Download
1 / 20
230 likes | 293 Vues
Chapter 17 Lipids. 17.4 Chemical Properties of Triacylglycerols. Chemical Properties of Triacylglycerols. The chemical reactions of triacylglycerols are similar to those of alkenes and esters.
E N D
Chapter 17 Lipids 17.4Chemical Propertiesof Triacylglycerols
Chemical Properties of Triacylglycerols The chemical reactions of triacylglycerols are similar to those of alkenes and esters. • Inhydrogenation, double bonds in unsaturated fatty acids react with H2 in the presence of a Ni or Pt catalyst. • In hydrolysis, ester bonds are split by water in the presence of an acid, a base, or an enzyme.
Hydrogenation of Oils Thehydrogenationof oils • converts double bonds to single bonds • adds hydrogen (H2) to the carbon atoms of double bonds • increases the melting point • produces solids such as margarine and shortening
Learning Check What product(s) is obtained from the complete hydrogenation of glyceryl trioleate? 1) glycerol and 3 oleic acids 2) glyceryltristearate 3) glycerol and 3 stearic acids
Solution What product(s) is obtained from the complete hydrogenation of glyceryl trioleate? 2) glyceryltristearate
Olestra, A Fat Substitute Olestradon’t consume much in a short term • used in foods as an artificial fat: potato chips, etc. • sucrose linked by ester bonds to several long-chain fatty chains • not broken down, not absorbed (too big) in the intestinal tract:Side effects: abdominal cramp, carries fat-soluble Vitamin (A, D, E & K) with it to waste them.
Cis and Trans Fatty Acids Unsaturated fatty acids can be • Cis with bulky groups on the same side of C═C • Trans have bulky groups on opposite sides of C═C
Trans Fatty Acids and Hydrogenation Trans fatty acids • are formed during hydrogenation when cisdouble bonds are converted to transdouble bonds • in the body behave like saturated fatty acids • are estimated to make up 2 to 4% of our total calories • in several studies are reported to raise LDL-cholesterol and lower HDL-cholesterol
Trans Fats In vegetable oils, • the unsaturated fats usually contain cis double bonds • during hydrogenation, some cisdouble bonds are converted to transdouble bonds (more stable), causing a change in the fatty acid structure • a label states “partially” or “fully hydrogenated” if the fats contain trans fatty acids
Learning Check Identify each statement as true (T) or false (F). A. There are more unsaturated fats in vegetable oils. B. Vegetable oils have higher melting points than fats. C. Hydrogenation of oils converts some cis-double bondstotrans-double bonds. D. Animal fats have more saturated fats.
Solution Identify each statement as true (T) or false (F). T A. There are more unsaturated fats in vegetable oils. F B. Vegetable oils have higher melting points than fats. T C. Hydrogenation of oils converts some cis-double bondsto trans-double bonds. T D. Animal fats have more saturated fats.
Hydrolysis In acid or enzyme hydrolysis, • water adds to the ester bonds • triacylglycerols split into glycerol and three fatty acids • an acid or enzyme catalyst is required
Base Hydrolysis (Saponification ) In base hydrolysis (saponification), • a triacylglycerol reacts with a strong base • a triacylglycerol splits into glycerol and the salts of fatty acids • soaps (salts of fatty acids) form
Learning Check What products are obtained from the complete hydrolysis of glyceryl trioleate? 1) glycerol and 3 oleic acids 2) glyceryl tristearate 3) glycerol and 3 stearic acids
Solution What products are obtained from the complete hydrolysis of glyceryl trioleate? 1) glycerol and 3 oleic acids
Learning Check Draw the condensed structural formula of the product of the following reaction.
