SN1 vs SN2

Question 1

Define nucleophile

Answer 1

A species prepared to donate a pair of e- to form a bond

Question 2

BRIEFLY describe a SN2 reaction

Answer 2

X leaves as Nu attaches

Question 3

Is SN2 uni- or bi- molecular? Why?

Answer 3

Bimolecular. Rate depends on the concentration of two (both) reactants

Question 4

What step is the slow step in a SN2 reaction?

Answer 4

The only step

Question 5

What are 3 important things to note about SN2 reactions?

Answer 5

1. Nu attacks from a specific direction (back), most δ+ region
2. Causes inversion to occur
3. Is stereospecific

Question 6

How many enantiomers are produced in a SN2 reaction? What does this mean for polarised light?

Answer 6

One enantiomer .: will rotate polarised light

Question 7

BRIEFLY describe a SN1 reaction

Answer 7

X leaves then Nu:– attaches

Question 8

What step is the slow step in a SN1 reaction?

Answer 8

The first step (leaving group leaving)

Question 9

Why is the removal of the leaving group the slow step?

Answer 9

Because energy needed to break bond

Question 10

What role does the nucleophile play in the slow step of an SN1 reaction?

Answer 10

Nucleophile plays no role. SN1 reactions are independent of Nu strength - just has to be a nucleophile (prepared to donate e-)

Question 11

What is the fast step of a SN1 reaction? Why?

Answer 11

Addition of the nucleophile.

Because attraction

Question 12

Describe the hybridisation and shape of a carbocation

Answer 12

sp2 hybridised trigonal planar

Question 13

How many enantiomers form from a SN1 reaction? Why? What does this mean for polarised light?

Answer 13

Flat carbocation = equal probability of Nu attacking from one side or the other
□ Equal probability of forming each enantiomer
□ Inversion and retention of shape
□ No net rotation of polarised light

Question 14

Why are tertiary C more likely to react via SN1 than primary or methyl C?

Answer 14

Because first transition state can be stabilised.
In 3º molecules, CH3 groups attached are e- donating, e- push in, diminish charge on central C = lower Ea, rate increases.
This stabilisation does not occur (as much) in primary or methyl C molecules

Question 15

What is an absolute requirement for SN1 reactions?

Answer 15

A good leaving group (strength of Nu:- is irrelevant)

Question 16

What requirement of SN2 reactions can be made up for by a strong nucleophile?

Answer 16

Having a good leaving group

Question 17

What determines how good a leaving group is?

Answer 17

How strong the C-X bond is /bond length

Question 18

What does protonating an -OH group do?

Answer 18

O becomes +vely charged, e- closer to O, bond even more polarised, weaker
= Increases likelihood of leaving, :. becomes good leaving group

Question 19

What is an absolute requirement for SN2 reactions?

Answer 19

A good nucleophile

Question 20

What makes a good/strong nucleophile?

Answer 20

• Small (has to get in past any other groups attached)
• negatively charged
• Not necessarily a strong base (poor nucleophile if large/hindered)

Question 21

Define protic

Answer 21

Protic = able to form H bonds

Question 22

Is water a protic or an aprotic solvent? Why?

Answer 22

Protic as able to from H-bonds

Question 23

What solvent types is SN1 fastest in?

Answer 23

Polar solvents, especially if H bonding (= polar protic)

Question 24

How is the first transition state of an SN1 reaction stabilised in water? What effect does this have on the Ea of the reaction?

Answer 24

• polar effect around C
  § δ- O end attracted to δ+ C, pulls towards
  § Helps stabilize emerging carbocation
  § Lowers Ea
• H-bonding to leaving group
  § δ+ H interacts with emerging -ve charge
  § Helps stabilize leaving group leaving as well
  § Lowers Ea

Question 25

What solvents are best for SN2 reactions? Why?

Answer 25

Polar APROTIC solvents, so solvent cage doesn’t form around Nu, inhibit its attack