L1 Electrophiles and regioselectivity

Both termini of an epoxide are electrophilic. Which end is most frequently attacked in an Sn2 reaction

The less hindered end is preferentially attacked - regioselectivity

Recap Grignard reactivity with carbonyls

Difference between direct addition and conjugate addition

Direct addition is 1,2-addition in which the carbonyl is attacked by the nucleophile.

Conjugate addition results from resonance in which the beta carbon can be seen to be electrophilic. It is also called 1,4-addition

Labels given to the carbons and oxygen in enones

Either of these can be used in literature and textbooks.

What affects which carbon the nucleophile reacts with

Type of nucleophile - whether it is stabilised, hard/soft

Nature of the compound

Sterics, solvent, temperature

Which carbonyl groups prefer which type of addition

This follows the reactivity trend for carbonyls - the more electrophilic the carbonyl carbon the more likely that direct addition occurs. If the carbonyl is less reactive conjugate addition becomes more likely.

How can steric factors influence addition

The left side with the bulkier group on the alkene part is going to be less favoured for addition.

What determines enone regioselectivity

Hard nulceophiles prefer the carbonyl while soft nucleophiles prefer the alkene.

How can reaction conditions affect regioselectivity e.g. in this enone 

Changing the temperature of this reaction will affect the major productt of the reaction.

How does changing the temperature affect the major product and why

At low temperatures, the kinetic product with the more stable transition state forms as the activation energy is lower. If the temperature increases sufficiently, the reaction to form the kinetic product begins to reverse and the thermodynamic product which is lower energy will form instead.

Essentially, the kinetic product reaction is fast but reversible, while the thermodynamic product is slower but not reversible.