Question

Give the detailed mechanism of the reaction of bromomethane and 2 bromo-2-methylpropane with aqueous \(\mathrm{NaOH}\).

Short answer

Bromomethane undergoes an S_N2 reaction with NaOH, forming methanol, while 2-bromo-2-methylpropane undergoes an S_N1 reaction, forming tert-butanol.

Step-by-step solution

Understanding the Reactants and Conditions

Bromomethane (CH₃Br) and 2-bromo-2-methylpropane ((CH₃)₃CBr) are both alkyl halides, and they react with aqueous sodium hydroxide (NaOH). Aqueous NaOH provides hydroxide ions (OH⁻), which act as nucleophiles in substitution reactions. The solvent is polar and protic, favoring substitution reactions.

Identifying the Reaction Mechanism for Bromomethane

For bromomethane (a primary alkyl halide), the reaction with NaOH follows an S_N2 mechanism. In this mechanism, the nucleophile (OH⁻) attacks the carbon atom bonded to the bromine atom, forming a transition state where the carbon is simultaneously bonded to both OH⁻ and Br. This results in the displacement of the bromide ion, forming methanol (CH₃OH) as the product.

Explaining S_N2 Reaction Process for Bromomethane

1. The hydroxide ion approaches the carbon atom from the side opposite the bromine atom. 2. A transition state forms with the carbon atom partially bonded to both the OH⁻ and the Br atom. 3. The bromide ion leaves, resulting in the formation of methanol (CH₃OH). This is a concerted reaction, meaning that bond formation and bond breaking occur simultaneously.

Identifying the Reaction Mechanism for 2-Bromo-2-methylpropane

2-bromo-2-methylpropane is a tertiary alkyl halide, which generally reacts via an S_N1 mechanism with aqueous NaOH. In this mechanism, the C-Br bond breaks first, forming a tertiary carbocation (stabilized by the surrounding methyl groups) and a bromide ion. The OH⁻ then attacks the carbocation, forming 2-methyl-2-propanol (tert-butanol).

Explaining S_N1 Reaction Process for 2-Bromo-2-Methylpropane

1. The C-Br bond in 2-bromo-2-methylpropane breaks spontaneously, forming a stable tertiary carbocation and a Br⁻. 2. The hydroxide ion (OH⁻) rapidly attacks the positively charged carbocation. 3. Formation of 2-methyl-2-propanol (tert-butanol) as the final product occurs after the nucleophilic attack.

Key concepts

S_N2 mechanism

In chemistry, the S_N2 mechanism is a type of nucleophilic substitution reaction. It's called "S_N2" because it's a substitution nucleophilic bimolecular reaction. This means that two molecular entities are involved in the rate-determining step. In an S_N2 reaction, the nucleophile attacks the substrate, forming a transition state where the substrate is bonded to both the nucleophile and the leaving group. The key idea here is that it happens simultaneously, which is why it's known as a concerted mechanism. This type of mechanism is typical for primary alkyl halides like bromomethane. Characteristics of S_N2 mechanisms include:

• Backside Attack: The nucleophile attacks from the side opposite the leaving group, like a strategic sneak play.
• Steric Hindrance: Because the nucleophile needs to approach the carbon from the backside, less crowded carbon atoms (usually primary ones) are more reactive.
• Inversion of Configuration: The S_N2 process often results in an inversion in the stereochemistry of the substrate.

These reactions are usually seen in primary or secondary alkyl halides where less steric hindrance promotes this mechanism.

S_N1 mechanism

S_N1 mechanisms are another form of nucleophilic substitution reaction. The "S_N1" stands for substitution nucleophilic unimolecular, indicating that one molecular entity is involved in the rate-determining step. This mechanism is different from the S_N2 because it involves two distinct steps. 1. First, the leaving group departs, forming a carbocation. This is the slowest and rate-limiting step. 2. Next, the nucleophile attacks the carbocation, leading to product formation. The S_N1 mechanism is favored in tertiary alkyl halides, like 2-bromo-2-methylpropane, where the carbocation can be stabilized by surrounding alkyl groups. Characteristics of S_N1 mechanisms include:

• Formation of Carbocation: A stable carbocation is key, often stabilized by surrounding groups through hyperconjugation or resonance.
• Racemization in Chiral Centers: As the nucleophile can attack from either side of the planar carbocation, a mixture of stereochemical outcomes (racemized products) is usually obtained.
• Polar Protic Solvent: These solvents stabilize the carbocation and the leaving group, promoting this pathway.

S_N1 reactions thus involve a two-step process where the formation and stability of the carbocation are crucial.

Alkyl halides

Alkyl halides, also known as haloalkanes, are compounds made up of an alkyl group attached to a halogen atom. Common halogens include fluorine, chlorine, bromine, and iodine. These compounds are known for their reactivity, primarily because of the polarized carbon-halogen bond. In this bond, the carbon atom has a slight positive charge (δ⁺) while the halogen has a small negative charge (δ^-). Important points about alkyl halides:

• Classification: They are classified based on the carbon atom bonded to the halogen. If the carbon is primary (connected to only one other carbon), secondary, or tertiary, it affects their reactivity in nucleophilic substitution.
• Influence on Reaction Mechanism: Primary alkyl halides often undergo S_N2 reactions, while tertiary alkyl halides favor S_N1 mechanisms.
• Solvent Effects: The choice of solvent can greatly influence whether a substrate undergoes an S_N1 or S_N2 reaction. Polar protic solvents are better for S_N1, while polar aprotic solvents favor S_N2 processes.

Alkyl halides are versatile in reactions, making them pivotal in organic synthesis applications.

Reaction mechanisms

Understanding reaction mechanisms is essential in chemistry as it describes the step-by-step sequence through which reactants turn into products. A reaction mechanism shows every single change in bonds, atoms, and electrons within the transformation process. Different types of mechanisms can explain similar reactions depending on the reactant's nature and conditions:

• Nucleophilic Substitution: This is a common type, where a nucleophile replaces a substituent in a molecule. The S_N1 and S_N2 mechanisms are classic examples.
• Factors Determining the Pathway: Factors such as steric hindrance, solvent effects, and the nature of the substrate influence whether a reaction will follow an S_N1 or S_N2 path.
• Transition States and Intermediates: These are the high-energy states and intermediates that occur during the reaction. For instance, the S_N2 mechanism involves a direct transition state, while S_N1 includes a stable intermediate carbocation.

Understanding these mechanisms allows chemists to predict reaction products, improve yields, and develop new synthetic pathways.