What is Q?

The correct answer is Option (4) → \(R-H\)

An alkyl bromide reacts with magnesium in dry ether to give a Grignard reagent, P = RMgBr. The C–Mg bond is highly polar, with a negative charge on carbon, so a Grignard reagent takes a proton from any source of proton and gives a hydrocarbon. Ethanol has an acidic O–H hydrogen:

    \( \ce{R-Br + Mg} \xrightarrow{\text{dry ether}} \ce{R-MgBr} \)

    \( \ce{R-MgBr + C2H5OH -> R-H + Mg(OC2H5)Br} \)

So Q is the alkane R–H. For the same reason, Grignard reagents are made and kept in dry conditions.

Why the other options are incorrect

• Option (1) \(R-OH\) — Incorrect. RMgX gives an alcohol when it adds to a carbonyl compound and the adduct is hydrolysed, not when it meets ethanol.

      \( \ce{HCHO + RMgX -> RCH2OMgX} \xrightarrow{\ce{H2O}} \ce{RCH2OH + Mg(OH)X} \)

• Option (2) \(R-Br\) — Incorrect. The C–Br bond is not formed again; the bromine ends up in \( \ce{Mg(OC2H5)Br} \).

• Option (3) \(R-R\) — Incorrect. R–R is the product of the Wurtz reaction, in which an alkyl halide reacts with sodium in dry ether:

      \( \ce{2RX + 2Na} \xrightarrow{\text{dry ether}} \ce{R-R + 2NaX} \)

NCERT (2026-27 edition):
Reaction with Metals – “… In the Grignard reagent, the carbon-magnesium bond is covalent but highly polar, with carbon pulling electrons from electropositive magnesium; the magnesium halogen bond is essentially ionic. Grignard reagents are highly reactive and react with any source of proton to give hydrocarbons. Even water, alcohols, amines are sufficiently acidic to convert them to corresponding hydrocarbons.”