Substitution And Elimination

Question 1

S_n1 reactivity (from most reactive to least)

Answer 1

Based on carbocation stability:
3° benzyl
3° allyl
2° allyl
3°
1° allyl
2°

1° and methyl cannot undergo S_n1 reaction

Question 2

S_n1 reaction product stereochemistry

Answer 2

Give racemic mixture of enaantiomers —> this is due to nucleophile being able to attack from either side

Question 3

Do S_n1 reactions involve rearrangement?

Answer 3

YES, because they involve carbocation intermediates, the carbocation can rearrange to give the most stable carbocation intermediate

Question 4

What is the rate of S_n1 reactions?

Answer 4

Rate = k[electrophile]

Question 5

What is the rate for S_n2 reactions?

Answer 5

Rate = k[electrophile][nucleophile]

Question 6

S_n2 reactivity (from most reactive to least reactive)

Answer 6

Methyl LG
1° LG
2° LG

Bezyls and allyls CAN undergo S_n2 depending on the nucleophile

3° LG cannot undergo S_n2 reactions

Question 7

S_n2 stereochemistry

Answer 7

Inversion of stereochemistry due to backside attack

Question 8

what mechanism would you use if a 1° carbon is bonded to the leaving group?

Answer 8

S_n2 or E2

Question 9

what mechanism would you use if a 2° carbon is bonded to the leaving group?

Answer 9

S_n1, S_n2, E1 or E2

Question 10

what mechanism would you use if a 3° carbon is bonded to the leaving group?

Answer 10

S_n1, E1 or E2

Question 11

1° bonded to leaving group
strong nucleophile or weak base

Answer 11

S_n2

Question 12

1° carbon bonded to leaving group
strong nucleophile or strong base

Answer 12

S_n2 (major)
E2 (minor)

Question 13

1° carbon bonded to leaving group
strong, bulky base

Answer 13

E2

Question 14

2° carbon bonded to leaving group
weak nucleophile or weak base

Answer 14

S_n1 or E1

Question 15

2° carbon bonded to leaving group
strong nucleophile or weak base

Answer 15

S_n2

Question 16

2° carbon bonded to leaving group
strong nucleophile or strong base

Answer 16

E2 (major, trans)
S_n2 (minor)

Question 17

2° carbon bonded to leaving group
strong bulky base

Answer 17

E2

Question 18

3° carbon bonded to leaving group
weak nucleophile or weak base

Answer 18

S_n1 or E1

Question 19

3° carbon bonded to leaving group
strong nucleophile or weak base

Answer 19

S_n1

Question 20

3° carbon bonded to leaving group
strong base

Answer 20

E2

Question 21

3° carbon bonded to leaving group
strong, bulky base

Answer 21

E2 (non-zaitsev)

Question 22

What are the three trends when ranking strength of nucleophiles?

Answer 22

1) Negative charges increase nucleophilicity
2) Nucleophilicity increases going left across a row
3) Moving down a column, nucleophilicity depends on the solvent

Question 23

Strength of nucleophiles in protic solvents:

Answer 23

Increases going down a column this is because protic solvents form a “hydrogen bond web” and solvate the nucleophile, making it harder for the nucleophile to attach —> a larger atom will break through the web better than a small, solvated atom

Size matters for protic solvents

Question 24

Strength of nucleophiles in aprotic solvents:

Answer 24

Decreases going down a column because aprotic solvents do not interact with the nucleophile —> only concentrated, negative charges matter in aparotic solvents

Question 25

Strength of nucleophile going across a row:

Answer 25

Increases going left across a row (becomes more basic) Ex: F- is a weaker nucleophile than NH₂-

Question 26

In nucleophilic addition reactions, EWG substituents (increase/decrease) the positive charge on the carbonyl carbon, thus (increasing/decreasing) reactivity

Answer 26

Increase Increasing An EWG pulls the electron density *away* from the carbonyl carbon, thus making the reactivity of the nucleophile and carbon *greater*

Question 27

In nucleophilic addition reactions, (aldehydes/ketones) are more reactive

Answer 27

Aldehydes

Question 28

In nucleophilic addition reactions, the carbon of the carbonyl group is _____________

Answer 28

Electron deficient- the electronegative oxygen pulls electron density *away* from the carbonyl carbon

Question 29

In nucleophilic addition reactions, bulky substituents attached to the carbonyl carbon will (increase/decrease) steric hinderance involved with nucleophilic attack, thus (increase/decreasing) reactivity

Answer 29

Increase Decreasing

Question 30

General rule for determining aprotic solvents:

Answer 30

Aprotic solvents **lack an acidic proton**, if the solvent lacks hydrogen directly bonded to *oxygen or nitrogen*, it can be considered aprotic

Question 31

What solvent should be used for Grignard reagents?

Answer 31

Grignard reagents are highly *basic* compounds that can act as a nucleophile to attack carbonyl-containing compounds. **Aprotic solvents** should be used with Grignard reagents because in protic solvents, the Grignard reagent will act as a base to deprotonate the solvent rather than attacking the nucleophile

Question 32

strong/weak nucleophile/base NaSCH₃

Answer 32

* strong nucleophile weak base

Question 33

strong/weak nucleophile/base NaOH

Answer 33

* strong nucleophile strong base

Question 34

strong/weak nucleophile/base tBu-OK

Answer 34

strong base weak nucleophile

Question 35

strong/weak nucleophile/base NaSH

Answer 35

weak base strong nucleophile

Question 36

strong/weak nucleophile/base NaCN

Answer 36

weak base strong nucleophile

Question 37

strong/weak nucleophile/base H₂O

Answer 37

weak base weak nucleophile

Question 38

strong/weak nucleophile/base NaH

Answer 38

strong base weak nucleophile

Question 39

a strong base would be able to ____ H atom easily

Answer 39

accept

Question 40

strong/weak nucleophile/base EtOH

Answer 40

weak nucleophile weak base

Question 41

strong/weak nucleophile/base H₂S

Answer 41

strong nucleophile weak base

Question 42

strong/weak nucleophile/base MeOH

Answer 42

weak nucleophile weak base

Question 43

strong/weak nucleophile/base NaN₃

Answer 43

strong nucleophile weak base

Question 44

strong/weak nucleophile/base KOH

Answer 44

strong nucleophile strong base

Question 45

strong/weak nucleophile/base KOH

Answer 45

strong nucleophile strong base

Question 46

strong/weak nucleophile/base NaI

Answer 46

strong nucleophile weak base

Question 47

strong/weak nucleophile/base NaOMe

Answer 47

strong nucleophile strong base