Alkenes

Question 1

What is an electrophile?

Answer 1

An electron pair acceptor.
Halides and H2 when reacting with alkenes due to induces dipole from e- dense C–C

Question 2

Why are alkenes more reactive than alkanes?

Answer 2

Pi bond is present. Pi e- are on outside of double bond means they are more exposed and are easier to break. So it is possible to ass atoms/groups to the alkene.

Question 3

What reactions do alkenes undergo?

Answer 3

Addition reactions

Question 4

What are the groups and atoms that react with alkenes?

Answer 4

• Hydrogen
• Halogens
• Hydrogen halides
• Steam (H2O (g) )

Question 5

What is the reaction of alkenes with hydrogen?

Answer 5

Hydrogenation.

Question 6

What is needed for hydrogenation reaction of alkenes? What is the product?

Answer 6

Ni catalyst and 150 degrees.
The pi bond breaks and 2 H atoms are added, producing an alkane.

Question 7

What is the reaction of alkenes with halogens and what are the conditions and product?

Answer 7

Halogenation.
Room temp, no catalyst. Pi bond breaks and 2 halogen atoms add across the C–C, making a dihaloalkane.

Question 8

What observations can be made when bromine reacts with alkenes?

Answer 8

It is decolourised in the halogenation reaction. This change can be used to test for the presence of unsaturated alkenes

Question 9

What is the reaction of alkenes with hydrogen halides?

Answer 9

Room temperature.
The pi bond breaks, adding the Hydrogen and halogen atoms across the double bond.

Question 10

What is the reaction of alkenes with steam called?

Answer 10

Hydration.

Question 11

What is the reaction of alkenes with steam and the conditions?

Answer 11

> 100 degrees and in the presence of a strong acid catalyst (e.g conc. H2SO4 or H3PO4).

PI bond breaks. H atom and OH group from steam add across double bond to form an alcohol.

Question 12

What is an electrophillic addition reaction?

Answer 12

An electrophile (eg partially positive end of Br2) accepts an electron pair. 2 reactants react to become 1 product

Question 13

When are there 2 possible products of a hydrogen halide and alkene reaction?

Answer 13

When there is an unsymmetrical alkene reacting. These are isomers. One is a major product (favoured) and one is a minor product.

Question 14

What decides which isomer of a product in a hydrogen halide and alkene reaction is the major is minor?

Answer 14

The stability of the carbocation in the intermediate stage. Alkyl groups (CH3, C2H5 etc) are e- releasing, spreading out charge on carbocation to help make it more stable. The more alkyl groups bonded to the positive carbon in the carbocation = more stable it is

Tertiary (3 alkyl groups) carbocation > secondary > primary