Basic principles

Question 1

What is the hybridisation of boranes?

Answer 1

sp2; 3 electrons in valence shell; can form up to 3 covalent bonds after hybridisation; majority are tricoordinate in monomeric state; vacant p AO orthogonal to 3 sp2 HAOs.

Question 2

Why are boranes lewis acidic?

Answer 2

Electron deficient boron with empty p orbital. Reaction with lewis base forms ‘ate’ complex (4 coord B is tetrahedral, 8 electrons on B, formal negative charge on B)

Question 3

Describe the Matteson rearrangement

Answer 3

Grignard reacts with most electrophilic centre (B); intermediate ate complex formed; migration of C from B to neighbouring C with displacement of Cl (proceeds with inversion of configuration)

Question 4

Why does the C migrate in preference to O in the Matteson rearrangement?

Answer 4

B-C bond weaker than B-O bond

Question 5

What effects influence lewis acidity?

Answer 5

Electronegativity; mesomeric

Question 6

Why does B have metallic character?

Answer 6

B is more electropositive than C and H

Question 7

Which elements does B form stronger bonds with?

Answer 7

To elements that can participate in p(pi)-p(pi) bonding e.g. O, N, F