Ch. 8 - Alkenes I: Properties and Electrophilic Additions

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

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Mullins 1st Edition

Ch. 8 - Alkenes I: Properties and Electrophilic Additions

Problem 76b

Chapter 7, Problem 76b

North American termite soldiers, when encountering enemy insects, contract their mandibular muscles, expelling a mixture of chemicals that essentially trap their enemies in a glue-like substance. This weapon, built into the face of the termite, is called the fontanellar gun. It releases a mixture of pinene (62%), myrcene (27%), and limonene (11%).

(b) Suggest an acid-catalyzed mechanism by which pinene could be produced from limonene.

Verified step by step guidance

Step 1: Recognize that the transformation involves an acid-catalyzed reaction. The presence of H₃O⁺ indicates that protonation will play a key role in the mechanism.

Step 2: Protonate the double bond in limonene. The π-electrons of the double bond will attack the proton (H⁺), forming a carbocation intermediate. This step increases the electrophilicity of the molecule.

Step 3: Consider the stability of the carbocation intermediate. The carbocation formed will undergo rearrangement if necessary to achieve a more stable tertiary carbocation. This rearrangement is facilitated by the cyclic structure of limonene.

Step 4: Once the carbocation is stabilized, a cyclization reaction occurs. The carbocation interacts with another double bond in the molecule, forming the bicyclic structure characteristic of pinene.

Step 5: Deprotonation occurs to restore neutrality, completing the formation of pinene. The acid catalyst (H₃O⁺) is regenerated in this step, allowing the reaction to proceed catalytically.

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Key Concepts

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

Acid-Catalyzed Reactions

Acid-catalyzed reactions involve the use of an acid to increase the rate of a chemical reaction. In organic chemistry, acids can protonate reactants, making them more electrophilic and facilitating nucleophilic attacks. This mechanism is crucial for transformations such as rearrangements and eliminations, which are often seen in the conversion of one hydrocarbon to another, like limonene to pinene.

Isomerization

Isomerization is a process where a molecule is transformed into one of its isomers, which have the same molecular formula but different structural arrangements. In the context of organic compounds like limonene and pinene, isomerization can involve the rearrangement of double bonds or the shifting of functional groups, leading to different chemical properties and reactivity. Understanding this concept is essential for predicting the products of acid-catalyzed reactions.

Terpene Structure and Reactivity

Terpenes are a large class of organic compounds, often characterized by their strong odors and derived from plants. They are built from isoprene units and can undergo various reactions, including cyclization and rearrangement. The specific structures of terpenes like pinene, myrcene, and limonene influence their reactivity and the types of reactions they can undergo, making it important to analyze their structures when proposing mechanisms for their interconversion.

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Related Practice

Textbook Question

The acid-catalyzed hydration we learned here in Chapter 8 is reversible:

(d) How might you shift the equilibrium to the right?

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Textbook Question

In light of your answer to Assessment 8.73, predict the product of the following oxymercuration–reduction reactions, each of which results in a single product. [Don't worry about the absolute stereochemistry, though these reactions are also stereoselective.]

(c) Oxymercuration–reduction was used in the stereoselective synthesis of the macrolactone core of neopeltolide, a marine macrolide isolated from a Caribbean sponge that has potent anticancer activity (Org. Lett. 2012, 14, 2346–2349).

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Textbook Question

The acid-catalyzed hydration we learned here in Chapter 8 is reversible.

(a) Propose a mechanism for the formation of an alkene from an alcohol.

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Textbook Question

North American termite soldiers, when encountering enemy insects, contract their mandibular muscles, expelling a mixture of chemicals that essentially trap their enemies in a glue-like substance. This weapon, built into the face of the termite, is called the fontanellar gun. It releases a mixture of pinene (62%), myrcene (27%), and limonene (11%). pinene myrcene limonene 62% 27% 11%

(a) Identify the isoprene units in pinene, myrcene, and limonene.

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Textbook Question

The acid-catalyzed hydration we learned here in Chapter 8 is reversible:

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Textbook Question

The formation of five-membered ring ethers is an important goal in synthetic organic chemistry because tetrahydrofurans are contained within a number of antitumor natural products. Toward that end, a one-pot synthesis of a bis-THF containing compound was developed (Eur. J. Org. Chem. 2010, 6263–6268). Suggest a mechanism for this transformation.

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