Ch. 16 - Aromatic Compounds

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

Wade 9th Edition

Ch. 16 - Aromatic Compounds

Problem 50

Chapter 16, Problem 50

Chlorophyll is the general name for a family of compounds present in algae and green plants. These molecules use the energy in sunlight to convert carbon dioxide and water into carbohydrates and other energy sources. At the heart of chlorophyll (shown below) is a large-ring magnesium complex called a chlorin. Circle each double bond in the large cyclic conjugated pi system that makes it aromatic. How many pi electrons are in this aromatic system?

Verified step by step guidance

Step 1: Identify the large cyclic conjugated pi system in the chlorin ring structure. This is the central ring system containing magnesium (Mg) and nitrogen atoms, along with alternating double bonds.

Step 2: Circle each double bond in the cyclic conjugated pi system. Look for alternating single and double bonds within the ring structure, which contribute to conjugation and aromaticity.

Step 3: Count the number of pi electrons in the conjugated system. Each double bond contributes 2 pi electrons, and lone pairs on nitrogen atoms may also contribute if they are part of the conjugated system.

Step 4: Apply Hückel's rule for aromaticity, which states that a molecule is aromatic if it has (4n + 2) pi electrons, where n is a non-negative integer. Verify if the total number of pi electrons satisfies this rule.

Step 5: Confirm the aromaticity of the chlorin ring system based on the conjugated pi electron count and Hückel's rule. This will determine if the system is aromatic.

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Key Concepts

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

Aromaticity

Aromaticity refers to a property of cyclic compounds that exhibit enhanced stability due to the delocalization of pi electrons across the ring structure. For a compound to be considered aromatic, it must follow Hückel's rule, which states that it should have a planar structure and contain a specific number of pi electrons (4n + 2, where n is a non-negative integer). This concept is crucial for understanding the stability and reactivity of compounds like chlorophyll.

Conjugated Pi System

A conjugated pi system consists of alternating single and double bonds, allowing for the overlap of p orbitals and the delocalization of electrons. In chlorophyll, the conjugated system contributes to its aromatic character and plays a significant role in its ability to absorb light. Identifying the double bonds in this system is essential for determining the number of pi electrons involved.

Pi Electrons

Pi electrons are the electrons found in the pi bonds of a molecule, which are formed by the sideways overlap of p orbitals. In the context of aromatic compounds, the count of pi electrons is critical for assessing aromaticity. In chlorophyll, counting the pi electrons in the cyclic conjugated system will help determine its stability and reactivity, as well as its role in photosynthesis.

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Related Practice

Textbook Question

Hexahelicene seems a poor candidate for optical activity because all its carbon atoms are sp² hybrids and presumably flat. Nevertheless, hexahelicene has been synthesized and separated into enantiomers. Its optical rotation is enormous: [α]_D = 3700°. Explain why hexahelicene is optically active, and speculate as to why the rotation is so large.

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Textbook Question

An unknown compound gives the following mass, IR, and NMR spectra. Propose a structure, and show how it is consistent with the spectra. Show the fragmentations that give the prominent peaks at m/z 127 and 155 in the mass spectrum.

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Textbook Question

Consider the following compound, which has been synthesized and characterized:

a. Assuming this molecule is entirely conjugated, do you expect it to be aromatic, antiaromatic, or nonaromatic?

b. Why was this molecule synthesized with three tert-butyl substituents? Why not make the unsubstituted compound and study it instead?

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