Week 7/8/9

Question 1

SN2 rate of rxn

Answer 1

• depends on [E+], [Nu]
• both E+ and Nu are involved in RDS
• second order rxn

Question 2

SN1 rate of rxn

Answer 2

• depends only on [E+]

- first order rxn

Question 3

Experimental determination of rxn order technique

Answer 3

• measure the rate before ~10% rxn completion

- simplifies kinetics (ignores reverse rxn, avoids side rxn)

Question 4

Steady state approximation

Answer 4

• assumes d[R+]/dt=0

- this occurs when ∆G‡ of formation step is large (unstable carbocation)

Question 5

Pre-equilibrium assumption

Answer 5

• fast pre equilibrium, final step is considered irreversible and rate determining
• consider the very large concentrations (ie. solvents) as constants, can be grouped with rate constant

Question 6

E1 reaction mechanism

Answer 6

• nucleophiles act as Bronsted-Lowry bases
• removal ß-H by Nu
• ß-H is acidic due to hyperconjugation with the empty p-orbital on alpha-C
• predominates when weak bases used and heat applied
• ie. H2O, EtOH

Question 7

E2 rxn mechanism

Answer 7

• favoured when strong base is present, heat applied

- concerted rxn (all steps happening at once)

Question 8

E vs. SN for 1˚ R-X

Answer 8

• SN1/E1 negligible due to R+ instability

- SN2 generally favoured over E2 (no steric hindrance to interfere w SN2 Nu attack)

Question 9

E vs. SN for 1˚ R-X w strong bulky base

Answer 9

• increase in steric hindrance of Nu/B favours E2 over SN2

- ß-H extraction less sensitive to steric hindrance then the Nu attack on alpha-C

Question 10

Strong bulky base examples

Answer 10

DBN
tert-BuOK
DBU

Question 11

E vs. SN for 3˚ R-X

Answer 11

• weak base=SN1/E1
• increased rxn temp, more E1 formed
• strong base=E2
• NO SN2 b/c steric hindrance

Question 12

E vs. SN for 2˚ R-X

Answer 12

• weak base/strong Nu, SN2 favoured (X-, R-S-, carboxylate anion, N3- (azide))
• with strong bases, favour E2
• with weak base/weak nucleophile, favour SN1/E1 (low temp: E1 < SN1, high temp: E1 > SN1)

Question 13

Why does E rate increase w temp

Answer 13

• arrhenius eqn

- slopes of ln(k) vs 1/T larger magnitude for E (slope proportional to activation energy)

Question 14

Regioselectivity of E1/E2

Answer 14

• more substituted alkenes are more stable (more opportunities for hyperconjugation)
• more substituted alkene formed when NO BULKY BASE present (vise versa for bulky base)

Question 15

E2 stereoselectivity

Answer 15

• 2 stereoisomers formed, but trans (E) favoured over cis (Z)
• needs ß-H and LG to be antiperiplanar to each other for C-H sigma and C-LG sigma* to be parallel (required to form new pi bond) and for the large lobe of C-LG sigma* to be the one close to the C-H sigma
• syn-periplanar does not react with E2 b/c large lobe of C-LG sigma* on the incorrect side

Question 16

Gauche vs. anti vs. eclipsed

Answer 16

• gauche has more steric clash than anti (high priority substituents near each other
• eclipsed is most energetically unstable
• anti is most stable configuration

Question 17

E2 stereospecificity

Answer 17

when only one ß-H present leading to the more substituted alkene, stereoisomer can only give Z or E, not both