Consider the reaction sequence given below :
Which of the following statements is true?
(a) Changing

Q: Why does Reaction 1 only depend on the substrate?

Because the rate-determining step is the slow ionization of the C-Br bond to form a carbocation. The nucleophile attacks in a subsequent fast step, so its concentration doesn't affect the overall speed.

Q: Why does Reaction 2 depend on both the substrate and the base?

Because it follows an $E_2$ mechanism, which is a concerted process. The base attacks the proton at the exact same time the leaving group departs. Both molecules must collide for the reaction to happen.

Q: Why does changing the base to $^\ominus OR$ affect Reaction 2?

An alkoxide ion ($^\ominus OR$) is generally a stronger base than a hydroxide ion ($OH^\ominus$). A stronger base will pull off the proton more aggressively, increasing the rate of the $E_2$ elimination.

Q: Why did the solvent change from $H_2O$ to $C_2H_5OH$ cause a change in mechanism?

Water is highly polar and protic, which excellently stabilizes the carbocation intermediate, favoring the $S_N1$ pathway. Ethanol is less polar, and when combined with a strong base, it favors the concerted $E_2$ elimination pathway.

Q: Can we force Reaction 1 to depend on the base?

No. As long as the reaction proceeds strictly via the $S_N1$ mechanism, the base is not involved in the slow step. To make it depend on the base, you would have to change the conditions to favor an $S_N2$ or $E_2$ mechanism.

Question

Consider the reaction sequence given below :
Which of the following statements is true?

Options:

(A) Changing the base from $OH^\ominus$ to $^\ominus OR$ will have no effect on reaction (2)

(B) Changing the concentration of base will have no effect on reaction (1)

(C) Doubling the concentration of base will double the rate of both the reactions

(D) Changing the concentration of base will have no effect on reaction (2)

Sigma Percentile · Accepted Answer

### The Tale of Two Pathways: $S_N1$ vs $E_2$

Imagine you have a molecule of tertiary butyl bromide ($t$-BuBr). It is a bulky, sterically hindered substrate. When you introduce a base like the hydroxide ion ($OH^\ominus$), two completely different chemical stories can unfold depending on the environment.

### The $S_N1$ Pathway: A Solo Journey

In Reaction 1, the solvent is water ($H_2O$), which is highly polar and protic. This environment stabilizes ions beautifully. The rate law given is:

$$	ext{Rate}_1 = k[t	ext{-BuBr}]$$

Notice something missing? The concentration of the hydroxide ion is nowhere to be found in the rate equation! This is the hallmark of a **first-order, unimolecular nucleophilic substitution ($S_N1$)** reaction.

In this mechanism, the rate-determining step is the slow, agonizing departure of the bromide ion to form a tertiary carbocation. The base is just waiting in the wings. Because the base is not involved in this critical slow step, **changing the concentration of the base will have absolutely no effect on the rate of Reaction 1**.

### The $E_2$ Pathway: The Concerted Attack

Now, let's look at Reaction 2. The solvent is ethanol ($C_2H_5OH$), which is less polar. The rate law changes dramatically:

$$	ext{Rate}_2 = k[t	ext{-BuBr}][OH^\ominus]$$

This is a **second-order, bimolecular elimination ($E_2$)** reaction. Here, the hydroxide ion doesn't wait. It aggressively attacks a beta-proton at the exact same time the bromide ion leaves. It is a concerted, synchronized dance. Because the base is actively participating in the rate-determining step, **any change in the concentration of the base will directly affect the rate of Reaction 2**.

### Evaluating the Options

Armed with this kinetic knowledge, the options become clear:

*   **(a)** Changing the base to a stronger alkoxide ($^\ominus OR$) would definitely speed up the $E_2$ reaction. (False)
*   **(b)** Changing the concentration of the base has no effect on the $S_N1$ reaction (Reaction 1) because the base is not in the rate law. **(True)**
*   **(c)** Doubling the base concentration will only double the rate of Reaction 2, not Reaction 1. (False)
*   **(d)** Changing the base concentration will absolutely affect Reaction 2. (False)

Therefore, the correct statement is that changing the concentration of the base will have no effect on Reaction 1.

Animated Solution for Chemistry - Organic Chemistry: Consider the reaction sequence given below : Which of the following statements is true?

Select Answer:

Visualized Solution

$A na l y z in g t h e G i v e n R e a c t i o n s$

$K in e t i cs o f R e a c t i o n 1$

$K in e t i cs o f R e a c t i o n 2$

$E v a l u a t in g t h e O pt i o n s$

$C o n c l u s i o n$

The Sigma Insight: Haloalkanes & Haloarenes

The Tale of Two Pathways: $S_{N} 1$ vs $E_{2}$

The $S_{N} 1$ Pathway

The $E_{2}$ Pathway

Evaluating the Options

Similar Questions

For the reaction sequence given below, the correct statement(s) is(are) (In the options, X is any atom other than carbon and hydrogen, and it is different in P, Q and R)

In the following reaction sequence, the correct structure(s) of X is (are)

The product $A$ and $B$ formed in above reactions are

Consider the following reactions : Which of these reactions are possible?

For the given reaction, What is A?

For the following compounds, the correct statement(s) with respect of nucleophilic substitution reactions is(are):

The product of the reaction given below is

Identify $A$ and $B$ in the chemical reaction.

The major product of the following reaction is

The major product of the following reaction is

Animated Solution for Chemistry - Organic Chemistry: Consider the reaction sequence given below : Which of the following statements is true?

Select Answer:

Visualized Solution

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Analyzing the Given Reactions

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Kinetics of Reaction 1

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Kinetics of Reaction 2

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Evaluating the Options

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Conclusion

The Sigma Insight: Haloalkanes & Haloarenes

The Tale of Two Pathways: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } SN​1 vs .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } E2​

The .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } SN​1 Pathway

The .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } E2​ Pathway

Evaluating the Options

Similar Questions

For the reaction sequence given below, the correct statement(s) is(are) (In the options, X is any atom other than carbon and hydrogen, and it is different in P, Q and R)

In the following reaction sequence, the correct structure(s) of X is (are)

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } The product A and B formed in above reactions are

Consider the following reactions : Which of these reactions are possible?

For the given reaction, What is A?

For the following compounds, the correct statement(s) with respect of nucleophilic substitution reactions is(are):

The product of the reaction given below is

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Identify A and B in the chemical reaction.

The major product of the following reaction is

The major product of the following reaction is

$A na l y z in g t h e G i v e n R e a c t i o n s$

$K in e t i cs o f R e a c t i o n 1$

$K in e t i cs o f R e a c t i o n 2$

$E v a l u a t in g t h e O pt i o n s$

$C o n c l u s i o n$

The Tale of Two Pathways: $S_{N} 1$ vs $E_{2}$

The $S_{N} 1$ Pathway

The $E_{2}$ Pathway

The product $A$ and $B$ formed in above reactions are

Identify $A$ and $B$ in the chemical reaction.