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Heat of hydrogenation of benzene is 51 kcal/mol and
its resonance energy is 36 kcal/mol. Then the heats
of hydrogenation of cyclohexadiene and cyclohexene
per mole respectively are
- a)58 kcal, 58 kcal
- b)25 kcal, 29 kcal
- c)29 kcal, 58 kcal
- d)58 kcal, 29 kcal
Correct answer is option 'D'. Can you explain this answer?
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Most Upvoted Answer
Heat of hydrogenation of benzene is 51 kcal/mol andits resonance energ...
Heat of hydrogenation of benzene and its resonance energy are given to find the heats of hydrogenation of cyclohexadiene and cyclohexene.
Heat of Hydrogenation
Heat of hydrogenation is defined as the heat evolved when one mole of an unsaturated compound is converted into a saturated compound by the addition of hydrogen. It is a measure of the stability of the unsaturated compound.
Resonance Energy
Resonance energy is defined as the difference between the actual energy of a molecule and the energy that would be expected if the molecule were completely described by a single Lewis structure. It is a measure of the stability of a molecule due to resonance.
Calculations
The heat of hydrogenation of benzene is 51 kcal/mol, which means that benzene is more stable than cyclohexene, which has a heat of hydrogenation of 29 kcal/mol. The resonance energy of benzene is 36 kcal/mol, which means that benzene is more stable due to resonance than the hypothetical cyclohexatriene, which would have a resonance energy of 0 kcal/mol.
Cyclohexadiene
Cyclohexadiene has two double bonds and can exist in two isomeric forms, namely, 1,2-cyclohexadiene and 1,4-cyclohexadiene. The heat of hydrogenation of 1,3-cyclohexadiene is the average of the heats of hydrogenation of 1,2-cyclohexadiene and 1,4-cyclohexadiene.
Heat of hydrogenation of 1,3-cyclohexadiene = (Heat of hydrogenation of 1,2-cyclohexadiene + Heat of hydrogenation of 1,4-cyclohexadiene)/2
= (2 x Heat of hydrogenation of benzene)/2
= Heat of hydrogenation of benzene
= 51 kcal/mol
Cyclohexene
Cyclohexene has one double bond and can exist in one form. The heat of hydrogenation of cyclohexene is half of the heat of hydrogenation of benzene.
Heat of hydrogenation of cyclohexene = (Heat of hydrogenation of benzene)/2
= 51/2
= 25.5 kcal/mol
Therefore, the heats of hydrogenation of cyclohexadiene and cyclohexene per mole respectively are 58 kcal and 29 kcal, which is option D.
Heat of Hydrogenation
Heat of hydrogenation is defined as the heat evolved when one mole of an unsaturated compound is converted into a saturated compound by the addition of hydrogen. It is a measure of the stability of the unsaturated compound.
Resonance Energy
Resonance energy is defined as the difference between the actual energy of a molecule and the energy that would be expected if the molecule were completely described by a single Lewis structure. It is a measure of the stability of a molecule due to resonance.
Calculations
The heat of hydrogenation of benzene is 51 kcal/mol, which means that benzene is more stable than cyclohexene, which has a heat of hydrogenation of 29 kcal/mol. The resonance energy of benzene is 36 kcal/mol, which means that benzene is more stable due to resonance than the hypothetical cyclohexatriene, which would have a resonance energy of 0 kcal/mol.
Cyclohexadiene
Cyclohexadiene has two double bonds and can exist in two isomeric forms, namely, 1,2-cyclohexadiene and 1,4-cyclohexadiene. The heat of hydrogenation of 1,3-cyclohexadiene is the average of the heats of hydrogenation of 1,2-cyclohexadiene and 1,4-cyclohexadiene.
Heat of hydrogenation of 1,3-cyclohexadiene = (Heat of hydrogenation of 1,2-cyclohexadiene + Heat of hydrogenation of 1,4-cyclohexadiene)/2
= (2 x Heat of hydrogenation of benzene)/2
= Heat of hydrogenation of benzene
= 51 kcal/mol
Cyclohexene
Cyclohexene has one double bond and can exist in one form. The heat of hydrogenation of cyclohexene is half of the heat of hydrogenation of benzene.
Heat of hydrogenation of cyclohexene = (Heat of hydrogenation of benzene)/2
= 51/2
= 25.5 kcal/mol
Therefore, the heats of hydrogenation of cyclohexadiene and cyclohexene per mole respectively are 58 kcal and 29 kcal, which is option D.
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Community Answer
Heat of hydrogenation of benzene is 51 kcal/mol andits resonance energ...
See first total energy to break 3 bond in benzene is 87 [51+36]
so to break one bond i.e. in cyclohexaene is 87/3=29kcal/mol
and to break two bond I.e. in cyclohexdiene is 87/3*2=58kcal/mol
so to break one bond i.e. in cyclohexaene is 87/3=29kcal/mol
and to break two bond I.e. in cyclohexdiene is 87/3*2=58kcal/mol
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Heat of hydrogenation of benzene is 51 kcal/mol andits resonance energy is 36 kcal/mol. Then the heatsof hydrogenation of cyclohexadiene and cyclohexeneper mole respectively area)58 kcal, 58 kcalb)25 kcal, 29 kcalc)29 kcal, 58 kcald)58 kcal, 29 kcalCorrect answer is option 'D'. Can you explain this answer?
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