Question

Benzyl bromide reacts with \(\mathrm{H}_{2} \mathrm{O}\) in formic acid solution to yield benzyl alcohol? the rate is independent of \(\left[\mathrm{H}_{2} \mathrm{O}\right]\). Under the same conditions \(\mathrm{p}\) -methylbenzyl bromide reacts 58 times as fast. Benzyl bromide reacts with ethoxide ion in dry alcohol to yield benzyl ethyl ether \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{OC}_{2} \mathrm{H}_{5}\right) ;\) the rate depends upon both \([\mathrm{RBr}]\) and \(\left[\mathrm{OC}_{2} \mathrm{H}_{5}^{-}\right]\). Under the same conditions \(\mathrm{p}\) -methylbenzyl bromide reacts \(1.5\) times as fast. Interpret these results. What do they illustrate concerning the effect of: (a) polarity of solvent, (b) nucleophilic power of the reagent, and (c) electron release by substituents?

Short answer

The given reactions illustrate that the rate of reaction is affected by the polarity of the solvent, nucleophilic power of the reagent, and electron release by substituents. Increased polarity of the solvent and nucleophilic power of the reagent leads to faster reaction rates, while electron-releasing substituents enhance reactivity. For the given reactants, p-methylbenzyl bromide shows a higher reactivity in a polar solvent (formic acid solution) compared to a less polar solvent (dry alcohol) and with electron-releasing substituents.

Step-by-step solution

Identify the rates of reaction

For each reaction, identify the rate and the factors affecting the rate: 1. Benzyl bromide reacts with water (H2O) in formic acid solution to yield benzyl alcohol. The rate is independent of [H2O]. Under the same conditions, p-methylbenzyl bromide reacts 58 times faster. 2. Benzyl bromide reacts with ethoxide ion in dry alcohol to yield benzyl ethyl ether (C6H5CH2OC2H5); the rate depends upon both [RBr] and [OC2H5-]. Under the same conditions, p-methylbenzyl bromide reacts 1.5 times as fast.

Effect of polarity of solvent

Analyze the solvents' polarity in each reaction. Formic acid solution creates a polar atmosphere for the reaction to happen with water. Dry alcohol is less polar as compared to formic acid solution. The p-methylbenzyl bromide reacts faster in a polar solvent (formic acid solution) compared to a less polar solvent (dry alcohol). Thus, the polarity of the solvent plays a role in affecting the rate of reaction, with increased polarity leading to an increase in reaction rate for the given reactants.

Effect of nucleophilic power of the reagent

Nucleophilic power refers to the ability of a nucleophile to donate lone pair of electrons to form a bond with the electrophile. In the given reactions, the nucleophiles are water (H2O) and ethoxide ion (OC2H5-). The reaction is faster with ethoxide ion as the nucleophile compared to water. This is because ethoxide ion is more nucleophilic than water due to the presence of negative charge on the oxygen atom in ethoxide ion. Hence, the nucleophilic power of the reagent affects the rate of reaction.

Effect of electron release by substituents

To analyze the effect of electron release by substituents, compare the reactions of benzyl bromide and p-methylbenzyl bromide in both cases (H2O/formic acid solution and dry alcohol/ethoxide ion). In the H2O/formic acid solution reaction, p-methylbenzyl bromide reacts 58 times faster than benzyl bromide. The presence of a methyl group in the p-methylbenzyl bromide adds to the electron density, thus making it more reactive. In the dry alcohol/ethoxide ion reaction, p-methylbenzyl bromide reacts 1.5 times faster than benzyl bromide. This indicates that the electron releasing effect of the substituent also plays a role in this reaction, although the difference in reactivity is not as pronounced as in the polar solvent. In conclusion, the given reactions illustrate the influence of factors such as polarity of the solvent, nucleophilic power of the reagent, and electron release by substituents on the rate of reaction. Each of these factors contributes to the overall rate and reactivity in the given chemical reactions.

Key concepts

Understanding Solvent Polarity

Solvent polarity is a critical factor in determining the rate of chemical reactions. A solvent's polarity is essentially a measure of how strongly its molecules pull on electrons—in scientific terms, its dielectric constant. When a solvent is polar, it allows charged particles, like ions, to disperse more readily, which can lower the energy barrier needed for a chemical reaction to occur.

In the exercise provided, formic acid solution is used as a solvent, creating a highly polar environment suitable for the reaction between benzyl bromide and water. The polar environment stabilizes the transition state, facilitating the substitution reaction to produce benzyl alcohol. The significant increase in reaction rate with p-methylbenzyl bromide in the polar formic acid solution (by 58 times) compared to the less polar dry alcohol, implies that the higher polarity of the solvent contributes to a faster rate.

The Role of Nucleophilicity

Nucleophilicity represents the propensity of a nucleophile, which is a chemical species with a lone pair of electrons, to form a bond with an electrophile—an electron deficient species. It is contingent on several factors, including charge, electronegativity, steric hindrance, and solvent effects. A highly nucleophilic reagent, such as an ethoxide ion (OC2H5-), is more reactive due to its negative charge as compared to a neutral molecule like water (H2O).

The contrasting reaction rates between water and ethoxide ion in the exercise are indicative of the difference in nucleophilicity. In anhydrous alcohol, the ethoxide ion's reactivity is highlighted as the rate depends on both the concentration of the alkyl bromide and the ethoxide ion. This corresponds to a bimolecular reaction mechanism (SN2), wherein the nucleophilic power greatly influences the reaction's rate, as observed in the faster reaction with p-methylbenzyl bromide.

Substituent Effects on Reaction Rate

Substituent effects encompass how the presence of various substituents attached to a molecular framework can influence a molecule's reactivity. Specifically, electron-donating or electron-withdrawing groups can either stabilize or destabilize the transition state of a reaction. In the given exercise, the presence of a methyl substituent on the para position of the benzyl bromide (forming p-methylbenzyl bromide) enhances the electron density around the aromatic ring, thereby facilitating a faster reaction rate.

In an aqueous formic acid solution, the p-methyl group's electron-donating effect enhances the reaction rate significantly, by 58 times compared to benzyl bromide. Yet, in the presence of a less polar solvent and a strong nucleophile (ethoxide ion), the substituent effect is still apparent but not as pronounced (1.5 times faster). This elucidates that while the substituent effect is invariably important, its influence can be modulated by factors like solvent polarity and the strength of the nucleophile.