Alkanols-Classification-sources-uses of Alkanols, Chemistry tutorial

Introduction:

Alkanol are the group of compounds in which the hydrogen atom(s) of alkanes have been substituted with the hydroxyl (-OH) functional group. Alkanols are usually termed to as 'alcohols'. We are as well familiar that the general formula of Alkanols is CnH2n+1OH and are named by replacing the 'e' of the alkane name via 'ol'. For example - methanol and ethanol from methane and ethane.

1884_Methanol and Ethanol.jpg

Fig: Methanol and ethanol

A simple general representation for Alkanols is R-OH; here 'R' symbolizes an alkyl group. Whenever essential, the position of the -OH group is illustrated numerically, example: propanol. The carbon atoms are numbered in the longest chain. The position of the OH group finds out the numbers. Let us study the structures shown below.

1555_Alkanols.jpg

Fig: Alkanols

The hydroxyl group is one of the most significant functional groups of naturally occurring molecules like carbohydrates and nucleic acids. These are complex alkanols however the main concern are simple alkanols which are made up from petroleum-derived hydrocarbons.

Classification of Alkanols:

Alkanols are categorized as primary, secondary or tertiary based on whether the OH functional group is linked to a primary, secondary or tertiary carbon atom, example:

2438_Classification of Alkanols.jpg

Fig: Classification of Alkanols

Sources of Alkanols:

General methods of preparation:

(a) Hydrolysis of halogenoalkanes or alkyl halides:

Alkyl halides are the compounds made by replacing one of the hydrogen of an alkane with a halogen (Cl or Br or I). Whenever alkyl halides are treated by dilute aqueous sodium hydroxide, the halogen is substituted by - OH groups therefore making an alkanol.

R-Cl + Na+ + OH- → R-OH + NaCl

Example: CH3CH2Cl + NaOH → CH3CH2-OH + NaCl

             Ethyl chloride                Ethanol

(b) Hydration of alkenes: 

The Alkenes react with steam whenever passed over phosphoric (v) acid catalyst at 300°C and 70 atmosphere. For illustration:

CH2 = CH2 + H2O → (H3PO4 at 300oc + 70 atms) → CH3-CH2-OH

The method, as illustrated in the equation, is mainly used industrially for the production of ethanol and some higher alkanols.

Manufacture of alkanols:

a) Methanol: Methanol is mainly made up from mixture of carbon (II) oxide and hydrogen (that is, obtained from either water gas or synthesis gas) in the presence of a catalyst.

C(coke) + H2O(g) → CO + H2 (water gas)

CH4 (natural gas) + H2O (g) → CO + 3H2 (synthesis gas)

CO + 2H2 → (ZnO + Cr2O3 at 400oc and 300 atm) → CH3OH

(b) Ethanol:

- From ethene: The main source of ethanol industrially is through hydration of alkenes as illustrated above.

- By fermentation: The fermentation method of obtaining ethanol from starch materials is still admired.

Fermentation is basically the decomposition of complex organic compounds example: carbohydrates, into simpler compounds via the action of enzymes.

Materials like - sugar, guinea-corn, rice, cassava, maize, potatoes, wheat, barley and so on can be employed as the source of glucose from which the ethanol is derived.

  • From starchy materials: The material is crushed and treated by steam to extract the starch from them.

Malt, made up from partially germinated barley, is added and then kept at around 50°C for one hour. The enzyme diastase, present in the malt, catalyses the conversion of starch to the maltose.

2(C6H10O5)n + nH2O → (diastase) → nC12H22O11

Starch                                                   maltose

The yeast is then added at room temperature. This causes fermentation, the enzyme maltase in the yeast, transforms the maltose to glucose and the other enzyme zymase in yeast, then decomposes the glucose into ethanol and carbon (iv) oxide.

C12H22O11 (aq) + H2O → (maltase) → 2C6H12O6 (aq)

                                                              Glucose

C6H12O6 → (zymase) → 2C2H5OH + 2CO2

                                        ethanol

  • From molasses: In countries such as Brazil, U.S.A; where the production of sugar cane is high, the main raw material for ethanol production is molasses, a syrupy liquid which remains after the crystallization of sugar. It includes sucrose, a kind of sugar. Whenever molasses is mixed by yeast, the enzyme invertase in yeast, transforms the sucrose to two simple isomeric sugars, glucose and fructose, which are both fermented to ethanol through zymase.

C12H22O11 + H2O → (invertase) → C6H12O6 + C6H12O6 → (zymase) → 4C2H5OH + 4CO2

                                                    Glucose     Fructose

  • From palm wine: In some portions of Africa example: Nigeria, South Africa and Ghana; local gin is obtained by fermenting palm wine which is rich in sugar. Fresh palm wine includes sugar and yeast, which on storage in large earthenware pots or drums for 3 to 4 days, is fermented to ethanol. The resultant solution is then distilled to get the clear ethanol generally termed as gin.

(c) Purification of ethanol:

The ethanol achieved from the fermentation method can only provide a maximum concentration of 18%, as the yeast cells die above this concentration. For commercial use, ethanol is needed in different compositions and to accomplish further concentration and purification, fractional distillation is used.

For the alcoholic beverages, different concentrations of ethanol are employed and their flavor differs with raw material fermented. The table illustrated below represents the percentage of ethanol and the method of preparation.

Table: Kinds of alcoholic beverages

Raw material

Alcohol beverage

% Ethanol (v/u)

Method of preparation

Barley, Wheat

Beer e.g. star

3 - 8

Fermentation

Grapes/Rice

Wines

8 - 18

Fermentation

Barley, Wheat, Grapes

Whisky/Spirit/Brandy

30 - 60

Fermentation & Distillation

The commercial ethanol is sold as pure spirit or rectified spirit or methylated spirit having 95% ethanol. Whenever sold to the public, 5% methanol, which is toxic, is added to prevent it being used as a drink.

Uses of ethanol:

1) This is used as solvent for resins, polishes, varnishes, liquid soaps, perfumes, drugs and paints.

2) It is employed as a fuel, either by itself or mixed by petrol, in rockets and racing cars.

3) It is employed as an anti-freeze in the automobile radiator.

4) It is present in numerous alcoholic beverages.

5) It is employed for sterilization and preservation of the specimens and food.

6) It is employed as raw material in the manufacture of chemical example: ethanol, trichloromethane and ethoxyethane.

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