Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions

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

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Mullins 1st Edition

Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions

Problem 58

Chapter 9, Problem 58

A chemist attempted to do the following acetylide alkylation reaction but was unsuccessful in several attempts, producing only the original starting materials in each case. Explain why the reaction didn't work.

Verified step by step guidance

Understand the reaction: Acetylide alkylation involves the reaction of a terminal alkyne with a strong base to form an acetylide ion, which then reacts with an alkyl halide to form a new carbon-carbon bond. The failure of this reaction suggests an issue with one or more of these steps.

Step 1: Analyze the base used. For the formation of the acetylide ion, a sufficiently strong base (e.g., sodium amide \( \text{NaNH}_2 \)) is required to deprotonate the terminal alkyne. If the base is not strong enough, the acetylide ion will not form, and the reaction will fail.

Step 2: Examine the alkyl halide. Acetylide ions are strong nucleophiles, and they react best with primary alkyl halides via an \( S_N2 \) mechanism. If the alkyl halide is secondary or tertiary, steric hindrance will prevent the \( S_N2 \) reaction, and elimination (\( E2 \)) may occur instead, leading to no desired product.

Step 3: Consider solvent effects. The reaction typically requires an aprotic solvent (e.g., tetrahydrofuran or dimethyl sulfoxide) to stabilize the acetylide ion and facilitate the \( S_N2 \) reaction. If a protic solvent is used, it may interfere with the nucleophile or promote side reactions.

Step 4: Check for competing reactions. If the alkyl halide is prone to elimination (e.g., due to the presence of a strong base or a bulky alkyl halide), the reaction may favor \( E2 \) elimination over \( S_N2 \) substitution, resulting in no formation of the desired product.

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Key Concepts

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

Acetylide Ion

An acetylide ion is a negatively charged species formed by deprotonating a terminal alkyne. It is a strong nucleophile, capable of attacking electrophiles in nucleophilic substitution reactions. Understanding the stability and reactivity of acetylide ions is crucial for predicting their behavior in alkylation reactions.

Nucleophilic Substitution Mechanisms

Nucleophilic substitution mechanisms, such as SN2 and SN1, describe how nucleophiles replace leaving groups in organic molecules. The success of an alkylation reaction involving acetylide ions depends on the nature of the substrate and the mechanism involved. Factors like steric hindrance and the stability of the leaving group can significantly influence the reaction outcome.

Steric Hindrance

Steric hindrance refers to the prevention of chemical reactions due to the spatial arrangement of atoms within a molecule. In the context of acetylide alkylation, if the electrophile is sterically hindered (e.g., tertiary alkyl halides), the acetylide ion may struggle to effectively attack, leading to a lack of reaction and the persistence of starting materials.

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Related Practice

Textbook Question

Ethinylestradiol is a synthetic hormone mimic used as a contraceptive for its ability to prevent ovulation. Suggest a mechanism for the synthesis using sodium acetylide and estrone, followed by quenching with acid.

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Textbook Question

An alternate method for the synthesis of alkynes relies on the double elimination of H―Br from a dihaloalkane under basic conditions. Suggest a mechanism for this reaction that we discuss in Chapter 12.

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Textbook Question

The addition of H―X to alkynes has been shown to occur predominately via anti addition:

Two chemists disagreed on whether or not anti addition would happen on terminal alkynes as well. Suggest an experiment through which you could resolve this dispute.

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Textbook Question

The synthesis of five-membered lactones (cyclic esters) has been accomplished using the electrophilic addition of I―Cl to an alkyne. Suggest a mechanism for this cyclization reaction. (Structure modification of Yao, T.; Larock, R.C. J. Org. Chem. 2005, 70, 1432–1437.)

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Textbook Question

In 1973, Caine and Tuller reported a synthesis of racemic oplapanone, a sesquiterpene isolated from Oplopanax japonicus (a deciduous shrub) involving a reaction we learned in this chapter. Predict the product of the reaction shown. (Caine, D.; Tuller, F. N. J. Org. Chem. 1973, 38, 3663.)

Textbook Question

When alkynes are treated with water and bromine a bromoketone is produced. Provide a plausible arrow-pushing mechanism that accounts for the formation of this product.

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