1.Rank the heats of hydrogenation for the following, 1 being most heat released

Question

1.Rank the heats of hydrogenation for the following, 1 being most heat released and 4 being least heat. 2 Rank the following carbocations in order of increasing stability (least>most): CH,CHCH CH,CHCH CHCH (CH)CCH2 3. Hydrogenation of cyclohexene releases 120 kJ/mol (28.6 kcal/mol) of heat. Which of the following most likely represents the observed heat of hydrogenation of 1.3- cyclohexadiene? (a) 232 kJ/mol (55.4 kcal/mol) (b ) 240 kJ/mol (57.2 kcal/mol (c ) 247 kJ/mol (59.0 kcal/mol) (d) 120 kJ/mol (28.6 kcal/mol) 4.Which of the following compounds give the same carbocation on ionization? Br Br (a) 1 and 3 (b) 2 and 4 (c) 1 and 2 (d) 1 and 4 5. For the following reactions the major products are shown: +25 CH.CHe-, These provide an example of-1control at low temperature and 2control at higher temperature. (a ] kinetie thermodynamic (b ) thermodynamickinetic (c) kinetic kinetic (d j thermodynamicthermodynamic

Explanation / Answer

1. Heat of hydrogenation of alkenes is a measure of the stability of the alkenes with lower heats of hydrogenation implying greater stability. Assuming all other factors are same for a set of alkenes, the stability increases with increasing substitution on the double bond. Also, due to resonance by conjugation, conjugated alkenes are more stable than isolated alkenes with all other factors being the same. Thus from left to right, the heats of hydrogenation of the alkenes given as ranked as 2, 4, 1 and 3.

2. The order of stability of carbocations is dependant on the number of substituents on the carbocation atom and the presence of absence of resonance. Thus primary is less stable than secondary which is less stable than allyl which is less stable than tertiary giving the order 3 > 2 > 1.

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