Section 18.4 Reactions of Carbonyl Compounds with Carbon Nucleophiles 741substituents come from the Grignard reagent. Alcohols (3) and (5) cannot be prepared in
this way because they do not have two identical substituents.SOLUTION TO 8b(2) Methyl propanoate and excess methylmagnesium bromide.PROBLEM 9Which of the following secondary alcohols can be prepared from the reaction of methyl
formate with excess Grignard reagent?Reaction with Acetylide Ions
We have seen that a terminal alkyne can be converted into an acetylide ion by a strong
base (Section 6.9).
An acetylide ion is another example of a carbon nucleophile that reacts with carbonyl
compounds. When the reaction is over, a weak acid (one that will not react with the
triple bond, such as pyridinium ion), is added to the reaction mixture to protonate the
alkoxide ion.
PROBLEM 10Show how the following compounds could be prepared, using ethyne as one of the starting
materials. Explain why ethyne should be alkylated before, rather than after, nucleophilic
addition.a. 1-pentyn-3-ol c. 2-methyl-3-hexyn-2-ol
b. 1-phenyl-2-butyn-1-olReaction with Hydrogen Cyanide
Hydrogen cyanide adds to aldehydes and ketones to form cyanohydrins. This reaction
forms a product with one more carbon atom than the reactant. In the first step of the
reaction, the cyanide ion attacks the carbonyl carbon. The alkoxide ion then accepts a
proton from an undissociated molecule of hydrogen cyanide.
Because hydrogen cyanide is a toxic gas, the best way to carry out this reaction is to
generate hydrogen cyanide during the reaction by adding HCl to a mixture of the alde-
hyde or ketone and excess sodium cyanide. Excess sodium cyanide is used in order to
make sure that some cyanide ion is available to act as a nucleophile.
NaNH 2
CH 3 CCH NH 3 CH 3 C
C−OHCH 3 CH 2 CHCH 3OHCH 3 CHCH 3OHCH 3 CHCH 2 CH 2 CH 3OHCH 3 CH 2 CHCH 2 CH 3O O+N
H
CH 3 CH 2 HCH 3 CCH 3 CH 2 CHCOHC CH 3 CH 2 CHC−C − CCH 3 CCH 3+acetone acetone cyanohydrinCO OCH 3+HCN
CCH 3 CCCH 3OHN CH 3 CC−− NN+ −CNCH 3 CH 3