Carbenes

Question 1

triplet carbenes

Answer 1

two unpaired electrons, R-R bond angle 130-150 due to unpaired electron needing less space than a substituent

Question 2

singlet carbenes

Answer 2

lone pair of electrons with spins paired, R-R bond angle 100-110 due to electron repulsion

Question 3

reactivity of carbenes

Answer 3

triplet less reactive than singlet, subs with lone pairs of conjugation increase stability of carbenes and decrease reactivity

Question 4

cyclopropane formation

Answer 4

alkene and CHCl3 and stong base (tBuOK), concerted for S0 carbenes so alkene stereochemistry is conserved, T0 carbenes act as a diradical therefore give mixture of isomers due to rotation around C-C bond

Question 5

photolysis in gas phase at low pressure

Answer 5

S0 carbene

Question 6

photolysis in soln phase

Answer 6

T0 carbene due to collisional mechanism

Question 7

Simmons-smith reaction

Answer 7

alkene with Ch2I and Zn/Cu forms cyclopropane, involves carbenoid and doesn’t work with substituted carbenes

Question 8

C-H insertion reactions

Answer 8

when there’s a carbene formed but no C=C about to react with

Question 9

C-H insertion of metal carbenoids

Answer 9

lithium carbenoids don’t insert
rhodium carbenoids insert

Question 10

Add of S0 carbene to alkene

Answer 10

cycloadditon therefore obeys WH rules, symmetrically allowed sideways approach for carbene add and CH insertion

Question 11

1,2-migration

Answer 11

rearrangement reaction, superbase (tBuOK/tBuLi) deprotonates primary chloride, a-elimination gives carbene, no C=C and C-H insertion would give a strained cyclopropane, aromatic CH stronger than alkyl CH so insertion is unfavourable, migration of easiest group