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290 CHAPTER 9 
 
 
9.21. 
(a) The desired product is an aldehyde, which can be 
prepared from the corresponding terminal alkyne, shown 
here: 
 
 
(b) The desired product is not an aldehyde, but it can be 
made directly from the following internal alkyne. This 
alkyne is symmetrical, so only one regiochemical 
outcome is possible: 
 
 
(c) The desired product is an aldehyde, which can be 
prepared from the corresponding terminal alkyne, shown 
here: 
 
 
9.22. 
(a) The starting material is a terminal alkyne and the 
product is a methyl ketone. This transformation requires 
a Markovnikov addition, which can be achieved via an 
acid-catalyzed hydration in the presence of mercuric 
sulfate. 
 
(b) The starting material is a terminal alkyne and the 
product is an aldehyde. This transformation requires an 
anti-Markovnikov addition, which can be achieved via 
hydroboration-oxidation. 
 
 
9.23. Compounds 1 and 3 have the same carbon skeleton. 
Only the identity and location of the functional groups 
have been changed. In compound 1, the following two 
highlighted positions are functionalized (each is attached 
to Br): 
 
 
So it is reasonable to propose an alkyne in which the 
triple bond is between these two carbon atoms: 
 
 
 
Indeed, we have learned a way to convert this alkyne 
into the desired methyl ketone (compound 3), via acid-
catalyzed hydration (Markovnikov addition): 
 
O
OH
O O
OH
HgSO4, H2SO4
H2O
2 3 
 
To complete the synthesis, we must propose reagents for 
the conversion of 1 to 2. As seen earlier in the chapter, 
this transformation can be achieved via two successive 
E2 reactions, requiring a strong base such as NaNH2. 
For this type of process, excess base is generally 
required. As shown below, the initial product of this 
general process is an alkynide ion, which is protonated 
upon treatment with a mild acid to give a terminal 
alkyne: 
 
 
In this case, the strongly basic conditions will also 
deprotonate the carboxylic acid to give a carboxylate 
group. This carboxylate group is protonated (together 
with the alkynide ion) upon treatment with a mild acid: 
 
 
 
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