Книга_5852_Библиотеки_им_академика_М_И_Перельмана

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C H A P T E R 12 u Alcohols

11. With aldehydes and ketones: Ethylene glycol undergoes condensation with aldehydes and ketones

in the presence of p-toluenesulphonic acid to form cyclic acetals and cyclic ketals, respectively.

461

PTS/∆

R H

PTS/∆

CH O

—

CH O

—

CH O

—

Cyclic acetal

—

Cyclic ketal

+ H O

12. Oxidation:

CH OH

—

CH OH

—

CH OH

+ O C

Aldehyde

+ O C

Ketone

—

a. With nitric acid: Oxidation of ethylene glycol with concentrated nitric acid gives glycolic and

oxalic acids. The other theoretically possible oxidation products are formed in small quantities

because they are more readily oxidized than glycol itself.

b. Fenton’s reagent (H2O2 + FeSO4) oxidizes ethylene glycol to glycolic aldehyde.

CH OH

—

CH OH

Ethylene

glycol

CHO

—

(O) (O)

CHO COOH

—

CH OH

Glycolic

aldehyde

COOH

—

CH OH

Glycolic

acid

(O)

COOH

—

CHO

Glyoxal Glyoxalic acid

CHO

COOH

Oxalic

acid

13. Oxidation with periodic acid, HIO4 (periodic acid) oxidation (Malaprade reaction): Com-

pounds containing two or more –OH groups attached to adjacent carbon atoms on oxidation with periodic acid undergo cleavage of carbon bonds. For example,

—

+ HIO

′

RCHO + R CHO + HIO + H O

OH OH

—

——

: :

—

′

—

+ HIO

HIO + R CO + R CHO

HO OH

—

C + HIO

—

′

—

No reaction since –OH groups are not on adjacent carbon atoms.

′

462

PH A R M A C EUT I C A L O R G ANIC C H E MIST RY

Oxidation by HIO4 resulting in cleavage in carbon–carbon bond is helpful in determining the structure of

1, 2-glycols. Oxidation by HIO

is qualitatively established by the formation of a white precipitate of AgIO3

on adding silver nitrate solution to the reaction mixture. As this oxidation is almost quantitative, valuable information is obtained from the quantity of periodic acid used and nature and amount of the products formed.

14. Oxidation with lead tetraacetate: Lead tetra acetate also oxidizes 1, 2-glycols.

—

(

Formaldehyde

Acetic

It is found that cis-glycols are cleaved much more readily than trans-glycols by periodic acid and lead

tetra-acetate.

15. Dehydration: Glycol undergoes dehydration to form different products depending on the conditions

and the dehydrating agent.

a. Action of heat: When ethylene glycol is heated alone at 773 K ethylene oxide is formed.

—

Ethylene

b. With anhydrous zinc chloride: On heating with dilute H2SO4 or anhydrous zinc chloride under

pressure, ethylene glycol is converted into acetaldehyde.

—

Vinyl alcohol

c. With sulphuric acid: When glycol is heated with concentrated sulphuric acid, dioxane is obtained,

Dioxane is used as an industrial solvent.

—

d. With phosphoric acid: It is quite interesting that a dehydrating agent like phosphoric acid gives

compounds known as polyethylene glycols. These are condensation polymers having both alcohol and ether as functional groups. These are excellent solvents for gums, resins, etc.

—

— —

—

C H A P T E R 12 u Alcohols

16. Pinacol-pinacolone rearrangement: This is a typical reaction of higher glycols involving formation

of rearranged products. It was rst observed in the case of pinacol, i.e. 2, 3-dimethyl-2, 3-butanediol

and hence the name pinacol rearrangement. It is found that on treatment with sulphuric acid, pinacol undergoes dehydration with molecular rearrangement to form methyl tert-butyl ketone, also known as pinacolone.

463

—

— — — —

—

Pinacolone

Methyl

—

-butyl

tert

pinacolone

Many other vicinal glycols are now known to undergo similar acid-catalysed reactions. All these reac-

tions are, therefore, collectively known as pinacol rearrangements.

Uses of ethylene glycol

1. It is used as antifreeze for automobile radiators and as a coolant for aeroplane aviation petrol under

the name prestone.

2. Its ethers like cellosolve, carbitol and diglyme are excellent solvents for oils, fats, waxes, lacquers,

enamels, etc.

—

Ethylene glycol

—

— — — — — —

—

Cellosolve

— —

Carbitol

Diglyme

—

3. Some esters of glycol are used in the manufacture of synthetic bres. The well-known synthetic bre

dacron is a polyester of ethylene glycol with terephthalic acid.

4. Glycol stearate is used as a lubricant.

5. Glyptal is obtained by condensing ethylene glycol with phthalic acid. It is used in paints and lacquers.

6. It is used in the manufacture of explosives (ethylene dinitrate).

464

PH A R M A C EUT I C A L O R G ANIC C H E MIST RY

MEMORY FOCUS

1. Some characteristic reactions of glycol are listed below:

CH OH

—

CH OH

Glycol

CH ONa

Na, 323 K

HCI, 573 K

(Red P + Br ), PBr

Pl , (Red P + I )

3 2

2CH COCI

2 3

PCl

HOOC — — COOH, H SO

—

CH OH

CH Cl

—

CH Cl

CH Br

—

CH Br CH I

—

CH I CH OCOCH

—

CH OCOCH

2 3

2 4

Phthalic acid

RCOR

Na, 433 K

–I

CH Cl

—

+ POCl + HCl

CH CI

Dacron (Terrylene)

Glyptal

—

CH O

—

CH O

(Cyclic ketal)

CH ONa

—

CH ONa

HNO

KMnO /H O

4 3

COOH

—

CH OH

Glycolic

COOH

—

acid

+ 2H O

COOH

—

COOH

CH ONa

CH OH

Na; 433 K

CH ONa

CH Cl

HCI; 573 K

(Red P + Br ); PBr

2 3

CH Br

2CH COCI

Pl ; (Red P + I )

3 2

CH I

–I

CH OCOCH

2 3

CH OCOCH

2 3

PCl

CH Cl

CH CI

+ POCl + HCl

HOOC — — COOH, H SO

2 4

Dacron (Terrylene)

Phthalic acid

Glyptal

RCOR

CH – O

(Cyclic ketal)

HNO

COOH

CH OH

COOH

Glycollic

acid

KMnO /H O

4 3

COOH

+ 2H O

Na; 323 K

—

C H A P T E R 12 u Alcohols

HIO

ZnCI , Heat

Heat, 775 K

2. The reaction of glycol with ketone is used in the protection of keto group in the reactions where it is to be protected.

3. Oxidation of glycol with lead tetraacetate gives formaldehyde.

CH OH

—

4. Glycol is used as an antifreeze for automobile radiators and as a coolant for aeroplane aviation petrol under the name prestone.

12.10 GLYCEROL (1,2,3-PROPANETRIOL)

+ (CH COO) Pb

3 4

2HCHO + HIO

CH CHO + H O

3 2

—

O + H O

Epoxyethane

2HCHO + 2CH COOH + (CH COO) Pb

3 3 2

Glycerol is known as glycerine. It is a trihydric alcohol containing three hydroxyl groups, hence has three, active hydrogens. Two hydroxyl groups are primary alcoholic groups while third is a secondary alcoholic group. Glycerol occurs in nature in almost all animal and vegetable oils and fats in the form of glycerides (esters of glycerol and higher carboxylic acids, mainly palmitic, stearic and oleic acids).

12.10.1 Preparation

465

Glycerol is usually prepared as described below:

1. From fats: Natural fats and oils are triesters of glycerol with long chain fatty acids. In other words,

they are triacyl glycerols and are commonly known as glycerides.

—

— —

—

O COR

COR

—

O COR

—

′

′′

The three alkyl groups (R, R´ and R´´) may be identical or different, and accordingly glycerides may be

simple or mixed as illustrated below.

466

(simple glyceride)

(mixed glyceride)

Glycerol

PH A R M A C EUT I C A L O R G ANIC C H E MIST RY

—

CH OCOC H

2 15 31

—

CH OCOC H

—

CH OCOC H

2 15 31

15 31

Glyceryl tripalmitate

Glyceryl palmito distrearate

—

CH OCOC H

2 17 35

—

CH OCOC H

—

CH OCOC H

2 17 35

15 31

—

CH OCOC H

2 17 33

—

CH OCOC H

—

CH OCOC H

2 17 31

17 35

Glyceryl oleo-stereo-palmitate

a. From spent lye: Hydrolysis of these fats or oils with sodium hydroxide (10% caustic soda solution

is called lye) gives sodium salts of fatty acids, which are known as soaps. The above hydrolysis reac- tion is called saponication. The process is employed for the commercial preparation of glycerol; soaps get precipitated and the mother liquor, which is known as spent lye, contains glycerol. Glycerol is then extracted from the spent lye.

—

Oil or fat

—

Sodium

b. From sweet lye or sweet water: When oil or fat is heated with superheated steam, fatty acids and

glycerol are liberated. On cooling, fatty acids get precipitated. The mother liquor left is known as sweet lye or sweet water and glycerol is extracted from it.

—

Oil or fat

Hydrolysis

—

Glycerol

Carboxylic

acid

c. By transesterication of fat: Glycerol can also be obtained from fats or oils by transesterication

with methyl alcohol in the presence of an acid or a base. The reaction leads to the formation of methyl esters of acids and glycerol.

—

C H A P T E R 12 u Alcohols

Glycerol

2. From propylene: On a large scale, glycerol is prepared from propylene (obtained from petroleum) in

the following steps:

467

—

Cl ,773

Propylene Allyl alcohol

–HCl

CH Cl

—

Allyl chloride

Aq. Na CO

423 K, 124 atm.

CH OH

—

HOCl

—

CH OH

—

NaOH

—

Monochlorohydrin

—

It should be noted that propylene undergoes substitution reaction when reacts with chlorine at higher

temperature 773 K.

3. Manufacture of glycerol from sugar: During the fermentation of sugar by yeast, appreciable quan-

tities of glycerol are produced along with ethyl alcohol. The proportion of glycerol in the product is considerably increased, while that of ethyl alcohol is decreased, if sodium sulphite (Na

) be added

2SO3

to the fermenting liquid. This fact is based on important process for the manufacture of glycerol. The

yield of glycerol by this method is from 15 to 60 percent of the sugar employed.

12.10.2 Physical Properties

1. It is a colourless, odourless, syrupy liquid having sweet taste. Because of three OH groups in it, glycerol undergoes extensive intermolecular hydrogen bonding. This leads to high viscosity.

2. It is miscible with water (due to hydrogen bond with water), alcohol and acetone but insoluble in

ether, benzene, chloroform, etc.

3. It boils at 563 K with slight decomposition. It is puried by vacuum distillation.

4. It is heavier than water (specic gravity is 1.265).

5. Because of large number of OH groups, it forms most extensive intermolecular H-bonding as compared to glycol or ethanol and highest surface tension among these alcohols.

6. On cooling, glycerol forms transparent crystals, which melt at 290 K.

12.10.3 Chemical Properties

Glycerol contains two primary and one secondary alcoholic groups. It, therefore, undergoes all the reactions characteristic of these groups.

1. Reaction with sodium: Glycerol reacts with sodium metal to form glycerolates. At room temperature, sodium reacts with one of the primary –OH groups of glycerol to form monosodium derivative. How-

ever, at higher temperature (375 K) both the primary –OH groups react to form disodium derivative.

468

PH A R M A C EUT I C A L O R G ANIC C H E MIST RY

CH – OH

—

CHOH

—

CH OH

Glycerol

+Na, Room temp.

–1/2 H

CH ON

—

CHOH

—

CH OH

Mono sodium

glycerolate

+Na / 375 K

–1/2 H

CH ON

—

CHOH

—

CH ON

Disodium

glycerolate

The negative charge on the oxygen atom in monoglycerolate repels electrons and thus decreases the

polarity of the second (1º) OH bond which requires stronger conditions for its reaction. Secondary alcoholic

group being less acidic is not attacked at all.

2. Reaction with hydrgoen chloride: Glycerol reacts with hydrogen chloride at 383 K to form mixture of two monoderivatives.

—

CHOH

—

CH OH

Glycerol

383 K

+ HCl

CH Cl

—

CHOH

—

CH OH

Glycerol

α-mono chlorohydrin

CH OH

—

CHC

—

CH OH

Glycerol

β-mono chlorohydrin

With excess of hydrogen chloride, glycerol yields a mixture of

CH OH

—

CHOH

—

CH OH

glycerol

HCl (excess)

383 K

CH Cl

CHOH

CH Cl

Glycerol

dichlorohydrin

αα-

—

a, a′

CH C

—

CHC

—

CH OH

Glycerol

dichlorohydrin

αβ-

and

a, b

dichlorohydrins.

3. With Hydrogen iodide: Different products are obtained under different conditions. Glycerol is re- duced to allyl iodide when it is heated with a small amount of hydrogen iodide.

CH OH

—

CHOH

—

CH OH

Glycerol

+ 3HI

∆

–3H O

Glycerol tri-iodide

CH I

—

CHI

—

CH I

(unstable)

+ I

—

CH I

Allyl iodide

C H A P T E R 12 u Alcohols

(Isopropyl iodide)

monoacetate

diacetate

When excess of hydrogen iodide is used, the main product formed is isopropyl iodide, which, in fact,

results by the subsequent action of HI on allyl iodide rst formed.

469

—

CH I

Allyl iodide Propene

CH I

—

CHI

—

Unstable

–

—

CH – I

—

2-Iodopropane

4. With PCl5 or PCl3 or thionyl chloride (SOCl2) glycerol gives 1,2,3-trichloropropane.

CH OH

—

CHOH

—

CH OH

Glycerol

3PC

CH Cl

—

CHCI

—

CH CI

1,2,3 Trichloropropane

3POCl 3HCl

5. Reaction with organic acid and acid derivatives:

a. With acetic acid: Glycerol reacts with acetic acid to form mono-, di- and tri-esters depending on

the amount of acid and reaction conditions employed.

CH OH

—

CHOH

—

CH OH

Glycerol

CH COOH

–H O

CH OOCCH

—

CHOH

—

CH OH

Glycerol

CH COOH

–H O

CH OOCCH

—

CHOH

—

CH OOCCH

CH COOH

–H O

CH OOCCH

—

CHOOCCH

—

CH OOCCH

Glycerol triacetateGlycerol

b. Similarly acetyl chloride or acetic anhydride reacts with glycerol to form glyceryl triacetate.

—

Glycerol

−

—

Glycerol triacetate

Note: Reactions 4 and 5 prove that there are three OH groups in a molecule of glycerine.

470

PH A R M A C EUT I C A L O R G ANIC C H E MIST RY

6. Reaction with nitric acid (Nitration): When added slowly to a mixture of concentrated nitric acid

and sulphuric acid maintained at 283–298 K, glycerol forms glyceryl trinitrate, which is commonly

but wrongly called nitroglycerine.

CH OH

—

CH OH

—

CH OH

3HNO

(HONO )

H SO

CH ONO

—

CHONO + 3H O

—

CH ONO

Glyceryl trinitrate

Nitroglycerine is one of the best explosives known to man. It was discovered by Alfred Nobel, the man

after whom the Nobel Prizes have been instituted. When two moles of HNO

are used, glyceryl dinitrate is

formed.

Glyceryl trinitrate is a colourless oily liquid. It explodes violently on heating or detonation. Alfred Nobel found that nitroglycerine can be stabilized by absorbing on kieselguhr (a kind of porous earth). A mixture of glyceryl trinitrate and glyceryl dinitrate absorbed on kieselguhr is called dynamite. Blasting gelatin is glycerol trinitrate + cellulose nitrate (gun cotton). Cordite is glycerol trinitrate + gun cotton + vaseline.

7. Reaction with oxalic acid:

i. When heated with oxalic acid at 383 K, it forms glycerol mono-oxalate. This loses CO2 to give

glycerol monoformate, which undergoes hydrolysis to yield formic acid.

CH OH

—

CHOH

—

CH OH

Glycerol

COOH

—

COOH

383 K –H O

CH OOC COOH

—

CHOH

—

CH OH

Glycerol

mono oxalate

—

Heat

–CO

CH OOCH

—

CHOH

—

CH OH

+H O

CH OH

—

CHOH HCOOH

—

Formic aci

CH OH

Glycerol

mono formate

ii. At 503 K, oxalic acid and glycerol form glyceryl dioxalate, which decomposes to give allyl alco-

hol and carbon dioxide.

CH OH

—

CHOH

—

CH OH

COOH

—

COOH

503 K

2H O

CH OOC

—

CHOOC

—

CH OH

Glyceryl

—

Heat

–2CO

—

CH OH

Allyl alcohol

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