[fai]

I want to make trans-3-phenylprop-2-enoic acid from acetone and benzaldehyde, however i'm not sure about my last step to make the carboxylic acid but it's the only way i know how. Can someone check this for me?

The steps
Acetone ---base---> enolate
enolate + benzaldehyde ---H2O---> aldol
aldol ---base---> α,β-unsaturated carbonyl product
α,β-unsaturated carbonyl product ---Cl2+base, then acid---> trans-3-phenylprop-2-enoic acid

Mechanism:

http://img687.imageshack.us/img687/3968/reaction.png

[tmartin]

Your final step looks correct to me.  You can look up the Haloform reaction for further info (I'm assuming that's what you did though  ).  However, your product of step 3 does not apear to be drawn correctly.  You refer to the product as an alpha,beta-unsaturated carbonyl, however that is not the product you drew, you'll want to double check that.