How many of the following five compounds are chiral (optically active as single enantiomers)? (i) penta-2,3-diene; (ii) 3-methylbuta-1,2-diene; (iii) spiro[3.3]heptane-2,6-dicarboxylic acid; (iv) hexa-2,3,4-triene (each geometric isomer); (v) 6,6'-dinitrobiphenyl-2,2'-dicarboxylic acid with rotation about the central bond blocked.

(i) is a chiral allene (each end has H and CH3). (ii) has two identical methyl groups on one end and is achiral. (iii) The two rings in a spiro system lie in perpendicular planes and each ring bears different groups at C2 and C6 positions, giving a chiral axis. (iv) A cumulene with an odd number (3) of C=C bonds is planar and achiral. (v) The hindered biphenyl is chiral. So (i), (iii) and (v): 3.