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Ch. 3 - Alkanes and Cycloalkanes: Properties and Conformational Analysis
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
Chapter 2, Problem 13a

There are five alkane constitutional isomers with the molecular formula C6H14. Draw them.

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Step 1: Understand the concept of constitutional isomers. Constitutional isomers are compounds with the same molecular formula but different connectivity of atoms. For the molecular formula C₆H₁₄, we are tasked with finding all possible unique structures where the carbon and hydrogen atoms are connected differently.
Step 2: Start with the simplest structure, the straight-chain alkane. Draw n-hexane, which is a straight chain of six carbon atoms (C-C-C-C-C-C) with hydrogens filling the remaining valences to satisfy the octet rule.
Step 3: Create isomers by branching the carbon chain. Begin by shortening the main chain to five carbons and adding one methyl group (-CH₃) as a branch. This gives you 2-methylpentane (branch on the second carbon) and 3-methylpentane (branch on the third carbon). Ensure that the branches are placed in unique positions to avoid duplicates.
Step 4: Further branch the carbon chain. Shorten the main chain to four carbons and add two methyl groups (-CH₃) as branches. Place the branches on different carbons to create unique structures. This results in 2,2-dimethylbutane (both branches on the second carbon) and 2,3-dimethylbutane (branches on the second and third carbons).
Step 5: Verify that you have all five isomers. Check that each structure has the molecular formula C₆H₁₄ and that no two structures are identical. The five isomers are n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane.

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Key Concepts

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

Alkanes

Alkanes are saturated hydrocarbons consisting only of carbon (C) and hydrogen (H) atoms, connected by single bonds. They follow the general formula CnH2n+2, where n is the number of carbon atoms. Alkanes are characterized by their relatively low reactivity and are commonly found in natural gas and petroleum.
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Constitutional Isomers

Constitutional isomers are compounds that have the same molecular formula but differ in the connectivity of their atoms. This means that the arrangement of atoms in the molecule varies, leading to different structural forms. For C₆H₁₄, the five constitutional isomers include straight-chain and branched forms, each with unique properties.
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Drawing Structural Formulas

Drawing structural formulas involves representing the arrangement of atoms in a molecule, including bonds between them. For alkanes, this includes depicting carbon chains and the hydrogen atoms attached to each carbon. Understanding how to accurately draw these structures is essential for visualizing isomers and predicting their chemical behavior.
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