Animated Solution for Chemistry - Organic Chemistry: Different possible thermal decomposition pathways for peroxyesters are shown below. Match each pathway from List-I with an appropriate structure from List-II.

Welcome to a beautiful exploration of radical chemistry! When we subject peroxyesters to thermal stress, they don't just fall apart randomly. They follow precise, predictable pathways dictated by the stability of the intermediates they form. The journey always begins at the weakest link: the oxygen-oxygen single bond. Because this bond is exceptionally weak, it undergoes homolytic cleavage upon heating, initiating the decomposition process.

Concerted vs. Stepwise

Once the $\text{O}-\text{O}$ bond is primed to break, the molecule faces a critical decision that depends entirely on the nature of the $\text{R}$ group attached to the carbonyl carbon.

Imagine this: if the $\text{R}$ group is capable of forming a highly stable radical—such as a resonance-stabilized benzyl radical (${\text{PhCH}}_2^{\bullet }$)—the molecule doesn't wait around. The carbon-carbon bond breaks at the exact same time as the oxygen-oxygen bond. This simultaneous rupture instantly releases carbon dioxide gas in what we call a concerted pathway.

However, if the $\text{R}$ group forms an unstable radical—like a phenyl radical (${\text{Ph}}^{\bullet }$), where the unpaired electron is trapped in an unyielding ${\text{sp}}^2$ orbital—the molecule is forced to take a slower, stepwise pathway. The $\text{O}-\text{O}$ bond breaks first to form a transient carboxylate radical (${\text{RCO}}_2^{\bullet }$), which only later loses ${\text{CO}}_2$.

While the left side of the molecule is busy losing ${\text{CO}}_2$, the right side forms an alkoxy radical (${\text{R}}^{\prime }{\text{O}}^{\bullet }$). If this radical is small and simple, like a methoxy radical (${\text{CH}}_3{\text{O}}^{\bullet }$), it remains intact.

But there is a catch! If the alkoxy radical is bulky and highly branched, it can undergo a fascinating fragmentation known as $\beta$-scission. It cleaves a carbon-carbon bond located one position away from the oxygen radical to form a stable carbonyl compound (like a ketone) and ejects a brand new radical (${\text{X}}^{\bullet }$). The golden rule of $\beta$-scission is simple: the bond that breaks will always be the one that produces the most stable radical possible.

Let's put these principles to the test with our four candidates:

Molecule 1: Features a benzyl $\text{R}$ group and a simple methyl ${\text{R}}^{\prime }$ group. The stable benzyl radical drives a concerted loss of ${\text{CO}}_2$. The resulting methoxy radical does not undergo $\beta$-scission. This direct formation of ${\text{R}}^{\bullet }$ and ${\text{R}}^{\prime }{\text{O}}^{\bullet }$ perfectly matches Pathway P.

Molecule 2: Features a phenyl $\text{R}$ group and a methyl ${\text{R}}^{\prime }$ group. The unstable phenyl radical forces a stepwise loss of ${\text{CO}}_2$. Again, the methoxy radical stays intact. This stepwise process without further fragmentation is Pathway S.

Molecule 3: Features a benzyl $\text{R}$ group, meaning a concerted start. The resulting bulky alkoxy radical must undergo $\beta$-scission. It has a choice between ejecting a methyl radical or a benzyl radical. Since benzyl is vastly more stable, it breaks the bond to the benzyl group, yielding acetone and a benzyl radical. A concerted start followed by $\beta$-scission is Pathway Q.

Molecule 4: Features a phenyl $\text{R}$ group, meaning a stepwise start. The resulting cumyloxy radical undergoes $\beta$-scission. It must choose between ejecting a phenyl radical or a methyl radical. Because a methyl radical is slightly more stable than a highly unstable phenyl radical, it ejects methyl, leaving behind acetophenone. A stepwise start followed by $\beta$-scission is Pathway R.

By meticulously analyzing the stability of the radicals formed at every step, we have successfully mapped the thermal destiny of each peroxyester. The final matrix match is a testament to the elegant logic of organic reaction mechanisms.