Using the table of bond dissociation enthalpies, calculate AH for the reaction A

Question

Using the table of bond dissociation enthalpies, calculate AH for the reaction A. (Use the entries for propene and propenyl halides for the calculation of cyclohexene and cyclohexenyl halides, respectively. Pay attention to the algebraic sign of your answer!) Table of bond dissociation enthalpies CX bondBDE (kJ/mol) X2 and HX bonds BDE (kJ/mol) CHs-Cl C2Hs-Cl (CHs)2CH-Cl 355 (CHs)sC-Cl C-H bondBDE (kJ/mol CH3-H 439 422 (CH3)2CH-H 414 405 CH2 CHCH2-H 372 464 CI-Cl Br-Br I-I H-CI H-Br H-I 247 192 151 431 368 297 351 355 (CHs)3C-H 355 CH2 CH-H Cl H +HCl kJ/mol

Explanation / Answer

Solution:-(i) Looking at the equation,one CH2=CHCH2-H and one Cl-Cl bonds are breaking and one CH2=CHCH2-Cl  and one H-Cl bonds are forming.

So, delta H0 = [(C-H) + (Cl-Cl)] - [C-Cl) + (H-Cl)]

delta H0 = [ 372 + 247] - [288 + 431]

delta H0 = 619 - 719

delta H0 = 100 kJ/mol

(ii) For the first propagation step, one CH2=CHCH2-H bond is breaking and H-Cl bond is forming. So,..

delta H0 = 372 - 431 = -59 kJ/mol

For the second propagattion step, one  Cl-Cl bond is breaking and one CH2=CHCH2-Cl bond is forming.

delta H0 = 247 - 288

delta H0 = -41 kJ/mol

Less is the energy more is the stability and so slow would be the step. second propagation step has less energy it means it is slow and hence the rate determining step.

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