Reactions of Alkenes
Chemistry: AQA A Level, topic 5.4.2
Alkenes
Alkenes are molecules with a double covalent bond. There is a high electron density between the carbon atoms because of the double bond.
Reactions of alkenes
- Alkenes react with several reagents in addition reactions.
- The alkenes are nucleophiles because of the high electron density between the carbon atoms.
- The mechanism is shown next for the reaction with hydrogen bromide.
- The mechanism is called the electrophilic addition mechanism (because an electrophile is added).
- They will react in a similar way with sulfuric acid, and bromine.
Electrophilic addition
- Above shows the reaction with HBr.
It's the same mechanism for sulfuric acid
With bromine
- The reaction can also be done with Br2.
- The partial charges are assigned to the bromine atoms because the high electron density of the double bond repels the bonding electrons and polarises the bond - this is shown in the mechanism.
Uses of Electrophilic Addition
Electrophilic addition reactions between alkenes and bromine as well as alkenes and sulfuric acid are useful.
With bromine
- The reaction with bromine is used as a test for alkenes.
- Bromine water is a light orange colour and will be decolourised in the presence of an alkene (light orange → colourless).
- This test works because bromine can add to alkenes via an electrophilic addition reaction.
With sulfuric acid
- The reaction with sulfuric acid is used to produce alcohols.
- The sulfate ion is removed by nucleophilic substitution.
Asymmetric Electrophilic Addition
Asymmetric alkenes can have multiple products in electrophilic addition reactions.
Multiple products
- If you have an asymmetric alkene, you can get multiple products.
- A reaction will favour one of the products over the other - we call this selectivity.
- This is shown for the reaction of propene with HBr.
Intermediate stability
- This selectivity is driven by carbocation intermediate stability:
- Tertiary carbocations > secondary carbocations > primary carbocations.
- The more alkyl groups there are next to the positive charge, the more stable the intermediate is.
- Alkyl groups push electrons and so help to stabilise adjacent positive charge.
Which product when?
- When considering which product is most likely, we must consider the intermediates formed.
- 2-bromopropane is the most likely product because the intermediate is a secondary carbocation, rather than a primary carbocation.
- This is because the secondary carbocation is more stable.
- It has more alkyl groups pushing electrons onto the positive charge.
- Both reactions will happen and you will get both products, but you will have one major and one minor.
Test yourself on Reactions of Alkenes
What is a typical reaction for alkenes?
Which of the following is true for alkenes?
What must be reacted together to give dibromopropane?
What is the end product in the sulfuric acid and alkene reaction (after water is added)?
What is produced by the reaction between an alkene and a Br2 molecule?