Q5. 4-Methylaniline (p-toluidine) is diazotised and the solution is mixed with cuprous bromide. Name the product.
4-Bromotoluene (p-bromotoluene), CH3C6H4Br: the diazonium group at C4 is replaced by –Br, and N2 is given off (reasoned from NCERT page 166).
Q6. What mass of iodobenzene (204 g mol−1) can at most be obtained from 0.050 mol of benzenediazonium chloride and excess KI?
One mole of diazonium salt gives one mole of C6H5I, so at most 0.050 × 204 = 10.2 g (reasoned from NCERT page 166).
Q7. What volume of nitrogen, at 273.15 K and 1 bar, is given off when 0.020 mol of benzenediazonium chloride is converted into chlorobenzene? (Molar volume 22.7 L mol−1.)
One N2 per diazonium ion: 0.020 mol × 22.7 L mol−1 = 0.454 L, that is 454 mL (reasoned from NCERT page 166).
Q8. NCERT says the ortho and para isomers from toluene are easily separated by their melting points. When a mixture of o- and p-chlorotoluene is cooled, which isomer crystallises first, and why?
The para isomer. The two isomers differ widely in melting point (NCERT page 166), and the para isomer is the higher-melting one, because its symmetrical molecules fit the crystal lattice better (NCERT Section 6.6, page 168, on dihalobenzenes; reasoned for the chlorotoluenes). So it freezes out while the ortho isomer stays liquid.