Organic Chemistry - Part VII

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5. Alcohols
Organic compounds which have the hydroxyl group (–OH) attached to carbon atoms are known as alcohols. Hydroxyl group is a functional group. As seen earlier a functional group is an atom or a group of atoms which defines the function or the mode of activity of a given carbon compound. The functional group determines all the properties of the compound.  Organic compounds with –OH group is also called as an alcoholic group. Methyl alcohol CH3OH is the simplest of the alcohols. This is also known as methanol. The next higher alcohol is ethyl alcohol or ethanol C2H5OH.

An alcohol is produced by replacing one hydrogen from an alkane and with an hydroxyl group. The two equations below show how methanol and ethanol are made.

Naming of alcohols
Since alcohols are derived from alkanes, the IUPAC names of  alcohols are also derived from the IUPAC names of alkanes.  The hydroxyl group is given a suffix “ol” which is then attached to the IUPAC name of the alkane. The suffix “ol” shows that there is a presence of an alcoholic group.  

For naming of the alcohols we will use the rules laid down by IUPAC :  

  • Find the alkane from which the alcohol is made. Name it similar to the nomenclature for straight chain hydrocarbons. This is called the parent hydrocarbon.

  • Find where the hydroxyl group is attached : in the branches or in the side chains.

  • Number the carbon atoms in the straight chain so that the hydroxyl groups of the side chain come attached to the smallest numbered carbon atom.

  • The position of the hydroxyl group gets the number of the carbon atom it is attached to.  

Examples below will clarify the way the IUPAC nomenclature is used.

Example 1 :  Let us see how methyl alcohol or methanol is named by IUPAC method.  

Methanol is made from methane by replacing one H by one –OH group.  So a suffix “ol” is attached at the end of methane, which becomes methanol.

Example 2 :  Let us see how ethyl alcohol or ethanol is named by IUPAC method.  

Ethanol is made from ethane by replacing one H by one –OH group.  So a suffix “ol” is attached at the end of ethane, which becomes ethanol.

Example 3 :  Let us see how propyl alcohol or propanol is named by IUPAC method.

The examples of methanol and ethanol were very simple and straightforward. But in case of propanol, the H can be replaced at any position. So there could be a straight-chained propanol or the OH group can be placed in the middle carbon atom. The structure of the two alcohols is very different. Hence the IUPAC names become very important to identify how and where the hydroxyl group is situated. The carbon chain is numbered such that the OH group is attached to the carbon with a smaller number.

Example 4 :  Find the IUPAC name for the following alcohol.

This compound can be simply written as  

The compound has 4 carbon atoms in a straight chain. Hence the parent alkane is a butane.  Now number the carbon atoms as 1,2,3,4 so that the hydroxyl group comes attached to the carbon with the lower number.

 3, butanol is an incorrect name according to IUPAC rules.  

2, butanol is the correct name according to the IUPAC nomenclature.  

Properties of alcohols

Physical Properties
1. Physical State : Alcohols are coloureless liquids at ordinary room temperatures.
2. Odour : Lower members of the alcoholic group have a characteristic fruity smell.
3. Density : Alcohols are lighter than water.
4. Solubility : Alcohols are completely soluble in water. Methanol and ethanol are completely miscible in water. Higher members of  alcohols tend to be less soluble in water.
5. Acidic nature : alcohols are neutral liquids and have no effect on litmus or other acid tests.
6. Conductivity : Alcohols are covalently bonded compounds and are hence non-ionic and non-conductors of electricity.  

Chemical Properties
1. Reaction with Sodium : Alcohols react violently with sodium metal and liberate hydrogen gas. The equation for this reaction is shown below.  

This reaction is a standard test for determining the presence of alcohols in a mixture of liquids.

2. Combustion :  The combustion of alcohols in air occurs very easily. Methanol for example burns with a blue flame. Alcohols burn in air to give carbon dioxide and water. A large amount of heat is also generated during this chemical reaction. In fact spirit lamps in laboratories are examples of burning of alcohol in air.  

3. Oxidation :  Oxidation as a process differs from combustion. In oxidation, oxygen is supplied in a controlled fashion. Oxidation of alcohols gives rise to carbonic acids and water.  The reaction of potassium dichloride and dilute suphuric acid can provide oxygen. This gives out atomic oxygen or nascent oxygen. Methanoic acid and water are the reaction products.  

Limited oxygen can be provided by copper pieces heated. The alcohol can be passed over the heated copper slowly. This gives rise to the reaction products : methanoic acid and water. Copper of course acts as a catalyst.  

4. Reaction with carboxylic acids : Carboxyl acids are nothing but organic acids. (We shall study about the carboxyl acids in the next section). Alcohols react with carboxyl acids to form sweet smelling organic compounds that are called esters (details about esters is given later in the chapter). The reaction of alcohols with carboxylic acids is called esterification. This is one of the major tests to see the presence of alcoholic group in a sample of liquid mixtures. When methanol is heated with ethanoic acid in the presence of concentrated sulphuric acid, methyl ethanoate, an ester and water is formed. Concentrated sulphuric acid acts as a catalyst for this reaction.

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