Which of the following reactions will not result in the formation of C-C bonds?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Cannizzaro reaction
Some named reactions build a bigger carbon skeleton because they form a new C–C bond. Others only change a functional group and leave the carbon skeleton as it was. To decide, look at whether a carbon atom of one reactant ends up bonded to a carbon atom of the other.
Option (1) Wurtz reaction — Forms a C–C bond. Two alkyl halides react with sodium in dry ether, and the two alkyl groups are joined by a new C–C bond:
\( \displaystyle \ce{2CH3-Br + 2Na} \xrightarrow{\text{dry ether}} \ce{CH3-CH3 + 2NaBr} \)
Option (2) Friedel-Crafts acylation reaction — Forms a C–C bond. Anhydrous AlCl3 gives the acylium ion, an electrophile, which attacks the ring. The acyl carbon bonds to a ring carbon:
\( \displaystyle \ce{C6H6 + CH3COCl} \xrightarrow{\text{anhyd. }\ce{AlCl3}} \ce{C6H5-CO-CH3 + HCl} \)
Option (3) Reimer-Tiemann reaction — Forms a C–C bond. Phenol is treated with chloroform and sodium hydroxide, and a –CHO group is put at the ortho position. The carbon of the –CHO group (from CHCl3) bonds to a ring carbon:
\( \displaystyle \ce{C6H5OH} \xrightarrow[\text{(ii) }\ce{H3O^+}]{\text{(i) }\ce{CHCl3} + \text{aq. }\ce{NaOH}} \ce{2-HO-C6H4-CHO} \)
Option (4) Cannizzaro reaction — Does not form a C–C bond. An aldehyde with no α-hydrogen, heated with concentrated alkali, undergoes self oxidation and reduction. One molecule is reduced to an alcohol and the other is oxidised to the salt of a carboxylic acid. Only a hydride ion moves from one molecule to the other; each product keeps the same number of carbon atoms as the aldehyde, so no C–C bond forms:
\( \displaystyle \ce{2HCHO} \xrightarrow{\text{conc. }\ce{KOH}} \ce{CH3OH + HCOOK} \)
\( \displaystyle \ce{2C6H5CHO} \xrightarrow{\text{conc. }\ce{NaOH}} \ce{C6H5CH2OH + C6H5COONa} \)
NCERT (2026-27 edition):
Other reactions – “(i) Cannizzaro reaction: Aldehydes which do not have an α-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.”