Alcohols with HX, PCl5 and PCl3: How Alcohols Become Alkyl Halides
Chemistry · Alcohols, Phenols And Ethers · NEET
Alcohols swap their -OH group for a halogen to make an alkyl halide (R-X). Three reagents do this: HX (like HCl, HBr, HI), PCl5, and PCl3. Remember the byproducts: PCl3 gives H3PO3 (phosphorous acid), and PCl5 gives POCl3 plus HCl. Memory hook: "PCl-THREE leaves acid H3PO-THREE; PCl-FIVE leaves POCl-three plus HCl."
One alcohol, three ways to make an alkyl halide. HX can rearrange the product; PCl3 leaves H3PO3; PCl5 leaves POCl3 + HCl. Memorising the red byproducts wins the NEET mark.
Your doubts, answered
What does an alcohol give with PCl3, and which acid is the byproduct?
An alcohol (ROH) reacts with PCl3 to give the alkyl chloride RCl. One PCl3 converts THREE alcohol molecules: 3 ROH + PCl3 -> 3 RCl + H3PO3. The byproduct is phosphorous acid, H3PO3 (NOT phosphoric acid H3PO4). NEET loves testing this: the '3' in PCl3 matches the '3' in H3PO3, so it is easy to remember.
What does an alcohol give with PCl5? Why is POCl3 formed?
ROH + PCl5 -> RCl + POCl3 + HCl. Here PCl5 reacts one-to-one with the alcohol. The oxygen of the -OH stays with phosphorus to form phosphoryl chloride, POCl3 (also called phosphorus oxychloride), and one HCl gas escapes. So PCl5 gives TWO byproducts (POCl3 and HCl), while PCl3 gives only H3PO3.
What is the reactivity order of HX (HI, HBr, HCl) with alcohols?
The order is HI > HBr > HCl. HI is the strongest acid and gives the best nucleophile (I-), so it reacts fastest. HCl is the slowest and usually needs anhydrous ZnCl2 (a Lewis acid catalyst) to react with primary and secondary alcohols. This ZnCl2 + conc. HCl mix is the Lucas reagent.
Which type of alcohol reacts fastest with HX, and why?
Tertiary (3 degree) > secondary (2 degree) > primary (1 degree). Tertiary alcohols react by an SN1 path: the -OH leaves (after protonation) to form a very stable tertiary carbocation, which the halide then grabs. Primary alcohols react by SN2 and are slowest. This reactivity order is the whole idea behind the Lucas test.
Why does the HBr product sometimes look 'wrong' or rearranged?
With HX, secondary and tertiary alcohols go through a carbocation (SN1). If a nearby carbon can give a MORE stable carbocation, a 1,2-hydride or 1,2-methyl shift happens first. So the halogen can end up on a different carbon than the original -OH. Always check if the first carbocation can rearrange to a more stable one before adding X-.
Why do we use PCl3, PCl5 or SOCl2 instead of just HX?
HX can cause rearrangement and side reactions because it makes a carbocation. PCl3, PCl5 and SOCl2 give cleaner alkyl chlorides without rearrangement. SOCl2 (thionyl chloride) is best because both byproducts, SO2 and HCl, are gases that escape, leaving pure RCl. This is called the Darzen's method.
⚠️ The NEET trap ✗ 3 ROH + PCl3 gives RCl + H3PO4 (phosphoric acid). ✓ 3 ROH + PCl3 gives 3 RCl + H3PO3 (phosphorous acid). With PCl5 you get POCl3 + HCl, not H3PO4 either. 🧠 PCl-THREE -> H3PO-THREE. The two 3's stick together. Phosphoric acid H3PO4 never appears in these alcohol reactions.
Real NEET questions
NEET 2024
The products A and B obtained in the following reactions, respectively, are: 3 ROH + PCl3 -> 3 RCl + A ; ROH + PCl5 -> RCl + HCl + B
A · POCl3 and H3PO4
B · H3PO4 and POCl3
C · H3PO3 and POCl3 ✓
D · POCl3 and H3PO3
Solution: With PCl3, three alcohol molecules give three alkyl chlorides and the phosphorus byproduct is phosphorous acid: 3 ROH + PCl3 -> 3 RCl + H3PO3, so A = H3PO3. With PCl5, the byproducts are HCl gas and phosphoryl chloride: ROH + PCl5 -> RCl + HCl + POCl3, so B = POCl3. Phosphoric acid (H3PO4) is never a product here.
NEET 2018
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 are, in order,
A · C2H5Cl, C2H6, C2H5OH
B · C2H5OH, C2H5Cl, C2H5ONa
C · C2H5OH, C2H6, C2H5Cl
D · C2H5OH, C2H5ONa, C2H5Cl ✓
Solution: A is ethanol (C2H5OH). With sodium it gives sodium ethoxide: 2 C2H5OH + 2 Na -> 2 C2H5ONa + H2, so B = C2H5ONa. With PCl5 the alcohol gives ethyl chloride: C2H5OH + PCl5 -> C2H5Cl + POCl3 + HCl, so C = C2H5Cl. The ethoxide (B) and ethyl chloride (C) then undergo Williamson synthesis to give diethyl ether: C2H5ONa + C2H5Cl -> C2H5-O-C2H5 + NaCl.
NEET 2023 Phase 1
Identify the major product P: 3-methylbutan-2-ol CH3-CH(CH3)-CH(OH)-CH3 reacts with HBr -> P
A · 2-bromo-2-methylbutane CH3-C(Br)(CH3)-CH2-CH3 ✓
B · 2-methylbut-2-ene CH3-CH=CH-CH3
C · 2-bromo-3-methylbutane CH3-CH(CH3)-CH(Br)-CH3
D · 1-bromo-2,2-dimethylpropane (CH3)3C-CH2Br
Solution: With HBr the secondary alcohol reacts by SN1. After protonation and loss of water, a secondary carbocation forms at C-2. A 1,2-hydride shift from C-3 converts it into a more stable TERTIARY carbocation on C-3 (which carries two methyl groups). Br- then captures this tertiary cation, giving 2-bromo-2-methylbutane (option A). Direct substitution without rearrangement would give option C, but the rearranged tertiary product is the major one.
Solved Alcohols, Phenols And Ethers NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Is the byproduct with PCl3 phosphorous acid or phosphoric acid?
It is phosphorous acid, H3PO3. Phosphoric acid (H3PO4) is a common wrong option in NEET. Just match the two 3's: PCl3 -> H3PO3.
How many alcohol molecules react with one PCl3?
Three. The balanced equation is 3 ROH + PCl3 -> 3 RCl + H3PO3. With PCl5 the ratio is 1:1.
What is the best reagent to make a pure alkyl chloride without rearrangement?
Thionyl chloride, SOCl2 (Darzen's method). ROH + SOCl2 -> RCl + SO2 + HCl. Both byproducts are gases that leave, so the alkyl chloride stays pure. HX can rearrange, so it is less clean.
Why does HCl need ZnCl2 to react with alcohols?
HCl is the weakest of the halogen acids and reacts slowly. Anhydrous ZnCl2 is a Lewis acid that helps the -OH leave, speeding up the reaction. This combination is the Lucas reagent, used to tell primary, secondary and tertiary alcohols apart.
Does the HX reaction ever give an alkene instead of an alkyl halide?
The main product is the alkyl halide (substitution). But because a carbocation forms, a small amount of alkene can appear from elimination, and rearrangement can move the halogen to a different carbon. NEET usually asks for the major, rearranged substitution product.
