List the following alkyl bromides in order of decreasing...

List the following alkyl bromides in order of decreasing reactivity in the indicated reactions. (I) <smiles>CCC(C)(C)Br</smiles> (II) $$ \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Br} $$ (a) $\mathrm{S}_{\mathrm{N}} 1$ reactivity, (b) $\mathrm{S}_{\mathrm{N}} 2$ reactivity, (c) reactivity with alcoholic $\mathrm{AgNO}_3$. (a) Reactivity for the $\mathrm{S}_{\mathrm{N}} 1$ mechanism is $3^{\circ}$ (I) $>2^{\circ}$ (III) $>1^{\circ}$ (II). (b) The reverse reactivity for $\mathrm{S}_{\mathrm{N}} 2$ reactions gives $\mathrm{I}^{\prime}$ (II) $>2^{\circ}$ (III) $>3^{\circ}$ (I). (c) $\mathrm{Ag}^{+}$catalyzes $\mathrm{S}_{\mathrm{N}} 1$ reactions and the reactivities are $3^{\circ}(\mathrm{I})>2^{\circ}$ (III) $>1^{\circ}$ (II).

List the following alkyl bromides in order of decreasing reactivity in the indicated reactions.
(I)
<smiles>CCC(C)(C)Br</smiles>
(II)
$$
\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Br}
$$
(a) $\mathrm{S}_{\mathrm{N}} 1$ reactivity, (b) $\mathrm{S}_{\mathrm{N}} 2$ reactivity, (c) reactivity with alcoholic $\mathrm{AgNO}_3$.
(a) Reactivity for the $\mathrm{S}_{\mathrm{N}} 1$ mechanism is $3^{\circ}$ (I) $>2^{\circ}$ (III) $>1^{\circ}$ (II).
(b) The reverse reactivity for $\mathrm{S}_{\mathrm{N}} 2$ reactions gives $\mathrm{I}^{\prime}$ (II) $>2^{\circ}$ (III) $>3^{\circ}$ (I).
(c) $\mathrm{Ag}^{+}$catalyzes $\mathrm{S}_{\mathrm{N}} 1$ reactions and the reactivities are $3^{\circ}(\mathrm{I})>2^{\circ}$ (III) $>1^{\circ}$ (II).

Key Concepts

Steric Hindrance

Steric hindrance refers to the physical crowding around the reactive center, which can hinder the approach of a nucleophile in SN2 reactions. A highly substituted center creates significant spatial resistance, reducing the reaction rate, whereas less substituted centers offer a more accessible target for the nucleophile.

Silver Ion Effects in Substitution Reactions

Silver ions (Ag+) can promote substitution reactions by interacting with the leaving group, often forming a precipitate that drives the reaction forward. This is particularly useful in reactions where the rate-determining step involves the departure of the leaving group and can help stabilize or sequester halide ions.

Carbocation Stability

Carbocation stability is a critical factor in determining the reactivity in SN1 reactions. Tertiary carbocations are stabilized by hyperconjugation and inductive effects from adjacent alkyl groups, which makes their formation more favorable compared to secondary or primary carbocations, thereby accelerating the reaction.

SN2 Reaction Mechanism

The SN2 mechanism is a bimolecular nucleophilic substitution that happens in a single concerted step where the nucleophile attacks the substrate from the backside as the leaving group is expelled. This reaction is sensitive to steric hindrance; less hindered (typically primary) substrates react faster because they allow more efficient nucleophilic attack.

SN1 Reaction Mechanism

The SN1 mechanism is a unimolecular nucleophilic substitution that occurs in two distinct steps: first, the leaving group departs to form a carbocation, and then the nucleophile attacks the carbocation. This pathway is highly dependent on the stability of the carbocation intermediate, which is usually greater in more substituted (such as tertiary) carbon centers.

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