In biaryls, assume that rotation about the bond joining the two rings is blocked when all four ortho positions (2, 6, 2', 6') carry substituents, and free otherwise. Which of the following is chiral?

Axial chirality needs blocked rotation and each ring unsymmetrical about the axis (its two ortho groups different). In the first compound each ring has $\mathrm{Br}$ at one ortho position and $\mathrm{NO}{}_2$ at the other, so the twisted form is chiral. In the tetranitro and tetrabromo compounds each ring has two identical ortho groups, so a mirror plane survives; the 4,4'-isomer has free rotation. Only the first is chiral.