carbanions - structure + stability

What is a carbanion?

An electron-rich carbon with 8 electrons around it: 6 bonding electrons and 2 non-bonding electrons, giving carbon a negative charge.

What is the electronic structure of an alkyl carbanion?

sp3 hybridised, with the negative charge/lone pair in a filled sp3 orbital.

What is the shape of an alkyl carbanion?

Trigonal pyramidal.

What are alkyl carbanions isoelectronic and isostructural with?

Amines with equivalent substituents.

Can alkyl carbanions readily invert?

Yes. Tetrahedral alkyl carbanions can undergo rapid inversion.

What is the electronic structure of a carbanion adjacent to a π-system?

sp2 hybridised, with the negative charge/lone pair in a filled p orbital.

Why is a carbanion adjacent to a π-system sp2 hybridised?

This places the lone pair in a p orbital so it can overlap with the adjacent π-system and delocalise the negative charge.

What is the shape of a carbanion adjacent to a π-system?

Planar at the carbanionic carbon.

Give examples of carbanions stabilised by an adjacent π-system.

Allylic carbanions, benzylic carbanions and enolates.

What is the electronic structure of an alkenyl carbanion?

sp2 hybridised, with the negative charge/lone pair in a filled sp2 orbital.

Where is the p orbital in an alkenyl carbanion?

It is involved in forming the C=C π bond, so the carbanion lone pair occupies an sp2 orbital instead.

What is the electronic structure of an alkynyl carbanion?

sp hybridised, with the negative charge/lone pair in a filled sp orbital.

What is the relationship between carbanion stability and conjugate-acid pKa?

The more stable the carbanion, the lower the pKa of its conjugate acid.

What does a lower conjugate-acid pKa tell you about its carbanion?

The carbanion is more stable.

What does a -I effect do to carbanion stability?

Stabilises the carbanion by withdrawing electron density away from the negatively charged carbon.

What does a +I effect do to carbanion stability?

Destabilises the carbanion by donating electron density towards an already negatively charged carbon.

What does a -M effect do to carbanion stability?

Stabilises the carbanion by allowing negative charge to be delocalised through a π-system.

What does a +M effect do to carbanion stability?

Destabilises the carbanion by donating additional electron density towards the negatively charged system.

Why does delocalisation stabilise a carbanion?

The negative charge is spread over multiple atoms, reducing the concentration of negative charge at one atom.

What are examples of mesomerically stabilised negative centres?

Allyl anion, benzyl anion, enolate and phenolate.

How is an enolate stabilised?

The negative charge is delocalised between the alpha-carbon and oxygen through the π-system.

Why is placing negative charge on oxygen favourable in an enolate?

Oxygen is more electronegative than carbon and therefore better stabilises negative charge.

How is a benzyl carbanion stabilised?

The filled p orbital on the benzylic carbon overlaps with the aromatic π-system, allowing the negative charge to delocalise.

How is an allylic carbanion stabilised?

The filled p orbital on the carbanionic carbon overlaps with the adjacent C=C π-system, delocalising the negative charge.

What is the order of carbanion stability based on hybridisation?

sp > sp2 > sp3.

What percentage s-character does an sp orbital have?

50%.

What percentage s-character does an sp2 orbital have?

33%.

What percentage s-character does an sp3 orbital have?

25%.

Why does increasing s-character stabilise a carbanion?

Greater s-character holds the negatively charged lone pair closer to the positively charged nucleus, lowering its energy.

Which adjacent atoms can stabilise carbanions?

Sulfur, phosphorus and silicon.

What orbital interaction helps stabilise a carbanion adjacent to sulfur?

The carbon lone pair donates into the antibonding sigma-star C-S orbital.

How is the orbital interaction for sulfur stabilisation written?

n(C) → σ*(C-S).

What is required for effective n(C) → σ*(C-S) stabilisation?

The orbitals must have the correct alignment for effective overlap.

What property of C-S bonds also contributes to carbanion stabilisation?

Their polarisability.

Is donation into empty sulfur 3d orbitals the favoured explanation for sulfur stabilisation?

No. Hyperconjugative donation from the carbon lone pair into σ*(C-S) is favoured.

How should you approach a question asking you to rank carbanion stability?

Check resonance/mesomeric effects, inductive effects, hybridisation and s-character, and stabilisation by adjacent S, P or Si.

How should you approach a question asking you to rank conjugate-acid pKa values?

First rank the stability of the corresponding carbanions, then reverse the order: the most stable carbanion has the lowest conjugate-acid pKa.