Question

(a) Discuss cis-trans isomerism with suitable example (Organic compounds)
(b) Account for the reducing nature of formic acid.

Answer 1

(a) Cis-trans isomerism:
(1) Isomerism that arises out of difference in the spatial arrangement of atoms or groups about the doubly bonded carbon atoms is called Geometrical isomerism. These isomers are not mirror images of each other. Rotation about $C=C$ is not possible at normal conditions and hence the isomers are isolable.
(2) If different groups (or) atoms are bonded to the $C=C$ bond in a molecule, more than one spatial arrangement is possible. for e.g., 2-butene exists in two isomeric forms. The isomer in which similar group lie on the same side is called cis-isomer and the other in which similar groups lie in opposite side called trans isomer. This isomerism is called cis-trans isomerism.
(3) Trans isomer is more stable than cis-isomer. In the cis-isomer because similar groups are very near each other, Vander Waals repulsion and Seric hindrance make the molecule much unstable. In trans isomer, similar groups are diagonally opposite to each other. Hence there is no such steric interaction. So trans isomer is more stable than cis-isomer.
(4) Reactivity of cis-isomer is higher than trans isomer.
(5) Though at room temperature, cis and trans isomers are stable and are not interconvertible, on heating to a certain temperature, trans isomer can be converted to cis isomer and vice-versa. Breaking of $C-C$ $\pi$ bond and its reformation is responsible for the interconversion.
$Transisomer\stackrel{\Delta }{\rightleftharpoons }Cisisomer$
(6) Dipolemoment studies is one of the best method of identifying cis-trans isomers. Generally cis isomers have larger dipolemoment than trans isomer. Often DPM of trans isomer is zero.
(b)
Formic acid is unique because it contains both an aldehyde group and carboxyl group also. Hence it can act as a reducing agent. It reduces Fehling's solution. Tollens reagent and decolourises pink coloured $KMN{O}_4$ solution.
(i) Formic acid reduces ammoniacal silver nitrate solution (Tollen's reagent) to metallic silver.
$HCOOH+{Ag}_{2}O\to H_{2}O+{CO}_2+2Ag\downarrow$(metallic silver)
(ii) Formic acid reduces Fehling's solution. It reduces blue coloured cupric ions to red coloured cuprous ions.
$HCO{O}^{-}+\underset{\left(blue\right)}{2{Cu}^{2+}}+5{OH}^{-}⟶{CO}_3^{2-}+\underset{\left(red\right)}{{Cu}_{2}O}+3H_{2}O$