Reaction Mechanisms

Question 1

Substitution (nucleophilic displacement)

Answer 1

Nuc: with unpaired e- attaches to the alkyl group and X is displaced from alkyl group
2 mechanisms: Sn1 or Sn2

Question 2

Sn2 mechanism

Answer 2

Simultaneous Nuc-C bind formation and C-X bond cleavage

Rate = k[Nuc:][RX]

Question 3

Sn1 mechanism

Answer 3

Heterolytic cleavage of the C-X bond, followed by the formation of a Nuc-C bond; carbocation formation

Rate = k[RX]

Question 4

Nucleophilic addition

Answer 4

Typical for aldehydes and ketones; the oxygen on carbonyl group get protonated m, which enhances partial (+) charge on carbonyl carbon and makes it more likely to be attacked by Nuc:

Question 5

Nucleophilic aromatic substitution

Answer 5

Nuc substitution when electron withdrawing group is on the ring

2 mechanisms

Question 6

Nucleophilic aromatic substitution mechanism 1

Answer 6

Addition of a Nuc: and formation of a carbanion intermediate, followed by the loss of the halide ion

Question 7

Nucleophilic aromatic substitution mechanism 2

Answer 7

In absence of electron withdrawing group substituents, very rare and proceeds through an enzyme intermediate

Question 8

Electrophilic addition

Answer 8

Initial attack by electrophile (on pi electron system) to form a carbocation, followed by reaction with the negative halide ion

Question 9

Electrophilic aromatic substitution

Answer 9

Electron rich pi-electron clouds are attacked by electrophiles

Question 10

Elimination

Answer 10

When a molecule loses atoms or ions

Two mechanisms: E2 and E1

Question 11

E2 elimination

Answer 11

Base removes a hydrogen atom; a C-H cleaves heterolytically; adjacent carbons form carbon-carbon double bind by ejecting the leaving group; no carbocation intermediate

Rate = k[base][substrate]

Question 12

Saytzeff’s rule

Answer 12

E2 obey this rule, which states that when more than one beta-carbon bears a hydrogen atom, it is the most substituted carbon that undergoes elimination

Question 13

E1 elimination

Answer 13

1. ) formation of carbocation through the loss of X-, without cleavage of C-H bond
2. ) weak base removes and H+ ion from carbon adjacent to the positive carbon

Rate = k[substrate]

Question 14

Duels alder reaction

Answer 14

A conjugated diene is heated and attacks an alkene or alkyne, called dienophiles to yield a six members ring

Question 15

Dienophile

Answer 15

Usually contains an electron-withdrawing group; electron-poor species

Question 16

Diene

Answer 16

electron rich species

Question 17

Hoffmann product

Answer 17

Less substituted alkene predominates due to most commonly steric hinderance caused by the: size of the attacking base and size of the group surrounding the leaving group