Animated Solution for Chemistry - Organic Chemistry: Tertiary alkyl halides are practically inert to substitution by $S_{N}2$ mechanism because of

To understand why certain molecules refuse to participate in an $S_{N}2$ reaction, we first need to understand the rules of the game. The $S_{N}2$ (Substitution Nucleophilic Bimolecular) mechanism is a concerted, single-step process. Imagine a perfectly choreographed dance: as the incoming nucleophile ($N{u}^{-}$) approaches the electrophilic carbon, the leaving group (like a bromide ion, $B{r}^{-}$) simultaneously departs.

For this to happen smoothly, the nucleophile must approach the carbon atom from the exact opposite side of the leaving group. This is famously known as the backside attack. The transition state requires the nucleophile, the central carbon, and the leaving group to align perfectly.

Now, let's look at our specific substrate: a tertiary ($3^{\circ }$) alkyl halide. In a tertiary alkyl halide, the central carbon atom is bonded to three other carbon-containing groups (alkyl groups, denoted as $-R$).

These alkyl groups are not just simple letters on a page; they are physical entities occupying three-dimensional space. They consist of carbon and hydrogen atoms, surrounded by their own dense clouds of electrons.

When a nucleophile attempts its mandatory backside attack on a tertiary alkyl halide, it runs into a massive physical barrier. The three bulky $-R$ groups act like bouncers at a club, completely blocking the entrance to the central carbon atom.

As the nucleophile tries to push through, its own electron cloud strongly repels the electron clouds of the alkyl groups. This physical crowding and the resulting severe repulsion is called steric hindrance.

Because the nucleophile simply cannot reach the central carbon to initiate the bond-forming process, the activation energy required to reach the $S_{N}2$ transition state skyrockets. Consequently, the reaction rate drops to near zero. This is why tertiary alkyl halides are practically inert to substitution via the $S_{N}2$ mechanism.

The $S_{N}1$ Alternative

Nature always finds a way. While the $S_{N}2$ door is firmly shut, tertiary alkyl halides are perfectly suited for the $S_{N}1$ mechanism. Instead of waiting for a nucleophile to force its way in, the leaving group simply leaves on its own, forming a highly stable tertiary carbocation intermediate. Once the bulky leaving group is gone, the nucleophile can easily attack the planar carbocation from either side.