Give differences between SN 1 and SN 2 transition states. 1. In the SN 1 TS there is considerabl

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Give differences between SN 1 and SN 2 transition states. 1....

Give differences between $\mathrm{S}_{\mathrm{N}} 1$ and $\mathrm{S}_{\mathrm{N}} 2$ transition states. 1. In the $\mathrm{S}_{\mathrm{N}} 1$ TS there is considerable positive charge on $\mathrm{C}$; there is much weaker bonding between the attacking group and leaving groups with $\mathrm{C}$. There is little or no charge on $\mathrm{C}$ in the $\mathrm{S}_{\mathrm{N}} 2 \mathrm{TS}$. 2. The $\mathrm{S}_{\mathrm{N}} 1$ TS is approached by separation of the leaving group; the $\mathrm{S}_{\mathrm{N}} 2$ TS by attack of :Nu${ }^{-}$or :Nu. 3. The $\Delta H^*$ of the $\mathrm{S}_{\mathrm{N}} \mathrm{I}$ TS (and the rate of the reaction) depends on the stability of the incipient $\mathrm{R}^{+}$. When $\mathrm{R}^{+}$ is more stable, $\Delta H^{\ddagger}$ is lower and the rate is greater. The $\Delta H^{\ddagger}$ of the $\mathrm{S}_{\mathrm{N}} 2$ TS depends on the steric effects. When there are more R's on the attacked $\mathrm{C}$ or when the attacking : $\mathrm{Nu}^{-}$is bulkier, $\Delta H^{\ddagger}$ is greater and the rate is less.

Give differences between $\mathrm{S}_{\mathrm{N}} 1$ and $\mathrm{S}_{\mathrm{N}} 2$ transition states.
1. In the $\mathrm{S}_{\mathrm{N}} 1$ TS there is considerable positive charge on $\mathrm{C}$; there is much weaker bonding between the attacking group and leaving groups with $\mathrm{C}$. There is little or no charge on $\mathrm{C}$ in the $\mathrm{S}_{\mathrm{N}} 2 \mathrm{TS}$.
2. The $\mathrm{S}_{\mathrm{N}} 1$ TS is approached by separation of the leaving group; the $\mathrm{S}_{\mathrm{N}} 2$ TS by attack of :Nu${ }^{-}$or :Nu.
3. The $\Delta H^*$ of the $\mathrm{S}_{\mathrm{N}} \mathrm{I}$ TS (and the rate of the reaction) depends on the stability of the incipient $\mathrm{R}^{+}$. When $\mathrm{R}^{+}$ is more stable, $\Delta H^{\ddagger}$ is lower and the rate is greater. The $\Delta H^{\ddagger}$ of the $\mathrm{S}_{\mathrm{N}} 2$ TS depends on the steric effects. When there are more R's on the attacked $\mathrm{C}$ or when the attacking : $\mathrm{Nu}^{-}$is bulkier, $\Delta H^{\ddagger}$ is greater and the rate is less.

Key Concepts

Transition State Structure

The SN1 transition state is characterized by the beginning of the bond-breaking process, where the leaving group departs to form a carbocation intermediate. In contrast, the SN2 transition state is a concerted process where bond formation with the nucleophile and bond breaking with the leaving group occur simultaneously in a single, concerted step.

Charge Distribution

In the SN1 transition state, there is a significant development of positive charge on the carbon atom because the leaving group is departing and the carbocation character is emerging. Conversely, in the SN2 transition state the charge distribution is more balanced, as the nucleophile begins to bond with the carbon while the leaving group is leaving, resulting in less charge accumulation on the carbon.

Activation Energy Determinants

For SN1 reactions, the activation energy depends largely on the stability of the incipient carbocation; a more stable carbocation lowers the activation barrier and speeds up the reaction. In SN2 reactions, the activation energy is influenced by steric effects around the electrophilic carbon; increased steric hindrance raises the energy barrier by making the approach of the nucleophile more difficult.

Mechanistic Pathways

SN1 reactions proceed via a two-step mechanism where the rate-determining step is the formation of the carbocation intermediate. On the other hand, SN2 reactions follow a single-step, bimolecular mechanism where bond making and bond breaking occur simultaneously, directly leading to the product.

Steric Effects

Steric hindrance has a minimal impact on SN1 reactions since the key step involves the formation of a planar carbocation intermediate. However, in SN2 reactions, steric effects are crucial because bulky groups around the carbon center hinder the backside attack by the nucleophile, thereby slowing down the reaction.

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