Ch. 5 - Alkenes: Structure, Nomenclature, and an Introduction to Reactivity • Thermodynamics and Kinetics

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 5 - Alkenes: Structure, Nomenclature, and an Introduction to Reactivity • Thermodynamics and Kinetics

Problem 24a

Chapter 6, Problem 24a

What alkene would you start with if you wanted to synthesize
a. pentane?

Verified step by step guidance

Understand the problem: To synthesize pentane, we need to identify an alkene that can undergo a reaction (such as hydrogenation) to produce pentane. Hydrogenation involves adding hydrogen (H₂) across the double bond of an alkene to form an alkane.

Recall the structure of pentane: Pentane is a straight-chain alkane with the molecular formula C₅H₁₂. It has no double bonds and consists of five carbon atoms in a continuous chain.

Determine the corresponding alkene: To form pentane, the starting alkene must have five carbon atoms and one double bond. This is because the double bond will be reduced during hydrogenation to form the single bonds present in pentane.

Choose the simplest alkene: The simplest alkene with five carbon atoms is 1-pentene (CH₂=CH-CH₂-CH₂-CH₃). Alternatively, 2-pentene (CH₃-CH=CH-CH₂-CH₃) could also work, as both will yield pentane upon hydrogenation.

Plan the reaction: Use a catalytic hydrogenation reaction, typically with H₂ gas and a metal catalyst like Pd, Pt, or Ni, to reduce the double bond in the chosen alkene (e.g., 1-pentene or 2-pentene) and produce pentane.

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Key Concepts

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

Alkenes and Their Reactivity

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. They are more reactive than alkanes due to the presence of this double bond, which can participate in various chemical reactions such as addition reactions. Understanding the reactivity of alkenes is crucial for predicting the products of reactions and for planning synthetic pathways.

Synthesis of Alkanes

Alkanes are saturated hydrocarbons with single bonds between carbon atoms. To synthesize an alkane like pentane, one can start with an alkene and perform hydrogenation or other reactions that add hydrogen atoms to the double bond, converting it into a single bond. Recognizing the relationship between alkenes and alkanes is essential for designing effective synthetic routes.

Chain Length and Isomerism

Pentane has five carbon atoms and can exist in different structural forms, known as isomers. When synthesizing pentane from an alkene, it is important to consider the chain length and the potential for branching, as these factors influence the choice of starting material. Understanding isomerism helps in selecting the appropriate alkene to achieve the desired alkane structure.

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Related Practice

Textbook Question

The same alkane is obtained from the catalytic hydrogenation of both alkene A and alkene B. The heat of hydrogenation of alkene A is 29.8 kcal/mol, and the heat of hydrogenation of alkene B is 31.4 kcal/mol. Which alkene is more stable?

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Textbook Question

a. For a reaction with ∆H° = -12 kcal/mol and ∆S° = 0.01 kcal mol^-1 K^-1, calculate the ∆G° and the equilibrium constant at:

1. 30 °C and 2. 150 °C.

b. How does ∆G° change as T increases?

c. How does K_eq change as T increases?

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Textbook Question

What alkene would you start with if you wanted to synthesize

b. ethylcyclopentane?

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Textbook Question