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 takes 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.
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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.
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