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A nucleophilic substitution replaces a leaving group by a nucleophile. In a concerted SN2 reaction, bond formation and departure occur in the same elementary step, commonly with inversion at the attacked stereocenter.

An SN1 reaction instead forms an intermediate before capture, often a carbocation, so rearrangement and a different stereochemical outcome may be possible. Substrate structure, solvent, leaving-group ability, and nucleophile properties influence which pathway is available. The substitution product alone does not uniquely identify the mechanism.

The primary chloride undergoes an SN2 reaction; the triphenylmethyl chloride undergoes an SN1 reaction. The primary site is accessible to back-side attack, whereas three phenyl groups make back-side attack at the triphenylmethyl site extremely hindered and stabilize its carbocation through resonance.

For the SN2 reaction, a suitable transition state isThe entering C and leaving Cl lie on opposite sides of the reacting carbon, with partial C–C and C–Cl bonds. The total charge is $-1$. This transition state is not a discrete reaction intermediate.

For the SN1 reaction, the approximately planar $\mathrm{P}{\mathrm{h}}_3{\mathrm{C}}^{+}$ carbocation is a genuine reaction intermediate. Assuming its formation by C–Cl heterolysis is rate determining and trapping is rapid,Thus the limiting rate law is independent of ethanol concentration. The successive attack and proton transfer occur after the slow ionization; a cation-stabilizing environment and efficient trapping underpin this idealized SN1 description.

The drawn aryl dichloride is expected to undergo no analogous substitution under ordinary ethoxide conditions. Its chlorines are attached directly to sp2 aromatic carbons, rather than to the benzylic sp3 carbon of the previous substrate.

A normal SN2 reaction cannot follow the accessible backside geometry available at a primary benzylic carbon. A simple SN1 reaction would require a highly unfavorable aryl cation. Addition–elimination aromatic substitution would need suitable ring activation, such as a strong electron-withdrawing group in an appropriate position, which the drawing lacks. Forcing aryne chemistry cannot be inferred from ethoxide alone.

The conclusion addresses the actual E-plus-ethoxide scheme shown. The accompanying reference to replacing product B is inconsistent with the preceding substrate labels; interpreting it literally would not define the intended substitution comparison.