Substitution reactions of alkanes

L.O.:

Describe the substitution of alkanes in the presence of ultraviolet radiation.

Describe how homolytic fission leads to the mechanism of radical substitution in alkanes.

Explain the limitations of radical substitution in synthesis

Free radical substitution: Cl2 + CH4

Other products of chain reactionsIf an alkane is more than two carbons in length then any of the hydrogen atoms may be substituted, leading to a mixture of different isomers. For example:

The mixture of products is difficult to separate, and this is one reason why chain reactions are not a good method of preparing halogenoalkanes.

1-chloropropane 2-chloropropane

Further substitution in chain reactionsSome chloromethane molecules formed during free radical substitution between methane and chlorine will undergo further substitution to form dichloromethane. Further substitution can occur until all hydrogens are substituted.

The further substituted chloroalkanes are impurities that must be removed. The amount of these molecules can be decreased by reducing the proportion of chlorine in the reaction mixture.

Task:

Bromine can react with ethane in UV light to give 1-bromoethane

1)Write the corresponding initiation, propagation and termination reactions.

Extension: a) write any other possible products from this reaction. b) Write the corresponding mechanisms.

AlkenesL.O.:

Understand that alkenes and cycloalkenes are unsaturated hydrocarbons.

Explain the overlap of adjacent p-orbitals to form a π-bond.

Explain the trigonal planar shape around each carbon making up the C=C bond of alkenes.

Ethene

Two kinds of bonding orbitals

Sigma bond = The one that makes up all the single bonds we need to know about

Pi bond = A double bond is made from two kinds of bond, one is a sigma bond, the other is a pi bond

Formation of the C=C double bond

A cross section of a pi bond

Shape of an ethene molecule

Cyclohexene

Describe addition reactions of alkenes with both hydrogen and halogens.

Describe the use of bromine as a test for unsaturation.

Typical addition reaction for an alkene

Hydrogenation of ethene

Halogenation of ethene with chlorine

Halogenation of propene with bromine

Week 15

© Pearson Education Ltd 2008This document may have been altered from the original

•Describe addition reactions of alkenes with both hydrogen halides and steam.

• Define an electrophile as an electron pair acceptor.

• Describe a curly arrow as the movement of an electron pair.

• Describe, using curly arrows, the mechanism of electrophilic addition in alkenes.

Addition reaction for ethene and HBr

Hydration of ethene

Addition reaction for an unsymmetrical alkene

Electrophilic reaction for ethene and hydrogen bromide

Curly arrows

Carbocation reacts with bromide ion

• Describe how heterolytic fission leads to electrophilic addition in alkenes, using bromine as an example.

• Recall the reactions of alkenes.

Electrophilic reaction for ethene with Br2

Carbocation reacts with bromide ion

Heterolytic fission of a bromine molecule

The structures of isoprene and myrcene

Reaction of myrcene with excess hydrogen to form a saturated product

Reactions of alkenes

But-2-ene takes part in the addition reactions