n-Butane is dichlorinated by free-radical substitution. How many structural isomers of $\mathrm{C}{}_4\mathrm{H}{}_8\mathrm{Cl}{}_2$ can be formed in total (stereoisomers not counted)? Give an integer.

Number the carbons C1-C4. Same-carbon pairs: 1,1 and 2,2. Different-carbon pairs: 1,2 ; 1,3 ; 1,4 ; 2,3. (2,3 gives a different compound from 1,3; 3,4 equals 1,2; 1,3 equals 2,4.) Total = 6, confirmed by substituting H twice and de-duplicating.