Question

In which of the following, the $ \Delta {H_h}^ \circ $ maximum?
(A) $ C{H_2} = C{H_2} $
(B) $ C{H_2} = C = C{H_2} $
(C) $ C = C - C = C $
(D) $ C = C - C - C = C $

Answer 1

Hint: The symbol $ \Delta {H_h}^ \circ $ represents the heat or enthalpy of hydrogenation that is linked with the stability of the different unsaturated organic compounds. The compounds containing double bonds can be hydrogenated to give saturated hydrocarbons and the amount of heat released in the process is a measure of the stability of the compound.

Complete Step By Step Answer:
Enthalpy of hydrogenation is the heat released in transforming a double bond into a single bond by adding hydrogens to it and is denoted by the symbol $ \Delta {H_h}^ \circ $ .
Different alkenes and double bonded compounds have different extent of stability that is influenced by the factors like hybridization and conjugation.
A conjugated alkene exhibits extra stability due to the delocalization of its double bonds. An allene $ C{H_2} = C = C{H_2} $ on the other hand contains two adjacent double bonds present on the same carbon that makes the molecule twisted (the two bonds are present in perpendicular planes) and unstable.
The enthalpy of hydrogenation is least for the most stable compounds as very small amounts of heat is released on hydrogenating a stable unsaturated compound. However, the enthalpy of hydrogenation is maximum for the least stable compounds.
 $ \Rightarrow $ Allene being highly unstable has the highest enthalpy of hydrogenation. Therefore option (b) is correct.

Note:
For a compound to be conjugated in nature it must contain two or more double bonds that are placed alternatively. Adjacent double bonds do not show resonance and are therefore categorized as allenes. Any compound with two or more double bonds may or may not be conjugated.