Week 3/4

Question 1

Hammond Postulate

Answer 1

• used to understand structure-reactivity relationships

- the structure of the TS resembles the adjacent GS species OR intermediate species that is closest in free energy

Question 2

Hyperconjugation

Answer 2

• weak e- donation from adjacent sigma bonds
• donating sigma orbital must be parallel to the empty orbital
• mimic a pi bond, leads to delocalization of +ve charge

Question 3

Allylic/benzylic carbocation

Answer 3

• +ve charge delocalized by conjugation
• 1˚ allylic carbocation more stable than 1˚ carbocation
• 3˚ benzylic carbocation more stable than 3˚ carbocation

Question 4

SN1 allylic mechanism

Answer 4

-rxn is favoured at the carbon that is less sterically hindered, Nu attack at end carbon

Question 5

SN1 Benzylic mechanism

Answer 5

-product is chosen so that aromaticity of the benzene is preserved

Question 6

What is a solvolysis rxn

Answer 6

-the solvent is one of the reagents

Question 7

what is a heteroatom

Answer 7

• O, N, S
• not metal, has LP e-
• help to delocalize +ve charge by donating LP to bond

Question 8

SN1 Ground state effects

Answer 8

• H3C-Cl has very little steric crowding in GS, so there is minimal steric relief to carbocation (sp3 to sp2)
• (CH3)3-C-Cl has major steric crowding in GS from Me groups, significant steric relief to carbocation, bulky groups can move apart

Question 9

SN2 Ground state effects

Answer 9

• GS to TS steric crowding increases (opposite to SN1)
• results in higher ∆G‡, also raises GS G˚
• ∆G‡ increase is more significant

Question 10

benzylic/allylic SN2 stability

Answer 10

• empty pi* orbitals accept e- density from the developing p-orbital of electrophilic carbon
• this delocalizes extra e- density built up on electrophilic C
• pi* must be aligned to p-orbital in TS to benefit

Question 11

SN2 TS

Answer 11

• electrophilic C is rehybridized from sp3 to sp2

- p-orbital develops

Question 12

SN2 adjacent carbonyl (=O)

Answer 12

• similar mech to allylic system
• empty pi* orbital accepts e- density that is building up in new p-orbital of TS
• must be parallel for delocalization of charge
• favours SN2 since electroneg O pulls more e- density from electrophilic C (LG does as well), which weakens LG-E+ bond, makes E+ more electropositive (ground state effect)

Question 13

SN2 adjacent oxygen (R-O-C)

Answer 13

• LP of e- on O donates e- density to sigma* E+-LG bond, weakens interaction, more susceptible to Nu attack
• GS effect (raises GS energy)

Question 14

SN1 ONLY

Answer 14

3˚ alkane

Question 15

SN2 ONLY

Answer 15

H3C-X, 1˚ alkane, alpha-halocarbonyl compound (directly adjacent C has =O)