Ch. 13 - Alcohols, Ethers and Related Compounds: Substitution and Elimination

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 13 - Alcohols, Ethers and Related Compounds: Substitution and Elimination

Problem 107

Chapter 12, Problem 107

Cleavage of the following ether produces the alcohol and haloalkane only, regardless of how much HBr is used. Thinking about the mechanism of the reaction, explain why bromobenzene is not also a product of this reaction.

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Identify the structure of the ether involved in the reaction. Ethers are compounds with an oxygen atom connected to two alkyl or aryl groups. In this case, the ether likely has a phenyl group and another alkyl group.

Understand the mechanism of ether cleavage with HBr. The reaction typically involves protonation of the ether oxygen, making it a better leaving group, followed by nucleophilic attack by bromide ion.

Consider the stability of the carbocation formed during the reaction. When the ether is cleaved, a carbocation is formed. The stability of this carbocation is crucial in determining the products of the reaction.

Recognize that phenyl cations are highly unstable. If the ether cleavage were to produce a phenyl cation, it would be very unstable and unlikely to form. Instead, the reaction favors the formation of a more stable carbocation.

Conclude that the reaction does not produce bromobenzene because the phenyl group remains attached to the oxygen, preventing the formation of a phenyl cation. The reaction instead produces an alcohol and a haloalkane, where the alkyl group forms a stable carbocation that reacts with bromide.

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Key Concepts

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

Ether Cleavage

Ether cleavage involves breaking the C-O bond in ethers, typically using strong acids like HBr. The reaction generally produces an alcohol and a haloalkane. The mechanism can vary depending on the structure of the ether, with primary ethers often undergoing SN2 reactions and secondary or tertiary ethers favoring SN1 reactions.

SN1 and SN2 Mechanisms

SN1 and SN2 are two types of nucleophilic substitution reactions. SN1 involves a two-step process with carbocation formation, favoring tertiary carbons, while SN2 is a one-step process with a backside attack, favoring primary carbons. The choice between these mechanisms affects the products formed during ether cleavage.

Stability of Aromatic Compounds

Aromatic compounds, like benzene, are highly stable due to resonance and delocalized π-electrons. This stability makes them less reactive in certain conditions, such as ether cleavage with HBr, where the aromatic ring remains intact, preventing the formation of bromobenzene as a product.

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Related Practice

Textbook Question

Triphenylphosphine and iodine can be used to convert alcohols to iodoalkanes. Suggest a mechanism for this reaction. [Triphenylphosphine first acts as a nucleophile in this reaction.]

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Textbook Question

Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (xi) HOCl, H₂O (xii) HIO₄ If no reaction occurs, write 'no reaction.'

(o)

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Textbook Question

Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (i) SOCl₂ ; (ii) PBr₃ ; (iii) SOCl₂ , NEt₃ (iv) 1. TsCl, Et₃N 2. NaCN; (v) 1. TsCl, Et₃N 2. NaOt-Bu (vi) H₂SO₄ (vii) HCl; (viii) HBr; (ix) PCC; (x) H₂CrO₄ , H₂O (xi) HOCl, H₂O (xii) HIO₄ If no reaction occurs, write 'no reaction.'

(o)

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Textbook Question

Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (vii) HCl; (viii) HBr; If no reaction occurs, write 'no reaction.'

(o)

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Textbook Question

Another method for converting alcohols to chloroalkanes makes use of chlorotrimethylsilane (TMSCl) and DMSO. Suggest a mechanism for this reaction to form (a) a 1° chloroalkane and (b) a 3° chloroalkane. [The reaction begins by the reaction of DMSO and TMSCl and is analogous to the Swern oxidation.]

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Textbook Question

We explain in Chapter 24 that bisphenols can be oxidized to quinones.

(a) Calculate the oxidation numbers of C1 and C₂ in going from reactant to product.

(b) Provide a mechanism for this transformation. [The reaction begins like the alcohol oxidations of Section 13.9.]

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