General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.1 Chapter 17 Lipids 17.4 Chemical Properties of Triacylglycerols.

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 1

Chapter 17 Lipids

17.4Chemical Propertiesof Triacylglycerols

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The chemical reactions of triacylglycerols are similar

to those of alkenes and esters.

In hydrogenation, 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.

Chemical Properties of Triacylglycerols

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Hydrogenation of Glyceryl Trioleate

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Hydrogenation of Oils

The hydrogenation of 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

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What product(s) is obtained from the completehydrogenation of glyceryl trioleate?

1) glycerol and 3 oleic acids

2) glyceryltristearate

3) glycerol and 3 stearic acids

Learning Check

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What product(s) is obtained from the completehydrogenation of glyceryl trioleate?

2) glyceryltristearate

Solution

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Olestra, A Fat Substitute

Olestra don’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.

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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

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Cis and Trans Fatty Acids

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Trans Fatty Acids and Hydrogenation Trans fatty acids are formed during hydrogenation when cis double

bonds are converted to trans double 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

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Trans Fats

In vegetable oils, the unsaturated fats usually contain cis double

bonds during hydrogenation, some cis double bonds are

converted to trans double 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

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Formation of Trans Fats

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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 bonds to trans-double bonds.

D. Animal fats have more saturated fats.

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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 bonds to trans-double bonds.

T D. Animal fats have more saturated fats.

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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

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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

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Saponification

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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

Learning Check

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What products are obtained from the complete hydrolysis of glyceryl trioleate?

1) glycerol and 3 oleic acids

Solution

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Learning Check

Draw the condensed structural formula of theproduct of the following reaction.

O

O

CH2

O

O

CH

CH2 O

O

C

C

C

(CH2)7 CH CH

CHCH(CH2)7

CHCH(CH2)7

(CH2)5 CH3

(CH2)5 CH3

(CH2)5 CH3

+ 3H2

Ni

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Solution

CH2 (CH2)14

CH2

CH O C

O

O C

O

O

O

C

(CH2)14 CH3

(CH2)14 CH3

CH3