Alcohols,diols and triols

Preparation

(1) Hydrolysis of alkyl halides This reaction is useful only with reactants that do not undergo E2 elimination readily. It is really

used for the synthesis of alcohols where alkyl halides are primary in nature.

(2) Reaction of Grignard reagents with aldehydes and ketones A method that allows for alcohol preparation with formation of

new carbon bonds. Primary, secondary, and tertiary alcohols can all be prepared and stepping up to two carbon atom also can t akes

place by the help of cyclic epoxides.

(3) Reaction of organolithium reagents with aldehydes and ketones Organolithium reagents react with aldehydes and ketones in

a manner similar to that of Grignard reagents to form alcohols. Other reagents which are less reactive than alkyl lithium is dialkyl

cadmium, R2Cd where active species is carbanian, and forms similar kind of product.

(4) Reaction of Grignard reagents with esters Produces tertiary alcohols in which two of the substituents on the hydroxyl-

bearing carbon are derived from the Grignard reagent. If we want to stop this reaction at carbonyl compound we must use less

reactive reagent like dialkyl cadmium R2Cd. Remember the alkyl group in ester goes out finally as alcohol.

(5) Acid catalyzed hydration of alkenes The elements of water add to the double bond according to the markonikov’s rule, by

intermediate carbocation so rearrangement occurs.

Oxymercuration-demercuration of alkenes

(6) Oxymercuration-demercuration of alkenes Markonikov’s addition of the elements of water to the double bond occurs. This

is a useful synthetic reaction where rearrangements do not occur due to cyclic mercurium ion.

(7) Hydroboration oxidation of alkenes by B2H6 followed by H2O2 Overall reaction is addition of water across the double bond

opposite to that of markovnikov’s rule This is a very good synthetic method, addition is syn, and no rearrangements are obser ved.

(8) Reaction of an amine with nitrous acid

(9) Reduction of compounds by NaBH4 and LiAlH4 (Synthesis of alcohol)

(a) NaBH4

Sodium borohydride is a reducing agent. It reduces by transfer of hydride ions to substrates as in the case of lithium alumin ium

hydride.

Applications

1. Reduction of carbonyl compounds and its derivatives into alcohol

In the above list it will be seen that only the carbonyl group is reduced and the other functional groups remain unaffected. Highly

selective behaviour of NaBH4 makes it the preferred reagent for the reduction of carbonyl groups in sensitive polyfunctional group

containing compounds.

Lithium Aluminium Hydride(LiAlH4)

Lithium aluminium hydride (LAH) is also one of the very important and useful synthetic reagent for the reduction of carbonyl

compounds along with many other compounds.

The reagent is prepared by slowly adding a calculated amount of anhydrous aluminium chloride to a paste of lithium hydride in dry

ether or THF.

The reactants and their reduction products with LiAlH4 are listed below.

From epoxides

(10) From epoxides

Although the chemical reactions of epoxides will not be covered in detail until the following chapter, we shall introduce their use

in the synthesis of alcohols here.

Grignard reagents react with ethyleneoxide to yield primary alcohols containing two more carbon atoms then the alkyl halide from

which the organometallic compound was prepared epoxides are open by charge separation of carbon and oxygen, then attach with

grignard reagent.

Organolithium reagents react with epoxides in a similar manner. These reactions are used to convert lower alcohol into higher

alcohol.

Puzzle: Prepared the following compounds by the help of tetrahydrofuran.

(1) Succinic acid (2) adipic acid (3) Glutaric acid

Solution:

Epoxide rings are readily opened with cleavage of the carbon-oxygen bond when attacked by serving as sources of nucleophilic

carbon so this method can also be used for stepping up to two carbon atom. For example

This kind of chemical reactivity of epoxides is rather general. Nucleophiles other than Grignard reagents react with epoxides, and

epoxides more elaborate than ethylene oxide may be used.

Reactions of alcohols :

Alcohols are versatile starting materials for the preparation of a variety of organic functional groups. Several reactions of alcohols

have already been seen in earlier chapters and are summarized in Table .

Alcohols can undergo reactions involving various combinations of the bonds of carbon and oxygen. The ionization of alcohols

when they act as weak acids and the formation of alkoxides when they react with metals are reactions that take place at the O–H

bond.

The carbon-oxygen bond of alcohols is cleaved when alcohols are converted to alkyl halides or undergo acid-catalyzed

dehydration.

Some of the new reaction of alcohols that we will examine in this chapter occur by O–H bond breaking and some by C–O bond

breaking. Additionally, primary and secondary alcohols can exhibit a third reaction type in which a carbon -oxygen double bond is

formed by cleavage of both an O–H bond and a C–H bond:

Chemical properties of alcohol

(i) Acidic nature of alcohol

Even alcohol are neutral to litmus and do not reacts with alkali like NaOH but contain active hydrogen atom so reacts with Na or K

metal.

Reaction with Thionyl chloride and phosphorus Halides

(ii) Reaction with Thionyl chloride and phosphorus Halides

Other reagents can also be used to avoid carbocation intermediates and minimize rearrangements in the conversion of alcohols to

alkyl halides. One of these is thionyl chloride, SOCl2. 3-Pentanol will react with thionyl chloride in pyridine to give 3-

chloropentane, and no 2-chloropentane.

This reaction is often accompanied by elimination reactions, so it does not give high yields and a good leaving group. An

intermediate chlorosulfite ester, which decomposes to give the alkyl halide. In the presence of organic bases such as pyridine,

inversion of configuration is seen for chiral compounds.

Another reagent that is used to substitute a halogen for a hydroxyl group with a minimum of rearrangement is phosphorus

tribromide. For example, it is used to convert the primary alcohol isobutyl alcohol into isobutyl bromide.

The mechanism suggested for the conversion of an alcohol to an alkyl halide using phosphorus tribromide resembles the one

proposed for the conversion using thionyl chloride. Its first step is the formation of a phosphorus- oxygen bond between the

alcohol and phosphorus tribromide, giving a phosphorus ester that has a good leaving group.

(iii) Reaction of alcohol with Hydrogen Halides

Alkyl halides, which participate in many different substitution reactions, may be prepared from alcohols in different ways. The

hydroxyl group in an alcohol can be converted into a good leaving group so that it can be displaced by an incoming halide ion . For

example, dissolving gaseous hydrogen bromide in 1-heptanol converts the alcohol to 1-bromoheptane.

(iv) Haloform test only for 2–alkanone (by three equivalents of halogen)

(v) Dehydration of alcohols

Alcohols when are reacts with any dehydrating reagent to form alkenes with removal of water. Intermediate carbocation formation

takes place where rearrangement occurs by alkyl shift, hydride shift and ring expansion.

In smaller ring always ring expansion takes place due to molecular strain and they tends to convert to high stability with large ring.

In Dehydration of alcohols

(vi) Alcoholation of alkyne

When alcohols reacts with alkyne in the presence of BF3 and HgO, according to markonikoff’s rule they forms ethers.

Acylation of alcohol

(vii) Acylation of alcohol

When alcohol reacts with acylhalide and anhydride substitution of hydrogen atom by acyl group is known as acylation of alcohols.

(viii) Reaction with CH2N2 or CH2CO

(ix) Esterification

When acid reacts with alcohol in the presence of of ester’s takes place with fruity smell. Striking feature is removal

of –OH group from acid so water contains isotopic oxygen comes from acid molecule.

(x) Methods of distinguishing between primary, secondary and tertiary alcohols

(a) Oxidation test

(b) Lucas test

(c) Victer meyer test (best method)

(d) Catalytic dehydrogenation by Cu/300°C.

(a) Oxidation with sodium Dichromate (Na2Cr2O7)

Primary and secondary alcohols are easily oxidized by reagents containing chromium (VI). For low-molecular-weight alcohols that

are water-soluble, water solutions of sodium dichromate in the presence of acid are used. A primary alcohol such as n-butyl alcohol

is oxidized first to an aldehyde, which is oxidized further by the same reagent to a carboxylic acid.

This process is not of practical use for preparing aldehydes from primary alcohols because it undergo further oxidation (except for

a few cases where the aldehyde that is formed has a boiling point low enough that it can be distilled out of the reaction mixture).

A secondary alcohol, for example sec-butyl alcohol, is oxidized to a ketone, which usually does not undergo further oxidation due

to absence of reducing hydrogen. For a ketone to be oxidized, carbon-carbon bonds would have to be broken because there are no

more carbon-hydrogen bonds available on the carbonyl carbon atom.

The tertiary alcohol tert-butyl alcohol is not oxidized by acidic sodium dichromate solution. The carbon atom bearing the hydroxyl

group in this alcohol does not have a hydrogen atom on it. Here, too, carbon-carbon bonds would have to be broken to allow

oxidation to take place.

The oxidizing solution in the above examples, an acidic solution of sodium dichromate, contains a variety of species in

equilibrium. Most important of these is the acid chromate ion, HCrO4¯.

(b) Lucas test: Alcohols react with concentrated hydrochloric acid in presence of anhydrous zinc chloride to form alkyl halides.

The three types of alcohols undergo this reaction at different rates. The rates of reaction with Lucas reagent

[conc. HCl + ZnCl2 (anhydrous) follow the following order:

Tertiary alcohol > Secondary alcohol > Primary alcohol

Lucas test is based on this order because formation of intermediate carbocation takes place. An unknown alcohol (monohydric) is

mixed with conc. HCl and ZnCl

2

at room temperature. The alkyl chloride formed is insoluble in the medium; thus the solution becomes cloudy before it separates a s

a distinct layer.

The following observations are made:

(a) If cloudiness appears immediately, the alcohol is tertiary.

(b) If cloudiness appears within 5–10 minutes, the alcohol is secondary.

(c) If the solution remains clear, i.e., on cloudiness is formed the alcohol is primary.

Note:

Primary alcohols do not react with Lucas reagent at room temperature. It requires high temperature.

(c) Victor Meyer’s test:

In this test, the following steps are involved:

(i) Alcohol is reacted with conc. HI or red phosphorus and iodine to form the corresponding alkyl iodide.

(ii) Alkyl iodide is treated with silver nitrite when corresponding nitroalkane is formed.

(iii) Nitro alkane is treated with nitrous acid (NaNO

2

+ H

2

SO

4

) and the solution is made alkaline by addition of excess of caustic soda.

Catalytic dehydrogenation: (d) Catalytic dehydrogenation: It involves the passing of vapours of alcohol over reduced copper at 300°C and the product thus

formed is identified.

(i) Primary alcohols give aldehydes (dehydrogenation).

(ii) Secondary alcohols give ketones (dehydrogenation).

(iii) Tertiary alcohols form olefins (alkenes). This is dehydration.

Manufacture of Ethyl Alcohol by Fermentation (Alcoholic Fermentation)

The conversion of sugar into ethyl alcohol by yeast is called alcoholic fermentation. In alcoholic fermentation cane sugar or

glucose is the fermenting material. Therefore, any natural product which contains these sugars or can be easily converted into

them, becomes a source of ethyl alcohol. The raw materials for alcohol industry are:

(a) Materials containing sugars like cane juice, beets, molasses and fruit juices. The cheapest source is molasses.

(b) Materials containing starch like potato, rice, barley and maize.

1. From molasses: Molasses is a dark coloured syrupy mass left after the crushing of cane sugar or beet sugar crystals from the

concentrated juice. It still contains 30% of uncrystallised sucrose and about 32% of invert sugar - a mixture of glucose and

fructose. It forms an excellent cheap source of industrial ethyl alcohol. The various steps involved in the manufacture are:

(i) Dilution: The molasses is diluted with water until a concentration of 8 - 10% sugar is obtained in solution. A small amount of

dilute sulphuric acid is added as to adjust the pH value of solution about 4-4.5. The acidity is favourable for the growth of yeast but

unfavourable for the growth of undesirable bacteria. To this solution a small quantity of ammonium sulphate is added which acts as

a food for the yeast cells.

(ii) Alcoholic fermentation The diluted solution is taken in big fermentation tanks and some yeast is added (5% by volume of the

liquid). The mixture is allowed to stand for a few days. The temperature is kept about 30°C. Fermentation sets in and the enzyme

invertase converts sucrose into glucose and fructose which are further converted into ethyl alcohol by another enzyme, zymase.

The fermentation is complete in three days and carbon dioxide is the by-product. The fermented liquor is filtered.

(iii) Fractional distillation The fermented liquor is technically called as wash or wort. The approximate composition of wash is: 6-

10% ethyl alcohol, 3-5% glycerol, higher alcohols (fusel oil), acetaldehyde, etc. Wash is subjected to fractional distillation in a

coffee still. Each coffee still consists of two fractionating columns known as analyser and rectifier which are provided with

perforated plates. It works on counter-current principle.

The wash is preheated by circulating it through the coil round the rectifier and then introduced into the analyser. A current of steam

is passed from the bottom of the analyser where by the alcohol and other volatile constituents present in the wash rise up along

with steam and enter the rectifier. In the upward passage steam goes on condensing while the vapours of alcohol leaving from the

top of the rectifier are condensed and collected. This is know as row spirit and contains about 90% alcohol. The liquid collected at

the bottom of analyser is called spent wash.

(iv) Rectification Ethyl alcohol obtained above contains in addition to water various other impurities which are removed by

careful fractional distillation. The following fractions are obtained:

Primary fraction - It contains low boiling liquids like acetaldehyde.

Secondary fraction - It contains 93-95% ethyl alcohol and is called rectified spirit.

Last fraction - It contains water and fusel oil. Fusel oil is a mixture of n-propyl, n-butyl, n-amyl, isoamyl and optically active amyl

alcohols.

2. From starch: The manufacture of ethyl alcohol from starchy materials like potatoes, barley, maize, etc., involves the following

steps:

(i) Saccharification: The starch is first converted into maltose. The process of conversion is known as saccharification. It

completes in three steps:

(a) Mashing : The process of liberating starch from starchy materials is called mashing .The starchy material is treated with super

heated steam as to break the cell walls and to form a paste like mass called mash.

(b) Malting : Moist barley is allowed to germinate in dark at 15°C. Germinated barley called malt is heated to 60°C to stop the

further growth. It is crushed and extracted with water. It is filtered. The filtrate containing diastase enzyme is known as malt

extract.

(c) Hydrolysis : Malt extract is added to mash at 50-60°C. The enzyme diastase hydrolyses the starch into maltose in about half an

hour’s time.

(ii) Alcoholic fermentation: The hydrolysed starch solution containing maltose is then fermented by yeast for 2-3 days at 30°C.

The enzyme maltase present in the yeast hydrolyses maltose into glucose. The other enzyme, zymase present in the yeast conver ts

glucose to ethyl alcohol.

(iii) Distillation and rectification of the fermented liquor: This is done in a similar way as described in the first method.

By-products of alcohol industry: The important by-products obtained during manufacture of ethyl alcohol are:

(a) Carbon di oxide : Large quantities of carbon dioxide are evolved during fermentation. It is usually solidified and sold as dry

ice.

(b) Acetaldehyde : During rectification, acetaldehyde is obtained in the first fraction.

(c) Fusel oil : It is obtained in the last fraction of the rectification step. It is used for the preparation of amyl alcohols and amyl

acetate.

(d) Spent wash : This is the residue left in the still after distillation. It contains nitrous matter and is used as cattle feed.

(e) Argol (potassium hydrogen tartrate) : This separates out as a crust during fermentation. It consists of crude potassium hydrogen

tartrate which is used to manufacture tartaric acid.

Preparation of Absolute Alcohol

Preparation of Absolute Alcohol

The rectified spirit contains 95.5% ethyl alcohol and 4.5% water. It is a constant boiling mixture and water cannot be removed

simply by distillation. Hence some special methods are adopted for removing water from rectified spirit as to obtain 100% ethyl

alcohol, i.e., absolute alcohol.

Laboratory method: Rectified spirit is kept in contact with quick lime (CaO) for a day and then distilled over quick lime. Final

traces of water are removed by adding anhydrous copper sulphate. It is again distilled to get absolute alcohol.

Industrial method: This method is based on azeotropic distillation. Excess of benzene is added to the rectified spirit. Benzene

forms a ternary mixture with water and alcohol which distills at 64.8°C till whole of the water is removed.

The remaining benzene then forms another low boiling binary mixture which distills over at 68.2°C till pure alcohol is left behind.

The absolute alcohol boils at 78.3°C.

Methylated spirit or denatured alcohol: In order to make the industrial alcohol unfit for drinking purposes, the rectified alcohol

is denatured by adding various poisonous compounds. In most of the countries, the denaturing is done by adding 5 to 10% of

methyl alcohol and 0.5% rubber distillate (caoutchoucine ) and 0.5% pyridine bases. This denatured spirit is sold in the market

under the name methylated spirit.

Power alcohol: Like petrol, absolute ethyl alcohol can be used in internal combustion engines to derive power. But ethyl alcohol

alone is not sufficiently volatile and is thus generally mixed with other volatile combustible substances like petrol, benzene, etc.

When alcohol is used in this form, it is known as power alcohol. Since rectified spirit does not mix with petrol, absolute alcohol is

used for this purpose, A mixture containing 25% absolute alcohol and 75% petrol acts as a good fuel for motor cars and is sold in

the market as power alcohol.

Alcoholic Beverages and alcoholometry

The colour, flavour and taste of a particular beverage depends on the materials used, process of fermentation and subsequent

treatments. The following list includes some common beverages with their alcoholic content and source form which it is obtained.

Alcoholic beverages when taken in small quantity act as stimulants but when taken regu larly in excess, may decrease the energy of

the body, retard the mental powers and damage the liver causing cirrhosis of liver.

The process of determining the percentage of alcohol in a given sample is known as alcoholometry. This is done by

measuring the specific gravity of the sample with the help of hydrometer and then reading the percentage of alcohol against this

value of specific gravity from a standard reference table.

An alcohol-water mixture having specific gravity 0.91976 at 15°C and containing 57.1% of ethyl alcohol by volume or 49.3% by

weight is called proof-spirit. A sample having higher percentage of ethyl alcohol in comparison to proof-spirit is referred to as

over-proof (O.P.) and the one having lower alcohol content than proof-spirit is known as under-proof (U.P). Thus 15 U.P. means

that 100 ml. of the sample contains as much alcohol as 85 ml. of proof spirit. Similarly, 15 OP means that 100 ml. of the sample

contains as much of alcohol as 115 ml. of proof spirit.

Glycerol or glycerine, HOCH2CH(OH)CH2OH

Glycerol is a trihydric alcohol. It may be considered as derivative of propane, obtained by replacement of three hydrogen atoms

from different carbon atoms by three hydroxyl group. In industry, its known as glycerine.

Manufacture

(i) From fats and oil: On hydrolysis of fats and oils, glycerol and higher fatty acids are formed.

The hydrolysis is carried out either by alkali solution (Lye) or by super heated steam.

In the manufacture of soap (sodium salt of higher fatty acids) and in the manufacture of fatty acids required in candle industry,

glycerol is obtained as by-product.

(ii) By fermentation of sugars: During alcoholic fermentation of sugar about 3% glycerol is formed. However, if the fermentation

is done in presence of sodium sulphite, the yield can be increased to 25%.

(iii) Synthesis (from propene): Today much of glycerol is obtained from propene (a product of catalytic cracking of petroleum).

Physical properties

(i) It is a colourless, odourless, viscous and hygroscopic liquid, sweet in taste and non-toxic in nature.

(ii) It is soluble in water and ethyl alcohol but insoluble in ether.

(iii) It has high boiling point, i.e., 290°C. The high viscosity and high boiling point of glycerol are due to association through

hydrogen bonding purified in the lab by reduced pressure distillation or vacuum distillation.

Chemical properties

Glycerol molecule contains two primary – OH groups and one secondary – OH group. Thus, it shows characteristics of both

primary and secondary alcohols. The carbon atoms in glycerol are indicated as

In general, primary – OH groups are more reactive than secondary – OH group.

(i) Reaction with sodium: Only primary alcoholic groups are attacked one by one and secondary alcoholic group is not attacked,

Sodium forms monosodium glycerolate at room temperature and disodium glycerolate at higher temperature.

(ii) Reaction with PCI5: All three OH groups are replaced by CI atoms.

Reaction with HCI or HBr:

(iii) Reaction with HCI or HBr: When HCI is passed into glycerol at 110°C, both , monochlorohydrins are

formed. If the HCI gas is passed for sufficient time, glycerol , and glycerol , are

formed.

Same reactions occur with HBr.

(iv) Reaction with HI: Glycerol reacts with HI in two ways:

(a) When glycerol is warmed with a small amount of hydrogen iodide, allyl iodide is formed. First tri iodide is formed but due to

large size of iodine atom I2 comes out from product.

(b) When glycerol is heated with a large amount of HI, the allyl iodide first formed is reduced to propene, which in presence of

excess of HI forms iso-propyl iodide.

(v) Reaction with nitric acid: When one part of glycerol in a thin stream is added to three times concentrated nitric acid and five

parts of concentrated sulphuric acid, nitro-glycerine (glyceryl trinitrate) is formed.

Glyceryl trinitrate is an yellow oily liquid. It is poisonous and causes headache. It explodes violently when heated rapidly or

subjected to sudden shock. It releases large volume of gases which occupy 10,900 times the volume of nitro glycerine.

It becomes a safer explosive when absorbed on kieselguhr. In this form, it is known as dynamite. Dynamite was discovered by

Alfred Nobel in 1867. The other important explosives prepared from glyceryl trinitrate are:

(a) Blasting gelatin - A mixture of glyceryl trinitrate and cellulose nitrate (gun cotton).

(b) Cordite - It is obtained by mixing glyceryl trinitrate with gun cotton and vaseline. It is

a smokeless explosive.

(vi) Reaction with acetic acid, acetic anhydride or acetyl chloride: Mono-, di- and tri-esters are formed.

(vii) Reaction with oxalic acid: Different products are formed under different conditions.

(viii) Dehydration: Glycerol when heated alone or with dehydrating agents such as potassium hydrogen sulphate or phosphorus

penta oxide or concentrated sulphuric acid, acrolein or acrylaldehyde is formed which has a characteristic bad smell. This reaction

can be used as a test of glycerol.

Oxidation:

(ix) Oxidation: Glycerol gives different oxidation products depending on the nature of

oxidising agent. The following products may be obtain during oxidation of glycerol.

(a) Dilute HNO3 gives mainly glyceric acid.

(b) Conc. HNO3 oxidises glycerol into glyceric acid and tartronic acid.

(c) Bismuth nitrate gives mainly meso oxalic acid.

(d) Fenton’s reagent (H2O2 + FeSO4) or NaOBr or Br2- water in presence of Na2CO3

oxidises glycerol into a mixture of glyceraldehyde and dihydroxy acetone (or glycerose).

(e) With solid KMnO4, glycerol is oxidised to oxalic acid and carbon dioxide. The reaction is

violent and explosion occurs.

Uses: Glycerol is used:

(a) As a sweetening agent in confectionery, beverages and medicines being nontoxic in

nature.

(b) As antifreeze in automobile radiators.

(c) In the preparation of good quality of soap, hand lotions, shaving creams and tooth pastes.

(d) As a lubricant in watches and preservative.

Analytical Tests of Glycerol

(i) Acrolein test: When glycerol is heated with potassium hydrogen sulphate a very

offensive odour is produced due to formation of acrolein. Its aqueous solution restores the

colour of Schiff’s reagent and reduces Fehling’s solution and Tollen’s reagent.

(ii) Dunstan’s test: A drop of phenolphthalein is added to approximately 6 ml. of borax

solution. The pink colour reappears on heating and disappears on cooling again.

(iii) To an equimolar mixture of phenol and glycerol, few drops of conc. H2SO4 are added

and heated to 130°C. It is then boiled by adding water. Addition of ammonium hydroxide to

this solution produces red colour.