Textbook Question
Draw the products of the following reactions:
a.
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Ch. 17 - Reactions at the Alpha-Carbon
Bruice8th EditionOrganic ChemistryISBN: 9780135213711
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Table of contents
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 1)31
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 2)65
- Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry84
- Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure183
- Ch. 4 - Isomers: The Arrangement of Atoms in Space121
- Ch. 5 - Alkenes: Structure, Nomenclature, and an Introduction to Reactivity • Thermodynamics and Kinetics83
- Ch. 6 - The Reactions of Alkenes • The Stereochemistry of Addition Reactions175
- Ch. 7 - The Reactions of Alkynes • An Introduction to Multistep Synthesis119
- Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene148
- Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides212
- Ch. 10 - Reactions of Alcohols, Ethers, Epoxides, Amines, and Sulfur-Containing Compounds131
- Ch. 11 - Organometallic Compounds60
- Ch. 12 - Radicals90
- Ch. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis Spectroscopy62
- Ch. 14 - NMR Spectroscopy95
- Ch. 15 - Reactions of Carboxylic Acids and Carboxylic Acid Derivatives123
- Ch. 16 - Reactions of Aldehydes and Ketones • More Reactions of Carboxylic Acid Derivatives141
- Ch. 17 - Reactions at the Alpha-Carbon101
- Ch. 18 - Reactions of Benzene and Substituted Benzenes144
- Ch. 19 - More About Amines • Reactions of Heterocyclic Compounds49
- Ch. 20 - The Organic Chemistry of Carbohydrates80
- Ch. 21 - Amino Acids, Peptides, and Proteins57
- Ch. 22 - Catalysis in Organic Reactions and in Enzymatic Reactions35
- Ch. 23 - The Organic Chemistry of the Coenzymes—Compounds Derived from Vitamins1
- Ch. 28 - Pericyclic Reactions58
Chapter 18, Problem 51
The 1H NMR chemical shifts of nitromethane, dinitromethane, and trinitromethane are at δ6.10, δ4.33, and δ7.52. Match each chemical shift with the compound. Explain how chemical shift correlates with pKa.
Verified step by step guidance1
Understand the relationship between chemical shift and electron density: In 1H NMR spectroscopy, the chemical shift is influenced by the electron density around the hydrogen atoms. A higher electron density results in shielding, leading to a lower chemical shift (downfield), while a lower electron density results in deshielding, leading to a higher chemical shift (upfield).
Analyze the compounds: Nitromethane (CH3NO2), dinitromethane (CH2(NO2)2), and trinitromethane (CH(NO2)3) have progressively more electron-withdrawing nitro (-NO2) groups attached to the central carbon. Nitro groups are highly electronegative and withdraw electron density from the hydrogen atoms, causing deshielding and increasing the chemical shift.
Match the chemical shifts: The compound with the highest number of nitro groups (trinitromethane) will have the most deshielded hydrogens and the highest chemical shift (δ 7.52). The compound with the fewest nitro groups (nitromethane) will have the least deshielded hydrogens and the lowest chemical shift (δ 6.10). Dinitromethane, with an intermediate number of nitro groups, will have a chemical shift between the two (δ 4.33).
Correlate chemical shift with pKa: The pKa of a compound is influenced by the electron-withdrawing ability of substituents. More nitro groups increase the acidity of the compound (lower pKa) by stabilizing the conjugate base through resonance and inductive effects. Thus, trinitromethane has the lowest pKa, followed by dinitromethane, and then nitromethane. This trend aligns with the chemical shift values, as more acidic compounds tend to have more deshielded hydrogens.
Summarize the matching: Based on the analysis, nitromethane corresponds to δ 6.10, dinitromethane corresponds to δ 4.33, and trinitromethane corresponds to δ 7.52. The chemical shift values reflect the increasing deshielding effect of the nitro groups and correlate with the decreasing pKa values of the compounds.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Chemical Shift in NMR Spectroscopy
Chemical shift is a key parameter in nuclear magnetic resonance (NMR) spectroscopy that indicates the resonance frequency of a nucleus relative to a standard reference. It is measured in parts per million (ppm) and reflects the electronic environment surrounding the nucleus. Different functional groups and substituents can cause variations in chemical shifts, allowing chemists to deduce structural information about the compound.
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1H NMR Chemical Shifts
pKa and Acidity
pKa is a measure of the acidity of a compound, representing the negative logarithm of the acid dissociation constant (Ka). A lower pKa value indicates a stronger acid, meaning it more readily donates protons (H+). The relationship between pKa and chemical shift arises because electron-withdrawing groups, such as nitro groups, can stabilize the negative charge of the conjugate base, influencing the chemical environment and thus the NMR chemical shifts.
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Electron-Withdrawing Effects
Electron-withdrawing groups (EWGs) are substituents that pull electron density away from the rest of the molecule, often through inductive or resonance effects. In the context of nitromethane and its derivatives, the presence of nitro groups significantly affects the electron density around the hydrogen atoms, leading to shifts in their NMR signals. This alteration in electron density correlates with changes in acidity, as stronger EWGs typically increase acidity by stabilizing the conjugate base.
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Related Practice
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