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

29. Amino Acids

Proteins and Amino Acids

Organic Chemistry

29. Amino Acids

Proteins and Amino Acids

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Problem 75a

Textbook Question

Show how valine can be prepared by
a. a Hell–Volhard–Zelinski reaction.

Verified step by step guidance

Step 1: Begin by identifying the starting material for the synthesis of valine. Valine is an α-amino acid with a branched alkyl side chain. To prepare valine using the Hell–Volhard–Zelinski (HVZ) reaction, start with a carboxylic acid that has the appropriate carbon skeleton, such as isobutyric acid (CH₃CH(CH₃)COOH).

Step 2: Apply the Hell–Volhard–Zelinski reaction to the carboxylic acid. This reaction involves treating the carboxylic acid with bromine (Br₂) in the presence of phosphorus tribromide (PBr₃). This step selectively brominates the α-carbon of the carboxylic acid, forming an α-bromo acid. The reaction mechanism includes the formation of an acyl bromide intermediate, which facilitates α-bromination.

Step 3: Convert the α-bromo acid into an α-amino acid. This is achieved by nucleophilic substitution, where ammonia (NH₃) is used to replace the bromine atom with an amino group (-NH₂). The reaction proceeds via an SN2 mechanism, resulting in the formation of valine.

Step 4: Purify the product. After the substitution reaction, the crude mixture may contain unreacted starting materials and byproducts. Use techniques such as recrystallization or chromatography to isolate and purify valine.

Step 5: Verify the structure and purity of valine. Use spectroscopic methods such as NMR (nuclear magnetic resonance) and IR (infrared spectroscopy) to confirm the presence of the amino group and the carboxylic acid functional group, as well as the correct branching in the side chain.

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Key Concepts

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

Hell–Volhard–Zelinski Reaction

The Hell–Volhard–Zelinski (HVZ) reaction is a method for the halogenation of carboxylic acids at the alpha position. This reaction involves the formation of an acyl halide from the carboxylic acid, followed by halogenation at the alpha carbon using a halogen and a base. It is particularly useful for synthesizing alpha-halo acids, which can be further transformed into amino acids like valine.

Alpha-Amino Acids

Alpha-amino acids are organic compounds that contain both an amino group (-NH2) and a carboxyl group (-COOH) attached to the same carbon atom, known as the alpha carbon. Valine, an essential branched-chain amino acid, is one such compound. Understanding the structure and properties of alpha-amino acids is crucial for grasping how they can be synthesized and modified in organic chemistry.

Synthesis of Valine

Valine can be synthesized through various methods, including the HVZ reaction, which allows for the introduction of a halogen at the alpha position of a carboxylic acid. Following the halogenation, further reactions such as hydrolysis and amination can convert the halogenated compound into valine. Familiarity with these synthetic pathways is essential for understanding how to prepare specific amino acids in organic chemistry.

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Related Practice

Textbook Question

A decapeptide undergoes partial hydrolysis to give peptides whose amino acid compositions are shown. Reaction of the intact decapeptide with Edman's reagent releases PTH-Gly. What is the sequence of the decapeptide?

1. Ala, Trp

2. Val, Pro, Asp

3. Pro, Val

4. Ala, Glu

5. Trp, Ala, Arg

6. Arg, Gly

7. Glu, Ala, Leu

8. Met, Pro, Leu, Glu