Decarboxylation, Kolbe Electrolysis and Hell-Volhard-Zelinsky (HVZ) Reaction
Chemistry · Aldehydes, Ketones And Carboxylic Acid · NEET
These are 3 special reactions of carboxylic acids. Decarboxylation removes the -COOH group as CO2 using soda lime (NaOH + CaO), giving an alkane with one carbon less. Kolbe electrolysis passes current through the salt solution and joins two alkyl groups, doubling the carbon number. HVZ uses X2 (Cl2 or Br2) with red phosphorus to put a halogen on the alpha carbon (the carbon next to -COOH). Memory hook: "Decarb loses 1 C, Kolbe doubles C, HVZ hits the alpha."
Side-by-side comparison: decarboxylation removes one carbon (soda lime), Kolbe electrolysis doubles the carbons, and HVZ puts a halogen on the alpha carbon (Br2/red P). Knowing the carbon-count change is the key to NEET matching questions.
Your doubts, answered
What exactly is decarboxylation and what reagent do I use?
Decarboxylation means removing the -COOH group as carbon dioxide (CO2). You take the SODIUM SALT of the acid (like CH3COONa) and heat it with SODA LIME. Soda lime is a mixture of sodium hydroxide (NaOH) and calcium oxide (CaO) in a 3:1 ratio. The product is an alkane with ONE carbon atom less than the acid. Example: sodium acetate (2 carbons) gives methane (1 carbon). NEET loves this because you must count carbons correctly.
Why is the carbon count one less after decarboxylation?
The whole -COOH carbon leaves as CO2 gas (trapped as Na2CO3). So if the acid had n carbons, the alkane has (n-1) carbons. CH3-CH2-COONa (propanoate, 3 C) gives CH3-CH3 ethane (2 C). Always remember: the carboxyl carbon is lost, so subtract one.
How is Kolbe electrolysis different from soda-lime decarboxylation?
Both remove CO2, but the result is opposite. Soda-lime decarboxylation gives an alkane with ONE carbon LESS. Kolbe electrolysis passes electric current through an aqueous solution of the sodium/potassium salt and JOINS two alkyl groups together, giving an alkane with DOUBLE the carbons of the alkyl part. Two CH3COONa give CH3-CH3 (ethane) at the anode, plus H2 at the cathode. So: soda lime = one less carbon, Kolbe = twice the alkyl carbons.
What is the Hell-Volhard-Zelinsky (HVZ) reaction and what does it need?
HVZ puts a halogen atom on the ALPHA carbon of a carboxylic acid. You treat the acid with Cl2 or Br2 (written as X2) plus a SMALL amount of RED PHOSPHORUS. The product is an alpha-halo carboxylic acid, for example CH3-CH2-COOH gives CH3-CHBr-COOH. Red phosphorus is the catalyst: it converts the acid to an acyl halide, which enolises easily so the halogen goes to the alpha position. After the reaction, water (H2O) is added to get the acid back.
Which carbon is the alpha carbon, and does every acid react in HVZ?
The alpha (a) carbon is the carbon DIRECTLY attached to the -COOH group. HVZ only works if there is at least one alpha HYDROGEN to replace. Formic acid (HCOOH) has NO alpha carbon and NO alpha hydrogen, so it does NOT undergo HVZ. Acids like acetic acid and propanoic acid have alpha hydrogens and react normally.
What does the arrow condition X2/Red P then H2O mean in NEET reactions?
When you see -COOH with '(i) X2/red P (ii) H2O' above the arrow, that is a code for the HVZ reaction. Step (i) X2/red P adds the halogen at the alpha carbon. Step (ii) H2O is just the work-up that gives back the -COOH group. The final product is an alpha-halo acid carrying both -COOH and the halogen (e.g. -Br) on neighbouring carbons.
⚠️ The NEET trap ✗ In soda-lime decarboxylation of CH3-CH2-COONa, students write the product as propane (same 3 carbons) because they think the -COONa just becomes -CH3. ✓ The whole -COOH carbon leaves as CO2. So CH3-CH2-COONa (3 carbons) loses one carbon and gives ETHANE, CH3-CH3 (2 carbons), plus Na2CO3. Always subtract ONE carbon. 🧠 Decarboxylation = 'De-carbon' = lose one carbon. Soda lime = NaOH + CaO (3:1).
Real NEET questions
NEET 2021
Consider the reaction: CH3CH2COONa (heated with NaOH + ?) gives CH3CH3 + Na2CO3. Identify the missing reagent.
A · CaO ✓
B · DIBAL-H
C · B2H6
D · red phosphorus
Solution: Heating the sodium salt of a carboxylic acid with soda lime causes decarboxylation. Soda lime is a mixture of NaOH and CaO (3:1). Since NaOH is already given, the missing part is CaO. Sodium propanoate loses its carboxyl carbon as CO2 (trapped as Na2CO3) and gives ethane, an alkane with one carbon less. Correct answer: (A) CaO.
Solution: (a) Benzene with CO+HCl over AlCl3/CuCl gives benzaldehyde = Gattermann-Koch (ii). (b) Methyl ketone with NaOX gives haloform CHX3 = Haloform reaction (iii). (c) Alcohol + acid with conc. H2SO4 = Esterification (iv). (d) Acid with X2/red P then H2O adds halogen at the alpha carbon = Hell-Volhard-Zelinsky (i). So the match is (a)-ii, (b)-iii, (c)-iv, (d)-i. Correct answer: (B).
NEET 2023 Phase 2
Identify the final product: cyclobutyl chloride -> (i) KCN (ii) H2O/HCl, heat -> (iii) Br2/red P (iv) H2O -> Product.
A · Cyclobutane bearing -Cl and -Br
B · Cyclobutane bearing two -Br
C · Cyclobutane bearing -COBr
D · Cyclobutane bearing -COOH and an alpha -Br (alpha-bromocyclobutanecarboxylic acid) ✓
Solution: Step (i): KCN replaces -Cl to give cyclobutyl cyanide (-CN). Step (ii): acidic hydrolysis turns the nitrile into cyclobutanecarboxylic acid (-COOH). Steps (iii)-(iv): Br2/red P is the HVZ reagent, which brominates the alpha carbon; water work-up gives the alpha-bromo acid carrying both -COOH and an alpha -Br. Correct answer: (D).
Solved Aldehydes, Ketones And Carboxylic Acid NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Soda lime is NaOH and CaO mixed in a 3:1 ratio. In NEET, if NaOH is shown, the 'missing reagent' is CaO.
Does formic acid (HCOOH) undergo HVZ?
No. Formic acid has no alpha carbon and no alpha hydrogen to replace, so it cannot undergo the Hell-Volhard-Zelinsky reaction.
What is the role of red phosphorus in HVZ?
Red phosphorus is a catalyst. It reacts with X2 to form PX3, which converts the acid to an acyl halide. The acyl halide enolises easily so the halogen adds to the alpha carbon.
What products does Kolbe electrolysis give?
At the anode you get an alkane with double the alkyl carbons (two acetate ions give ethane) plus CO2. At the cathode you get hydrogen gas and hydroxide ions.
Is the carbon number more or less after Kolbe electrolysis?
More. Kolbe joins two alkyl groups, so the carbon count doubles compared to the alkyl part of one acid molecule. This is opposite to soda-lime decarboxylation, which reduces carbon count by one.
What is the product of HVZ on propanoic acid?
Propanoic acid (CH3-CH2-COOH) with Br2/red P gives 2-bromopropanoic acid (CH3-CHBr-COOH), where Br is on the alpha carbon.
