Alkenes and Alkynes- Addition of HX to alkenes

Question 1

Describe the structure of an alkene

Answer 1

1. Contain a carbon-carbon double bond
2. Sp2 hybridised orbitals on each carbon make up 3 sigma bonds
3. Remaining p-orbital on each carbon overlap to form a new pi-bond
4. Rotation around pi-bond is not possible- can exist in diastereomeric forms

Question 2

What types of reactions do alkenes undergo

Answer 2

1. Simple alkenes are electron rich and act as nucleophiles
2. Alkenes react with a range of electrophiles
3. Interaction of the pi-bond (HOMO) of the alkene with the LUMO of the electrophile forms addition products
4. From planar sp2 hybridised carbon atoms on alkene to tetrahedral sp3 hybridised carbon atoms in alkane product
5. Reactions can be regioselective and stereoselective

Question 3

Describe the mechanism for addition of HX to alkenes

Answer 3

1. Alkenes are nucleophilic so first react with the electrophilic proton to form a carbocation intermediate
2. X can then act as a nucleophile
3. X can attack either face of the planar carbocation intermediate

Question 4

What would the product be for addition of HX to alkenes if the alkene is chiral

Answer 4

1. The product would be racemic

Question 5

What is the rate limiting step for addition of HX to alkenes

Answer 5

1. Formation of the carbocation intermediate is the rate-limiting step

Question 6

What happens for addition of HX to alkenes if the alkene is unsymmetrical

Answer 6

1. The hydrogen attaches to the least substituted carbon atom

Question 7

Describe Markovnikov’s rule

Answer 7

1. On addition of HX to an alkene, the hydrogen adds to the least substituted carbon atom

Question 8

What is the regioselectivity for addition of HX to alkenes correlated with

Answer 8

1. Stability of the intermediate carbocation
2. Tertiary most stable etc
3. Stability of carbocation intermediate is reflected in stability of transition state leading to its formation (Hammond Postulate)

Question 9

What happens if an allyl cation is formed

Answer 9

1. Resonance

2. Nucleophilic attack at least hindered end

Question 10

Is the protonation of alkenes to give carbocations reversible

Answer 10

1. Yes

Question 11

How can alkenes be isomerised

Answer 11

1. They are isomerised through reversible addition of a proton
2. E.g. a proton is added to a di-substituted alkene
3. A tertiary carbocation intermediate is formed
4. Then a Trisubstituted alkene forms which is more stable
5. Allows the more stable alkene (thermodynamic product) to be formed

Question 12

What is the difference in alkene formed for the addition of HX to alkenes if a peroxide is present

Answer 12

1. The least-substituted alkyl halide is formed as the major product
2. Anti-Markovnikov product

Question 13

How can the formation of the anti-Markovinikov product in the presence of a peroxide be explained

Answer 13

1. Radical chemistry

Question 14

What is a radical

Answer 14

1. Atoms or molecules containing an unpaired electron

Question 15

How are radicals formed

Answer 15

1. Formed by bond homolysis initiated by high temperatures or light

Question 16

What controls radical reactions

Answer 16

1. Controlled by the strength of bonds

2. In contrast, to ionic reactions where polar effects are more important

Question 17

How do you represent the movement of a single electrion

Answer 17

1. Use a half-headed arrow

2. Radical is represented by a dot

Question 18

Show the equation for the homolysis of X-Y

Answer 18

1. X-Y –> X’ + Y’

Question 19

What are alkyl radicals stabilised by

Answer 19

1. Hyper conjugation with adjacent sigma-bonds- tertiary most stable
2. Conjugation with adjacent pi-systems- allyl/benzylic
3. The single electron is delocalised and resonance structures can be drawn

Question 20

What do alkyl radicals look like

Answer 20

1. Close to planar and sp2 hybridised

2. Contain a singly occupied molecular orbital- SOMO

Question 21

What are the steps in radical addition to alkenes

Answer 21

1. Initiation
2. Propagation
3. Termination

Question 22

Describe the initiation step in radical addition to alkenes

Answer 22

1. Radical generation through homolysis of a suitable initiator such as a peroxide with a weak O-O bond
2. Alkoxy radical abstracts hydrogen from HBr (OR other HX) to generate a bromine radical

Question 23

Describe the propagation step in radical addition to alkenes

Answer 23

1. Bromide radical reacts with the alkene to form the most stable tertiary radical
2. Lower energy transition state for formation of Hammond Postulate- primary radicals not formed
3. Alkyl radical abstracts hydrogen from HBr, generating another bromide radical to continue the chain this also forms the product
4. Radical addition is regioselective and leads to the anti-Markovnikov product

Question 24

Describe the chain termination step in radical addition to alkenes

Answer 24

1. Radical chain processes are not 100% efficient

2. Any two radicals can react together to terminate the chain reaction