Alkenes

Question 1

Stereochemistry of alkene addition reactions

Answer 1

1. Syn addition: The reagent XY adds to the double bond from the same side. Syn addition occurs in hydroboration
2. Anti-addition: X and Y add from the opposite sides
   Anti addition occurs in halogenation and halohydrin formation
3. Both syn and anti addition occur when carbocations are intermediates
4. Syn and anti addition occur in hydrohalogenation hydration.

Question 2

Hydrohalogenation

Answer 2

1. Addition of HX (X=Cl,Br,I)
2. The mechanism has two steps
3. carbocations are formed as intermediates
   4 Carbocation rearrangements are possible
4. Markovnikov’s rule is followed. H bonds to the less substituted C to form the more stable carbocation
5. Syn and anti addition occur

Question 3

Hydration

Answer 3

1. Addition of H20 or ROH
2. The mechanism has three steps
3. Carbocations are formed as intermediates
4. Carbocation rearrangements are possible
5. Markovnikov’s Rule is followed. H bonds to less substituted C to form the more stable carbocation
6. Syn and anti addition occur

Question 4

Halogenation

Answer 4

1. The mechanism has two steps
2. Bridged halonium ions are formed as intermediates
3. No rearrangements occur
4. Anti addition occurs

Question 5

Halohydrin formation

Answer 5

1. Addition of OH and X (X=Cl,Br)
2. The mechanism has three steps
3. Bridged halonium ions are formed as intermediates
4. No rearrangements occur
5. X bonds to the less substituted C
6. Anti addition occurs
7. NBS in DMSO and H2O adds Br and OH in the same fashion.

Question 6

Hydroboration-oxidation

Answer 6

1. Addition of H20
2. Has a one-step mechanism
3. No rearrangements occur
4. OH bonds to the less substituted C
5. Syn addition of H2O results