Engineering Chemistry S Datta

162 ENGINEERING CHEMISTRY
3.5 Kcal
4.4–6.1 Kcal
Potential energy
0.9 Kcal
CH 3 CH 3
CH 3
CH 3
CH 3
CH 3 CH3
CH 3
CH 3
CH 3
Anti Gauche Eclipsed Gauche
Angle of rotation
Fig. 7.4 (b) Potential energy changes during rotation about C 2
—C 3
bond of n-butane.
From the potential energy diagram, we see that the anti-conformation of n-butane is
more stable (CH 3
-groups are as apart as possible, 180°). In two Gauche conformations, the
—CH 3
-groups are 60° apart and they are less stable than the anti-form and more stable than
the eclipsed form, the eclipsed conformation is least stable due to severe crowding.
Conformations of Cyclohexane
Factors influencing the stability of conformations: To go deep into the chemistry
i.e., the stability of the cyclic compound and its reactivity, we must take help of conformational
analysis.
• Any deviations from the normal, i.e., sp 3 hybridised carbon (bond angle 109.5°) are
accompanied by angle strain.
• Any pair of tetrahedral carbon attached to each other tend to have their bonds staggered
(i.e., ethane-like). Any deviation from the staggered conformation is accompanied by
torsional strain.
• Any two atoms or groups that are not bonded to each other can interact. This nonbonded
interaction can either be repulsive or attractive. And the result can be either
destabilisation or stabilisation of the conformation.
In the case of cyclohexane the following conformations are common [Fig. 7.4(c)].
H A H A H A
H B
H B H B
Chair
Conformer
Twist
Conformer
Boat
Conformer
Fig. 7.4 (c) Conformers of cyclohexane.