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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3:36 minutes

Problem 20a,b

Textbook Question

Draw a perspective structure or a Fischer projection for the products of the following S_N2 reactions.
(a) trans-1-bromo-3-methylcyclopentane + KOH
(b) (R)-2-bromopentane + KCN

Verified step by step guidance

Analyze the reaction type: This is an SN2 reaction, which involves a backside attack by the nucleophile, leading to inversion of configuration at the carbon center where substitution occurs. Keep this stereochemical inversion in mind for both parts of the problem.

For part (a): trans-1-bromo-3-methylcyclopentane + KOH: Identify the leaving group (Br) and the nucleophile (OH⁻). The OH⁻ will attack the carbon bonded to the bromine from the opposite side, displacing the bromine and inverting the stereochemistry at that carbon. Draw the resulting product with the hydroxyl group replacing the bromine and ensure the stereochemistry is inverted.

For part (b): (R)-2-bromopentane + KCN: Start by identifying the leaving group (Br) and the nucleophile (CN⁻). The CN⁻ will perform a backside attack on the carbon bonded to the bromine, displacing the bromine and inverting the stereochemistry. Since the starting compound is in the (R)-configuration, the product will have the (S)-configuration. Draw the product with the cyano group (-CN) replacing the bromine and ensure the stereochemistry is inverted.

For both parts, decide whether to use a perspective structure or a Fischer projection. For cyclic compounds like in part (a), a perspective structure is more common. For part (b), you can use either a perspective structure or a Fischer projection, depending on the context of the problem.

Double-check your drawings: Ensure that the stereochemistry is correctly inverted for both products and that the nucleophile has replaced the leaving group in each case. Verify that the products are consistent with the SN2 mechanism and the starting materials provided.

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Key Concepts

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

SN2 Reactions

SN2 (Substitution Nucleophilic Bimolecular) reactions involve a nucleophile attacking an electrophile, resulting in the substitution of a leaving group. This reaction occurs in a single concerted step, where the nucleophile approaches the electrophile from the opposite side of the leaving group, leading to an inversion of configuration at the chiral center. Understanding the mechanism and stereochemical implications is crucial for predicting the products.

Fischer Projections

Fischer projections are a two-dimensional representation of three-dimensional organic molecules, particularly useful for depicting stereochemistry. In a Fischer projection, vertical lines represent bonds that project away from the viewer, while horizontal lines represent bonds that project towards the viewer. This format is essential for visualizing the stereochemical outcomes of reactions, especially for chiral compounds.

Stereochemistry

Stereochemistry is the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In the context of SN2 reactions, the stereochemical outcome is significant, as the inversion of configuration at a chiral center can lead to different optical isomers. Recognizing the stereochemical implications of reactants and products is vital for accurately drawing the resulting structures.

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Related Practice

Textbook Question

For each pair of compounds, state which compound is the better S_N2 substrate.

c. 2-bromobutane or isopropyl bromide

d. 1-chloro-2,2-dimethylbutane or 2-chlorobutane

e. 1-iodobutane or 2-iodopropane

Textbook Question

Triethyloxonium tetrafluoroborate, (CH₃CH₂)₃O⁺ BF₄^–, is a solid with melting point 91–92°C. Show how this reagent can transfer an ethyl group to a nucleophile (Nuc:⁻) in an S_N2 reaction. What is the leaving group? Why might this reagent be preferred to using an ethyl halide? (Consult Table 6-2)

Textbook Question

Explain why a much better yield of primary amine is obtained from the reaction of an alkyl halide with azide ion (^-N₃), followed by catalytic hydrogenation. (Hint: An alkyl azide is not nucleophilic.)

Textbook Question

Consider the reaction of 1-bromobutane with a large excess of ammonia (NH₃). Draw the reactants, the transition state, and the products. Note that the initial product is the salt of an amine (RNH₃⁺Br⁻), which is deprotonated by the excess ammonia to give the amine.

Textbook Question

Draw a perspective structure or a Fischer projection for the products of the following S_N2 reactions.

(c)

(d)

Textbook Question

Predict the major products of the following reactions.

(d) the tosylate of cyclohexylmethanol + excess NH₃

(e) n-butyl tosylate + sodium acetylide, H–C≡C:^{– +}Na

Textbook Question

Rank the following compounds in decreasing order of their reactivity toward the S_N2 reaction with sodium ethoxide (Na⁺ ^–OCH₂CH₃) in ethanol.

methyl chloride

tert-butyl iodide

neopentyl bromide

isopropyl bromide

methyl iodide

ethyl chloride

Textbook Question

When (±)−2,3−dibromobutane reacts with potassium hydroxide, some of the products are (2S,3R)-3-bromobutan-2-ol and its enantiomer and trans-2-bromobut-2-ene. Why is no cis-2-bromobut-2-ene formed?

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Draw a perspective structure or a Fischer projection for the products of the following SN2 reactions.(a) trans-1-bromo-3-methylcyclopentane + KOH(b) (R)-2-bromopentane + KCN

Key Concepts

SN2 Reactions

Fischer Projections

Stereochemistry

Watch next

Draw a perspective structure or a Fischer projection for the products of the following S_N2 reactions.
(a) trans-1-bromo-3-methylcyclopentane + KOH
(b) (R)-2-bromopentane + KCN