Nucleophilic substitution with loss of carbonyl oxygen Flashcards
Draw the mechanism for the nucleophilic substitution with loss of carbonyl
-
How do you make something a better leaving group?
Protonate it
What is Keq for aldehydes?
Is this favourable or unfavourable?
0.01
Unfavourable
What is the overall equation for acetal formation and how can this reaction become more favourable?
Aldehyde + 2ROH acetal + H2O
Remove water as it formed to drive eq to the RHS
Why is Keq so low for acetal formation?
Both entropically and enthalpically unfavourable
How is acetal formation entropically unfavourable?
From 3 molecules to 2 molecules
Why do the entropy and enthalpy factors affect Keq?
They cause ΔG to be positive which is not favourable
Which conditions are acetals hydrolysed under?
Acidic
H+ , H2O
What are acetals stable to?
Base hydrolysis
HO-, H2O
Why is it good to convert an aldehyde to an acetal?
Acetal acts as a protecting group
What is an imine?
Nitrogen equivalent of a carbonyl compound
What does imine formation require?
acid catalysis
How does an imine compare to amine and ketone?
Unstable
Keq < 1
How can equilibrium be driven to form the imine?
Remove water as it is formed
What reaction are imines involved in? How are they involved?
Reduction of amides through a double hydride addition
There is an iminium ion intermediate
What reagent is required for the reduction of amides?
LiAlH4
What is the Wittig reaction?
Substitution of C=O for C=C
What is a ylide?
Molecule with adjacent positive and negative charges
What reagent is involved in the Wittig reaction?
Ph3P–CH2
Which bit of the reagent is swapped out in the Wittig reagent? What for?
-CH2 for =O
What is a ylid?
Molecule with adjacent positive and negative charges
What is the driving force for the Wittig reaction?
P=O bond strength as it is extremely strong which creates an irreversible reaction
How is a phosphonium ylid formed?
Using Me - I to form a phosphonium salt and then a strong base
