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Chapter 15: Problem 40

Predict the products likely to be formed on cleavage of the following ethers with hydroiodic acid: a. \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{O}-\mathrm{CH}_{3}\) b. \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{O}-\mathrm{CH}=\mathrm{CH}_{2}\) c. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2}-\mathrm{O}-\mathrm{CH}_{3}\) d. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COCH}_{3}\)

Short Answer

Expert verified
(a) Allyl alcohol, methyl iodide. (b) Ethyl alcohol, vinyl iodide. (c) Isobutyl alcohol, methyl iodide. (d) Tert-butyl iodide, methanol.

Step by step solution

01

Understand Ether Cleavage with HI

When ethers react with hydroiodic acid, they undergo cleavage, forming alcohol and alkyl iodide. The reaction follows a nucleophilic substitution mechanism, typically involving the attack on the less hindered or more stable carbocation center.
02

Determine Products for Compound (a)

For compound (a), \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{O}-\mathrm{CH}_{3}\), HI would preferentially cleave at the \(\mathrm{O}-\mathrm{CH}_{3}\) bond due to the formation of a primary alcohol and an alkyl iodide as major products. Thus, the products are \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2}\text{-OH}\) (allyl alcohol) and \(\mathrm{CH}_{3}\mathrm{I}\) (methyl iodide).
03

Determine Products for Compound (b)

For compound (b), \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{O}-\mathrm{CH} =\mathrm{CH}_{2}\), HI would cleave at the \(\mathrm{CH}_{2}\)-\(\mathrm{O}\) bond, producing \(\mathrm{CH}_{3}\mathrm{CH}_{2}\text{-OH}\) (ethyl alcohol) and \(\mathrm{CH}_{2} = \mathrm{CH}\mathrm{I}\) (vinyl iodide).
04

Determine Products for Compound (c)

For compound (c), \(\left(\mathrm{CH}_{3}\right)_{3}\mathrm{CCH}_{2}-\mathrm{O}-\mathrm{CH}_{3}\), the cleavage occurs at the \(\mathrm{C}-\mathrm{O}-\mathrm{CH}_{3}\) bond. The more stable tertiary carbocation is not formed here directly; conversely, the \(\mathrm{CH}_{3}\mathrm{I}\) leaves, forming \(\left(\mathrm{CH}_{3}\right)_{3}\mathrm{CCH}_{2}\text{-OH}\) as the product, along with \(\mathrm{CH}_{3}\mathrm{I}\).
05

Determine Products for Compound (d)

For compound (d), \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COCH}_{3}\), preferential cleavage occurs at the \(\mathrm{OCH}_{3}\) because the tert-butyl carbocation formed as an intermediate is very stable. Thus, the products are \(\left(\mathrm{CH}_{3}\right)_{3}\mathrm{C}\text{-I}\) (tert-butyl iodide) and \(\mathrm{CH}_{3}\mathrm{OH}\) (methanol).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Nucleophilic Substitution
Nucleophilic substitution is a fundamental reaction in organic chemistry where a nucleophile replaces a leaving group on a carbon atom. In the context of ether cleavage with hydroiodic acid (HI), this process typically follows one of two primary mechanisms:
  • SN1 mechanism involves a carbocation intermediate and is favored in tertiary systems where carbocation stability is high.
  • SN2 mechanism occurs when a nucleophile attacks the electrophilic carbon directly, leading to bond cleavage. This is more common in primary or less hindered systems.
When ethers are subjected to HI, the iodide ion (I-) acts as the nucleophile, attacking the ether's carbon-oxygen bond. Simultaneously, the hydrogen ion (H+) protonates the oxygen, promoting the cleavage of the C-O bond. Whether the reaction proceeds via the SN1 or SN2 route depends on factors such as the nature of the carbon attached to the oxygen and steric hindrance.
Carbocation Stability
Understanding carbocation stability is crucial when predicting the outcome of electrophilic and nucleophilic reactions in organic chemistry, including ether cleavage. A carbocation is a positively charged carbon atom, and its stability greatly influences reaction pathways. The order of carbocation stability is typically:
  • Tertiary (3掳) carbocations are the most stable, due to hyperconjugation and the inductive effects of surrounding alkyl groups.
  • Secondary (2掳) carbocations are stable but less so than tertiary ones.
  • Primary (1掳) carbocations are the least stable and are rarely formed during reactions.
In the ether cleavage process, if a carbocation intermediate can form, the reaction is more likely to proceed via the SN1 mechanism, particularly if a tertiary carbocation can be formed, as seen in compound (d). Here, the tert-butyl carbocation's stability facilitates the reaction.
Hydroiodic Acid Reaction
The reaction with hydroiodic acid (HI) is a classical method for the cleavage of ethers. HI is a strong acid that dissociates into H+ and I- ions in solution.
  • The hydrogen ion (H+) is responsible for protonating the ether oxygen, enhancing its leaving group's ability.
  • The iodide ion (I-) acts as the nucleophile, facilitating the cleavage of the C-O bond and forming an alkyl iodide.
Consider the example of compound (a). The HI attacks the less hindered methoxy group ( O- CH3), resulting in the formation of allyl alcohol and methyl iodide. This cleavage mechanism may vary depending on the steric and electronic nature of the ethers involved.
Organic Chemistry Reactions
Organic chemistry involves numerous reactions, each with distinct mechanisms and pathways. Understanding ether cleavage in the context of organic chemistry reactions highlights essential principles like nucleophilic substitution and reaction conditions. These reactions are foundational in understanding more complex organic synthesis.
  • Ether cleavage demonstrates the interplay of charge, reactivity, and reaction conditions in organic processes.
  • Each reaction typically involves analyzing variables such as substituents, leaving groups, and the functional group environment to predict the products accurately.
In ether cleavage, we particularly explore how different structural elements and conditions, such as in the presence of hydroiodic acid, lead to varying products, emphasizing the broader applicability of organic reaction principles.

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Most popular questions from this chapter

Triethyloxonium fluoroborate can be prepared from 1-chloromethyloxacyclopropane and a \(\mathrm{BF}_{3}\) -etherate according to the equation The boron in the complex boron anion ends up as \(\mathrm{BF}_{4}^{-}\), but the details of this reaction need not concern you. Write the steps that you expect to be involved in the reaction to form \(\mathrm{R}_{3} \mathrm{O}^{\oplus}\) and that you can support by analogy with other reactions discussed in this chapter.

An alternative and plausible mechanism for esterification of carboxylic acids is shown by the following steps: This mechanism corresponds to an \(S_{\mathrm{N}} 2\) displacement of water from the methyloxonium ion by the acid. How could you distinguish between this mechanism and the addition-elimination mechanism using heavy oxygen \(\left({ }^{18} \mathrm{O}\right)\) as a tracer?

The diethyl ester of cis-butenedioic acid can be prepared by heating the corresponding anhydride with ethanol and concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) in benzene in a mole ratio of perhaps \(1: 2.5: 0.25 .\) Write the steps that occur in this reaction and explain how the use of benzene and more than a catalytic amount of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) makes the formation of the diethyl ester thermodynamically more favorable than with just a catalytic amount of \(\mathrm{H}_{2} \mathrm{SO}_{4}\).

a. Explain why trimethyloxonium salts will convert alcohols to methyl ethers at neutral or acidic \(\mathrm{pH}\), whereas either methyl iodide or dimethyl sulfate requires strongly basic reaction conditions. b. What products would you expect from the reactions of trimethyloxonium fluoroborate with (1) ethanethiol, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{SH}\), (2) diethylamine, \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{NH}\), and (3) hydrogen bromide.

What type of infrared absorption bands due to hydroxyl groups would you expect for trans-cyclobutane-1,2-diol and butane-1,2-diol (a) in very dilute solution, (b) in moderately concentrated solution, and (c) as pure liquids? Give your reasoning.
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