[phillyj]

In lab, we did the aldol condensation experiment of 4 chlorobenzaldehyde to 4,4'-chlorobenzalacetone

4 chlorobenzaldehyde was reacted with acetone and NaOH solution.

The limiting reagent was the aldehyde (0.79 mmol) and acetone was 0.80 mmol.

It turns out that after I added amounts of the chemicals, the limiting reagents were changed. I added 0.85 mmol of aldehyde and 0.82 mmol of acetone.

The reaction was completed and the product was a yellow sticky, gluey substance that was hard to get out of the vial and spin vane. That didn't seem like the correct product.

Then we filtered it by vacuum filtration and the filtrate was basic (11 pH). Washing more of the product with water was supposed to turn the filtrate to neutral but it didn't change.

1)Why did this happen?
2)Was it because acetone became the limiting reagent?
3)What is the dibenzalacetone (4,4'-chlorobenzalacetone) supposed to look like?

Tell me if more info is needed. Thank you

[agrobert]

What other products/side reactions can you propose?  How can you say the yellow goo is not the product?  What was your order of addition?  Why do you think the acetone was supposed to be limited?  What color are highly conjugated molecules?  What is the effect of the chlorine in the para position?

[phillyj]

well, does anyone know what dibenzalacetone is supposed to look like? A crystal, powder...?

The instructor did not seem to think that was the product. And If the goo was the product, then the filtrate after water washes was supposed to be neutral, not basic.

I think you misunderstood about the limiting reagent. the benzaldehyde was to be the limiting reagent and the acetone was to be in excess but when i added the reactants, the amounts were not correct and acetone became the limiting. I believe that led to the sticky product. Any ideas?

[agrobert]

The amounts you used should be fine.  If anything acetone is meant to be limiting so that you get a solid product.  Either way you get a product (yellow goo-which is the right product according to your reaction) but with excess acetone you will dissolve your product making a yellow liquid.  The purpose of excess benzaldehyde is the ensure that you form aldol products on both sides of acetone.  Really you should be able to answer the questions I gave you.  This will help us to help you.

And do some of your own research

http://www.mnstate.edu/jasperse/Chem365/Aldol%20Reaction.doc.pdf

http://en.wikipedia.org/wiki/Dibenzylideneacetone

YELLOW?