807 / 912
Chapter 25 799
Copyright © 2017 Pearson Education, Inc.
33.
All triacylglycerols do not have the same number of asymmetric centers. If the carboxylic acid components
at C-1 and C-3 of glycerol are not identical, the triacylglycerol has one asymmetric center (C-2). If the
carboxylic acid components at C-1 and C-3 of glycerol are identical, the triacylglycerol has no asymmetric
centers.
34.
a.
There are three triacylglycerols in which one of the fatty acid components is lauric acid and two are
myristic acid. Myristic acid can be at C-1 and C-3 of glycerol, in which case the triacylglycerol does
not have any asymmetric centers. If myristic acid is at C-1 and C-2 of glycerol, C-2 is an asymmetric
center, and consequently, the compound can have a pair of enantiomers.
b.
There are six triacylglycerols in which one of the fatty acid components is lauric acid, one is myristic
acid, and one is palmitic acid. The three possible arrangements are shown below (with the fatty acid
components abbreviated as L, M, and P). Because each has an asymmetric center, each can exist as a
pair of enantiomers for a total of six triacylglycerols.
CH
2
O L
*CH O M
CH
2
O P
CH
2
O L
*CH O P
CH
2
O M
CH
2
O M
*CH O L
CH
2
O P
35.
CH
2
CH
OH
OH
CH
2
OH
P
O
HO O
−
O
−
2
3
−
O R
1
C
O
−
O R
2
C
O
−
O R
3
C
O
−
O R
4
C
O
36.
CH
2
O C CH
3
O
CH O C CH
3
O
CH
2
O C CH
3
O
The structure to the left has a molecular formula = C
9
H
14
O
6
and a molecular
weight = 218.
Subtracting 218 from the total molecular weight gives the molecular weight con-
tribution from the methylene (CH
2
) groups in the triacylglycerol.
722 − 218 = 504
Dividing 504 by the molecular weight contribution from a methylene group (14)
gives the number of methylene groups.
504
14
= 36
Because there are 36 methylene groups, each fatty acid in the triacylglycerol
has 12 methylene groups.
CH
2
CH
O
O
CH
2
O
C
C
C
O
O
O
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
3
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
3
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
3
nutmeg