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Chemistry Notes Organic Chemistry: Alcohols The homologous series alcohols have the general formula CnH2n+1OH. Alcohols can be represented as R-OH, since the OH group is the functional group that takes part in reactions. R, represents the rest of the molecule which is unaffected. The work alcohol is used to refer to ethanol (beers, liquor, etc). Manufacture of Ethanol There are 2 processes for commercially manufacturing ethanol: Hydration of ethaneWater is added to ethane by passing a mixture of steam and ethane over a phosphoric acid in silica gel catalyst at 300°C, 60 atm. Fermentation of carbohydratesFermentation is the partial breakdown of glucose by anaerobic respiration. It is done in controlled conditions using a carbohydrate (starch or sugar) and the unicellular fungi, yeast. The yeast provides enzymes (amylase and maltase) that hydrolyse complex carbohydrates into simple fermentable sugars (glucose and fructose). Ethanol is toxic to yeast, so about 12% ethanol the yeast dies and the reaction stops. To produce a greater concentration of ethanol, the mixture is fractionally distilled. Physical Properties of Alcohols Alcohol molecules are covalent by nature. However, they contain the strongly polar O-H bond. Alcohol structure consists of alcohol molecules held together by a combination of weak Van-der-Waals forces and stronger hydrogen bonds. This has significant effects on the physical properties of alcohols. Boiling points, viscosity and density of these substances are higher than expected. State is also affected, as C1 – C12 are all liquids and the rest are solids. As molecules get larger, the effect of the OH group becomes less significant. It is important to know that the solubility of alcohols is also affected by the presence of the OH group. Reaction of Alcohols Reactions are based on the presence of the functional group –OH. Combustion Alcohols burn with a clean, blue flame in air or oxygen to produce carbon dioxide and water. Reaction with reactive metals Alcohols react with highly reactive metals (Na, K, Li, Mg and Ca) to give a salt and hydrogen gas. Dehydration Alcohols can be dehydrated in two ways: 1) Heating to about 170°C with concentrated sulphuric acid. 2) Passing ethanol vapour over aluminium oxide heated to 450°C. Oxidation Powerful oxidizing agents such as acidified potassium manganate (VII) and acidified potassium chromate (VI) can oxidize alcohols to organic acids. The reaction is accompanied by the characteristic colour change of the oxidizing agent. Esterification Alcohols react with organic acids to produce sweet-smelling liquids called esters. We will discuss more on a later date.