Q.53 - Organic Chemistry

Question

For which reaction mechanisms—${\mathbf{\text{S}}}_{\mathbf{\text{N}}}{\mathbf{\text{1, S}}}_{\mathbf{\text{N}}}\mathbf{\text{2}}$, E1, or E2—are each of the following statements true? A statement may be true for one or more mechanisms.

a. The mechanism involves carbocation intermediates.

b. The mechanism has two steps.

c. The reaction rate increases with better leaving groups.

d. The reaction rate increases when the solvent is changed from ${\mathbf{\text{CH}}}_{\mathbf{3}}\mathbf{\text{OH}}$ to ${\mathbf{\text{(CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{)}}}_{\mathbf{\text{2}}}\mathbf{\text{SO}}$.

e. The reaction rate depends on the concentration of the alkyl halide only.

f. The mechanism is concerted.

g. The reaction of ${\mathbf{\text{CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}\mathbf{\text{Br}}$ with NaOH occurs by this mechanism.

h. Racemization at a stereogenic center occurs.

i. Tertiary (3°) alkyl halides react faster than 2° or 1° alkyl halides.

j. The reaction follows a second-order rate equation.

Step-by-Step Solution

Answer

The reactions that go with the mentioned statements are written below:-

a.  ${\text{S}}_{\text{N}}\text{1}$and E1 involves a carbocation intermediate.

b. ${\text{S}}_{\text{N}}\text{1}$ and E1 involves two steps.

c.  ${\text{S}}_{\text{N}}\text{2}$,  ${\text{S}}_{\text{N}}\text{1}$, E1 and E2 reaction rate increases with the better leaving group.

d.  ${\text{S}}_{\text{N}}\text{2}$ and E2 reaction rate increases when the solvent is changed from polar protic solvent to polar aprotic solvent, i.e., from  ${\text{CH}}_2{\text{OH to (CH}}_3{\text{)}}_2\text{SO}$

e.  The reaction rate depends on the concentration of the alkyl halide only in the case of  ${\text{S}}_{\text{N}}\text{1}$ and E1.

f. The mechanism is concerted for ${\text{S}}_{\text{N}}\text{2}$ and E2.

g. The reaction of   with NaOH occurs by this mechanism is ${\text{S}}_{\text{N}}\text{2}$ and E2.

h. Racemization at a stereogenic center occurs at ${\text{S}}_{\text{N}}\text{1}$ .

i. Tertiary  alkyl halides react faster than $\text{2°}$  or  $\text{1°}$ alkyl halides in the case of ${\text{S}}_{\text{N}}\text{1}$, E1, and E2.

j. The reaction follows a second-order rate equation for ${\text{S}}_{\text{N}}\text{2}$ and E2.

S N 1 Reaction

• This is a two-steps process reaction where the formation of carbocation takes place in the intermediate step. The reaction follows first-order kinetics.
• Due to the carbocation formation in the intermediate step, the products formed are a racemic mixture. The process through which a racemic mixture is formed is known as racemization.
• The process is favorable when a weak nucleophile is used.
• The process is favorable when the solvent used is a polar protic solvent such as water.
• The rate of the reaction increases and is favorable when the reactant is a tertiary halide followed by secondary and primary halide.
• The rate of the reaction increases too when it has a better leaving group.

S N 2 Reaction

• They follow a concerted mechanism to carry out their substitution. The reaction follows second-order kinetics.
• The product formed is inverted concerning the reagent, i.e., Inversion takes place in these kinds of reactions.
• The process is favorable when a strong nucleophile is used.
• The process is favorable when the solvent used is a polar aprotic solvent such as DMSO.
• The rate of the reaction increases and is favorable when the reactant is a primary halide followed by secondary and tertiary halide.
• The rate of the reaction increases when it has a better leaving group.

E1 reaction

• This is a single-step process reaction where the formation of carbocation takes place in the intermediate step, followed by a loss of a proton and a leaving group to give an alkene as an elimination product.
• The reaction follows first-order kinetics.
• The process is favorable when a weak base is used.
• The process is favorable when the solvent used is a polar protic solvent such as water.
• The rate of the reaction increases and is favorable when the reactant is a tertiary halide followed by secondary and primary halide.
• The rate of the reaction increases too when it has a better leaving group.

E2 reaction

• This is a two-steps process reaction where the loss of a proton and a leaving group takes place to give an alkene as an elimination product. The reaction follows second-order kinetics.
• The process is favorable when a strong base is used.
• The process is favorable when the solvent used is a polar aprotic solvent such as DMSO.
• The rate of the reaction increases and is favorable when the reactant is a tertiary halide followed by secondary and primary halide.
• The rate of the reaction increases when it has a better leaving group.

Explanation

By taking into account all the key points, the following answers are done.

a. ${\text{S}}_{\text{N}}\text{1}$ and E1 involves a carbocation intermediate.

b. ${\text{S}}_{\text{N}}\text{1}$ and E1 involves two steps.

c.  ${\text{S}}_{\text{N}}\text{2}$,  ${\text{S}}_{\text{N}}\text{1}$, E1, and E2 reaction rate increases with the better leaving group.

d.  ${\text{S}}_{\text{N}}\text{2}$ and E2 reaction rate increases when the solvent is changed from polar protic solvent to polar aprotic solvent, i.e., from  ${\text{CH}}_2{\text{OH to (CH}}_3{\text{)}}_2\text{SO}$

e.  The reaction rate depends on the concentration of the alkyl halide only in the case of  ${\text{S}}_{\text{N}}\text{1}$ and E1.

f. The mechanism is concerted for ${\text{S}}_{\text{N}}\text{2}$ and E2.

g. The reaction of   with NaOH occurs by this mechanism is ${\text{S}}_{\text{N}}\text{2}$ and E2.

h. Racemization at a stereogenic center occurs at  ${\text{S}}_{\text{N}}\text{1}$.

i.  Tertiary  alkyl halides react faster than  $\text{2°}$ or  $\text{1°}$ alkyl halides in the case of  ${\text{S}}_{\text{N}}\text{1}$, E1, and E2.

j. The reaction follows a second-order rate equation for ${\text{S}}_{\text{N}}\text{2}$ and E2.