# How to decide what pathway a reaction will follow, elimination, addition to a carbonyl or nucleophilic substitution?

There is a substrate such as this,

and it is treated with alcoholic KOH.

I know that a hydrogen in $\alpha$-position to the carbonyl group will be abstracted, leaving a carbanion. My query lies in deciding what pathway will the substrate follow from hereon. I see three options: An E1cB elimination of the bromide, aldol condensation by nucleophilic addition to the carbonyl group and SN2 with the 1 degree bromide. Which pathway will be favored and why?

I'd be glad if someone could specify the conditions for one pathway being predominantly favored over another (even if I haven't provided information regarding that, like temperature, solvent, etc) or some empirical data on one pathway being faster or slower. Any degree of help will be appreciated, thank you.

Edit: The answer in the book where this question is from says that nucleophilic addition should predominate (providing neither justification nor data), which contradicts my guess that elimination would dominate because of it being an internal mechanism.

• Two of the possible pathway require the carbanion to interact with a second molecule, one does not. Which do you think will happen quicker? – Waylander Feb 18 '18 at 16:05
• I actually thought of that as well, but the textbook with this problem stated that addition would be faster so I am confused. (Should I edit this into the original post?) – Shikhar_Mohan Feb 18 '18 at 17:37
• Yes, add that to the question – Waylander Feb 18 '18 at 18:09
• Can you tell the name of the book? – Apoorv Potnis Feb 24 '18 at 2:07
• It's a problem book for preparation for IIT-JEE, Resonance Rank Booster. – Shikhar_Mohan Feb 24 '18 at 9:28

## 1 Answer

1. The nucleophilic substitution with another equivalent of the primary bromide will never be favourable as the nucleophilicity of the enolate generated is pretty low (the charge is delocalized!)

2. Even if E1cb was a possible route, you would end up with an extremely reactive vinyl aldehyde, which would likely participate in a Michael addition with an enolate in the reaction medium. So, isolating the product from this route would be very unlikely, even though I cannot back my claim up immediately.

If you ask me, both the Michael addition and the Aldol reaction should compete, given the nature of the base the problem has used. Had it been something like LDA (or any other directed Aldol conditions), the greater stability of the enolate would probably have precluded E1cb as a possible route.