Alkenes: Addition Reactions of Alkenes

Question 1

What are electrophiles?

Answer 1

• Electron-pair acceptors

- Electron deficient species

Question 2

What happens during an electrophilic addition reaction?

Answer 2

• Double bond in an alkene opens up and atoms are added to carbon atoms

Question 3

Why do alkenes undergo electrophilic addition reactions?

Answer 3

• Double bond has plenty of electrons so is easily attacked by electrophiles

Question 4

What are the most common electrophiles?

Answer 4

• HBr
• Br2
• H2SO4

Question 5

What compounds are formed from electrophilic addition reactions?

Answer 5

• Alcohols (from reactions with water)

- Halogenoalkanes (from reactions with hydrogen halides)

Question 6

Describe a mechanism for an electrophilic addition reaction

Answer 6

• The pi bond repels the electrophile, inducing a temporary dipole with 𝛿+ and 𝛿- charges
• Two electrons from C=C attack the 𝛿+ charged atom, breaking the bonds within the electrophile (X-Y)
• 𝛿+ atom from electrophile (X) bonds to a carbon, leaving other carbon with a positive charge - carbocation
• The Y- ion from electrophile has lone pair of electrons
• Y- ion attacks carbocation, forming new bond with other carbon

Question 7

What is a carbocation?

Answer 7

• An ion with a positively-charged carbon

Question 8

How many products can be formed from addition of hydrogen halides to unsymmetrical alkenes?

Answer 8

• 2

Question 9

What factor affects the amount of each product formed from addition of hydrogen halides to alkenes?

Answer 9

• 2 products not formed in equal quantities - major and minor product
• Depends on how stable the carbocation intermediate formed is

Question 10

What are the different types of carbocations?

Answer 10

• Primary carbocation - 1⁰

• When a carbon atom with a positive charge has one R group bonded to it
• R group is alkyl group - alkane with H removed, CH3

• Secondary carbocation - 2⁰
- When a carbon atom with a positive charge has 2 R groups bonded to it

• Tertiary carbocation - 3⁰
- When a carbon atom with a positive charge has 3 R groups bonded to it

Question 11

Which carbocations are more stable and why?

Answer 11

• Carbocations with more alkyl groups are more stable
• The alkyl groups move their negatively charged electrons closer to carbocation, which helps to stabilise the positive charge

Question 12

Which carbocations are more likely to form?

Answer 12

• More stable carbocations are more likely to form than less stable carbocations
• More stable carbocation - major product
• Less stable carbocation - minor product

Question 13

How do you test for unsaturation/alkenes?

Answer 13

• When you shake an alkene with orange bromine water, the solution turns from orange to colourless
• Bromine is added across C=C to form a colourless dibromoalkane - happens via electrophilic addition

Question 14

Describe the mechanism and conditions involved in a reaction between an alkene and water to form an alcohol

Answer 14

• H+ ion from sulfuric acid acts as electrophile
• C=C is broken after it attacks H+ ion and forms a carbocation
• Oxygen in water has lone pair it donates to carbocation
• Oxygen has 3 bonds and positive charge
• Oxygen from HSO4 donates electrons to H from water bonded to carbon
• Forms alcohol and reforms sulfuric acid
• Sulfuric acid catalyst
• Sulfuric acid reforms

Question 15

How would you turn the product (from sulfuric acid + ethene) into an alcohol and how does this show that sulfuric acid catalyses the addition of water to an alkene?

Answer 15

• React sulphuric acid and ethene to make ethyl hydrogensulfate
• Add water to ethyl hydrogensulfate (CH3CH2OSO2OH)
• H2SO4 reforms, showing it catalyses the hydration of alkenes