Some Commercially Important Alcohols Preparation and Properties
→ Methanol : Chemical formula of methanol is CH3OH. It is also known as wood spirit because it was originally prepared by destructive distillation of wood.
→ Commercially methanol is produced by catalytic hydrogenation of carbon monoxide at high temperature and pressure. Mixture of carbon monoxide and hydrogen at 573 K temperature and 200 atm pressure are passed over (Copper, Zinc and Chromium) oxide, as a result methanol is formed.
→ For commercial production of methanol CO and H2 are obtained from water gas or by partial oxidation of methane.
→ Properties : Methanol is a colourless liquid with b.p. 337.5 K. It is miscible with water. Its relative density is 0.795.
It is poisonous in nature and when taken internally it causes blindness or even death when taken in excess.
It gives most of the general reaction of alcohols (alkonols).
- As a solvent for paints, varnishes and celluloide.
- In the manufacture of formaldehyde.
- For denaturing ethanol.
- As an antifreeze for automobile radiators.
- In the formation of organic compounds for e.g., lodo methane.
→ Ethanol : Its chemical formula is C2H5OH. It is main constituent of alcoholic beverages. It is also known as grain alcohol because it can be prepared from starchy grains. Commercially it can be manufactured by fermentation of molasses or starchy substances.
→ A process in which complex organic substance break down into simple molecules by nitrogen containing complex organic compounds (enzyme) is known as fermentation. It is an exothermic reaction. In this reaction CO2 gas is evolved and it appears as boiling of solution hence it is known as fermentation.
→ Formation of Ethanol from Molasses : When sugar crystals are separated from juice of sugarcane, then red-brown coloured mother liquid is left behind, it is known as molasses. It is a mixture of sugar, glucose and fructose.
Ethanol is obtained from molasses in given steps:
→ Dilution : Molasses is diluted with water. In one volume molesses five volume of water is added.
→ Addition of Acid and Ammonium SaIt : Dil. H2SO4 is added to the dilute aqueous solution of molasses to adjust the pH in between 4-5 so that undesirable bacteria do not grow. A small quantity of ammonium sulphate is added which acts as a food for the yeast cells.
→ Addition of Yeast : Above solution is taken in a wooden container and yeast is added. Then it is allowed to stand for some days at about 298K-303K temperature. The Invertase enzyme present in yeast convert the sugar into glucose and fructose.
→ Other enzyme present in yeast converts glucose and fructose into ethanol and CO2 gas evolve.
→ The solution thus obtained by fermentation contains 10-15% ethanol. This solution is known as wash.
→ Distillation of Wash : Distillation of wash is carried out in a special apparatus called coffey’s still. By it 90% ethanol is obtained. Ethanol thus obtained is again subjected to fractional distillation. The collected distillate at 351.2 K contains 95.5% ethanol, it is known as rectified spirit.
→ Formation of Ethanol from Starchy Materials Starch is present in wheat, rice, maize, barley, potatoes. Starch is obtained from starchy materials in the following steps:
→ Saccharification : To convert starch into maltose Sugar : Formation of maltose sugar from starch occurs in following steps:
→ Mash . Malt (Maltose solution) Small pieces of wheat, rice maize, barley, potatoes etc.. starchy material are heated with the help of steam under high pressure and temperature. Due to which starch forms milky slurry with water which is known as mash.
→ Diastase enzyme is present in barley. Germinated barley contains high amount of diastase. To germinate barley it is spreaded in a dark room at 288 K temperature. Barley germinates in two-three days, this is known as malt. A germinated barley is grinded with water and then filtered. Filtered liquid is called malt extract.
→ It contains high amount of diastase enzyme. Mash and malt extract both are mixed at temperature 323-333 K. The enzyme diastase present in malt extract, hydrolyses starch which is present in mash and converts it into maltose. This solution of maltose is known as wart.
→ Conversion of Maltose into Glucose : Yeast is added in wart (maltose solution) and temperature of the solution is kept approx, at 303 K. Maltase enzyme present in yeast hydrolyses maltose into Glucose.
→ Conversion of Glucose into Ehtnaol : Zymase enzyme present in yeast converts glucose into ethanol The solution obtained after fermentation contains 10-15% ethanol which is knwon as wash.
→ Distillation : Distillation of wash is done in coffey’s still and approx 95% pure ethanol is obtained.
→ Properties : This is a colourless, volatile liquid with characteristic pleasant odour. It has boiling point of 351,2 K It is soluble in water. It gives all general reactions of alcohols. Absolute Alcohol : 100% pure ethanol is known as absolute alcohol.
→ Rectified Alcohol : Mixture of ethanol (95.5%) and water (4.5%) which distillates at constant temperature 351.2 K. It is known as rectified alcohol.
→ Power Alcohol : It has been found that absolute ethyl alcohol mixed with petrol and benzene can be used as a motor fuel. Alcohol thus used for generating power is called power alcohol (20%), ethanol +80% gasoline (petrol).
→ Denatured Alcohol : To make rectified alcohol unfit for drinking purposes, in it methanol, dyes or base like pyridine is added. Alcohol thus obtained is called denatured alcohol.
- As a solvent for paints, varnishes etc.
- For the preparation of organic compounds like ester, ether, chloral, chloroform, iodoform etc.
- In the manufacture of medicines. Fore.g, in formation of tincture.
- In the form of antiseptic and dehydrating agent.
- In the manufacture of alcoholic beverages.
- In the manufacture of transparent soap, colour polish, perfumes etc.
- In the manufacture of Terylene.
- It is used in the form of preservative.
- In the form of lubricant.
- In the manufacture of drugs like boroglycerine, glycerophosphoric acid.
- Due to its hygroscopic nature, it is used in shaving soap, toothpaste, cream, lipstick like cosmetics.
- In the preparation of plastic and artificial fibres.
- In the form of lubricant.
- As an antifreeze in automobile radiators.