Organic and Biological Chemistry

4.10 Triglycerides

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TriglyceridesFats and oils from plants and animals

Tri-esters of propan-1,2,3-triol (glycerol)

Three long straight chain carboxylic acids

(fatty acids) form ester linkages with each

glycerol molecule

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Triglycerides

3

C

H

H

OH C

H

O

H

C

H

H

O H

C

H

H

OH C

H

O

H

C

H

H

O H

Propan-1,2,3-triol

R C

O

O

H

R C

O

O

H

Carboxylicacid

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5

TriglyceridesFatty acids contain an even

number of carbon atoms (between 12 and 20). The carbon chain can be saturated or unsaturated.

If there is more than 1 double bond the molecule is referred to as polyunsaturated

Naturally occurring tri-esters of propan-1,2,3-triol generally will contain three different fatty acids

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Examples of Fatty Acids

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8

Oleic Acid

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Olive Oil

Triglycerides

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Triglycerides can be hydrolysed to produce glycerol and three fatty acid molecules.

In biological organisms that can utilise triglycerides for energy this reaction is catalysed by lipase enzymes

In the laboratory concentrated acid or alkali combined with heating can be used to break down the triglyceride

TriglyceridesThe state of an edible fat or oil at room

temperature can be used to determine its source.

Edible fats are solids at 25oC and generally are obtained from land animals

Edible oils are liquids at 25oC and are obtained from plants or marine animals

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TriglyceridesMelting points of fats and oilsAs the length of the carbon chain increases

so does the tm increase, due to increased dispersion forces.

As the degree of unsaturation increases (i.e. number of C=C bonds increases) the tm

decreases. The molecules can’t pack together as closely and so don’t solidify

Fat contains a greater percentage of saturated fatty acids than unsaturated fatty acids and as a result it is solid at room temperature

Oils contain a greater percentage of unsaturated fatty acids and are liquids

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Reactions of TriglyceridesThe degree of unsaturation of a

triglyceride can be determined by its reaction with bromine or iodine

C

H

H

C

H

H

C

H

H

C

H

H

+

Br Br

Br Br

C

H

H

Br C

H

H

Br

C

H

H

Br C

H

H

Br

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This is referred to as an Addition reaction

The orange colour of the bromine disappears as the reaction occurs. (The products are colourless)

To enable the bromine to mix with the triglyceride both the bromine and triglyceride are dissolved in a non polar solvent (cyclohexane)

Reactions of TriglyceridesBy titrating a standard solution of bromine

(burette) with a known volume of a standard solution of fat or oil the degree of unsaturation can be measured.

The greater the amount of bromine required the greater the degree of unsaturation.

The end point is indicated by the first permanent orange colour in the flask.

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Reactions of TriglyceridesIodine numberThe degree of unsaturation of a fat

or oil is often described in terms of the iodine number

The iodine number is the mass of iodine that reacts with 100g of the fat or oil

The greater the iodine number the greater the degree of unsaturation

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Olive oil Iodine Number =75–94 (virgin and refined)

Reactions of TriglyceridesLiquid oils can be converted to solid fats by

catalytic hydrogenationThe vegetable oil is heated with hydrogen gas

under pressure in the presence of a nickel catalyst. These conditions increase the rate of reaction.

Sufficient hydrogen is added to produce a product that is solid at room temperature

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C

H

H

C

H

H

C

H

H

C

H

H

+

H H

H H

Ni High PNi

Ni

High P

High P

C

H

H

H C

H

H

H

C

H

H

H C

H

H

H

Organic and Biological Chemistry

4.11 Carbohydrates

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CarbohydratesGeneral formula is usually CxH2yOy

This can often be written as Cx(H2O)y

Carbohydrates are polyhydroxyaldehydes or polyhydroxyketones or compounds that produce these when hydrolysed

Can be monosaccharides, disaccharides or polysaccharides depending on the number of simple sugars in the molecule

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MonosaccharidesMonomersGeneral formula CxH2xOx where x = 3 to 8Water soluble (Hydrogen bond with water)Simple sugars eg. Glucose, FructoseSolids at room temperatureSweet

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GlucoseC6H12O6

Can exist as a chain or ring structureThese structures are in equilibrium in

aqueous solution

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D-Glucose α-D-Glucose

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GlucoseIn the chain form the aldehyde can be

oxidised by Tollen's reagent forming the silver mirror and the carboxylate ion but there is no reaction with the ring form

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H O C

H

H

C

H

O

H

C

H

O

H

C

O

H

H

C

H

O

H

C

O

H

H O C

H

H

C

H

O

H

C

H

O

H

C

O

H

H

C

H

O

H

C

O

H

TollensTollens

Tollens

H O C

H

H

C

H

O

H

C

H

O

H

C

O

H

H

C

H

O

H

C

O

O–

H O C

H

H

C

H

O

H

C

H

O

H

C

O

H

H

C

H

O

H

C

O

O–

This reaction causes the equilibrium to favour the formation of the chain structure

DisaccharidesTwo monosaccharides per moleculeFormed by a condensation reaction

(eliminating water)Can be hydrolysed to form

monosaccharidesWater soluble compounds (Hydrogen bond

with water)

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Sucrose

Lactose

Maltose

PolysaccharidesLarge polymers of monosaccharidesFormed by condensation reaction and

broken down to monosaccharides by hydrolysis

(C6H10O5)n +½nH2O ½nC12H22O11

polysaccharide disaccharideC12H22O11 + H2O 2C6H12O6

disaccharide monosaccharideOverall(C6H10O5)n + nH2O nC6H12O6

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PolysaccharidesInsoluble in water. Although hydrogen

bonding can occur the large molecular size prevents mixing with water

Will absorb water

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PolysaccharidesCelluloseStructural material in plantsMade up of approx. 3000 glucose unitsStraight chain polymer

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PolysaccharidesStarchMade up of amylose 250-2000 glucose

units in a straight chain and amylopectin which contains hundreds of thousands of glucose in a branched chain structure.

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AmyloseAmylopectin

PolysaccharidesGlycogenStorage molecule for glucose in liver and

muscleBranched chain polymer

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