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Solutions for Carbohydrates and Nucleic Acids
74
base
OH OH
O
OC
(a)
phenolphthalein
(b) Trityl groups are specific for 1° alcohols for steric 
reasons: the trityl group is so big that even a 2° alcohol is 
too crowded to react at the central carbon. It is possible for 
a trityl to go on a 2° alcohol, but the reaction is exceedingly 
slow and in the presence of a 1° alcohol, the reaction is done 
at the 1° alcohol long before the 2° alcohol gets started.
(c) Reactions happen faster when the product or intermediate is stabilized. Each OCH3 group stabilizes 
the carbocation by resonance as shown here; two OCH3 groups stabilize more than just one, increasing 
the rate of removal. The color comes from the extended conjugation through all three rings and out onto 
the OCH3 groups. Compare the DMT structure with that of phenolphthalein, the most common acid 
base indicator, that turns pink in its ring open form shown here. Phenolphthalein is simply another trityl 
group with different substituents.
C
O
CH3
OCH3
One of the major
resonance contributors
shows the delocalization
of the positive charge on
the oxygen.
C
O
H
OH
O
O
C
O
CH3
OCH3
One of the resonance 
contributors shows the 
delocalization of the 
positive charge onto the 
central carbon.
plus many more resonance forms
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