Chemistry · Haloalkanes And Haloarenes · NEET
These are two different reactions with different by-products, and NEET loves this. With PCl3: 3 ROH + PCl3 -> 3 RCl + H3PO3 (phosphorous acid). With PCl5: ROH + PCl5 -> RCl + POCl3 + HCl (phosphorus oxychloride). Memory tip: PCl3 (odd, fewer Cl) leaves H3PO3; PCl5 leaves POCl3 + HCl. Mixing these up is the single most common mistake here.
The reaction is R-OH + SOCl2 -> R-Cl + SO2(gas) + HCl(gas). Both by-products, SO2 and HCl, are gases that escape into the air. So you are left with pure alkyl chloride and nothing to separate. The other methods leave liquid or solid by-products (like H3PO3 or POCl3) that contaminate the product. This is why SOCl2 is called the Darzen method and is preferred when you want a clean, pure haloalkane.
Reactivity with HX follows: HI > HBr > HCl (and HF barely reacts). For the alcohol itself: tertiary (3 degree) > secondary (2 degree) > primary (1 degree), because this goes through a carbocation (SN1) and 3 degree carbocations are most stable. Since primary and secondary alcohols react slowly with HCl, we add anhydrous ZnCl2 - this mixture (conc. HCl + ZnCl2) is Lucas reagent, and the ZnCl2 acts as a catalyst to speed it up. Tertiary alcohols react instantly even without ZnCl2.
When you use HX on secondary or tertiary alcohols, the reaction forms a carbocation. Carbocations can rearrange (a hydride or methyl group shifts) to become more stable. So the halogen may end up on a different carbon than the -OH was. This is why SOCl2, PCl3, and PCl5 are safer for pure products - they mostly avoid rearrangement. NEET has directly asked a question where HBr gives a rearranged tertiary bromide instead of the expected secondary one.
Yes - SOCl2, PCl3, and PCl5 put a chlorine in. For bromides you use HBr, PBr3 (or red phosphorus + Br2). For iodides you use HI, or red phosphorus + I2. In practice, for Br and I compounds, chemists often make the P-halide in the flask by mixing red phosphorus with Br2 or I2. So remember: SOCl2 = only chloride; use HX or P + X2 for the others.
The products A and B obtained in the following reactions, respectively are: 3 ROH + PCl3 -> 3 RCl + A ; ROH + PCl5 -> RCl + HCl + B
The compound A on treatment with Na gives B, and with PCl5 gives C. B and C react together to give diethyl ether. A, B and C, in order, are:
In the sequence: CH3CH2CH2OH + PCl3 -> CH3CH2CH2Cl + X + HCl ; then 1-chloropropane with alc. KOH gives Y ; Y + HBr (peroxide) gives Z. X and Z respectively are:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
SOCl2 (thionyl chloride), the Darzen method. Its by-products SO2 and HCl are both gases that escape, so the alkyl chloride is left pure with nothing to separate out.
Lucas reagent is conc. HCl mixed with anhydrous ZnCl2. It converts alcohols to chloroalkanes and is used to tell apart 1 degree, 2 degree and 3 degree alcohols by how fast a cloudy layer appears - tertiary reacts at once, secondary in a few minutes, primary only on heating.
For the halogen acid: HI > HBr > HCl. For the alcohol: tertiary (3 degree) > secondary (2 degree) > primary (1 degree), because a more stable carbocation forms more easily in the SN1-type path.
Use HBr or PBr3 (red P + Br2) for bromides, and HI or red P + I2 for iodides. PCl3, PCl5 and SOCl2 all put in chlorine only.
Alcohol-to-haloalkane steps appear almost every year, usually hidden inside a reaction-sequence question. Knowing the exact by-products (H3PO3 vs POCl3) and that SOCl2 is cleanest often decides one full mark.
