Question

From the following pairs, select the compound that would react more rapidly with bromomethane in acetone solvent. a) water or hydroxide ion b) \(\mathrm{CH}_{3} \mathrm{~S}^{-}\) or \(\mathrm{CH}_{3} \mathrm{OH}\) c) \(\mathrm{CH}_{2} \mathrm{~S}^{-}\) or \(\mathrm{CH}_{3} \mathrm{SH}\) d) acetate ion or hydroxide ion e) diethyl sulfide or diethyl ether f) dimethylamine or diethylether g) trimethylamine or 2,2 -dimethylpropane

Short answer

a) Water or hydroxide ion b) \(\mathrm{CH}_{3} \mathrm{~S}^{-}\) or \(\mathrm{CH}_{3} \mathrm{OH}\) c) \(\mathrm{CH}_{2} \mathrm{~S}^{-}\) or \(\mathrm{CH}_{3} \mathrm{SH}\) d) Acetate ion or hydroxide ion e) Diethyl sulfide or diethyl ether f) Dimethylamine or diethylether g) Trimethylamine or 2,2-dimethylpropane Answer: a) Hydroxide ion b) \(\mathrm{CH}_{3} \mathrm{~S}^{-}\) c) \(\mathrm{CH}_{2} \mathrm{~S}^{-}\) d) Hydroxide ion e) Diethyl sulfide f) Dimethylamine g) Trimethylamine

Step-by-step solution

Identify nucleophiles in each pair

Identify the nucleophilic atom in each compound of the given pairs, which is the atom with a lone pair of electrons or a negative charge capable of donating electrons to bromomethane (acting as an electrophile).

Compare nucleophilic atoms based on factors affecting nucleophilicity

Compare the nucleophilic atoms in each pair based on the following factors: row in periodic table (higher row indicates higher nucleophilicity), charge (a more negative charge indicates higher nucleophilicity), and solvent effects. Use these factors to determine which compound in each pair will react more rapidly with bromomethane.

Determine the compound that reacts more rapidly with bromomethane in each pair

Using the factors affecting nucleophilicity, select the compound in each pair that reacts more rapidly with bromomethane in acetone solvent. Here are the determinations for each pair: a) In the pair water or hydroxide ion, hydroxide ion has a higher nucleophilicity due to a negative charge. b) In the pair \(\mathrm{CH}_{3} \mathrm{~S}^{-}\) or \(\mathrm{CH}_{3} \mathrm{OH}\), \(\mathrm{CH}_{3} \mathrm{~S}^{-}\) has a higher nucleophilicity due to the negative charge and sulfur being in a higher row in the periodic table than oxygen. c) In the pair \(\mathrm{CH}_{2} \mathrm{~S}^{-}\) or \(\mathrm{CH}_{3} \mathrm{SH}\), \(\mathrm{CH}_{2} \mathrm{~S}^{-}\) has a higher nucleophilicity due to the negative charge. d) In the pair acetate ion or hydroxide ion, hydroxide ion has a higher nucleophilicity due to higher charge density on the oxygen atom. e) In the pair diethyl sulfide or diethyl ether, diethyl sulfide has a higher nucleophilicity due to sulfur being in a higher row in the periodic table than oxygen. f) In the pair dimethylamine or diethylether, dimethylamine has a higher nucleophilicity due to nitrogen being a more nucleophilic atom than oxygen in ethers. g) In the pair trimethylamine or 2,2-dimethylpropane, trimethylamine has a higher nucleophilicity due to nitrogen being a more nucleophilic atom than carbon.

Key concepts

Reactivity of Nucleophiles

Understanding the reactivity of nucleophiles is crucial in organic chemistry, especially during substitution and elimination reactions. Nucleophiles are species that donate an electron pair to an electrophile to form a chemical bond. The reactivity of a nucleophile is influenced by several factors, including the presence of negative charge, the electronegativity of the nucleophilic atom, and steric hindrance. Generally, nucleophilic reactivity increases with a higher electron density (i.e., a stronger negative charge) on the nucleophilic atom.

The solvent plays a role as well; polar protic solvents can stabilize nucleophile charges through hydrogen bonding, which can reduce their reactivity. Conversely, polar aprotic solvents like acetone don't form hydrogen bonds with the nucleophile and thus can enhance the nucleophile’s reactivity by not solvating the nucleophile as effectively. Reactions in these solvents are typically faster due to the less-stable, more reactive nucleophile.

Other molecular attributes affecting nucleophilicity include the size of the atom and orbital characteristics—the smaller the atom or the more s-character in the orbital holding the electron pair, the more nucleophilic the species will be.

Bromomethane Reactions

Bromomethane, also known as methyl bromide, is an alkyl halide that commonly undergoes nucleophilic substitution reactions (S_N1 and S_N2). The bromine atom is a good leaving group due to its size and electronegativity, making bromomethane highly reactive toward nucleophiles. In a S_N2 reaction, the nucleophile attacks the carbon bonded to bromine from the side opposite the leaving group, resulting in the inversion of configuration at the carbon atom.

Factors Affecting the Reaction with Bromomethane

When reacting with bromomethane, a nucleophile’s strength as well as steric effects can dictate the reaction rate. Stronger nucleophiles with less steric hindrance will react more rapidly. Therefore, in a given pair, the compound with a more localized negative charge and less steric bulk will generally be a better reactant with bromomethane.

For the substitution to occur effectively, the solvent must be able to dissolve both the nucleophile and the substrate, and thus the choice of solvent can greatly influence the reaction's rate.

Acetone as a Solvent

Acetone is a polar aprotic solvent, meaning it has a polar bond but no acidic hydrogen capable of hydrogen bonding. This characteristic makes it an excellent solvent for organic reactions involving nucleophiles. Acetone can dissolve a wide range of organic compounds and can stabilize the transition state of the reaction, thus speeding up the rate.

Why Use Acetone?

• It has a relatively high dielectric constant, which allows it to dissolve ionic compounds effectively.
• Acetone can stabilize carbocations in S_N1 reactions and promote the departure of the leaving group.
• In S_N2 reactions, acetone doesn’t solvate nucleophiles heavily, resulting in increased nucleophilicity and faster reaction rates.

In the context of the exercise, acetone would not hydrogen bond with negatively charged nucleophiles, such as hydroxide ion, thus maintaining their high reactivity for bromomethane substitution reactions.

Periodic Table Trends in Nucleophilicity

Nucleophilicity trends within the periodic table correlate with electron density and atomic size. As we move from right to left across a period, nucleophilicity generally increases since the electronegativity decreases, allowing nucleophiles to donate electrons more readily. Conversely, as we move down a group, nucleophilicity also increases because larger atoms can more easily donate an electron pair due to their less tightly held valence electrons.

Down the Group

For example, within the same group, a sulfide ion will generally be more nucleophilic than an oxide ion because sulfur is lower down the group than oxygen, meaning it is less electronegative and has a larger radius.

It's important to note that these trends can sometimes be overridden by solvent effects or the presence of other functional groups that can either hamper or enhance nucleophilicity.

In the given exercise, these periodic table trends were used to ascertain the more nucleophilic species between pairs of compounds, considering the solvent is acetone.