The number of –CH₂– (methylene) groups in the product formed from the following reaction sequence is______ 

The correct answer is 0.

Let us analyze the reaction sequence step-by-step to find the final product and count the number of methylene (-CH₂-) groups present in it.

Starting Material:

As shown in the image, the starting compound is oct-3-ene:

${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-\text{CH}=\text{CH}-{\text{CH}}_2-{\text{CH}}_3$

Step 1: Ozonolysis (${\text{O}}_3\text{, Zn /}{\text{H}}_2\text{O}$)

Reductive ozonolysis cleaves the double bond to yield two aldehydes:

1. Butanal: ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-\text{CHO}$

2. Propanal: ${\text{CH}}_3-{\text{CH}}_2-\text{CHO}$

Step 2: Oxidation with ${\text{KMnO}}_4$

Potassium permanganate oxidizes the aldehydes into their corresponding carboxylic acids:

1. Butanoic acid: ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-\text{COOH}$

2. Propanoic acid: ${\text{CH}}_3-{\text{CH}}_2-\text{COOH}$

Step 3: Kolbe's Electrolysis (NaOH, electrolysis)

Neutralizing the acids with NaOH forms sodium carboxylate salts ($\text{R-COONa}$). Electrolysis of the mixture of sodium butanoate and sodium propanoate undergoes decarboxylative coupling to form alkanes. Considering the major alkane formed from the coupling of the larger alkyl chain (propyl radicals, ${\text{CH}}_3{\text{CH}}_2{\text{CH}}_2·$):

$2{\text{CH}}_3{\text{CH}}_2{\text{CH}}_2{\text{COO}}^{-}\stackrel{\text{electrolysis}}{\to }{\text{CH}}_3{\text{CH}}_2{\text{CH}}_2{\text{CH}}_2{\text{CH}}_2{\text{CH}}_3+2{\text{CO}}_2$

The resulting major alkane product is n-hexane (${\text{C}}_6{\text{H}}_{14}$).

Step 4: Aromatization / Dehydrocyclization (${\text{Cr}}_2{\text{O}}_3\text{, 770 K, 20 atm}$)

Heating n-hexane in the presence of chromium oxide (${\text{Cr}}_2{\text{O}}_3$) at 770 K and high pressure causes dehydrogenation and cyclization to yield benzene (${\text{C}}_6{\text{H}}_6$):

${\text{C}}_6{\text{H}}_{14}\stackrel{{\text{Cr}}_2{\text{O}}_3\text{, 770 K, 20 atm}}{\to }{\text{C}}_6{\text{H}}_6+4{\text{H}}_2$

Analysis of the Final Product:

The final product formed is benzene. The structural formula of benzene consists of a 6-membered ring with alternating double bonds, where every carbon atom is ${\text{sp}}^2$ hybridized and attached to one hydrogen atom (-CH- groups).

Therefore, there are 0 methylene (-CH₂-) groups present in the final product.