Textbook Question
In the second propagation step in the bromination of toluene, Br2 is only attacked by a radical on the substituent carbon. Why?
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Ch. 24 - Benzene II: Reactions Influenced by the Aromatic Ring
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471
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Table of contents
- Ch. 2 - General Chemistry Translated: Finding the Electrons114
- Ch. 3 - Alkanes and Cycloalkanes: Properties and Conformational Analysis109
- Ch. 4 - Acids and Bases: Electron Flow144
- Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics118
- Ch. 6 - Stereoisomerism: Arrangement of Atoms in Space112
- Ch. 7 - Principles of Green (Organic) Chemistry3
- Ch. 8 - Alkenes I: Properties and Electrophilic Additions158
- Ch. 9 - Alkenes II: Oxidation and Reduction150
- Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions101
- Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions108
- Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes172
- Ch. 13 - Alcohols, Ethers and Related Compounds: Substitution and Elimination239
- Ch. 14 - Structural Identification I: Infrared Spectroscopy and Mass Spectrometry54
- Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy93
- Ch. 16 - Metals in Organic Chemistry48
- Ch. 17 - Carbonyl Addition Reactions: Aldehydes and Ketones45
- Ch. 18 - Nucleophilic Acyl Substitution I: Carboxylic Acids18
- Ch. 19 - Nucleophilic Acyl Substitution II: Carboxylic Acid Derivatives39
- Ch. 20 - Enolates: Carbonyl Addition and Substitution13
- Ch. 21 - Conjugated Systems I: Stability and Addition Reactions56
- Ch. 22 - Conjugated Systems II: Pericyclic Reactions54
- Ch. 23 - Benzene I: Aromatic Stability and Substitution Reactions64
- Ch. 24 - Benzene II: Reactions Influenced by the Aromatic Ring62
- Ch. 25 - Amines: Structure, Reactions, and Synthesis27
- Ch. 26 - Amino Acids, Proteins, and Peptide Synthesis9
- Ch. 27 - Carbohydrates, Nucleic Acids, and Lipids54
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
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Mullins 1st EditionCh. 24 - Benzene II: Reactions Influenced by the Aromatic RingProblem 40
Mullins 1st EditionCh. 24 - Benzene II: Reactions Influenced by the Aromatic RingProblem 40Chapter 23, Problem 40
The benzylic bromide shown undergoes neither SN1 nor SN2 substitution reactions. Explain.
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Identify the structure of the benzylic bromide. A benzylic bromide has a bromine atom attached to a carbon that is directly bonded to a benzene ring. This position is known as the benzylic position.
Analyze why the SN1 mechanism does not occur. The SN1 reaction involves the formation of a carbocation intermediate. In this case, the benzylic carbocation would be highly unstable if the benzene ring is substituted with electron-withdrawing groups, which destabilize the positive charge.
Analyze why the SN2 mechanism does not occur. The SN2 reaction requires a backside attack by the nucleophile. If the benzylic carbon is sterically hindered (e.g., due to bulky substituents on the benzene ring or the benzylic carbon), the nucleophile cannot approach effectively, preventing the reaction.
Consider the electronic effects of the benzene ring. If the benzene ring has substituents that withdraw electron density (e.g., nitro groups), they can reduce the reactivity of the benzylic carbon toward both SN1 and SN2 mechanisms.
Conclude that the lack of reactivity in both SN1 and SN2 mechanisms is due to a combination of steric hindrance and electronic effects, which make the benzylic bromide unreactive under typical substitution conditions.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
SN1 and SN2 Mechanisms
SN1 (unimolecular nucleophilic substitution) and SN2 (bimolecular nucleophilic substitution) are two fundamental mechanisms of nucleophilic substitution reactions. SN1 involves the formation of a carbocation intermediate and is favored in polar protic solvents, while SN2 involves a direct attack by the nucleophile and is favored in polar aprotic solvents. The structure of the substrate significantly influences which mechanism predominates.
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Drawing the SN1 Mechanism
Carbocation Stability
Carbocation stability is crucial in determining the feasibility of SN1 reactions. Benzylic carbocations, which are stabilized by resonance with the aromatic ring, are generally stable. However, if the benzylic bromide does not form a stable carbocation due to steric hindrance or other factors, the SN1 pathway may be unfavorable, preventing the reaction from occurring.
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Steric Hindrance
Steric hindrance refers to the prevention of reactions due to the spatial arrangement of atoms within a molecule. In the case of SN2 reactions, bulky groups around the reactive center can hinder the nucleophile's approach, making the reaction less likely. If the benzylic bromide has significant steric hindrance, it may not undergo SN2 substitution, contributing to the lack of reactivity in both SN1 and SN2 pathways.
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Related Practice
Textbook Question
In Chapter 13, we learned that epoxide opening can give different products, depending on whether the reaction occurs under acidic or basic conditions. Explain why the epoxide shown opens identically under either set of conditions.
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Textbook Question
When (R)-(1-bromoethyl)benzene is treated with sodium cyanide, a single enantiomer is produced. However, upon treatment of the same molecule with water, a mixture of two enantiomers is obtained.
(a) Explain these results.
(b) Why is only partial racemization sometimes observed?
Textbook Question
Beginning with benzene, synthesize the benzyl bromide shown.
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Textbook Question
Oxidation of the phenol shown gives a single quinone product. Predict this product and explain why it is the only one formed.
Textbook Question
The following reaction proceeds in good yield, despite requiring high temperature and high concentration. Identify the product you’d expect to obtain in this reaction.
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