Question

a) Calculate the steric energy or heat of formation for one single bond isomer of trans-benzalacetone using the usual energy minimization procedure. The result should be a planar molecule. (b) Then deliberately hold the dihedral angle defined by atoms 1, 2, 3, and 4 at 0°, 90°, and 180° and again calculate the energies of the molecule. What is the approximate barrier to rotation about the single bond?

Answer 1

Factors effecting steric energy: i) Streaching of bond, ii) bond angle change , iii) torsional strain(rotation of bond) repulsion due to bonded pair of electrons, iv) electrostatic interaction, v)Van der Wall energy.

Benzalacetone is an unsaturated compound.Only trans isomer is observed.Its formula is C₆H₅CH=CHCOCH₃

When dihedral angle is 0⁰ than there will be maximum torsional strain and that particaular conformar will be eclipsed means unstable.

When dihedral angle is 90⁰ than there will be comparable torsional strain and that particular conformar will be partially eclipsed means stable than 0⁰ and less stable than conformer having angle of separation between different group is 180⁰.

When dihedral angle is 180⁰ than there will be minimam torsional strain and that particaular conformar will be anti means stable.