Before drawing all the isomers of butane (C₄H₁₀), let’s quickly recall that isomers are compounds that have the same molecular formula but different connectivity of atoms. In simpler words, the composition is the same, but the atoms are connected in different ways, and that is why we also call them constitutional isomers.
Butane has two constitutional isomers. In one, called n-butane, four carbon atoms are joined in a continuous, unbranched chain. The “n” in n-butane stands for “normal” and indicates that the carbon skeleton is not branched. The second isomer has a branched carbon chain and is called isobutane:
Sometimes, molecules are represented by condensed structural formulas, where the carbon atoms are written one after another with the appropriate number of hydrogen atoms attached to each carbon.
CH₃CH₂CH₂CH₃ (n-butane) and CH₃CH(CH₃)CH₃, or equivalently (CH₃)₃CH (isobutane).
So, n-butane contains two methyl (CH₃) groups at the ends and two methylene (CH₂) groups in the middle. Isobutane, on the other hand, contains three methyl groups attached to a central carbon atom.
If you have already covered the bond-line notation, where we omit the carbon atoms, you can also show these isomers like this:
Bonding in both molecules follows the same general pattern seen in methane, ethane, and propane. All carbon atoms are sp³-hybridized, all bonds are σ bonds, and the bond angles are close to the ideal tetrahedral 109.5o. This description applies to all alkanes, regardless of chain length or branching:
The Physical Properties of Butane
Because of this structural difference, n-Butane and isobutane also have different physical properties. Both are gases at room temperature, but n-butane boils at -0.4 °C, whereas isobutane boils at about -10.2 °C. Similarly, their melting points differ significantly, with n-butane melting at -139 °C and isobutane at −160.9 °C.
This is consistent with what we learned about the boiling and melting point patterns of straight and branched alkanes. Remember, the boiling point decreases quite significantly as we move towards the more branched isomers. And this is a demonstration of a direct relationship between the surface area and the boiling point.
Butane is unbranched and provides a large surface for intermolecular interactions. Isobutane has one substituent, so it is a little more branched than butane, which reduces the surface for intermolecular interactions and lowers the boiling point of the molecule.
Check Also
- Naming Alkanes by IUPAC Nomenclature Rules Practice Problems
- Naming Bicyclic Compounds
- Naming Bicyclic Compounds-Practice Problems
- How to Name a Compound with Multiple Functional Groups
- Primary, Secondary, and Tertiary Carbon Atoms in Organic Chemistry
- Constitutional or Structural Isomers with Practice Problems
- Degrees of Unsaturation or Index of Hydrogen Deficiency
- The Wedge and Dash Representation
- Sawhorse Projections
- Newman Projections with Practice Problems
- Staggered and Eclipsed Conformations
- Conformational Isomers of Propane
- Newman Projection and Conformational Analysis of Butane
- Newman Projection of Chair Conformation
- Gauche Conformation
- Gauche Conformation, Steric, Torsional Strain Energy Practice Problems
- Ring Strain
- Steric vs Torsional Strain
- Conformational Analysis
- Drawing the Chair Conformation of Cyclohexane
- Ring Flip: Drawing Both Chair Conformations with Practice Problems
- 1,3-Diaxial Interactions and A value for Cyclohexanes
- Ring-Flip: Comparing the Stability of Chair Conformations with Practice Problems
- Cis and Trans Decalin
- IUPAC Nomenclature Summary Quiz
- Alkanes and Cycloalkanes Practice Quiz