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

8. Elimination Reactions

Cumulative Substitution/Elimination

Organic Chemistry

8. Elimination Reactions

Cumulative Substitution/Elimination

Previous problemNext problem

5:46 minutes

Problem 71a,b

Textbook Question

Explain how each of the following changes affect the rate of the reaction of 1-bromobutane with ethoxide ion in DMF.
a. The concentration of both the alkyl halide and the nucleophile are tripled.
b. The solvent is changed to ethanol.

Verified step by step guidance

The reaction of 1-bromobutane with ethoxide ion in DMF is an example of an SN2 reaction. In an SN2 reaction, the rate is determined by the rate law: \( \text{Rate} = k[\text{1-bromobutane}][\text{ethoxide ion}] \). This means the reaction is second-order overall, depending on the concentrations of both the alkyl halide and the nucleophile.

a. If the concentration of both the alkyl halide (1-bromobutane) and the nucleophile (ethoxide ion) are tripled, substitute the new concentrations into the rate law. The new rate will be \( \text{Rate}_{\text{new}} = k(3[\text{1-bromobutane}])(3[\text{ethoxide ion}]) \). Simplify this to \( \text{Rate}_{\text{new}} = 9k[\text{1-bromobutane}][\text{ethoxide ion}] \). This shows that the rate of the reaction will increase by a factor of 9.

b. Changing the solvent to ethanol affects the reaction because ethanol is a protic solvent, whereas DMF is an aprotic solvent. In an SN2 reaction, aprotic solvents like DMF stabilize the nucleophile less, allowing it to remain more reactive. Protic solvents like ethanol, on the other hand, can hydrogen bond with the nucleophile, reducing its reactivity. This means the rate of the reaction will decrease when the solvent is changed to ethanol.

To summarize, the rate of the reaction is directly proportional to the concentrations of both the alkyl halide and the nucleophile, and the solvent choice significantly impacts the nucleophile's reactivity in an SN2 mechanism.

For part (a), the rate increases by a factor of 9 when concentrations are tripled. For part (b), the rate decreases when the solvent is changed to ethanol due to reduced nucleophilicity in a protic solvent.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above

Video duration:

Play a video:

Was this helpful?

Key Concepts

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

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions involve the replacement of a leaving group in an organic molecule by a nucleophile. In the case of 1-bromobutane reacting with ethoxide ion, the reaction mechanism can be either SN1 or SN2, depending on factors like steric hindrance and solvent. Understanding the mechanism is crucial for predicting how changes in concentration and solvent will affect the reaction rate.

Reaction Rate and Concentration

The rate of a chemical reaction is influenced by the concentrations of the reactants. For a bimolecular reaction like the SN2 mechanism, tripling the concentration of both the alkyl halide and the nucleophile will increase the reaction rate significantly, as the rate is directly proportional to the product of the concentrations of the two reactants. This relationship is described by the rate law for the reaction.

Effect of Solvent on Reaction Mechanism

The choice of solvent can greatly influence the rate and mechanism of a nucleophilic substitution reaction. DMF (dimethylformamide) is a polar aprotic solvent that stabilizes ions and enhances nucleophilicity, favoring SN2 reactions. In contrast, ethanol is a polar protic solvent that can solvate nucleophiles and reduce their reactivity, potentially favoring an SN1 mechanism and slowing the reaction rate.

Watch next

Master Intro to Substitution/Elimination Problems with a bite sized video explanation from Johnny

Start learning

Related Videos

Related Practice

Textbook Question

Protonation converts the hydroxy group of an alcohol to a good leaving group. Suggest a mechanism for each reaction.

(b)

Textbook Question

Draw the products of the following intramolecular reactions:

Textbook Question

After a proton is removed from the OH group, which compound in each pair forms a cyclic ether more rapidly?

Textbook Question

cis-4-Bromocyclohexanol and trans-4-bromocyclohexanol form the same elimination product but a different substitution product when they react with HO⁻.

c. How many stereoisomers does each of the elimination and substitution reactions form?

Textbook Question

Explain how the following changes affect the rate of the reaction of 2-bromo-2-methylbutane with methanol:

a. The alkyl halide is changed to 2-chloro-2-methylbutane.

b. The alkyl halide is changed to 2-chloro-3-methylbutane.

Textbook Question

The following substitution reaction, between a strong base and a 1° haloalkane, occurs in a single step via backside displacement. Yet it is not technically an S_N2 reaction. Why?

Textbook Question

Propose a mechanism for each of the following reactions:

Textbook Question

Which of the following two compounds eliminates HBr more rapidly in a basic solution?

Show more practices