Alkenes and Alkynes- Hydration and Oxidation of Alkenes

Question 1

Under what conditions can water add across alkenes

Answer 1

1. Under acidic conditions

2. Reverse of E1 elimination of tertiary alcohol

Question 2

What substrates can water be added to

Answer 2

1. Only possible for substrates that can form a stabilised tertiary carbocation intermediate

Question 3

What is uncertainty about addition of water under acidic conditions to an alkene

Answer 3

1. Difficult to predict whether aqueous acid will hydrate alkene or dehydrate alcohol
2. Alkenes can also polymerise under acidic conditions

Question 4

What is a sequence that can be used to hydrate alkenes selectively

Answer 4

1. Oxymercuration/demercuration sequence
2. Add Hg(OAc)2 + H2O which breaks C=C
3. Then add NaBH4 which breaks the HgOAc-C weak bond producing an alcohol

Question 5

Why is oxymercuration/demercuration not used now

Answer 5

1. Very toxic

Question 6

Describe the mechanism for the oxymercuration/demercuration reaction

Answer 6

1. Reaction proceeds via a mercurinium ion intermediate, which ring-opens at the most substituted position
2. Weak C-Hg bond is cleaved under reductive conditions

Question 7

What method can be used to form alcohols with anti-Markovnikov’s regioselectivity from alkenes

Answer 7

1. Hydroboration of alkenes followed by oxidation

Question 8

Describe borane derivatives

Answer 8

1. Contain B-H bond
2. They are electrophilic due to empty p-orbital on boron atom which can accept e-
3. Boron has 3 valence electrons
4. Less electronegative than hydrogen

Question 9

Give 3 examples of common boranes

Answer 9

1. B2H6- diborane
2. H3B.THF
3. H3B.SMe2

Question 10

Describe the regioselectivity of hydroboration

Answer 10

1. Hydroboration is concerted with a four-membered trnasition state
2. Regioselectivity is determined by combination of steric and electronic factors
3. Electronics- partial positive charge builds up on more substituted end of alkene
4. Sterics- boron adds to the least hindered end of the alkene
5. First borane formed is still electrophilic so undergoes two more hydroboration reductions to form trialkylborane

Question 11

What happens once the trialkylborane has formed

Answer 11

1. It can be oxidised into the alcohol using hydrogen peroxide and base

Question 12

Describe the oxidation of trialkylborane using hydrogen peroxide and base

Answer 12

1. Deprotonated hydrogen peroxide adds to empty p-orbital on boron atom
2. Boronate undergoes bond migration
3. Breaks weak O-O bond to form strong B-O bond
4. Occurs 3 times to form trialkylborate

Question 13

What happens once the trialkylborate is formed

Answer 13

1. In the presence of the hydroxide, the trialkylborate undergoes hydrolysis to release the alcohol product and boric acid side product

Question 14

Describe the stereochemistry of hydroboration

Answer 14

1. It is a stereospecific process with hydrogen and boron adding to the same face of the alkene- syn-addition
2. Oxidation does not affect stereochemistry ast eh migration step occurs with retention

Question 15

How can you form epoxides from alkenes

Answer 15

1. Treating alkenes with peroxycarboxylic acid (peracid)

2. Often meta-chloroperbenzoic acid (mCPBA) is a commercially available oxidant- as it contains a weak O-O sigma bond

Question 16

Describe what happens in the formation of an epoxide from alkenes

Answer 16

1. HOMO of alkene (piC-C) interacts with LUMO of mCPBA (sigma* O-O)

Question 17

Describe the stereochemistry of epoxidation

Answer 17

1. It is stereospecific
2. Oxygen added to one face of the alkene- syn-addition
3. Racemic but either top or bottom or meso compound

Question 18

Describe epoxide ring-opening under basic conditions

Answer 18

1. Under basic conditions, epoxides undergo regioselective ring-opening with good nucleophiles at the least hindered positions
2. SN2 transition state- steric hindrance controlling factor

Question 19

Describe epoxide ring-opening under acidic conditions

Answer 19

1. The selectivity of ring-opening is lower

2. Both SN2 and loose SN2 transitions states are accessible leading to a mixture of regioisomers

Question 20

How else can epoxides be ring-opened

Answer 20

1. With hydroxides to form 1,2-diols
2. Stereospecific SN2 ring-opening leading to inversion of configuration at one centre
3. Racemic or meso epoxide could give racemic or meso diol

Question 21

Describe example of epoxide ring-opening in nature

Answer 21

1. Marine ladder polyethers are highly complex natural products
2. While the biosynthesis is not clear, one suggestion is a dramatic epoxide ring-opening cascade