91Ó°ÊÓ

Consider the following two-step reaction:

a. How many bonds are broken and formed in Step [1]? Would you predict $\mathbf{∆}\mathbf{H}\mathbf{°}$of Step [1] to be positive or negative?

b. How many bonds are broken and formed in Step [2]? Would you predict the $\mathbf{∆}\mathbf{H}\mathbf{°}$of Step [2] to be positive or negative?

c. Which step is rate-determining?

d. Draw the structure for the transition state in both steps of the mechanism.

e. If ${\mathbf{∆}\mathbf{H}\mathbf{°}}_{\mathbf{overall}}$is negative for this two-step reaction, draw an energy diagram illustrating all of the information in parts (a)–(d).

Indicate which factors affect the rate of a reaction.

1. $\mathbf{∆}\mathbf{G}\mathbf{°}$
   
2. $\mathbf{∆}\mathbf{H}\mathbf{°}$
3. ${\mathbf{E}}_{\mathbf{a}}$
4. Temperature
5. concentration
6. width="28">Keq
   
7. k
8. Catalysts

The following is a concerted, bimolecular reaction:${\mathbf{CH}}_{\mathbf{3}}\mathbf{+}\mathbf{NaCN}\mathbf{→}{\mathbf{CH}}_{\mathbf{3}}\mathbf{CN}\mathbf{+}\mathbf{NaBr}$.

a. What is the rate equation for this reaction?

b. What happens to the rate of the reaction if$\left[{\mathrm{CH}}_3\mathrm{Br}\right]$ is doubled?

c. What happens to the rate of the reaction if [NaCN] is halved?

d. What happens to the rate of the reaction if $\left[{\mathrm{CH}}_3\mathrm{Br}\right]$ and [NaCN] are both increased by a factor of five?

Follow the curved arrows and draw the products of the following reaction.

The conversion of acetyl chloride to methyl acetate occurs via the following two-step mechanism:

a. Add curved arrows to show the movement of the electrons in each step.

b. Write the rate equation for this reaction, assuming the first step is rate-determining.

c. If the concentration of were increased 10 times, what would happen to the rate of the reaction?

d. If the concentrations of both and were increased 10 times, what would happen to the rate of the reaction?

e. Classify the conversion of acetyl chloride to methyl acetate as an addition, elimination, or substitution.

Label each statement as true or false. Correct any false statement to make it true.

a. Increasing temperature increases reaction rate.

b. If a reaction is fast, it has a large rate constant.

c. A fast reaction has a large negative $\mathbf{∆}\mathbf{G}\mathbf{°}$value.

d. When ${\mathbf{E}}_{\mathbf{a}}$is large, the rate constant k is also large.

e. Fast reactions have equilibrium constants > 1.

f. Increasing the concentration of a reactant always increases the rate of a reaction.

The Diels–Alder reaction, a powerful reaction discussed in Chapter 16, occurs when a 1,3- diene such as A reacts with an alkene such as B to form the six-membered ring in C.

a. Draw curved arrows to show how A and B react to form C.

b. What bonds are broken and formed in this reaction?

c. Would you expect this reaction to be endothermic or exothermic?

d. Does entropy favor the reactants or products?

e. Is the Diels–Alder reaction a substitution, elimination, or addition?

Explain why $\mathbf{HC}\mathbf{≡}\mathbf{CH}$is more acidic than ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{3}}$, even though the C-H bond in $\mathbf{HC}\mathbf{≡}\mathbf{CH}$has a higher bond dissociation energy than the C-H bond in ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{3}}$.

What carbon radical is formed by homolysis of the$\mathbf{C}\mathbf{-}{\mathbf{H}}_{\mathbf{a}}$ bond in propylbenzene? Draw all reasonable resonance structures for this radical.

What carbon radical is formed by homolysis of therole="math" localid="1648540916945" $\mathbf{C}\mathbf{-}{\mathbf{H}}_{\mathbf{b}}$bond in propylbenzene? Draw all reasonable resonance structures for this radical.

The bond dissociation energy of one of the C-H bonds is considerably less than the bond dissociation energy of the other. Which C-H bond is weaker? Offer an explanation.

Which bond in each pair has the higher bond dissociation energy?

a.

b.