Download CHE 297 Organic Workshop

CHE 297 Organic Workshop
Problem Set 7B
March 5, 2007
Although the aldol condensation has not yet been covered in lecture, today’s problem set should teach you how
to do it.
A. Translate the following text into a mechanism with structures and conventional curved arrows.
Step 1 A base (use hydroxide ion) removes the acidic alpha hydrogen from a molecule of acetaldehyde
(ethanal) to form an enolate anion and a molecule of water.
Step 2 The enolate anion attacks the carbonyl group of a second molecule of acetaldehyde. That
intermediate is then protonated to form a beta-hydroxyaldehyde (3-hydroxybutanal).
Step 3 The beta hydroxylaldehyde undergoes an elimination reaction (OH is the unusual leaving group
here) to form an alpha, beta-unsaturated aldehyde called crotonaldehyde (2-butenal).
B. Complete the following reactions. All except #2 are examples of the aldol condensation or variations on the
same theme. In #2 the enolate anion formed with LDA is alkylated by benzyl bromide in an S N2 reaction. Write
a mechanism for #5.
H
1.
2
KOH, H2O
heat
O
2.
1) LDA
O
2) PhCH2Br
O
3.
O
NaOH, H2O
H
+
heat
O
O
4.
Ph
Ph +
Ph
Ph
heat
O
O
KOH
5.
heat
O
NaOH