Fig: Addition reaction
All unsaturated hydrocarbons reacts with molecules like H2, X2, H2O etc. to form another saturated compounds are called addition reactions.
Unsaturated hydrocarbons add hydrogen in the presence of catalysts such as nickel or palladium to give saturated hydrocarbons.
Note: Catalysts are substances that cause a reaction to occur or proceed at a different rate without the reaction itself being affected.
Addition of hydrogen to ethene:
Addition of hydrogen to ethyne:
Addition of hydrogen to an unsaturated carbon compound is called hydrogenation reaction.
Certain vegetable oils such as ground nut oil, cotton seed oil and mustard oil, contain double bonds (C = C) and are liquids at room temperature. Because of the unsaturation, the vegetable oils undergo hydrogenation, like alkenes, to form saturated products called vanaspati ghee, which is semi-solid at room temperature.
Vegetable oils (Unsaturated oil) + Hydrogen Vanaspati Ghee (Saturated ghee)
The reactions in which one or more hydrogen atoms of a hydrocarbon are replaced by some other atoms or groups are called substitution reactions.
Methane reacts with chlorine (or bromine) in the presence of sunlight and undergoes substitution reaction. It is called photochemical reaction because it takes place in the presence of sunlight.
Some important carbon compounds:
Ethanol (Ethyl alcohol, C2H5OH):
Ethanol is the second member of the homologous series of alcohols.
Preparation: By the fermentation of carbohydrates (sugar or starch).
Ethanol is prepared on commercial scale by fermentation of sugar (molasses). Fermentation is allowed to take place at 298- 303 K in the absence of air. Ethanol (ethyl alcohol) gets oxidised to form ethanoic acid (acetic acid) in the presence of air.
Physical state/colour and odour: Pure ethanol is a colourless liquid having a pleasant smell and a burning taste.
Boiling and Freezing points: It is a volatile liquid with a boiling point of 78.1°C, and freezing point is 118°C.
Density: Ethanol is lighter than water as its density is 0.79 g ml-1 at 293 K.
Solubility: Ethanol is miscible with water in all proportions, due to the formation of hydrogen bonds with water molecules.
Ethanol containing 5% water is called rectified spirit and containing 0% water is called absolute alcohol.
Conductivity: Ethanol is a covalent compound and does not ionise easily in water, hence it is a neutral compound.
Action on Litmus: Ethanol is a neutral compound. So, it has no effect on the colour of litmus.
Chemical properties of ethanol:
Combustion (or burning): Ethanol is highly inflammable liquid and readily burns in air with blue flame to form water vapour, carbon dioxide and evolving heat. Thus, combustion of ethanol is an exothermic reaction.
Reaction with sodium metal: Ethanol reacts with sodium metal to produce sodium ethoxide and hydrogen gas is evolved.
Reaction with ethanoic acid (Esterification reaction):
The reaction in which an alcohol reacts with acetic acid in the presence of conc. H2SO4 to form an ester is called esterification.
Note: Ester are sweet-smelling compounds and are used for making perfumes.
Reaction with conc. sulphuric acid (Dehydration):
Ethanol when heated with excess of concentrated sulphuric acid at 443 K, gets dehydrated to give ethene.
Note: The concentrated sulphuric acid can be regarded as a dehydrating agent which removes water from ethanol.
Uses of ethanol:
Harmful effects of Alcohols:
Ethanol to which certain poisonous and nauseating substances like methyl alcohol, pyridine, copper sulphate etc. have been added is termed denatured alcohol.
Note: To prevent the misuse of ethanol (Alcohol), industrial alcohol is coloured blue so that it can be recognised easily.
Harmful effects of denatured alcohol:
Ethanoic acid (acetic acid) CH3COOH:
Ethanoic acid is commonly called acetic acid and belongs to the homologous series of carboxylic acid and is represented as CH3COOH.
5-8% solution of acetic acid in water is called vinegar and is used for preserving foods like sausage, pickles.
Reaction with alcohols (Esterification reaction):
Ethanoic acid reacts with ethanol in the presence of conc. H2SO4 to form ethyl ethanoate which is an ester.
The reaction of carboxylic acid with an alcohol to form an ester is called "esterification reaction".
Note: Ester can be hydrolysed in the presence of an acid or a base to give back the parent carboxylic acid and the alcohol.
(i) Ethyl ethanoate on acid hydrolysis gives ethanoic acid and ethanol.
CH3COOC2H5(l) + H2O(l) → CH3COOH(aq.) + C2H5OH
(ii) Hydrolysis of ester in the presence of base (alkali) is called "Saponification reaction".
Note : Alkaline hydrolysis of higher esters is used in the manufacture of soaps.
Reaction with sodium carbonate and sodium hydrogen carbonate:
Ethanoic acid decomposes sodium hydrogen carbonate and sodium carbonate with a rapid evolution of carbon-dioxide gas.
Note: Reactions of ethanoic acid with NaOH, NaHCO3, Na2CO3 and active metals show that the hydrogen present in the carboxyl (COOH) group is acidic in nature.
Uses of ethanoic acid: