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 109

Chapter 12, Problem 109

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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Identify the role of triphenylphosphine (PPh₃) as a nucleophile. In this reaction, triphenylphosphine will attack the iodine molecule (I₂), forming a phosphonium iodide intermediate.

Write the initial nucleophilic attack: PPh₃ + I₂ → PPh₃I⁺ + I⁻. This step involves the formation of a positively charged phosphonium ion and a negatively charged iodide ion.

Consider the alcohol (R-OH) as the next reactant. The alcohol will react with the phosphonium iodide intermediate. The oxygen atom in the alcohol acts as a nucleophile, attacking the phosphonium ion.

Illustrate the formation of an alkoxyphosphonium intermediate: R-OH + PPh₃I⁺ → R-O-PPh₃⁺ + I⁻. This step involves the displacement of the iodide ion by the alcohol, forming an alkoxyphosphonium ion.

Finally, describe the elimination step where the alkoxyphosphonium ion undergoes a nucleophilic substitution by the iodide ion, resulting in the formation of the iodoalkane (R-I) and triphenylphosphine oxide (PPh₃=O).

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Key Concepts

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

Nucleophilic Substitution

Nucleophilic substitution is a fundamental organic reaction where a nucleophile replaces a leaving group in a molecule. In this context, triphenylphosphine acts as a nucleophile, attacking the iodine molecule to form a phosphonium iodide intermediate, which facilitates the conversion of alcohols to iodoalkanes.

Role of Triphenylphosphine

Triphenylphosphine is a versatile reagent in organic chemistry, often used as a nucleophile due to its lone pair of electrons on phosphorus. In this reaction, it initiates the process by attacking iodine, forming a reactive intermediate that aids in the transformation of alcohols into iodoalkanes.

Formation of Iodoalkanes

The conversion of alcohols to iodoalkanes involves replacing the hydroxyl group with an iodine atom. This transformation typically proceeds through a series of steps, including the formation of an intermediate, which then reacts with the alcohol to produce the iodoalkane, facilitated by the phosphonium iodide formed earlier.

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Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (v) 1. TsCl, Et₃N 2. NaOt-Bu (vi) H₂SO₄ If no reaction occurs, write 'no reaction.'

(o)

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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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In Chapter 12, we learned that crown ethers were used to increase the rate of S_N2 reactions (Assessment 12.80). Suggest a synthesis of 15-crown-5 using the reactions learned here in Chapter 13.

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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

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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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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