ORGANIC CHEMISTRY 2

ATIKAHBENNY YODI SDESY INDRA WHIDAYATULLAH

MAYSAROH

LIPIDS

INTRODUCTIONLipids are compounds of biological origin that dissolve in nonpolar solvents, such as

chloroform and diethyl ether. The name lipid comes from the Greek word lipos, for fat.

DEFINITIONLipids can best be defined as

biomolecules which are soluble to a great extent in nonpolar solvents.

Lipids can be categorized as hydrolyzable or nonhydrolyzable

• Hydrolyzable lipids can be cleaved into smaller molecules by hydrolysis with water. Most hydrolyzable lipids contain an ester unit. We will examine three subgroups: waxes, triacylglycerols, and phospholipids.

• Nonhydrolyzable lipids cannot be cleaved into smaller units by aqueous hydrolysis. Nonhydrolyzable lipids tend to be more varied in structure. We will examine four different types: fat-soluble vitamins, eicosanoids, terpenes, and steroids.

WAXES

Waxes are the simplest hydrolyzable lipids. Waxes are esters (RCOOR') formed from

a high molecular weight alcohol (R'OH) and a fatty

acid (RCOOH).

The structure of this compound shows how small the ester

group is compared to the long hydrocarbon chains.

Wax is a generic term that encompasses materials that have some specific properties such as:

• Solid at 20C, varying from soft / plastic to brittle / hard

• A melting point > 40C without decomposing

• A relatively low viscosity slightly above the melting point

• Transparency to opaque, but not glass-like

• Buffable under slight pressure

TRIACYLGLYCEROLS

Triacylglycerols, or triglycerides, are triesters that produce glycerol and three molecules of fatty acid

upon hydrolysis. Simple triacylglycerols are composed of

three identical fatty acid side chains, whereas mixed

triacylglycerols have two or three different fatty acids.

The characteristics of fatty acids

• All fatty acid chains are unbranched, but they may be saturated or unsaturated.

• Naturally occurring fatty acids have an even number of carbon atoms.

• Double bonds in naturally occurring fatty acids generally have the Z configuration.

• The melting point of a fatty acid depends on the degree of unsaturation.

• Fats and oils are triacylglycerols; that is, they are triesters of glycerol and these fatty acids. – Fats have higher melting points, making

them solids at room temperature. – Oils have lower melting points, making

them liquids at room temperature.

As the number of double bonds increases, the melting point decreases, as it does for the constituent fatty acids as well.

NOMENCLATURE

• Short-hand notations can be written for the fatty acids which indicate the number of carbons/ number of double bonds/ positions of double bonds from the carboxyl end of the molecule;

for example, linoleic acid would be C18:2∆9, 12. • Another way to describe unsaturated fatty acids

denotes the position of the first double bond from the alkyl end of the molecule; for example, linoleic acid would be ω6.

Hydrolysis of triacylglycerols

Hydrogenation of unsaturated fatty acids

Oxidation of unsaturated fatty acids

Saturated fat acid reaction

Unsaturated fat acid reaction

Saponification

Soap and detergent

Structure of Soaps

• A soap molecule has a nonpolar, alkyl end and a polar, salt end. Because of this dual polarity, it is called amphipathic. This hydrophobic/hydrophilic nature is essential to the function of such molecules.

Mechanism of Soap Action

• The cleaning action of soap involves lowering the surface tension of water by disrupting hydrogen bonds at the surface and the formation of micelles within the volume of water present. Micelles are aggregrations of soap molecules arranged so that the hydrophobic “tails” are oriented towards each other away from the water solvent and the hydrophilic “heads” are pointed into the water.

Detergents

Detergents are amphipathic molecules which have enhanced solubility and biodegradability properties compared to soaps. Instead of having a sodium salt in the polar portion of the molecule, other ionic and polar groups are used giving rise to what are called "cationic", "anionic" and "nonionic" detergents.

PHOSPOLIPIDS

Phospholipids are hydrolyzable lipids that contain a phosphorus atom. There are two common types of phospholipids: phosphoacylglycerols and sphingomyelins.

Phosphoacylglycerols

• When R'' = CH2CH2NH3+, the compound is called a phosphatidylethanolamine or cephalin.

• When R'' = CH2CH2N(CH3)3+, the compound is called a phosphatidylcholine, or lecithin.

Sphingomyelins

• Other notable features of a sphingomyelin include:

= • A phosphodiester at C1. = • An amide formed with a fatty acid at C2.

FAT-SOLUBLE VITAMINS

EICOSANOIDS

The word eicosanoid is derived from the Greek word eikosi, meaning 20.

TERPENES

Terpenes are lipids composed of repeating five-carbon units called isoprene units. An iso- prene unit has five carbons: four in a row, with a one-

carbon branch on a middle carbon.

• An isoprene unit may be composed of C– C σ bonds only, or there may be π bonds at any position.

• Isoprene units are always connected by one or more carbon–carbon bonds.

• Each carbon atom is part of one isoprene unit only. • Every isoprene unit has five carbon atoms.

Heteroatoms may be present but their presence is ignored in locating isoprene units.

STEROIDS

Steroids are important “biological regulators” that nearly always show dramatic physiological effects when they are administered to living organisms. Among these important

compounds are male and female sex hormones, adrenocortical hormones, D vitamins, the bile acids, and

certain cardiac poisons. Steroids are derivatives of the following perhydrocyclopentanophenanthrene ring

system:

REACTION

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