What is commonly referred to as alcohol is just the most widely known example of a group of organic compounds which are collectively known as alcohols. The alcohol we drink is correctly called ethanol or ethyl alcohol and it has the chemical formula of C2H5OH. The alcohol functional group contains an -OH, or hydroxyl functional group, attached to carbon. Phenols are a subgroup of alcohol in which the hydroxyl groups are attached directly to the carbon atoms of a ring structure. Phenols have markedly different characteristics from the rest of the alcohols and the phenol group is discussed in a separate entry. Aliphatic dihydric alcohols are known as glycols.
If a compound possesses only one hydroxyl group then it is said to be a monohydric alcohol. Di-, tri-, and polyhydric alcohols also exist. Monohydric alcohols derive their names from the appropriate hydrocarbon with an ending of -ol. For example, the alcohol derivative of methane is methanol and of ethane is ethanol. If the hydroxyl group is bound to a carbon atom that is itself bound to three other carbon atoms then the resultant alcohol is a tertiary alcohol. A secondary alcohol has the hydroxyl group bound to a carbon atom bound to two other carbon atoms. A primary alcohol (such as ethanol) has the hydroxyl attached to a carbon atom which is itself attached to either one or no other carbon atoms.
The production of alcohols is carried out in different ways depending on the start products and the alcohol required. When unsaturated organic compounds are used as the starting point the alcohol is made by the addition of water to the double or triple bond. Methanol can be made by mixing hydrogen and carbon monoxide at high temperatures and pressure over a zincoxide-zinc chromate catalyst.
One of the simplest ways to manufacture alcohol is by fermentation. Sugar is taken and in the presence of an enzyme (a biological catalyst) ethyl alcohol and carbon dioxide are produced. When alcohol is produced for human consumption, zymase--provided by yeast--is used as the catalyst.
The lower alcohols (those with only a small number of carbon atoms) are readily soluble in water. As the size of the alkyl group increases, i.e. as the number of carbon atoms increases, the water solubility decreases.
The boiling point of an alcohol is always higher than the hydrocarbon from which it is derived. This is due to the hydrogen bonding that occurs between the hydroxyl groups. Extra energy must be added to the system to overcome these extra bonds in order for boiling to occur. With alcohols containing several hydroxyl groups the level of hydrogen bonding is much higher and consequently the level of energy required to move from the liquid to the gaseous phase is yet higher. The hydrogen bonding can still occur in the gaseous phase, giving paired molecules of alcohol. This can then give a molecule with twice the expected molecular weight when analysis, such as gas chromatography, is undertaken.
When an alcohol is reacted with a strongly electropositive metal, such as sodium, the products are hydrogen and a crystalline, ionic salt of the metal, called an alkoxide. In this type of reaction primary monohydric alcohols are the most reactive.
Oxidation occurs with alcohols, and the products depend upon the alcohol reacted and the conditions under which the reaction occurs. Primary alcohols lose two hydrogen atoms to produce an aldehyde. Further oxidation, by the addition of oxygen, converts the aldehyde to a carboxylic acid. For example ethanol loses two hydrogen atoms to give ethanal and when an oxygen atom is added to this the product is ethanoic (acetic) acid. Secondary alcohols undergo the same process but ketones are produced. Tertiary alcohols are oxidized with difficulty, but the result of the first stage is a mixture of ketone and carboxylic acid. These oxidation reactions can be used as a test to distinguish between the different classes of alcohols. The most regularly employed oxidizing agent for this set of reactions is a mixture called chromic acid, a mixture of sodium or potassium dichromate and sulfuric acid. It is also possible to oxidize primary alcohols using atmospheric oxygen with platinum or copper as a catalyst. In the case of ethanol, oxidation can occur under the action of certain species of bacteria. This reaction is responsible for the souring of wine and it is commercially employed in the manufacture of vinegar.
Reduction of an alcohol produces the corresponding alkane, by loss of the hydroxyl group. This is carried out using an aqueous solution of hydrogen iodide reacting in the presence of red phosphorus. This mixture is one of the most powerful reduction agents for use on organic compounds. The red phosphorus is present to make more hydrogen iodide from the iodine gas that would be otherwise liberated.
Complete combustion of alcohols produces carbon dioxide and water. Alcohols can be dehydrated in two distinct manners. If the alcohol (with the exception of methanol) is heated with aluminum oxide, phosphoric acid or excess sulphuric acid, an alkene is formed along with a molecule of water. The second type of alcohol dehydration occurs when the alcohol is reacted, in excess, with sulphuric acid. This reaction produces an ether and water.
Esterification of alcohols can occur with either an organic or an inorganic acid. Both types of reaction are reversible and produce an ester and water. This reaction is catalyzed by hydrogen ions. The hydrogen is usually supplied by sulfuric acid which also acts as a dehydrating agent, thus giving the maximum yield of ester.
Alcohols are used for a number of commercial and industrial processes. Methanol is manufactured chiefly for conversion into formaldehyde. The formaldehyde is then reacted with urea or phenol to make a number of plastics, including bakelite. Methanol is also used as a paint solvent, antifreeze, and as an additive to ethanol to make it undrinkable. Ethanol is used to provide the alcohol content in alcoholic drinks. It is also used as an additive to gasoline to make a compound called gasohol. Gasohol has been extensively used in some countries in South America to decrease their reliance on imported oil.
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