Ch26 - Chapter 25: Carbohydrates Solutions 732 . C hapter...

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Chapter 16: Lipids Solutions 733 CHAPTER 26 Solutions to the Problems Problem 26.1 (a) How many constitutional isomers are possible for a triglyceride containing one molecule each of palmitic acid,. oleic acid, and stearic acid? There are three constitutional isomers possible, the difference being which fatty acid is in the middle of the molecule: o o II II o--C-palmitic /o--C-stearic H'C/ ' 2 0 H 2 C 0 I* II 1* II Hr--o--C-oleic Hr--o-C-palmitic l'h C " H 2 C" o-C-stearic o-C-oleic II II 000 Each of these molecules has one chiral center as indicated by the asterisk, so each constitutional isomer shown llbove can exist as a pair of enantiomers. Thus there are 2 x 3 = 6 total isomers possible. Note that for oleic acid, the ca~bon.carbon double bond is assumed to have the Z (cis) configuration only. (b) Which of these constitutional isomers are chiral? They are all chiral because they each have a chiral, center at the middle carbon of the glycerol moiety. Problem 26.2 Define the term hydrophobic. A good working definition of the term hydrophobic is "dislikes water". Hydrophobic materials are nonpolar, and do not dissolve in water. Because they are not solvated, hydrophobic surfaces of molecules aggregate in polar solvents like water. This aggregation behavior of hydrophobic surfaces is an extremely important phenomenon that underlies a number of important aspects Qf biological chemistry such as membrane formation, surfactant action, protein folding, and protein-protein recognition. Problem 26.3 Identify the hydrophobic and the hydrophilic region(s) of a triglyceride. Hydrophilicregion 0----- ,...--. . (CH 2 )14 CH 3 H 2 C I HC Hydrophobic region, - II o--C- - 0 II r-o-C o-C- II 0 - H)'drophobic region (CH 2 )14 CH 3' I H 2 C. '---' (CH 2 )14 CH 3 Note that the vast majority of the, molecule is hydrophobic, thus explaining why triglycerides are so hydrophobic overall. ' ' Problem 26.4 Explain why the melting points of unsaturated fatty acids are lower than those of saturated fatty acids. When fatty acids pack together better, the attractive dispersion forces between molecules are stronger, thereby increasing the melting point. Saturated fatty adds can adopt a much more regular, compact structure . compared to unsaturated fatty acids that have a kink induced by the ci$ double bond. The more regular compact saturated fatty acids can pack together better, so their melting points are higher. Problem 26.5 Which would you expeC\ to have the higher melting point, glyceryl trioleate or glyceryl trilinoleate? Cis double bonds disrupt chain packing and thus lower melting points. The triglyceride with fewer cis double bond,s will have the higher melting point. Each oleic acid unit has only one cis double bond, while each linoleic acid has two (Please ,see Table 26.1). Glycerol trioleate will have the higher melting point.
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734 Solutions Chapter 26: Lipids Problem 26.6 Draw a structural formula for methyllinoleate. Be certain to show the COITect configuration of groups about,
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Ch26 - Chapter 25: Carbohydrates Solutions 732 . C hapter...

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