What does the '1' in $S_N1$ represent regarding the reaction kinetics?

The '1' stands for unimolecular, meaning the rate of the reaction depends on the concentration of only one reactant (the substrate). This occurs because the rate-determining step involves only the heterolytic fission of the substrate's bond.

How does the number of steps differ between $S_N1$ and $S_N2$ mechanisms?

$S_N1$ is a two-step mechanism involving a carbocation intermediate, whereas $S_N2$ is a one-step, concerted mechanism where bond-breaking and bond-forming happen simultaneously.

Why are tertiary halogenoalkanes more likely to undergo $S_N1$ reactions than primary ones?

Tertiary halogenoalkanes form tertiary carbocations, which are stabilized by the positive inductive effect of three alkyl groups. Primary carbocations lack this stabilization and are too high in energy to form easily.

What is the rate equation for an $S_N2$ reaction involving bromoethane and hydroxide ions?

The rate equation is $Rate = k[CH_3CH_2Br][OH^-]$. Because it is a bimolecular reaction, the rate depends on the concentrations of both the halogenoalkane and the nucleophile.

What is a common mistake students make when interpreting the '2' in $S_N2$?

Students often incorrectly assume the '2' means the reaction occurs in two steps. In reality, $S_N2$ occurs in a single concerted step; the '2' refers to the two molecules colliding in that step.

In an $S_N2$ reaction, why is the 'backside attack' necessary?

The nucleophile must attack from the side opposite the leaving group to avoid electronic repulsion from the leaving group's lone pairs and to properly align orbitals for the new bond to form as the old one breaks.

Which halogenoalkane reacts fastest in nucleophilic substitution: fluoroethane or iodoethane?

Iodoethane reacts fastest. The $C-I$ bond is the longest and weakest among the halogens, requiring the least energy to break during the rate-determining step.

What happens to the rate of an $S_N1$ reaction if the concentration of the nucleophile is tripled?

The rate remains unchanged. In an $S_N1$ mechanism, the nucleophile is not involved in the slow, rate-determining step, so its concentration does not affect the overall reaction velocity.

Define 'heterolytic fission' in the context of $S_N1$ reactions.

Heterolytic fission is the breaking of a covalent bond where both electrons from the bond are taken by one of the atoms (the leaving group), resulting in the formation of a cation and an anion.

Why does steric hindrance inhibit $S_N2$ reactions in tertiary substrates?

Tertiary substrates have three bulky alkyl groups surrounding the central carbon, which physically block the nucleophile from reaching the carbon atom to initiate the backside attack.

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Chemistry

15. Halogen Compounds

SN1 & SN2

Summary

Nucleophilic substitution reactions ( $S_N1$ and $S_N2$ ) are fundamental mechanisms in organic chemistry where a nucleophile replaces a leaving group on a carbon atom. The choice between these pathways is primarily determined by the structure of the substrate, the stability of potential intermediates, and the concentration of reactants, which dictates the kinetic order of the reaction.

1. Definition & Core Concepts

2. The $S_N1$ Mechanism (Unimolecular)

Two-Step Process: The reaction occurs in two distinct stages. First, the bond between the carbon and the leaving group breaks heterolytically to form a carbocation intermediate; second, the nucleophile attacks this carbocation.
Rate-Determining Step: The first step (formation of the carbocation) is the slowest. Consequently, the rate depends only on the concentration of the halogenoalkane: $Rate = k[RX]$ .
Substrate Preference: This mechanism is favored by tertiary halogenoalkanes. The three surrounding alkyl groups stabilize the positive charge on the carbocation through the positive inductive effect, where electron density is pushed toward the electron-deficient center.

Energy profile diagram of an SN1 reaction showing two transition states and a stable carbocation intermediate.

3. The $S_N2$ Mechanism (Bimolecular)

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions