14.2 alkenes

State the ways in which alkenes can be produced

1. Elimination of HX from halogenoalkane

2. Dehydration of alcohol

3. Cracking of longer chain alkane

Élimination of HX

Elimination of halogenoalkane by heating with ethanolic NaOH to form alkene salt and water

Dehydration of alcohol

Alcohol vapour is passed over a hot catalyst of aluminium oxide powder (Al₂O₃) to produce alkene and water

Cracking of longer chain alkanes

• An aluminium oxide catalyst and 600’C are used to speed up this reaction.

State the ways in which alkenes can react via electrophilic addition

Electrophilic addition of:

Hydrogen

Steam

Hydrogen halides

Halogen

Hydrogenation

Reaction alkene with hydrogen to form alkane

Heat and Ni catalyst

Hydration

React alkene with steam to form alcohol

Use H3PO4 catalyst and heat

Add HX

React alkene with HX to form halogenoalkane

RTP

Halogenation

React alkene with halogen to form dihalogenoalkane

Oxidation of alkenes

By cold dilute KMnO4

Forms diol

Pale purple to colourless

How do alkenes with C atom attached to 2 hydrogens react when oxidised by hot concentrated KMnO4.

Oxidised to CO2

Each carbon treated separately

How do alkenes with C atom attached to 1 hydrogen and an R group react when oxidised by hot concentrated KMnO4.

Oxidised to aldehydes then carboxylic acid

How do alkenes with C atom attached to 2 R groups react when oxidised by hot concentrated KMnO4.

Oxidised to ketone

How to test for unsaturated compounds

• unknown compound is shaken with the bromine water

• If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise

Draw the mechanism for electrophilic addition

What is the major carbocation intermediate

The most stable intermediate

Define positive inductive effect

The alkyl groups attached to the positively charged carbon atoms are ‘electron donating groups’

Which carbocation intermediate is most stable and thus the major intermediate?