The S_N1 mechanism involves the formation of a carbocation intermediate in the rate-determining step. The most stable carbocation will produce the fastest reaction. We can immediately eliminate any answer choices that will produce primary or secondary carbocations, since a tertiary carbocation will be much more stable. When comparing tertiary carbocations, larger and more electronegative substituents will allow for more charge stabilization.

Since the tertiary carbocation formed by the dissociation of iodide from will the be most stable, this substrate will react the fastest.

The rate of an  reaction is determined only by the concentration of the substrate. Unlike an  reaction, where the addition occurs in one step and requires the activity of the substrate and the nucleophile, an  reaction occurs in two steps and is only limited by the activity (i.e. leaving ability) of the substrate. Once the leaving group leaves the substrate, the nucleophile does not hesitate to attack the exposed carbocation.

Keep in mind that after the aldehyde is reduced into an alcohol, the molecule can undergo an intramolecular reaction, as alcohol is a good nucleophile and the halogen is a stellar leaving group.

A strong nucleophile is not required for SN1. A weak nucleophile may be used. Remember that the SN1 mechanism goes through a carbocation intermediate.

Rearrangements are possible for SN1 reactions (not SN2). A rearrangement will occur to create a more stable intermediate in the mechanism. For example, if the carbocation is secondary, a methyl shift may occur to make the carbocation intermediate tertiary.

A racemic mixture of products occurs when with the nucleophile may attack the carbocation from either the top face or bottom face. When a reaction goes through a carbocation intermediate, as in SN1, there may be a racemic mix of products.

SN1 is unimolecular, and the rate of the reaction is determined by the substrate and reaction constant.

SN1 reactions result in racemization when the nucleophile has a 50% chance of attacking the carbocation intermediate from the top face, and a 50% chance of attacking from the bottom face. SN1 reactions are favored in polar protic solvents, such as ethanol.

E2 and E1 are incorrect as they are elimination reaction mechanisms, and we are looking for a substitution mechanism. SN2 reactions result in inversion, not racemization. Additionally we know that SN2 is incorrect because SN2 is favored in polar aprotic solvents.

S_N2 reactions involve a backside nucleophilic attack on an electrophilic carbon. As a result, less steric congestion for this backside attack results in a faster reaction, meaning that S_N2 reactions proceed fastest for primary carbons. In addition, beta-branching next to a primary carbon results in a slower reaction, as does a poorer leaving group (i.e. chloride instead of bromide).

1-bromopentane has a good leaving group (bromine) attached to a primary carbon with no beta-branching, meaning it will proceed the fastest.

The reason that the alkyl halide is preferred to be primary is because the mechanism for these reactions is SN2. SN2 indicates a substitution reaction that takes place in one step. A primary alcohol is preferred to prevent steric congestion caused by the simultaneous binding of the nucleophile and release of the leaving group. This reaction mechanism is faster because it omits the formation of a carbocation intermediate.

In contrast, SN1 reactions take place in two steps and involve the formation of a carbocation intermediate.

S_N1 reactions are characterized by two distinct steps. The first step, which determines the rate of the reaction, is the dissociation of the leaving group. This step leaves behind a carbocation intermediate.

As opposed to S_N2 reactions, in which nucleophilic substitution occurs in one step, the temporary formation of a carbocation in S_N1 reactions allows for carbocation rearrangement, which serves to stabilize the positive charge.

The major product, molecule IV, results from the shift of a hydrogen atom from the adjacent carbon, moving the positive charge to a carbon with greater alkyl substitution. Electron density is inducted to the secondary carbocation (bound to two alkyl groups), stabilizing the positive charge. Carbocation rearrangement occurs extremely fast, usually before a nucleophile (in this case water) may bind.

As a result, molecule IV is the major product instead of molecule II (the S_N2 product).

This is an SN2 reaction. When there is a methyl halide with a strong nucleophile, the nucleophile will force the halide group to leave. Strong nucleophiles dictate SN2 reaction mechanisms.

The sterochemistry should be inverted for an SN2 reaction. The product is has S chirality , so the starting material should have R.