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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6:55 minutes

Problem 5

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.

Verified step by step guidance

Step 1: Recall the key factors that influence the rate of an SN2 reaction. The SN2 mechanism involves a single-step nucleophilic substitution where the nucleophile attacks the electrophilic carbon and displaces the leaving group. The rate of the reaction is highly dependent on steric hindrance around the electrophilic carbon. Less steric hindrance leads to faster reactions.

Step 2: Analyze the structure of each alkyl bromide. For 1-bromo-2-methylbutane, the bromine is attached to a primary carbon, but there is steric hindrance from the adjacent methyl group. For 1-bromo-3-methylbutane, the bromine is attached to a primary carbon with less steric hindrance compared to 1-bromo-2-methylbutane. For 2-bromo-2-methylbutane, the bromine is attached to a tertiary carbon, which has significant steric hindrance. For 1-bromopentane, the bromine is attached to a primary carbon with no adjacent bulky groups, making it the least hindered.

Step 3: Rank the alkyl bromides based on steric hindrance. The least hindered compound will be the most reactive in an SN2 reaction, while the most hindered compound will be the least reactive. Based on the analysis, 1-bromopentane has the least steric hindrance, followed by 1-bromo-3-methylbutane, then 1-bromo-2-methylbutane, and finally 2-bromo-2-methylbutane.

Step 4: Consider the leaving group (bromine) in all compounds. Bromine is a good leaving group, so the reactivity differences are primarily determined by steric factors rather than the leaving group itself.

Step 5: Finalize the ranking based on steric hindrance and SN2 reactivity. The order from most reactive to least reactive is: 1-bromopentane > 1-bromo-3-methylbutane > 1-bromo-2-methylbutane > 2-bromo-2-methylbutane.

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Key Concepts

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

SN2 Mechanism

The SN2 (substitution nucleophilic bimolecular) mechanism involves a single concerted step where a nucleophile attacks the electrophilic carbon, displacing a leaving group. This reaction is characterized by a backside attack, leading to inversion of configuration at the carbon center. The rate of the reaction depends on both the concentration of the nucleophile and the substrate, making sterics a crucial factor.

Steric Hindrance

Steric hindrance refers to the crowding around a reactive center that can impede the approach of a nucleophile. In SN2 reactions, primary alkyl halides are generally more reactive than secondary or tertiary ones due to less steric hindrance. The presence of bulky groups near the electrophilic carbon can significantly slow down the reaction rate.

Leaving Group Ability

The ability of a leaving group to depart from the substrate is critical in determining the reactivity of alkyl halides in SN2 reactions. Good leaving groups, such as bromide, stabilize the transition state and facilitate the reaction. The strength of the bond between the carbon and the leaving group, as well as the stability of the leaving group after departure, influences the overall reactivity of the alkyl halide.

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Related Practice

Textbook Question

Predict the compound in each pair that will undergo the S_N2 reaction faster.

(a)

(b)

Textbook Question

Draw the substitution product formed by each of the following S_N2 reactions:

a. trans-1-iodo-4-ethylcyclohexane and methoxide ion

b. cis-1-chloro-3-methylcyclobutane and ethoxide ion

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

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

Textbook Question

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

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.

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