My question is; If using the same two aldehyde's (CH3CH2CHO) for both alpha substitution and condensation reactions I always get the same end product, basically a Beta-hydroxy aldehyde. Is the only difference in these reactions simply the base used to deprotonate? I get that you need a full equivalent of something fairly strong such as Na+OEt- for the alpha and a catalytic amount of something fairly week such as -OH for the condensation. Is that all it is, or is there some fundamental flaw in my reaction mechanism?

You will certainly proceed through a beta-hydroxy carbonyl intermediate, however if heat is specified for the reaction, the result is beta-elimination affording the enal. Additionally, OH- or EtO- is not always used as there are advantages to using LDA to obtain kinetic control.

 

Please read the articles below:

 

Aldol Condensation Part I: The Basics

 

Aldol Condensation Part II: Kinetic Control

 

Aldol Condensation Part III: Stereochemistry

 

 

Good luck!