Alcohol forms a homologous series with general formula $C_nH_{2n+1}-OH$ and -OH group as functional group. Ethanol is commonly called alcohol and is used in alcoholic drinks. It is a good solvent, used in medicines, cough syrups, tonics etc.

(a) Write structural formula and name of 4th member of alcohol homologous series.

(b) What happens when ethanol is heated with alkaline $KMnO_4$? Write chemical equation involved.

(c) Write the chemical equation of reaction of ethanol with ethanoic acid in the presence of concentrated $H_2SO_4$. Write the name of this reaction.

OR

(c) What happens when ethanol is heated with excess concentrated sulphuric acid at 443 K? Write chemical equation involved. What is the role of concentrated sulphuric acid in this reaction?

This case-based question tests the alcohol homologous series and the characteristic reactions of ethanol as given in the NCERT chapter (Section 4.2.4 Homologous Series and Section 4.4.1 Properties of Ethanol / Reactions of Ethanol).

(a) The chapter defines a homologous series as one 'in which the same functional group substitutes for hydrogen in a carbon chain,' with successive members differing by a $-CH_2-$ unit. For alcohols ($C_nH_{2n+1}OH$), the first four members are methanol ($n=1$), ethanol ($n=2$), propanol ($n=3$) and butanol ($n=4$, the 4th member), consistent with Activity 4.2's method of building the homologous series of alcohols.

(b) The chapter states directly (Section 4.3.2 Oxidation, Activity 4.5) that 'alkaline potassium permanganate or acidified potassium dichromate are oxidising alcohols to acids, that is, adding oxygen to the starting material. Hence they are known as oxidising agents.' The colour of $KMnO_4$ persists momentarily then fades as ethanol is progressively oxidised to ethanoic acid; with excess ethanol the oxidising agent is consumed and no permanent colour remains, but with excess $KMnO_4$ its colour persists because all the ethanol has already reacted.

(c) The main answer uses the esterification reaction explicitly given in Section 4.4.2 (Reactions of ethanoic acid, Activity 4.8): ethanoic acid + ethanol in the presence of an acid catalyst (conc. $H_2SO_4$) gives the ester (ethyl ethanoate) and water, described in the text as producing a 'sweet-smelling' substance used in perfumes/flavouring agents.

The alternative (OR) answer uses the dehydration reaction explicitly given in Section 4.4.1 (Reaction to give unsaturated hydrocarbon): heating ethanol at 443 K with excess concentrated $H_2SO_4$ dehydrates it to ethene, with concentrated sulphuric acid described in the text as 'a dehydrating agent which removes water from ethanol.' Both reaction equations and the roles of concentrated $H_2SO_4$ (catalyst/dehydrating agent in esterification vs. dehydrating agent in ethene formation) are drawn directly from the chapter.