Ch. 7 - Structure and Synthesis of Alkenes; Elimination

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 7 - Structure and Synthesis of Alkenes; Elimination

Problem 35a

Chapter 7, Problem 35a

Show the product(s) you expect from dehydration of the following alcohols when they are heated in sulfuric or phosphoric acid. In each case, use a mechanism to show how the products are formed.
(a)

Verified step by step guidance

Step 1: Recognize that the reaction involves the dehydration of alcohols using sulfuric acid (H₂SO₄) and heat. This is an elimination reaction (E1 mechanism) where the hydroxyl group (-OH) is removed, forming an alkene.

Step 2: Protonation of the hydroxyl group occurs first. The sulfuric acid donates a proton (H⁺) to the alcohol, converting the -OH group into a better leaving group, water (H₂O). This step increases the reactivity of the molecule.

Step 3: The water molecule leaves, forming a carbocation intermediate. In this case, the carbocation formed is a benzylic carbocation, which is highly stable due to resonance with the benzene ring.

Step 4: The carbocation undergoes elimination of a proton (H⁺) from an adjacent carbon atom to form the double bond. The most stable alkene product is formed, following Zaitsev's rule, which states that the more substituted alkene is favored.

Step 5: Draw the final product, which is the alkene formed after elimination. For this molecule, the product is styrene (C₆H₅-CH=CH₂), as the double bond forms between the benzylic carbon and the adjacent carbon.

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Key Concepts

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

Dehydration of Alcohols

Dehydration of alcohols involves the removal of a water molecule from the alcohol, typically in the presence of an acid catalyst like sulfuric or phosphoric acid. This process leads to the formation of alkenes. The reaction can proceed via either an E1 or E2 mechanism, depending on the structure of the alcohol and the reaction conditions.

E1 and E2 Mechanisms

The E1 mechanism is a two-step process where the alcohol first ionizes to form a carbocation intermediate, followed by the loss of a proton to form the alkene. The E2 mechanism is a one-step process where the base abstracts a proton while the leaving group (water) departs simultaneously, resulting in the formation of the alkene. The choice between these mechanisms depends on the substrate and reaction conditions.

Regioselectivity in Elimination Reactions

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others. In dehydration reactions, the more substituted alkene is often favored due to greater stability, following Zaitsev's rule. Understanding regioselectivity is crucial for predicting the major product when multiple elimination pathways are possible.

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Related Practice

Textbook Question

Show the product(s) you expect from dehydration of the following alcohols when they are heated in sulfuric or phosphoric acid. In each case, use a mechanism to show how the products are formed.

(b)

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Textbook Question

Show the product(s) you expect from dehydration of the following alcohols when they are heated in sulfuric or phosphoric acid. In each case, use a mechanism to show how the products are formed.

(c)

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Textbook Question

The dehydrogenation of butane to trans-but-2-ene has ΔH° = +116 kJ/mol (+27.6 kcal/mol) and ΔS° = +117J/kelvin-mol (+28.0 cal/kelvin-mol).

a. Compute the value of ΔG° for dehydrogenation at room temperature (25 °C or 298 °K). Is dehydrogenation favored or disfavored?

HINT: When you are doing synthesis problems, avoid using these high-temperature industrial methods. They require specialized equipment, and they produce variable mixtures of products.

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Textbook Question

Propose mechanisms for the following reactions.

(d)

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Textbook Question

Propose mechanisms for the following reactions.

(c)

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