Basic Principles

Question 1

Describe the electron configuration of Si

Answer 1

sp3 hybridisation; 4 electrons in valence shell so can form 4 covalent bonds (after hybridisation). The availability of relatively low energy empty 3d AOs allows the possibility for higher coordination number (hypervalent Si compounds) and expansion of the valence site of Si.

Question 2

Describe substitution at Si

Answer 2

Very fast (much faster than C-X analogues). SN2-like (back side attack) with inversion of config. at stereogenic Si centre. Addition-elim mechanism, pentacoordinate species is a short-lived intermediate. SN2-like sub at tertiary Si possible due to more space around Si than tertiary C (Si larger and longer C-Si bond lengths). RDS involves Nu- and substrate (bimolecular).

Question 3

Describe the stabilisation of B-carbocations

Answer 3

When C-Si can align parallel with vacant p AO, filled orbital can overlap with empty in net bonding interaction (hyperconjugative stabilisation).

Question 4

Describe the stabilisation of a-carbanions

Answer 4

Carbanions with alpha Si group more stable than their C analogues

Question 5

Describe the bond strength of Si bonds

Answer 5

Bonds to O and F (lone pairs) stronger than with C. Si-H and Si-C (no lone pairs) weaker than C-C and C-H. Si-O and Si-F bonds strengthened by partial p pi-d pi bonding

Question 6

Si is more electropositive than C and so exerts a weak +I effect. This should destabilise alpha-negative charge but how is this effect overriden?

Answer 6

Si in period 3, empty 3d AOs allows d pi-sp3 pi bonding. Larger coefficient on Si atom in antibonding MO improves orbital overlap. Si large and so readily polarised. Induced dipoles will also help stabilise proximal negative charge (most important factor)

Question 7

What are silyl ethers and how are they used?

Answer 7

Protecting groups for alchohol functional groups. Silyl ethers bearing large bulky alkyl groups tend to be the most stable towards hydrolysis.

Question 8

How are silyl ethers hydrolysed?

Answer 8

Can be done with base or acid but alternative is using F- as this is a chemoselective route. Orthogonal protection stategy. Fluoride-induced deprotection proceeds by Nu- addition-elim and is thermodynamically driven by high strength of Si-F bond.