Rates of reaction, redox reactions? 10 points?

An structural isomer of bromobutane (C4H9Br) can be hydrolysed using aqueous sodium hydroxide to produce butanol. This can be represented by the following equation:

C4H9Br(l) + OH-(aq) > C4H9OH(l) + Br-(aq)

This reaction was investigated experimentally and the following results
were obtained:

Experiment A -
Initial [bromobutane], mol dm-3 = 0.01
Initial [OH-], mol dm-3 = 0.01
Initial rate, mol dm-3s-1 = 4.3 x 10 -4^

Experiment B
Initial [bromobutane], mol dm-3 = 0.01
Initial [OH-], mol dm-3 = 0.02
Initial rate, mol dm-3s-1 = 8.6 x 10-4^

Experiment C -
Initial [bromobutane], mol dm-3 = 0.02
Initial [OH-], mol dm-3 = 0.02
Initial rate, mol dm-3s-1 = 1.7 x 10-3^

(b) (i) Deduce the overall order of the reaction and write the rate equation for this reaction

(ii) What is meant by the rate-limiting step of a reaction mechanism?

(iii) What can you say about the rate-limiting step for the above reaction?

(iv) Suggest which isomer of bromobutane is likely to react via this rate-limiting step. Explain your answer fully.

Look at the three experiments. Note that A and B have the same concn bromobutane but OH is twice as much in B as in A. Note the rate change is by a factor of two also. 2^x = 2. Of course x = 1 so the reaction is first order with respect to OH^-. Do the same for bromobenzene. Then the overall order is the sum of the two orders.

The rate equation is
rate = k[bromomenzene][OH^-]

why do the same for bromomenzene? it sayes to do so for bromobutane?

My typo. My brain got bromobutane and bromobenzen mixed up and bromobutane got lost.

You have the order for OH. Find the order for bromobutane the same way. Then
rate = k[bromobutane]^x[OH^-]^y
where x and y are the orders. I've already shown y is 1. You need to do x. The overall order is x+y.

Thank you soo much! Really appreciate it. Could you also help with iv please? Thank you.

To answer the questions (b) (i) to (iv), we need to analyze the given experimental data and understand the concepts of rates of reaction and the rate-limiting step.

(b) (i) Deduce the overall order of the reaction and write the rate equation for this reaction:

To determine the overall order of the reaction, we need to examine the effect of changing the concentrations of the reactants on the initial rate.

Let's compare experiments A and B, where the concentration of bromobutane is the same (0.01 mol dm-3) but the concentration of hydroxide ion (OH-) is different. The initial rate doubles when the concentration of OH- doubles. This indicates that the rate is directly proportional to the concentration of OH-, so the reaction is first order with respect to OH-.

Next, let's compare experiments A and C, where the concentration of OH- is the same (0.01 mol dm-3), but the concentration of bromobutane doubles. The initial rate quadruples when the concentration of bromobutane doubles. This suggests that the rate is proportional to the square of the concentration of bromobutane, so the reaction is second order with respect to bromobutane.

Since the overall order of a reaction is the sum of the individual orders, the overall order for this reaction is first order with respect to OH- and second order with respect to bromobutane.

Thus, the rate equation for this reaction is: Rate = k * [C4H9Br]^2 * [OH-]

(ii) What is meant by the rate-limiting step of a reaction mechanism?

The rate-limiting step is the slowest step in a reaction mechanism that determines the overall rate of the reaction. It is usually characterized by having the highest activation energy or the slowest reaction rate among all the steps.

(iii) What can you say about the rate-limiting step for the above reaction?

Without knowing the complete reaction mechanism of the hydrolysis of bromobutane, we cannot definitively identify the rate-limiting step. However, based on the given information, we can make an assumption.

In the rate equation, the concentration of OH- appears to be first order, while the concentration of bromobutane appears to be second order. This suggests that the step involving OH- is likely to be faster than the step involving bromobutane.

(iv) Suggest which isomer of bromobutane is likely to react via this rate-limiting step. Explain your answer fully.

Since we assumed that the step involving OH- is faster, it is reasonable to assume that the hydrolysis of bromobutane occurs via an SN2 (substitution nucleophilic bimolecular) mechanism. In SN2 reactions, the rate is influenced by steric hindrance. Bulky groups near the site of substitution hinder the reaction by making it difficult for the nucleophile (OH-) to approach the carbon atom.

In general, neopentyl bromide (2,2-dimethylpropyl bromide) tends to react slower compared to other isomers of bromobutane because it has four bulky methyl groups attached to the carbon atom bearing the bromine. The steric hindrance caused by these groups makes it more difficult for OH- to approach the carbon atom, hence reducing the rate of the reaction.

Therefore, the isomer of bromobutane likely to react via the rate-limiting step is neopentyl bromide (2,2-dimethylpropyl bromide).