Alkenes and Alkynes- Addition of X2 to alkenes

Question 1

What is the difference between rectangle blocks and triagle blocks when drawing a moelcule to show stereochemistry

Answer 1

1. Rectangle- relative stereochemistry

2. Triangle- absolute stereochemistry

Question 2

Why can is bromine good to use for addition to alkenes

Answer 2

1. It is a good electrophile with a low energy sigma* (Br-Br’)
   2.

Question 3

Describe the addition of bromine to an alkene (E-but-2-ene)

Answer 3

1. First concerted addition of bromine to form a bromonium ion intermediate
2. Then nucleophilic ring-opening by SN2 substitution
3. Forms a single diastereomer product
4. Plane of symmetry, so achiral (meso)

Question 4

What is the evidence for bromonium ions

Answer 4

1. Very sterically hindered alkenes form bromonium ions that are resistant to nucleophilic attack
2. Allowed bromonium ion intermediate to be isolated and characterisation by X-ray crystallography

Question 5

Describe the stereochemistry for the addition of bromine to an alkene

Answer 5

1. Stereospecific as the geometry of alkene determines diastereoselectivity
2. Bromine adds to one face of the alkene- 2 faces of the alkene are the same so equal addition to both
3. Interact HOMO of alkene (pi C-C) with LUMO of Br2 (sigma* Br-Br)
4. SN2 ring-opening of bromonium ion, with nucleophile approaching at 180 degrees to the leaving group leading to inversion of configuration

Question 6

What is formed when E-but-2-ene reacts with bromine in methanol as solvent

Answer 6

1. OMe attaches and so does bromine to the alkene
2. HBr is also formed
3. Product is a racemic
4. Methanol is acting as an external nucleophile

Question 7

Describe the mechanism for addition of bromine to alkene in methanol solvent

Answer 7

1. Addition of bromine to form a bromonium intermediate

2. The bromonium ion is ring-opened by methanol (present in large excess)- SN2

Question 8

Describe the regioselectivity of ring-opening

Answer 8

1. Nucleophile adds to the most substituted end of the bromonium ion
2. Stability of transition state for ring opening determines regioselectivity
3. Loose SN2 transition state with partial positive charge better stabilised on more substituted carbon atom

Question 9

What is an alternative to bromine

Answer 9

1. Bromine is a noxious brown liquid that fumes when opened
2. NBS is a crystalline solid that is an alternative source of electrophilic bromine
3. NBS provides a constant, low concentration of Br2 through reaction with trace HBr

Question 10

What effect does the alkene have on reaction rate

Answer 10

1. More substituted alkenes are more reactive
2. Z-alkenes are more reactive than E-alkenes
3. 1,1-Disubstituted alkenes are more reactive than 1,2-disubstituted alkenes

Question 11

What are more substituted alkenes

Answer 11

1. More reactive and more stable

Question 12

Why are more substituted alkenes more reactive and more stable

Answer 12

1. Substituted alkenes are stabilsed by hyperconjugation
2. Overlap of sigma(C-H) with pi(C-C)
3. Sigma(C-H) and pi*(C-C) are also close enough in energy to interact in a bonding sense
4. The combined energy of electrons is lower through hyperconjugation- more stable
5. But HOMO is raised by hyperconjugation compared with original pi-C-C - more reactive

Question 13

How are bromohydrins formed

Answer 13

1. Bromonium ion intermediate can also be ring-opened by water to form bromohydrins
2. Ring-opening occurs with high regioselectivity at most substituted end of the bromonium ion

Question 14

What happens when a base is added to a bromohydrin

Answer 14

1. Bromohydrin alcohol is deprotonated and undergoes a rapid intramolecular substitution to form an epoxide

Question 15

Describe the hydrogenation of alkenes

Answer 15

1. Hydrogen can be added across alkenes to form alkanes
2. Reaction is thermodynamically favourable
3. Large activation energy means no reaction occurs under standard conditions and requires the use of a catalyst

Question 16

What catalysts are used in the hydrogenation of alkenes

Answer 16

1. Palladium on charcoal (black powder) is an insoluble heterogenous catalyst
2. Rh(PPh3)3Cl (Wilkinson’s catalyst) is a soluble homogenous catalyst

Question 17

Describe the stereochemistry of the hydrogenation of alkenes

Answer 17

1. Both hydrogen atoms add to the same face of the alkene (syn-addition)
2. Hydrogen gas is absorbed onto the metal surface- catalyst
3. Hydrogen is then transferred onto one face of the alkene
4. Products are single diastereoisomers (either racemic or meso)