Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

7. Substitution Reactions

SN2 Reaction

Organic Chemistry

7. Substitution Reactions

SN2 Reaction

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4:40 minutes

Problem 4

Textbook Question

Does increasing the energy barrier for an S_N2 reaction increase or decrease the magnitude of the rate constant for the reaction?

Verified step by step guidance

Understand the relationship between the energy barrier and the rate constant: The rate constant (k) for a reaction is related to the activation energy (Ea) through the Arrhenius equation:

k = A e^{- \frac{E}{a}}

, where A is the pre-exponential factor, R is the gas constant, and T is the temperature.

Recognize that the energy barrier corresponds to the activation energy (Ea): In an SN2 reaction, the energy barrier is the energy required to reach the transition state from the reactants. A higher energy barrier means a larger Ea.

Analyze the effect of increasing Ea on the rate constant: From the Arrhenius equation, as Ea increases, the exponential term

e^{- \frac{E}{a}}

decreases, which in turn decreases the value of k.

Conclude the relationship: Increasing the energy barrier (Ea) decreases the magnitude of the rate constant (k) for the SN2 reaction because the reaction becomes slower as it requires more energy to overcome the activation energy.

Relate this to reaction conditions: To counteract the effect of a higher energy barrier, one could increase the temperature (T) in the Arrhenius equation, as a higher temperature would increase the rate constant despite a higher Ea.

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Key Concepts

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

SN2 Reaction Mechanism

The SN2 (substitution nucleophilic bimolecular) reaction is a type of nucleophilic substitution where a nucleophile attacks an electrophile, resulting in the simultaneous displacement of a leaving group. This reaction occurs in a single concerted step, meaning that bond formation and bond breaking happen simultaneously. The rate of an SN2 reaction depends on the concentration of both the nucleophile and the substrate, making it a second-order reaction.

Activation Energy and Rate Constant

Activation energy is the minimum energy required for a reaction to occur. According to the Arrhenius equation, the rate constant (k) of a reaction is inversely related to the activation energy; as the energy barrier increases, the rate constant decreases. This relationship highlights how higher activation energy makes it less likely for reactants to successfully collide with enough energy to form products, thus slowing the reaction.

Effect of Energy Barrier on Reaction Rate

The energy barrier of a reaction directly influences its rate. In the context of an SN2 reaction, increasing the energy barrier means that fewer molecules will have sufficient energy to overcome this barrier, leading to a decrease in the rate of reaction. Consequently, as the energy barrier rises, the magnitude of the rate constant decreases, indicating a slower reaction rate.

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Related Practice

Textbook Question

Draw the products obtained from the S_N2 reaction of:

c. (S)-3-chlorohexane and hydroxide ion.

d. 3-iodopentane and hydroxide ion.

Textbook Question

Which alkyl halide is more reactive in an S_N2 reaction with a given nucleophile?

Textbook Question

Rank the following alkyl bromides from most reactive to least reactive in an S_N2 reaction:

1-bromo-2-methylbutane, 1-bromo-3-methylbutane, 2-bromo-2-methylbutane, and 1-bromopentane.

Textbook Question

Which alkyl halide is more reactive in an S_N2 reaction with a given nucleophile?

Textbook Question

The rate of the reaction of methyl iodide with quinuclidine was measured in nitrobenzene, and then the rate of the reaction of methyl iodide with triethylamine was measured in the same solvent. The concentration of the reagents was the same in both experiments.

a.Which reaction had the larger rate constant?

Textbook Question

Draw a transition state for the following substitution reaction.

Textbook Question

Predict the major products of the following reactions.

(a) ethyl tosylate + potassium tert-butoxide

(b) isobutyl tosylate + NaI

Textbook Question

Practice your electron-pushing skills by drawing a mechanism for the following S_N2 reactions.

(a)

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