Question

Why is the heat of hydrogenation of benzene less than expected?

Answer 1

Hint: We know that heat of hydrogenation of an alkene is the standard enthalpy of synergist hydrogenation of an alkene. Reactant hydrogenation of an alkene is consistently exothermic. Along these lines, the warmth of hydrogenation of alkenes is consistently negative.

Complete answer:
We need to know that the heat of hydrogenation is not exactly expected on the grounds that benzene is settled by resonance. Cyclohexene has one twofold bond, and its warmth of hydrogenation is \[ - 120kJ/mol\]. Benzene has three twofold bonds, so we may anticipate that its heat of hydrogenation should be \[ - 360kJ/mol\]. Its deliberate warmth of hydrogenation is just \[ - 208kJ/mol\]. Benzene is steadier than anticipated by \[152kJ/mol\]. This distinction is called its reverberation energy. Heat of hydrogenation is straightforwardly relative to the number of π bonds and conversely corresponds to solidness. The \[HOH\] of benzene is even not exactly that of cyclohexadiene. The moderately more modest Heat of hydrogenation for benzene when contrasted with that for $1,3$-cyclohexadiene is because of the aromaticity of the first.
Heat of hydrogenation of alkenes is a proportion of the steadiness of carbon-carbon twofold bonds. All else being something very similar, the more modest the mathematical worth of warmth of hydrogenation of an alkene, the more steady the twofold bond in that. In light of warm hydrogenation of alkenes, the pattern in the dependability of carbon-carbon twofold bonds is tetrasubstituted > trisubstituted > disubstituted > monosubstituted > unsubstituted.

Note:
As well as having engineered applications, synergist hydrogenation is valuable for logical and thermochemical purposes. The examination of a substance for the quantity of carbon-carbon twofold bonds it contains is done by estimating the take-up of hydrogen for a known measure of test. Estimation of the warmth developed in the hydrogenation of alkenes gives data concerning the general secure qualities of alkenes, given that the distinctions in \[\Delta {S_0}\] esteems are little.