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Ch.9 - Molecular Geometry and Bonding Theories
All textbooksBrown 14th EditionCh.9 - Molecular Geometry and Bonding TheoriesProblem 120b
Chapter 9, Problem 120b

The organic molecules shown here are derivatives of benzene in which six-membered rings are 'fused' at the edges of the hexagons.
(b) Suppose you are given a sample of one of the compounds. Could combustion analysis be used to determine unambiguously which of the three it is?

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1
Understand the concept of combustion analysis: Combustion analysis is a method used to determine the elemental composition of a compound by burning the sample and analyzing the resulting products, typically carbon dioxide and water.
Recognize the output of combustion analysis: This technique provides the ratios of carbon, hydrogen, and sometimes other elements like nitrogen or sulfur in the compound, but it does not provide structural or isomeric information.
Consider the structural similarities of the derivatives: If the compounds are isomers (same molecular formula but different structures), combustion analysis will yield the same carbon to hydrogen ratio for each compound, making it impossible to distinguish between them based solely on this analysis.
Evaluate the need for additional analytical techniques: To unambiguously identify which derivative of benzene you have, consider using spectroscopic methods such as NMR (Nuclear Magnetic Resonance) or IR (Infrared Spectroscopy), which can provide information about the molecular structure.
Conclude the suitability of combustion analysis: Determine that while combustion analysis can confirm the elemental composition, it cannot differentiate between structural isomers, and thus, it cannot be used alone to unambiguously identify the specific derivative of benzene in the sample.

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Key Concepts

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

Combustion Analysis

Combustion analysis is a technique used to determine the elemental composition of organic compounds. In this method, a sample is burned in excess oxygen, producing carbon dioxide and water. By measuring the amounts of these products, one can calculate the ratios of carbon and hydrogen in the original compound, which can help identify the compound's structure.
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Isomerism

Isomerism refers to the phenomenon where compounds have the same molecular formula but different structural arrangements or spatial orientations. In the context of fused benzene derivatives, isomers can exhibit distinct physical and chemical properties, which may affect their combustion products. Understanding isomerism is crucial for interpreting combustion analysis results.
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Molecular Formula vs. Structural Formula

The molecular formula indicates the number and types of atoms in a compound, while the structural formula shows how these atoms are arranged. In combustion analysis, knowing the molecular formula is essential, but distinguishing between different structural isomers is necessary to determine the specific compound. This distinction is vital when analyzing combustion products to identify the original compound.
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Related Practice
Textbook Question
Many compounds of the transition-metal elements containdirect bonds between metal atoms. We will assumethat the z-axis is defined as the metal–metal bond axis.(d) Sketch the energyleveldiagram for the Sc2 molecule, assuming that only the3d orbital from part (a) is important.
Textbook Question
The organic molecules shown here are derivatives of benzenein which six-membered rings are 'fused' at the edgesof the hexagons.

(e) Benzene, naphthalene, and anthraceneare colorless, but tetracene is orange. What does this implyabout the relative HOMO–LUMO energy gaps in these molecules?See the 'Chemistry Put to Work' box on orbitalsand energy.
Textbook Question

Methyl isocyanate, CH3NCO, was made infamous in 1984 when an accidental leakage of this compound from a storage tank in Bhopal, India, resulted in the deaths of about 3800 people and severe and lasting injury to many thousands more. (a) Draw a Lewis structure for methyl isocyanate.