# What determines whether an aldol reaction will occur again or eliminate?

The product of an aldol reaction is a $\beta$-hydroxy ketone. This can go on to eliminate to form an alkene thus:

However, the species on the far left of the above diagram (the $\beta$-hydroxy ketone product of the first aldol reaction) can react through an aldol reaction again given that it has two more available protons at the $\alpha$ position.

How is it possible to tell which reaction pathway will occur? Is this the issue that is referred to when the Aldol reaction is referred to as "hard to control"?

Partially yes. The big problem with aldol reactions in general is that there are a lot of species that can react together to form stable compounds resulting in low yield. There are numerous ways to counter this:

• Specific enolate equivalants, like lithium enolates, silyl enol ethers, aza enolates, enamines, etc. These are among the most important.

• Lack of proton in $\alpha$-position.

• Strong thermodynamic favoring of certain products in combination with reversibility of the reaction.

• etc.

As for your example the $\beta$-hydroxy ketone is more sterically hindered than acetone and is therefore not very likely to be the major contributor (although polymerization can still happen of course).

In general you cannot predict whether or not elimination of $\ce{H2O}$ occurs after the aldol reaction, but the likeliness can be increased by forcing certain reaction conditions.