Chemistry · Hydrocarbons · NEET
Two molecules of an alkyl halide (R-X) are treated with sodium metal in dry ether. The sodium removes the halogens, and the two alkyl parts join to form a bigger alkane: 2 R-X + 2 Na -> R-R + 2 NaX. Example: 2 CH3-CH2-Br + 2 Na -> CH3-CH2-CH2-CH3 (butane) + 2 NaBr. Because two identical alkyl groups join, the product always has an even number of carbons. The ether must be dry (moisture-free) or the reaction fails.
Sodium metal reacts violently with water. If any moisture is present, the sodium reacts with water instead of with the alkyl halide, and you get an alcohol or hydroxide instead of the alkane. That is why NCERT says 'dry ethereal solution, free from moisture.' Always remember: dry ether + sodium metal are the two must-have conditions for Wurtz.
Decarboxylation means removing CO2 from a carboxylic acid. You take the sodium salt of the acid (R-COONa) and heat it with soda lime, which is a mixture of NaOH and CaO. The CO2 leaves as sodium carbonate, and you get an alkane with one carbon less than the acid. Example: CH3-COONa + NaOH --(CaO, heat)--> CH4 + Na2CO3. CaO is added so the NaOH does not absorb moisture and can be handled easily.
The carbon that leaves is the carbon of the -COOH (carboxyl) group. It goes away as CO2 (inside Na2CO3). So the rest of the chain becomes the alkane, which is one carbon shorter. Acetate (CH3COONa, 2 carbons) gives methane (CH4, 1 carbon). Propanoate (CH3CH2COONa, 3 carbons) gives ethane (C2H6, 2 carbons).
Wurtz joins two alkyl groups. If both are the same, the carbon count is always even (2+2=4, 3+3=6). To make an odd-carbon alkane like heptane (7 C), you would need two DIFFERENT alkyl halides (say 3-carbon + 4-carbon). But then you get a messy mixture of three products, so the yield of the alkane you want is poor. This is why n-heptane cannot be made in good yield by Wurtz. (Covered fully in the next concept.)
Wurtz: alkyl halide + Na + dry ether -> bigger alkane (carbons ADD up, even number). Decarboxylation: sodium salt of acid + soda lime + heat -> smaller alkane (ONE carbon lost as CO2). One builds up the chain, the other cuts it down by one.
Identify the missing reagent/chemical in the reaction: CH3CH2COONa --(NaOH, ?, heat)--> CH3CH3 + Na2CO3
Which of the following alkane cannot be made in good yield by the Wurtz reaction?
Hydrocarbon (A) reacts with bromine by substitution to form an alkyl bromide which, by Wurtz reaction, is converted to a gaseous hydrocarbon containing less than four carbon atoms. (A) is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Sodium metal in dry ether (moisture-free ethereal solution). The alkyl halide reacts with sodium to couple two alkyl groups into a longer alkane.
Soda lime is a mixture of sodium hydroxide (NaOH) and calcium oxide (CaO). It is used in decarboxylation to convert the sodium salt of a carboxylic acid into an alkane.
It gives methane (CH4) plus sodium carbonate (Na2CO3). The carboxyl carbon leaves as CO2, so the alkane has one carbon less than the acid.
It increases it. Two alkyl groups combine, so the product alkane has more carbons than the starting halide, and the number is always even for identical halides.
The carbon of the -COOH group leaves as carbon dioxide (as part of Na2CO3). The remaining chain becomes the alkane, so it is shorter by exactly one carbon.
Yes. Wurtz reaction and decarboxylation appear regularly in NEET (2018, 2020, 2021). Questions test the reagents (Na/dry ether, soda lime = NaOH + CaO), the even-carbon rule, and the one-carbon-less rule.
