Question

Br CHM product the major (one compound) the minc (one compou reactive conformation 2. Rank the following as indicated by placing the appropriate letter in the respective box a. Decreasing bond length of the indicated bond. dune Single SInge Fople В C A 13 longest bond b.Increasing HOMO energy shortest bond Hас-сHз A lowest HOM0 highest HOMO

Answer 1

2

(a) Decreasing bond length :

Butane (C-C single bond) > Butadiene (C₂-C₃ : its bond order is around 1.5 : it is between 2 double bonds there are evidences of shorter bond length than C-C) > 2-Butene (C=C bond of 2-butene : smaller than C-C) > 2-Butyne (C$\equiv$C is the shortest bond , bond order : 3)

(b) Increasing HOMO (Highest Occupied Molecular Orbital) energy :

   D<B<A<C

General order of energy : $\sigma$ < $\pi$

Ethane HOMO is $\sigma$ orbital which is lower in energy than $\pi$.

C (1,3,5-hexatriene) : It has three double bonds in conjugation, and HOMO $\pi$₃ ; 1,3-Butadiene has 2 double bond in conjugation and HOMO $\pi$₃ and Ethene's HOMO is  $\pi$. with increasing no. of double bonds in conjugation HOMO-LUMO decreases as  $\pi$-HOMO and  $\pi$*-LUMO become closer , energy level of HOMO increases , so order of energy for HOMO of these alkenes :

Ethene< 1,3-Butadiene< 1,3,5-hexatriene.