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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6:16 minutes

Problem 53

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)

Verified step by step guidance

Step 1: Understand the role of triethyloxonium tetrafluoroborate ((CH3CH2)3O+ BF4-) in the reaction. This compound acts as an ethylating agent, capable of transferring an ethyl group to a nucleophile (Nuc:−) via an SN2 mechanism. The positively charged oxonium ion ((CH3CH2)3O+) is highly electrophilic, making it susceptible to nucleophilic attack.

Step 2: Analyze the SN2 reaction mechanism. In an SN2 reaction, the nucleophile (Nuc:−) attacks the electrophilic carbon in the ethyl group of the oxonium ion. This attack occurs in a single step, where the bond between the ethyl group and the oxygen is broken simultaneously as the nucleophile forms a new bond with the ethyl group.

Step 3: Identify the leaving group. In this reaction, the leaving group is diethyl ether ((CH3CH2)2O), which is formed when the nucleophile displaces one ethyl group from the oxonium ion. Diethyl ether is a neutral and stable molecule, making it an excellent leaving group.

Step 4: Compare triethyloxonium tetrafluoroborate to ethyl halides. Ethyl halides, such as ethyl bromide (CH3CH2Br), are commonly used for ethylation reactions. However, triethyloxonium tetrafluoroborate is preferred because it avoids the use of halides, which can be toxic or environmentally harmful. Additionally, the oxonium ion is more reactive, allowing for faster and cleaner ethylation reactions.

Step 5: Consider the physical properties of triethyloxonium tetrafluoroborate. Unlike ethyl halides, which are often volatile liquids, triethyloxonium tetrafluoroborate is a solid with a melting point of 91–92 °C. This makes it easier to handle and store, reducing risks associated with volatility and flammability.

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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 fundamental mechanism in organic chemistry where a nucleophile attacks an electrophile, resulting in the substitution of a leaving group. This reaction occurs in a single concerted step, meaning that bond formation and bond breaking happen simultaneously. The reaction rate depends on the concentration of both the nucleophile and the substrate, making it a bimolecular process.

Leaving Group

In nucleophilic substitution reactions, the leaving group is the atom or group that departs with a pair of electrons, allowing the nucleophile to bond with the substrate. A good leaving group is typically stable after departure, such as halides (e.g., Cl-, Br-, I-) or other groups like water. In the case of triethyloxonium tetrafluoroborate, the leaving group is the tetrafluoroborate ion (BF4-), which is stable and facilitates the transfer of the ethyl group.

Advantages of Triethyloxonium Tetrafluoroborate

Using triethyloxonium tetrafluoroborate as a reagent for ethyl group transfer offers several advantages over traditional ethyl halides. It provides a more stable and less reactive source of the ethyl group, reducing side reactions and improving selectivity. Additionally, the tetrafluoroborate ion is a non-nucleophilic leaving group, which minimizes unwanted reactions that can occur with more reactive leaving groups found in alkyl halides.

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Related Practice

Textbook Question

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

(e)

Textbook Question

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

(f)

Textbook Question

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

(c)

(d)

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

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.

(a) trans-1-bromo-3-methylcyclopentane + KOH

(b) (R)-2-bromopentane + KCN

Textbook Question

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

(c)

(d)

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