Animated Solution for Chemistry - Organic Chemistry: For the given reaction, What is A?

When you first look at this reaction, you might be tempted to think about electrophilic aromatic substitution. After all, we have a benzene ring and bromine. But wait! The presence of UV light ($h u$) completely changes the game.

UV light is the classic trigger for a free radical substitution mechanism. Instead of attacking the electron-rich benzene ring, the reaction will target the alkyl side chain. This is a crucial distinction that often trips up students. The UV light provides the exact energy needed to homolytically cleave the $Br-Br$ bond, generating highly reactive bromine free radicals ($B{r}^{\bullet }$).

Finding the Sweet Spot

Now that we know we are dealing with a free radical pathway, the next logical question is: Where will the radical form?

Our substrate, 3-ethylbenzonitrile, has an ethyl group ($-C{H}_{2}C{H}_3$) attached to the ring. This gives us two potential sites for hydrogen abstraction: the terminal methyl carbon ($C{H}_3$) or the benzylic carbon ($C{H}_2$) directly attached to the ring.

Nature always favors the path of least resistance, which in chemistry means the path that leads to the most stable intermediate. If a hydrogen is abstracted from the benzylic carbon, the resulting secondary radical is highly stabilized by resonance with the adjacent benzene ring. The unpaired electron can delocalize across the $\pi$-system of the ring, drastically lowering the energy of the intermediate.

Conversely, a radical on the terminal methyl group would only be a primary radical with no resonance stabilization. Therefore, the bromine radical will selectively abstract a hydrogen from the benzylic position.

The Major Product

Once the stable benzylic radical is formed, it rapidly reacts with another molecule of $B{r}_2$. The radical plucks a bromine atom, propagating the chain reaction and forming our final product: 1-bromo-1-(3-cyanophenyl)ethane.

In this molecule, the bromine atom is securely attached to the benzylic carbon. When we compare this structure to our given options, it perfectly matches option (c). The $-CN$ group remains untouched at the meta position, quietly observing the radical chemistry happening on the side chain. It's a beautiful example of how reaction conditions dictate the mechanism and ultimately, the product!