C3H6. Butane is C4H10 and cyclobutane is C4H10.

•The names of the cyclic structures use the prefix cyclo in from of the alkane name for the carbon chain

More Cycloalkanes

Cyclopentane CH2

CH2 CH2

CH2 CH2

Cyclohexane

CH2

CH2 CH2

CH2 CH2

CH2

Shapes of alkanes

represents a bond inthe plane of the paper

represents a bond ina direction behindthe plane of the paper

represents a bond ina direction in front ofthe plane of the paper

Pairs of electrons in the bonds repel each other so in all covalent compounds the bonds are as far away from each other as possible. The bond angle for H-C-H is 109

Shapes of alkanes

a simpler way of drawingethane which shows theshape less accurately

Ethane

Each carbon atom is at the centre at of a tetrahedral arrangement

Shapes of alkanesHydrocarbon chains are not really straight but a zig-zag of carbon atoms. All bond angles are 109◦

Reaction of alkanes

Oxidation are v. unreactive unaffected by acids and alkalis and oxidising agents

When they do react it is usually in the gas phase and energy needs to be supplied to get the reaction started

Reaction of alkanes

Combustion

alkane + O2 CO2 + H2O + heat

C6H14 + 9 1/2 O2 6CO2 + 7H2O + heat

If air supply is limited combustion in incomplete and products include CO and C (soot) along with partially oxidised hydrocarbons

Action of heat on alkanes When alkanes fractions are heated under different conditions,3 different

reactions can occur, isomerisation, reforming and cracking