Ch. 6 - The Reactions of Alkenes • The Stereochemistry of Addition Reactions

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 6 - The Reactions of Alkenes • The Stereochemistry of Addition Reactions

Problem 72

Chapter 7, Problem 72

The second-order rate constant (in units of M^-1s^-1) for acid-catalyzed hydration at 25 °C is given for each of the following alkenes:

a. Calculate the relative rates of hydration of the alkenes. (Hint: Divide each rate constant by the smallest rate constant of the series: 3.51 × 10^-8.)
b. Why does (Z)-2-butene react faster than (E)-2-butene?
c. Why does 2-methyl-2-butene react faster than (Z)-2-butene?
d. Why does 2,3-dimethyl-2-butene react faster than 2-methyl-2-butene?

Verified step by step guidance

Step 1: To calculate the relative rates of hydration for the alkenes, divide each rate constant by the smallest rate constant in the series, which is 3.51 × 10⁻⁸ M⁻¹s⁻¹. For each alkene (W, X, Y, Z), perform the division: relative rate = (rate constant of alkene) / (smallest rate constant).

Step 2: (Z)-2-butene reacts faster than (E)-2-butene because the (Z)-isomer has substituents on the same side of the double bond, leading to increased steric hindrance. This steric hindrance destabilizes the alkene and makes it more reactive toward acid-catalyzed hydration.

Step 3: 2-methyl-2-butene reacts faster than (Z)-2-butene because the methyl group provides additional hyperconjugation and inductive effects, stabilizing the carbocation intermediate formed during the reaction. This stabilization lowers the activation energy, increasing the reaction rate.

Step 4: 2,3-dimethyl-2-butene reacts faster than 2-methyl-2-butene because it has two methyl groups on each side of the double bond. These groups provide even greater hyperconjugation and inductive effects, further stabilizing the carbocation intermediate and increasing the reaction rate.

Step 5: Summarize the trends: The rate of acid-catalyzed hydration increases with the degree of substitution around the double bond due to increased carbocation stability and steric effects. Additionally, (Z)-isomers are generally more reactive than (E)-isomers due to steric hindrance.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Second-Order Kinetics

Second-order kinetics refers to reactions where the rate is proportional to the square of the concentration of one reactant or the product of the concentrations of two reactants. In this context, the second-order rate constant (k) is expressed in units of M⁻¹s⁻¹, indicating how the concentration of alkenes affects the rate of hydration. Understanding this concept is crucial for calculating relative rates based on the provided rate constants.

Stereochemistry and Reactivity

Stereochemistry involves the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In the case of alkenes, the configuration (cis or trans) can influence the rate of reaction due to steric hindrance and electronic effects. Recognizing how the stereochemical configuration of (Z)-2-butene and (E)-2-butene impacts their reactivity is essential for answering the question about their relative rates.

Substituent Effects on Stability

The stability of carbocations formed during the hydration of alkenes is influenced by the presence of alkyl substituents. More substituted carbocations are generally more stable due to hyperconjugation and inductive effects. This concept is key to understanding why 2-methyl-2-butene and 2,3-dimethyl-2-butene react faster than their less substituted counterparts, as the stability of the intermediate carbocations directly affects the reaction rate.

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Related Practice

Textbook Question

Draw the product or products that will be obtained from the reaction of cis-2-butene and trans-2-butene with each of the following reagents. If a product can exist as stereoisomers, show which stereoisomers are formed.

3. a peroxyacid

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Textbook Question

Draw the products of the following reactions. If the products can exist as stereoisomers, show which stereoisomers are formed.

1. HCl

A student was about to turn in the products he had obtained from the reaction of HI with 3,3,3-trifluoropropene when he realized that the labels had fallen off his flasks and he did not know which label belonged to which flask. His friend reminded him of the rule that says the electrophile adds to the sp² carbon bonded to the most hydrogens. In other words, he should label the flask containing the most product 1,1,1-trifluoro-2-iodopropane and label the flask containing the least amount of product 1,1,1-trifluoro-3-iodopropane. Should he follow his friend’s advice?

Textbook Question

1-Methylcyclohexene forms two products when it reacts with bromine in methanol.

a. Draw the mechanism for the formation of the products.

b. Describe their stereochemical relationship—that is, are they enantiomers or diastereomers?

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Textbook Question

Draw the products of the following reactions. If the products can exist as stereoisomers, show which stereoisomers are formed.

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