Unit 3 - Reactivity of conformations

Question 1

What is the Wittig reaction?

Answer 1

Olefination of an aldehyde or a ketone using a triphenyl phosphonium ylide (Wittig reagent)

Question 2

How can the Wittig reaction be manipulated?

Answer 2

More stable ylide forms the E isomer and less stable forms the Z isomer

Question 3

Why does a less stable ylide form the Z isomer?

Answer 3

Forming the cis intermediate is fast because R on opposite sides in the TS. Reaction is reversible because intermediate has more unfavourable gauche interactions. Therefore, if yilde is stable, intermediate will degenerate.

Question 4

Why does a more stable ylide form the E isomer?

Answer 4

Forming the trans intermediate causes clashing R groups in TS so step is slow. However, intermediate has more favourable interactions and so is more stable. If ylide is more stable cis intermediate degenerates before reaction is complete.

Question 5

What groups stabilise the ylide and how?

Answer 5

Electron withdrawing groups because they create resonance structures

Question 6

Which AD mix attacks the ‘top’ of a compound?

Answer 6

AD mix beta

Question 7

Which AD mix attacks the ‘bottom’ of a compound?

Answer 7

AD mix alpha

Question 8

Which DIPT isomer attacks the ‘top’ of a compound?

Answer 8

(-)-DIPT

Question 9

Which DIPT isomer attacks the ‘bottom’ of a compound?

Answer 9

(+)-DIPT

Question 10

Why do stereospecific reactions work?

Answer 10

The added compound is also chiral and so slightly affects the compounds reactivity`

Question 11

How many Hz is a coupling constant separated by 60 degrees on different carbons be?

Answer 11

3-4 Hz

Question 12

How many Hz is a coupling constant separated by 120 degrees on the same carbon be?

Answer 12

8-12 Hz

Question 13

How many Hz is a coupling constant separated by 180 degrees on different carbons be (antiperiplanar)?

Answer 13

10-12 Hz