What are the three geometric isomers of dibenzalacetone?

1 Answer
Ernest Z.
May 24, 2016

The three isomers of dibenzalacetone are cis-cis, cis-trans, and trans-trans.

Explanation:

Dibenzalacetone (trans, trans-1,5-diphenylpenta-1,4-dien-3-one) is prepared by the aldol condensation of acetone with excess benzaldehyde.

Initially, one mole of benzaldehyde reacts with one mole of acetone to form benzylidineacetone (4-phenylbut-3-en-2-one)

#"C"_6"H"_5"CHO" + "CH"_3"COCH"_3 stackrelcolor(blue)("NaOH"color(white)(m))(→) underbrace("C"_6"H"_5"CH=CHCOCH"_3)_color(red)("benzylidineacetone") + "H"_2"O"#

This newly-formed ketone still possesses alpha hydrogens, so it can undergo another aldol condensation with a second mole of benzaldehyde:

#underbrace("C"_6"H"_5"CH=CHCOCH"_3)_color(red)("benzylidineacetone")+ "CH"_3"COCH"_3 stackrelcolor(blue)("NaOH"color(white)(m))(→) underbrace("C"_6"H"_5"CH=CHCOCH=CHC"_6"H"_5)_color(red)("dibenzalacetone") + "H"_2"O"#

You can have cis-trans isomerism about each of the double bonds.

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The possible combinations are cis,cis, cis,trans, trans,cis, and trans,trans.

The cis,trans and the trans,cis forms, however, are identical, so there are only three isomers.

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