Engineering Chemistry S Datta

MECHANISM OF ORGANIC REACTIONS 163
Newman projection formula of chair conformer [Fig. 7.4(d)]:
Fig. 7.4 (d ) Cyclohexane (chair form) staggered form of ethane.
Here the three pairs of tetrahedral carbon of cyclohexane remain in staggered forms. So
the chair cyclohexane is more stable conformer of the cyclohexanes. For boat cyclohexane
the Newman projection formula is as Fig. 7.4(e).
Cyclohexane (boat form)
Eclipsed ethane
Fig. 7.4 (e)
The boat conformation is less stable than the chair conformation at an energy maximum,
it is thus not a conformer but a transition state between two conformers. Here the three pairs
of tetrahedral carbon of cyclohexane remain in eclipsed forms.
The twist form or skew boat form is a conformer lying at energy minimum. Between the
chair form and twist form there are half chair forms with angle strain and torsional strain. The
half chair form is a most unstable form which lies at energy maximum (see Fig. 7.5).
1.3 Kcal
Potential energy
11 Kcal
5.6 Kcal
Chair
form
Half
chair
Twist
form
Boat
form
Twist
form
Fig. 7.5 Potential energy diagram (conformations of cyclohexane).