Elimination- E2

Question 1

When does an elimination reaction occur

Answer 1

1. When a nucleophile acts as a base and attacks hydrogen atoms beta to the leaving group to form an alkene

Question 2

What is the beta position

Answer 2

1. Not attached to atoms adjacent to leaving group

2. Attached to the atom after

Question 3

Describe the structure of an alkene

Answer 3

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 pi-bond
4. Rotation around pi-bond is not possible

Question 4

What does the restricted rotation around the pi-bond mean

Answer 4

1. Substituted alkenes can exist in diastereomeric forms

2. Stereochemical descriptors E/Z are used to assign geometry

Question 5

Describe the reaction kinetics of an E2 reaction

Answer 5

1. Bimolecular- overall 2nd order
2. The reaction rate is proportional to the concentrations of both the base and electrophile (1st order each)
3. Both reactants are involved in the rate-limiting step

Question 6

Describe the basic mechanism of an E2 reaction

Answer 6

1. Simultaneous attack of the base and loss of the leaving group
2. Curly arrow from base lone pair to H
3. Arrow from H to C-C bond
4. Arrow from C-X bond to X
5. Forms alkene where C-C bond was and base.HX

Question 7

Describe the transition state for an E2 reaction

Answer 7

1. C-C solid line with dotted line above to show double bond forming
2. Dotted line to delta negative X- leaving group
3. Dotted line to H which has a dotted line to delta negative B
4. Anti-periplanar

Question 8

Write the rate equation for an E2 reaction of a halogenoalkane

Answer 8

1. Rate = k * [halogenoalkane] * [base]

Question 9

Describe the orbital requirements of an E2 reaction

Answer 9

1. The new pi-bond is formed from overlap of sigma(C-H) with sigma*(C-X)
2. The two orbitals must therefore overlap in the transition state
3. Maximum orbital overlap when the hydrogen atom and the leaving group are anti-periplanar to each other
4. Staggered conformation with an angle of 180 degrees between the hydrogen atom and the leaving group

Question 10

What factors affect the rate of an E2 reaction

Answer 10

1. Base
2. Electrophile structure
3. Leaving group
4. Temperature

Question 11

Why are E2 eliminations highly stereoslective

Answer 11

1. Due to the requirement for an anti-periplanar transition state

Question 12

What can be used to determine possible conformations for elimination

Answer 12

1. Newman projections

2. Least sterically hindered conformer is favoured

Question 13

What does a stereoselective reaction mean

Answer 13

1. Multiple reaction pathways, but one stereoisomer of product is favoured
2. Either the pathway with the lower activation energy is favoured and/or the most stable product is formed (thermodynamic control)
3. Select most favourable pathway

Question 14

What does a stereospecific reaction mean

Answer 14

1. Stereochemistry of the starting material determines the stereochemistry of the product as a consequence of the reaction mechanism

Question 15

What conformations of cyclohexanes can undergo E2 reactions

Answer 15

1. The requirement for an anti-planar transition state means only certain conformations of substituted cyclohexanes can undergo E2 reactions
2. Equatorial C-X bond means only C-C bonds are anti-periplanar so no E2 elimination
3. Axial C-X bond means C-H bonds anti-periplanar to C-X so E2 elimination possible
4. Leaving group must be axial for E2 elimination

Question 16

How can regioselectivity of E2 eliminations be influenced

Answer 16

1. By the steric demand of the base
2. Small base- more substituted alkene favoured
3. Hindered base- less substituted alkene favoured

Question 17

Why do hindered bases favour less substituted alkenes

Answer 17

1. Hindered bases attack the most accessible hydrogen atoms
2. Transition state to more substituted product has greater steric demand so becomes higher in energy- thermodynamically favourable?
3. Least substituted product forms faster (kinetic control)

Question 18

What is the formation of the most-substituted alkene called

Answer 18

1. Saytzeff product/elimination
2. Formation of the thermodynamically favourable, most-substituted alkene
3. Favoured with small bases

Question 19

What is the formation of the least-substituted alkene

Answer 19

1. Hofmann product/elimination
2. Formation of the kinetically favourable, least substituted alkene
3. Favoured with sterically demanding, hindered bases

Question 20

Is NaOMe a big or small base

Answer 20

1. Small

Question 21

Is KOCEt3 a big or small base

Answer 21

1. Massive

Question 22

What substrates can undergo E2 elimination

Answer 22

1. Primary alcohols can only undergo E2 as would form unstable primary carbocation
2. B-hydrogen atoms are more accessible than an alpha-carbon atom
3. This means tertiary alkyl substrates can undergo E2 as well as E1 whereas they can’t undergo SN2