Q.(a) Write notes on cis-trans isomerism with suitable example.

(a) Cis-trans (geometrical) isomerism occurs about a restricted (double) bond when each carbon of the bond bears two different substituents, illustrated by cis- and trans-2-butene. (b) Oxalic acid is prepared in the laboratory by nitric-acid oxidation of sucrose ($V_2O_5$ catalyst) and industrially from sodium formate via a dehydrogenation–double-decomposition–acidification sequence.

(a) Cis-trans isomerism

Geometrical (cis-trans) isomerism is a form of stereoisomerism that arises because rotation about a carbon–carbon double bond (or around the bonds of a ring) is restricted (the π-bond must break for rotation to occur). It is shown by compounds of the type $Cabc=Cabc$ type structure where each doubly-bonded carbon carries two different groups (if either carbon carries two identical groups, geometrical isomerism is not possible).

• In the cis isomer, the two identical (or the two reference) groups lie on the same side of the double bond.
• In the trans isomer, they lie on opposite sides of the double bond.

Example — but-2-ene (2-butene), $CH_3-CH=CH-CH_3$:

$cis$-2-butene: both $-CH_3$ groups on the same side of the $C=C$ bond (and both H's on the same side, opposite to the methyls). It is the less stable, slightly higher-energy isomer (due to steric strain between the two methyl groups on the same side) and has a small net dipole moment.

$trans$-2-butene: the two $-CH_3$ groups lie on opposite sides of the $C=C$ bond. It is the more stable isomer (methyls kept apart) and, being more symmetrical, has essentially zero (or very small) net dipole moment.

The two isomers are genuinely different compounds — they cannot be interconverted at room temperature without breaking the π-bond — and they differ measurably in physical properties such as melting point, boiling point and dipole moment. (The same type of isomerism is also classically shown by maleic acid (cis-butenedioic acid) and fumaric acid (trans-butenedioic acid), which even differ sharply in chemical behaviour, e.g. only maleic acid readily forms a cyclic anhydride on heating.)

(b) Preparation of oxalic acid, $(COOH)_2$

1. Laboratory method — oxidation of sucrose: Sucrose (cane sugar, $C_{12}H_{22}O_{11}$) is heated with concentrated nitric acid in the presence of a small quantity of vanadium pentoxide ($V_2O_5$), which acts as a catalyst. Vigorous oxidation of the sugar by the $HNO_3$ (itself reduced, with evolution of nitrogen oxide fumes) converts it into oxalic acid, which crystallises out on cooling the reaction mixture. $C_{12}H_{22}O_{11} \xrightarrow[V_2O_5]{\text{conc. } HNO_3} 6(COOH)_2 + \text{(oxides of nitrogen + water)}$
2. Industrial method — from sodium formate: This is the principal industrial route. …