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ALKANOLS (ALCOHOLS)
Alcohols are those organic compounds which are characterized by the presence of one, two or more hydroxyl groups (−OH) that are attached to the carbon atom in an alkyl group or hydrocarbon chain.
The simplest alkanol is methanol (CH3OH). Its common name wood alcohol comes from the fact that it was first obtained by heating wood in the absence of air. Methanol is very poisonous; consumption of less than two teaspoons can cause blindness. Of all the alkanols, methanol is produced in the greatest quantities in industry.
Ethanol (CH3CH2OH) or simply, alcohol in layman‟s terms has been prepared since antiquity by fermentation of sugars and starches, catalyzed by yeast. Sugars for fermentation come from a variety of sources, including grains, grape juice, various vegetables and agricultural wastes. The immediate product of fermentation is a water solution containing up to 15% ethanol (its not surprising that home brew can be so potent!). This value can be increased by distillation.
Classification of alcohols
Alcohols are differentiated based upon the presence of hydroxyl group attached. The location of this hydroxyl group as well will change the physical and chemical properties of any alcohol.
There are three types of alcohol. Alcohols are classified as primary, secondary or tertiary alcohols.
Types of Alcohols
The classification is done in accordance to where the carbon atom of an alkyl group is attached to the hydroxyl group.
Primary alcohols are those alcohols where the carbon atom of the hydroxyl group (OH) is attached to only one single alkyl group. Some examples of these primary alcohols include Methanol (propanol), ethanol, etc. The complexity of this alkyl chain is unrelated to the classification of any alcohol considered as primary. The existence of only one linkage among –OH group and an alkyl group and the thing that qualifies any alcohol as a primary.
Secondary alcohols are those where the carbon atom of the hydroxyl group is attached to two alkyl groups on either side. The two alkyl groups present may be either structurally identical or even different. Some examples of secondary alcohols are given below.
Tertiary alcohols are those which feature hydroxyl group attached to the carbon atom which is connected to 3- alkyl groups. The physical properties of these alcohols mainly depend on their structure. The presence of this -OH group allows the alcohols in the formation of hydrogen bonds with their neighbouring atoms. The bonds formed are weak, and this bond makes the boiling points of alcohols higher than its alkanes. The examples of tertiary alcohols include-
Most of the alcohols are known to be colourless liquids or even are said to behave as solid at room temperatures.
Alcohols with less molecular weight are said to be highly soluble in water; and with their increase in molecular weight, they tend to become less soluble and their vapour pressures, boiling points, densities, and the viscosities to increase.
Nomenclature of alkanols
The systematic name for an alkanol is obtained by following the rules listed below:
- Find the longest chain that contains the -OH group.The stem of the alkanols name is obtained from the parent alkane, with the -e being replacedby the suffix -ol.
- The position of the -OH group is specified by a numberchosen so that it is the smallest of the substituent numbers.
Properties of Alcohols
Alcohols are organic compounds in which a hydrogen atom of an aliphatic carbon is replaced with a hydroxyl group. Thus an alcohol molecule consists of two parts; one containing the alkyl group and the other containing functional group hydroxyl group. They have a sweet odour. They exhibit a unique set of physical and chemical properties.
The physical and chemical properties of alcohols are mainly due to the presence of hydroxyl group.
Physical Properties of Alcohol
- The Boiling Point of Alcohols
Alcohols generally have higher boiling points in comparison to other hydrocarbons having equal molecular masses. This is due to the presence of intermolecular hydrogen bonding between hydroxyl groups of alcohol molecules. In general, the boiling point of alcohols increases with an increase in the number of carbon atoms in the aliphatic carbon chain. On the other hand, the boiling point decreases with an increase in branching in aliphatic carbon chains the Van der Waals forces decreases with a decrease in surface area. Thus, primary alcohols have a higher boiling point.
- Solubility of Alcohols
The solubility of alcohol in water is governed by the hydroxyl group present. The hydroxyl group in alcohol is involved in the formation of intermolecular hydrogen bonding. Thus, hydrogen bonds are formed between water and alcohol molecules which make alcohol soluble in water. However, the alkyl group attached to the hydroxyl group is hydrophobic in nature. Thus, the solubility of alcohol decreases with the increase in the size of the alkyl group.
- The Acidity of Alcohols
Alcohols react with active metals such as sodium, potassium etc. to form the corresponding alkoxide. These reactions of alcohols indicate their acidic nature. The acidic nature of alcohol is due to the polarity of –OH bond. The acidity of alcohols decreases when an electron-donating group is attached to the hydroxyl group as it increases the electron density on the oxygen atom. Thus, primary alcohols are generally more acidic than secondary and tertiary alcohols. Due to the presence of unshared electrons on the oxygen atom, alcohols act as Bronsted bases too.
Chemical Properties of Alcohols
Alcohols exhibit a wide range of spontaneous chemical reactions due to the cleavage of the C-O bond and O-H bond. Some prominent chemical reactions of alcohols are:
- Oxidation of Alcohol
- Alcohols undergo oxidation in the presence of an oxidizing agent to produce aldehydes and ketones which upon further oxidation give carboxylic acids.
2. Dehydration of Alcohol
- Upon treatment with protic acids, alcohols undergo dehydration (removal of a molecule of water) to form alkenes. Dehydration of alcohol