Ethanol

Ethanol

Ethanol is also known as Ethyl alcohol. Ethanol is a colorless, inflammable liquid, b.p. 78.1^oC. it is miscible with water in all proportions, and is also miscible with most organic solvents. Ethanol and methanol resemble each other very closely, but they may be distinguished (i) by the fact that ethanol gives the haloform reaction (q.v.), whereas methanol does not; and (ii) ethanol gives acetic acid on oxidation; methanol gives formic acid. These two acids are readily distinguished from each other.

Ethanol cannot be dried by means of calcium chloride, since a compound (an alcoholate) is formed, e.g., CaCl₂.3C₃H₅OH (cf. methanol).  Distillation of rectified spirit over calcium oxide, and then over calcium, gives absolute alcohol. This method is often used in the laboratory, and was formerly used industrially.

Preparation of Ethanol

Ethanol is prepared industrially by several methods, e.g., ethylene (from cracked petroleum) is absorbed in concentrated sulphuric acid (98 per cent) at 75-80^oC, under pressure (17.5-35 kg/cm²). Ethyl hydrogen sulphate and ethyl sulphate are formed:

C₂H₄   +   (HO)₂SO₂    ⟶    C₂H₅OSO₂OH

         C₂H₅OSO₂OH     ⟶     (C₂H₅O)₂SO₂

The reaction mixture is then diluted with about an equal volume of water, and warmed. Hydrolysis takes place and ethanol together with some diethyl ether is formed:

     C₂H₅OSO₂OH   +   H₂O    ⟶    C₂H₅OH   +   H₂SO₄

    (C₂H₅O)₂SO₂   +   2H₂O    ⟶    2C₂H₅OH   +   H₂SO₄

(C₂H₅O)₂SO₂   +   C₂H₅OH    ⟶    (C₂H₅)₂O   +   C₂H₅OSO₂OH

The ether is kept to a minimum by separating the ethyl sulphate from the reaction products, and hydrolyzing it separately. The hydrolysed liquids are distilled, and the aqueous ethanol distillate is concentrated by fractional distillation.

Ethanol is also manufactured by the direct hydration of ethylene with steam under pressure in the presence of a suitable catalyst, e.g., phosphoric acid on a support.

C₂H₄   +   H₂O    ⟶   C₂H₅OH

Some ether (〜5 per cent) is formed as a by-product.

The earliest method of preparing ethanol is by fermentation, and this is still used for the manufacture of beer, wine, brandy, etc., and also as a source of ethanol. The starting material is starch, which is obtained from sources depending on the particular country: common sources of starch are wheat, barley, potato, etc. Molasses is also used as the starting material for ethanol the grain, e.g., wheat or barley, is mashed with hot water, and then heated with malt (germinated barley) at 50^oC for 1 hour. Malt contains the enzyme diastase which, by hydrolysis, converts starch into the sugar, maltose (q.v.):

If molasses is used, then this step is unnecessary, since it contains carbohydrates already present as sugars which can be fermented.

The liquid is cooled to 30^oC and fermented with yeast for 1-3 days. Yeast contains various enzymes, among which is maltase, which converts the maltose into glucose, and zymase, which converts the glucose into ethanol:

Structure of ethanol

Analysis and molecular-weight determinations show that the molecular formula of ethanol is C₂H₆O. Assuming that carbon is quadrivalent, oxygen bivalent, and hydrogen univalent, two structures are possible:

CH₃-CH₂-OH                         CH₃-O-CH₃

    (I)                                             (II)

(i)  Only one hydrogen atom in ethanol is replaceable by sodium or potassium. This indicates that one hydrogen atom is in a different state of combination from the other five. In (I), one hydrogen atom differs from the other five, but in (II) all the hydrogen atoms are equivalent.

(ii)  When ethanol is treated with hydrochloric acid or phosphorus pentachloride, one oxygen atom (bivalent) and one hydrogen atom (univalent) are replaced by one chlorine atom (univalent) to give ethyl chloride, C₂H₅Cl. This implies the presence of a hydroxyl group (cf. methanol).

(iii)  When ethyl chloride is hydrolysed with dilute alkali, ethanol is obtained. This reaction also indicates the presence of a hydroxyl group in ethanol.

(iv)  Ethanol may be prepared as follows:

The arrangement of the six hydrogen atoms in ethane is known, and it is reasonable to suppose that five retain their original arrangement in ethyl chloride and ethanol, since these five hydrogen atoms do not enter (presumably) into the above reactions. Thus there is an ethyl group C₂H₅-  in ethanol. This is so in (I), but not in (II).

(v)  Structure (II) is definitely eliminated, since it can be shown that it is the structure of di-methyl ether (q.v.), a compound that has very little resemblance, physically or chemically, to ethanol.

Hence (I) is accepted as the structure of ethanol, and it accounts for all the known properties of ethanol.

Use of Ethanol

Ethanol is used for the preparation of esters, ether, chloral, chloroform, etc. It is also used as a solvent for gums, resin, paints, varnishes, etc., and as a fuel.