Rebecca Melon Flashcards

1
Q

Two methods to calculate Lewis acidity

A
  • Gutmann Becket Test

- Childs method

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2
Q

Gutmann Becket test

A
  • The lewis acid in question forms an adduct with triethylphosphineoxide (Et3P=O).
  • The result is the removal of electron density from the P atom, which can be obsereved by a deshielding effect in the 31P NMR chemical shift.
  • From this a value of Lewis acidity can be calculated
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3
Q

Childs Method

A
  • Measure the downfield chemical shift of the H3 proton on crotonaldehyde, upon coordination to a lewis acid.
  • The more downfield the shift of H3, the stronger the lewis acid
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4
Q

Lewis Acid and lewis Base

A

Lewis Acid - Electron Pair acceptor (non-bonding, low energy sigma* orbital)

Lewis Base - Electron pair donator

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5
Q

Frustrated lewis pair (FLP)

A

A frustrated lewis pair is a compound or a mixture of two compounds that contains both lewis acidic and lewis basic sites which are prevented from adduct formation due to steric hinderance

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6
Q

Dehydrocoupling

A

Element-element bond formation

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7
Q

Hard/Soft acids and bases

A

Hard: Small, highly charged, weakly polarisable

Soft: Large, low charge, strongly polarisable

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8
Q

FLP reaction with alkynes (deprotonation or Nucleophilic attack)

A

-Reactivity depends on the nature of the phosphine
-More bronsted basic phosphines favour deprotonation
tBu3P = deprotonation
o-tol3P and PPh3 = 1,2 addition
-Can be proved through P NMR (presence of PH coupling)

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9
Q

Main group catalysed B-N dehydrocoupling (requirements)

A
  • A strong base to deprotonate the amine-borane (hydride, amido, alkyl)
  • A lewis acidic metal to act as a lone pair acceptor (from amine)
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10
Q

Which dehydrocoupling reaction works best Si-S or Si-O?

A

S-H bonds are weaker than O-H bonds and so sulphur will work better.

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11
Q

B-N dehydrocoupling: primary vs secondary amines

A
  • Primary amines you can loose up to 2H2 per B-N unit. Typically form borazine
  • Secondary amines loose 1H2 per B-N unit. Bulk R groups favour N=B unit where as small R groups favour dimer
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