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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4:14 minutes

Problem 19a,b

Textbook Question

For each pair of compounds, state which compound is the better S_N2 substrate.
a. 2-methyl-1-iodopropane or tert-butyl iodide.
b. cyclohexyl bromide or 1-bromo-1-methylcyclohexane

Verified step by step guidance

Step 1: Recall the key factors that influence the SN2 reaction mechanism. SN2 reactions are bimolecular nucleophilic substitution reactions that occur in a single step. The rate of the reaction depends on steric hindrance around the electrophilic carbon and the leaving group. Less steric hindrance leads to a faster SN2 reaction.

Step 2: Analyze the first pair of compounds: 2-methyl-1-iodopropane and tert-butyl iodide. Compare the steric hindrance around the electrophilic carbon. In 2-methyl-1-iodopropane, the electrophilic carbon is primary, meaning it is attached to only one alkyl group, which minimizes steric hindrance. In tert-butyl iodide, the electrophilic carbon is tertiary, meaning it is attached to three alkyl groups, creating significant steric hindrance. Therefore, 2-methyl-1-iodopropane is the better SN2 substrate.

Step 3: Analyze the second pair of compounds: cyclohexyl bromide and 1-bromo-1-methylcyclohexane. Cyclohexyl bromide has a primary electrophilic carbon, as the bromine is attached to a carbon that is part of the cyclohexane ring but not substituted with additional groups. In contrast, 1-bromo-1-methylcyclohexane has a tertiary electrophilic carbon due to the methyl group attached to the same carbon as the bromine. The tertiary carbon in 1-bromo-1-methylcyclohexane creates steric hindrance, making cyclohexyl bromide the better SN2 substrate.

Step 4: Summarize the reasoning for both pairs. For SN2 reactions, primary carbons are preferred over tertiary carbons due to reduced steric hindrance. This principle applies to both pairs of compounds analyzed.

Step 5: Conclude that steric hindrance is the primary factor in determining the better SN2 substrate. For pair (a), 2-methyl-1-iodopropane is better. For pair (b), cyclohexyl bromide is better.

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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 mechanism is a type of nucleophilic substitution reaction where a nucleophile attacks an electrophile, resulting in the simultaneous displacement of a leaving group. This reaction is characterized by a single concerted step, leading to an 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 and electronic factors crucial for determining the reactivity.

Steric Hindrance

Steric hindrance refers to the prevention of chemical reactions due to the spatial arrangement of atoms within a molecule. In SN2 reactions, bulky groups around the electrophilic carbon can hinder the approach of the nucleophile, making the substrate less reactive. Therefore, substrates with less steric hindrance are generally better SN2 candidates, as they allow for easier access to the reactive site.

Leaving Group Ability

The ability of a leaving group to depart from a substrate is a critical factor in nucleophilic substitution reactions. Good leaving groups, such as iodide or bromide, stabilize the negative charge after leaving, facilitating the reaction. In comparing substrates, the quality of the leaving group can influence the overall reactivity, but steric factors often play a more significant role in determining which compound is a better SN2 substrate.

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