Question

Define hydrogenation?

Answer 1

Hint:Alkenes are readily hydrogenated under pressure in presence of a catalyst.
\[R{\text{ }}-{\text{ }}CH{\text{ }} = {\text{ }}C{H_2}\; + {\text{ }}{H_2}\; \to R - C{H_2} - C{H_3}\]
Alkene shows chain, position, functional (ring chain), optical & geometrical isomerism and undergo addition reactions due to unsaturation.

Complete answer:
The addition of hydrogen to an alkene or alkyne in presence of a catalyst to form a saturated hydrocarbon, alkane is called hydrogenation. The addition of hydrogen to form a saturated compound is termed hydrogenation.
The following catalysts have been used satisfactorily in the above reaction.
(i) Finely divided platinum and palladium are effective at room temperature. Platinum or palladium black i.e., metals in a very finely divided state, may be prepared by reducing their soluble salts with (formaldehyde).
(ii) Nickel requires a temperature of\[200-300^\circ C\] . Raney nickel is effective at room temperature. Platinum or and atmospheric pressure. (The method for the preparations of Raney nickel has been described elsewhere in the module).
One molecule of hydrogen is adsorbed for each double bond present in the unsaturated compound. The order of rate of hydrogenation of alkenes at room-temperature and atmospheric pressure is
\[-{\text{ }}CH{\text{ }} = {\text{ }}C{H_2}\; > {\text{ }}-{\text{ }}CH{\text{ }} = {\text{ }}CH-\;\] or a ring double bond
Alkenes of the type \[{R_2}C = C{R_{2\;}}or{\text{ }}{R_2}C{\text{ }} = {\text{ }}CHR\] are difficult to hydrogenate under the above reaction conditions.

Note:
Alkynes can be reduced directly to alkanes by the addition of \[{H_2}\;\] in the presence of a catalyst. The addition reaction takes place in two steps. lt is not possible to isolate the mediate alkene under the above reaction conditions. By using Lindlar’s catalyst [Pd on\[CaC{O_3},{(OCOC{H_3})_2}\;Pb\]] nickel boride, alkynes can be partially hydrogenated to syn product.