Structural Isomers of Butane

Introduction

The first open-chain saturated hydrocarbon formula with two carbon skeletons is C₄H₁₀. Butane has an unbranched four-carbon path; 2-methylpropane has a three-carbon path with a one-carbon branch. They have equal atom inventories but different carbon neighbor relationships.

Core explanation

Butane is CH₃–CH₂–CH₂–CH₃. The two end carbons each connect to one carbon and have three H; the two inner carbons each connect to two carbons and have two H. Total H = 3 + 2 + 2 + 3 = 10. The longest connected carbon path has four atoms.

2-Methylpropane is CH₃–CH(CH₃)–CH₃. The middle carbon connects to three other carbons and has one H. Each outer carbon is CH₃. Total H = 3 + 1 + 3 + 3 = 10. The longest carbon path contains three atoms, so the parent is propane with a methyl group on carbon 2. Calling it “1-methylpropane” would merely create a four-carbon unbranched path and describe butane.

These structures are constitutional or structural isomers because their molecular formula C₄H₁₀ is the same while connectivity differs. Their different shapes affect intermolecular contact and physical properties. A molecule's formula does not capture this distinction. The name or a structural representation is needed to identify which one is meant.

Why are there exactly two in this class? For four connected carbons with no ring and only single bonds, a skeleton can form a path of four or a central carbon connected to three ends. A drawing with one carbon at a different paper position can usually be rotated or reflected into one of those two graphs. A four-carbon cycle would have C₄H₈, not C₄H₁₀, so it is excluded by the formula.

Careful enumeration avoids duplicate drawings. Draw the longest unbranched path first. Then create a branch point and check whether moving the branch along a three-carbon parent actually changes connectivity. The end positions are equivalent after reversing the chain, and attaching a methyl at an end lengthens the parent path. Only the middle-branch structure is new.

Three-dimensional conformations of butane are not additional chain isomers. Rotating around a C–C single bond changes relative positions of groups but preserves each atom's neighbors. A skeletal zig-zag, a straightened drawing and a rotated ball-and-stick model can all represent the same butane connectivity.

Step-by-step reasoning

1. Use C₄H₁₀ to restrict to an acyclic saturated four-carbon skeleton. 2. Draw a four-carbon path and fill H by valence. 3. Draw a three-carbon path with one carbon attached at the middle. 4. Check both formulas equal C₄H₁₀. 5. Test any other sketch for equivalence by carbon neighbor relationships.

Visual explanation

Draw four nodes in a line beside one central node with three outer nodes. Under them write CH₃CH₂CH₂CH₃ and CH₃CH(CH₃)CH₃. Circle the carbon with three carbon neighbors in the branched structure and show none in the unbranched path.

Real-world analogy

Four people can stand in a hand-holding line or one person can hold hands with three others. Moving the people around the room does not alter who holds whom. Carbon skeleton isomerism is about those connections, not the drawing's orientation.

Real-world example

Butane and 2-methylpropane are both gaseous hydrocarbons under ordinary ambient conditions but have different boiling points. Their formula equality makes shape and intermolecular-contact differences especially informative when comparing properties.

Why?

Why does branching not change H total here? Both four-carbon skeletons are connected, acyclic and saturated, so each has three C–C bonds. Those bonds use six of carbon's sixteen available bond orders, leaving ten C–H bonds.

Common misconception

“1-Methylpropane is a third C₄H₁₀ isomer.” A methyl group attached to the end of a three-carbon chain makes an unbranched four-carbon chain, which is butane.

Worked example

Check CH₃CH(CH₃)CH₃. Count four C atoms. Three CH₃ groups supply nine H, and the central CH supplies one, totaling ten. The central carbon has bonds to three carbons plus one H, satisfying valence four. Its carbon connectivity differs from CH₃CH₂CH₂CH₃, where no carbon has three carbon neighbors.

Quick check

1. How many distinct acyclic saturated structural isomers have formula C₄H₁₀? Answer: Two: unbranched butane and branched 2-methylpropane.

Exam focus

Show both structures, count H explicitly and identify the branch point. Reject duplicate reversed or rotated drawings. A ring is excluded because it would change the hydrogen count.

Advanced insight

Graph theory represents the two skeletons as nonisomorphic four-vertex trees: a path and a three-branch star. This gives a rigorous version of the neighbor-relationship test used in drawings.

Summary

C₄H₁₀ has two acyclic saturated constitutional isomers. Butane is a four-carbon path; 2-methylpropane has a central three-neighbor carbon. Formula and valence match, but connectivity differs. Rotation and redrawing do not make further isomers.

Practice questions

1. Write butane's condensed formula. Answer: CH₃CH₂CH₂CH₃. 2. Write 2-methylpropane's condensed formula. Answer: CH₃CH(CH₃)CH₃. 3. What is the central carbon's hydrogen count in 2-methylpropane? Answer: One H, because it has three C–C single bonds. 4. Why is cyclobutane not a C₄H₁₀ isomer? Answer: Its ring closure gives C₄H₈, a different molecular formula.