Textbook Question
Propose a mechanism for the coupling of acetic acid and aniline using DCC as a coupling agent.
Ch. 24 - Amino Acids, Peptides, and Proteins
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
Chapter 24, Problem 27a
(a) The isoelectric point (pI) of phenylalanine is pH 5.5. Draw the structure of the major form of phenylalanine at pH values of 1, 5.5, and 11.
Verified step by step guidance1
Step 1: Understand the concept of isoelectric point (pI). The isoelectric point is the pH at which a molecule, such as an amino acid, has no net charge. For phenylalanine, the pI is 5.5, meaning it exists as a zwitterion (neutral form) at this pH.
Step 2: Analyze the structure of phenylalanine. Phenylalanine is an amino acid with an amino group (-NH3+), a carboxylic acid group (-COOH), and a phenyl side chain. The protonation state of the amino and carboxylic acid groups changes with pH.
Step 3: Determine the structure of phenylalanine at pH 1. At this low pH, the environment is highly acidic, so both the amino group and the carboxylic acid group will be protonated. The structure will have a positively charged amino group (-NH3+) and a neutral carboxylic acid group (-COOH).
Step 4: Determine the structure of phenylalanine at pH 5.5. At this pH (the isoelectric point), phenylalanine exists as a zwitterion. The amino group will be protonated (-NH3+), and the carboxylic acid group will be deprotonated (-COO−), resulting in no net charge.
Step 5: Determine the structure of phenylalanine at pH 11. At this high pH, the environment is basic, so the amino group will lose a proton and become neutral (-NH2), while the carboxylic acid group remains deprotonated (-COO−). The overall charge of the molecule will be negative.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Isoelectric Point (pI)
The isoelectric point (pI) is the pH at which a molecule, such as an amino acid, carries no net electrical charge. For phenylalanine, the pI of 5.5 indicates that at this pH, the positive and negative charges balance out, resulting in a neutral molecule. Understanding pI is crucial for predicting the behavior of amino acids in different pH environments.
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Amino Acid Structure
Amino acids have a general structure consisting of a central carbon atom, an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R group). For phenylalanine, the side chain is a benzyl group, which influences its properties and behavior in solution. Recognizing how these groups interact with pH is essential for drawing the correct structures at different pH levels.
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Protonation and Deprotonation
Protonation and deprotonation refer to the addition or removal of protons (H+) from a molecule, affecting its charge and structure. At low pH (1), amino acids are typically protonated, leading to a positively charged form. At high pH (11), they are deprotonated, resulting in a negatively charged form. Understanding these processes is vital for determining the predominant form of phenylalanine at various pH levels.
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Related Practice
Textbook Question
Draw the structure of the phenylthiohydantoin derivatives of
(a) alanine.
1
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Textbook Question
Predict the products of the following reactions.
(b)
1
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Textbook Question
The isoelectric point of glutamic acid is pH 3.2. Draw the structures of the major forms of glutamic acid at pH values of 1, 3.2, 7, and 11. Explain why the side-chain carboxylic acid is a weaker acid than the acid group next to the α-carbon atom.
1
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Textbook Question
Draw the structure of the phenylthiohydantoin derivatives of
(c) lysine.
1
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Textbook Question
Predict the products of the following reactions.
(c)
1
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