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792 Chapter 25
Copyright © 2017 Pearson Education, Inc.
Solutions to Problems
1.
a.
Stearic acid has the higher melting point, because it has two more methylene groups (giving it a greater
surface area and, therefore, stronger London dispersion forces) than palmitic acid.
b.
Stearic acid has the higher melting point because stearic acid is saturated, so its molecules can pack
closer together than can the molecules of oleic acid that has a double bond.
c.
Oleic acid has the higher melting point because it has two more methylene groups than palmitoleic
acid (see part
a
).
d.
Oleic acid has the higher melting point, because it has one cis double bond, whereas linoleic acid has
two cis double bonds. The greater the number of double bonds, the harder for the molecules to pack
closely together.
2.
Glyceryl tripalmitate has a higher melting point, because the carboxylic acid components are saturated and
can, therefore, pack more closely together than the cis-unsaturated carboxylic acid components of glyceryl
tripalmitoleate.
3.
To be optically inactive, the fat must have a plane of symmetry. In other words, the fatty acid residues at
C-1 and C-3 must be identical. Therefore, stearic acid must form the esters at C-1 and C-3.
CH
2
O C (CH
2
)
16
CH
3
O
CH O C (CH
2
)
10
CH
3
O
CH
2
O C (CH
2
)
16
CH
3
O
4.
To be optically active, the fat must not have a plane of symmetry. Therefore, the two stearic acid residues
must be attached to adjacent alcohol groups (either C-1 and C-2 or C-2 and C-3).
CH
2
O C (CH
2
)
16
CH
3
O
CH O C (CH
2
)
16
CH
3
O
CH
2
O C (CH
2
)
10
CH
3
O
5.
Solved in the text.
6.
The identities of R
1
and R
2
have no effect on the configuration of the asymmetric center at C-2 because:
the group with priority #1 is always the carboxyl group on C-2;
the group with priority #2 is always the group that contains the phosphorus;
the group with priority #3 is always the group that contains the other carboxyl group;
the group with priority #4 is always the H on C-2.