2.3

Question 1

What’s a good leaving group?

Answer 1

• Weak bases with strong conjugate acids; gives anions happy to float in solution.
• Sulfonates.

Question 2

Is hydroxyl a good leaving group?

Answer 2

• Hydroxyl is not a good leaving group, but when hydrogenated to give water, water is a good leaving group.

Question 3

How can solvent be used to determine SN1 or SN2?

Answer 3

SN1: acidic conditions.

SN2: basic or neutral conditions.

• Polar protic vs polar aprotic

Polar protic: smaller halogens stronger.

Polar aprotic: larger halogens stronger.

Aprotic favours SN2

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Question 4

Does how good the nucleophile is effect the rate of substitution?

Answer 4

For SN1: No (needs non-nucleophilic solvent, or else will compete)
For SN2: Yes (needs nucleophile)

Question 5

Properties of racemic compounds?

Answer 5

• is a 1:1 mixture of the two enantiomers of a compound
• will not rotate plane polarised light
• will have the same melting point as the individual enantiomers
• has identical chemical reactivity as the individual enantiomers

Question 6

Relationship between quality of nucleophile + of leaving group?

Answer 6

Inverse: usually a good nucleophile is a bad leaving group.

Question 7

Effects of temp?

Answer 7

Cold favours substitution

Hot favours elimination.

Question 8

What can we rule out with a weak nucleophile?

Answer 8

SN2 and E2

Question 9

pH on elim mechanisms?

Answer 9

E1: acidic non-nucleophilic solvent to attract away leaving group to give, say, H-Cl.
E2: strong base to remove a hydrogen ion.

Question 10

Alkyl halides contain a [1] bond as the halogen is more [2] than the carbon atom it is attached to. This means that there is less [3] on the carbon atom and, because of this, alkyl halides contain an [4] carbon atom.

Answer 10

4 electrophilic

3 electron density

1 polar

2 electronegative

Question 11

In an SN2 reaction, the incoming nucleophile reacts with the electrophile on the opposite side to that of the leaving group. As the nucleophile enters and the leaving group leaves, the central carbon inverts its configuration. We say that in the middle of this reaction the species present is;

Answer 11

A transition phase

Question 12

Alkyl halides promote nucleophilic substitution reactions because they are electrophiles and;

Answer 12

They have a good leaving group

Question 13

Are carbocations nucleophiles or electrophiles?

Answer 13

ELECTROPHILES bc pos charge means needs e- to balance out.