Nucleophilic substitution reactions are fundamental processes in organic chemistry where a nucleophile (an electron-rich species) replaces a leaving group on a carbon atom. These reactions are categorized into two major types: unimolecular (\(\text{S}_{\text{N}}1\)) and bimolecular (\(\text{S}_{\text{N}}2\)) substitution. The \(\text{S}_{\text{N}}2\) mechanism is called “bimolecular” because both the nucleophile and the substrate are involved in the rate-determining step. A tertiary carbon is bonded directly to three other carbon atoms. The structure of this central carbon strongly influences which substitution pathway can occur, raising the question: can \(\text{S}_{\text{N}}2\) substitution successfully take place on a tertiary carbon atom?
Understanding the \(\text{S}_{\text{N}}2\) Mechanism
The \(\text{S}_{\text{N}}2\) reaction proceeds through a single, concerted step where bond-breaking and bond-forming occur simultaneously. The incoming nucleophile must attack the carbon atom from the side directly opposite the departing leaving group, a process known as backside attack. This specific geometry is required because the nucleophile must interact with the carbon-leaving group bond’s lowest unoccupied molecular orbital, which is located 180 degrees away from the leaving group.
This simultaneous action results in the formation of a high-energy, transient species called the transition state. In this state, the central carbon is temporarily bonded to five groups: the nucleophile, the leaving group, and the three other substituents. This gives the carbon a brief trigonal bipyramidal geometry.
Because the nucleophile approaches from the opposite side, the spatial arrangement of the three other groups on the carbon atom flips over. This change in configuration is referred to as Walden inversion, and the complete inversion of stereochemistry is a defining feature of the \(\text{S}_{\text{N}}2\) mechanism. The entire process requires a clear, unobstructed path for the nucleophile to reach the reaction center.
The Role of Steric Hindrance at Tertiary Carbons
The feasibility of the \(\text{S}_{\text{N}}2\) reaction depends heavily on the spatial availability around the carbon atom connected to the leaving group. This is due to the requirements of the backside attack and the formation of the five-coordinate transition state. Tertiary carbons are attached to three bulky alkyl groups, which physically crowd the reaction center.
These three alkyl groups create a massive physical barrier that effectively blocks the approach of the incoming nucleophile. The nucleophile must get close enough to the central carbon to begin forming the new bond, but the surrounding groups obstruct this path. This physical obstruction is known as steric hindrance.
The increased steric crowding significantly raises the energy of the required transition state. Forcing five substituents—including three large alkyl groups—into the small space required for the transition state makes the energy barrier prohibitively high. Consequently, the reaction rate for \(\text{S}_{\text{N}}2\) on a tertiary substrate is reduced to virtually zero.
The reactivity trend for the \(\text{S}_{\text{N}}2\) pathway shows methyl substrates reacting fastest, followed by primary, then secondary, with tertiary substrates showing almost no reaction. This trend correlates directly with the increasing size and number of alkyl groups surrounding the reaction site. Therefore, the \(\text{S}_{\text{N}}2\) reaction does not occur to any measurable extent on a typical tertiary carbon.
The Preferred Reaction: \(\text{S}_{\text{N}}1\) Substitution
Since the \(\text{S}_{\text{N}}2\) pathway is blocked by steric hindrance, the \(\text{S}_{\text{N}}1\) mechanism predominates for tertiary substrates. This reaction follows a two-step process that avoids the spatial constraints of the bimolecular pathway. The first and slowest step is the spontaneous departure of the leaving group to form a planar, positively charged carbocation intermediate.
Tertiary substrates favor this mechanism because of the stability of this intermediate. Tertiary carbocations are the most stable type, as they have three electron-donating alkyl groups attached to the positively charged carbon. This stability lowers the energy required for the first, rate-determining step of the \(\text{S}_{\text{N}}1\) reaction.
The second, much faster step involves the nucleophile attacking the planar carbocation. Because the nucleophile attacks an intermediate that has already lost the leaving group, it can approach from either the front or the back face of the planar structure. This means the \(\text{S}_{\text{N}}1\) mechanism is not affected by the steric bulk that prevented the \(\text{S}_{\text{N}}2\) reaction. The structure that makes the \(\text{S}_{\text{N}}2\) reaction impossible is precisely what makes the \(\text{S}_{\text{N}}1\) reaction highly favorable due to the resulting carbocation stability.