Nucleophilic substitution with loss of carbonyl oxygen

Question 1

Draw the mechanism for the nucleophilic substitution with loss of carbonyl

Answer 1

-

Question 2

How do you make something a better leaving group?

Answer 2

Protonate it

Question 3

What is Keq for aldehydes?

Is this favourable or unfavourable?

Answer 3

0.01

Unfavourable

Question 4

What is the overall equation for acetal formation and how can this reaction become more favourable?

Answer 4

Aldehyde + 2ROH acetal + H2O

Remove water as it formed to drive eq to the RHS

Question 5

Why is Keq so low for acetal formation?

Answer 5

Both entropically and enthalpically unfavourable

Question 6

How is acetal formation entropically unfavourable?

Answer 6

From 3 molecules to 2 molecules

Question 7

Why do the entropy and enthalpy factors affect Keq?

Answer 7

They cause ΔG to be positive which is not favourable

Question 8

Which conditions are acetals hydrolysed under?

Answer 8

Acidic

H+ , H2O

Question 9

What are acetals stable to?

Answer 9

Base hydrolysis

HO-, H2O

Question 10

Why is it good to convert an aldehyde to an acetal?

Answer 10

Acetal acts as a protecting group

Question 11

What is an imine?

Answer 11

Nitrogen equivalent of a carbonyl compound

Question 12

What does imine formation require?

Answer 12

acid catalysis

Question 13

How does an imine compare to amine and ketone?

Answer 13

Unstable

Keq < 1

Question 14

How can equilibrium be driven to form the imine?

Answer 14

Remove water as it is formed

Question 15

What reaction are imines involved in? How are they involved?

Answer 15

Reduction of amides through a double hydride addition

There is an iminium ion intermediate

Question 16

What reagent is required for the reduction of amides?

Answer 16

LiAlH4

Question 17

What is the Wittig reaction?

Answer 17

Substitution of C=O for C=C

Question 18

What is a ylide?

Answer 18

Molecule with adjacent positive and negative charges

Question 19

What reagent is involved in the Wittig reaction?

Answer 19

Ph3P–CH2

Question 20

Which bit of the reagent is swapped out in the Wittig reagent? What for?

Answer 20

-CH2 for =O

Question 21

What is a ylid?

Answer 21

Molecule with adjacent positive and negative charges

Question 22

What is the driving force for the Wittig reaction?

Answer 22

P=O bond strength as it is extremely strong which creates an irreversible reaction

Question 23

How is a phosphonium ylid formed?

Answer 23

Using Me - I to form a phosphonium salt and then a strong base