Draw line-angle formulas for the nine constitutional isomers with the molecular formula C7H16

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  1. Draw line-angle formulas for the nine constitutional isomers with the molecular formula C7H16.
  2. Write the IUPAC names for isobutane and isopentane.
  3. The condensed structural formula of butane is CH3CH2CH2CH3. Explain why this formula does not show the geometry of the real molecule.

The Correct Answer and Explanation is :

1. Line-Angle Formulas for the Nine Constitutional Isomers of C7H16

The molecular formula C7H16 corresponds to heptane and its structural isomers. Below are the line-angle formulas for the nine constitutional isomers:

  1. Heptane (n-heptane):
    • Structure: CH3-CH2-CH2-CH2-CH2-CH2-CH3
  2. 2-Methylhexane:
    • Structure: CH3-CH2-CH2-CH2-CH2-CH3 (methyl group at the second carbon)
  3. 3-Methylhexane:
    • Structure: CH3-CH2-CH2-CH2-CH2-CH3 (methyl group at the third carbon)
  4. 2,2-Dimethylpentane:
    • Structure: CH3-CH2-CH(CH3)2-CH2-CH3
  5. 2,3-Dimethylpentane:
    • Structure: CH3-CH(CH3)-CH(CH3)-CH2-CH3
  6. 3,3-Dimethylpentane:
    • Structure: CH3-C(CH3)2-CH2-CH2-CH3
  7. 2,4-Dimethylpentane:
    • Structure: CH3-CH2-CH(CH3)-CH2-CH3 (methyl groups at positions 2 and 4)
  8. 3-Ethylpentane:
    • Structure: CH3-CH2-CH2-CH(CH2CH3)-CH3
  9. 2,2,3-Trimethylbutane:
    • Structure: CH3-C(CH3)2-CH(CH3)-CH3

2. IUPAC Names for Isobutane and Isopentane

  • Isobutane is 2-methylpropane.
  • Isopentane is 2-methylbutane.

3. Explanation of Why the Condensed Structural Formula of Butane (CH3CH2CH2CH3) Does Not Show the Geometry of the Real Molecule

The condensed structural formula CH3CH2CH2CH3 represents butane, a straight-chain alkane, and is often used for simplicity. However, this formula does not convey the full 3D geometry of the actual molecule for several reasons:

  1. Representation of Bonding: The condensed formula shows the carbon atoms in a linear sequence with their attached hydrogen atoms but does not explicitly show the bonding angles or the arrangement of atoms in space. This format implies a linear structure, even though the atoms in a real molecule have specific geometric orientations. For example, the bond angles in butane are approximately 109.5° for each carbon atom due to the tetrahedral geometry of the sp³ hybridized orbitals.
  2. Lack of Spatial Arrangement: In butane, the carbon atoms are sp³ hybridized, meaning each carbon forms four bonds that arrange themselves in a tetrahedral geometry. The condensed formula doesn’t show this 3D shape, so it omits the fact that the molecule adopts a specific three-dimensional arrangement in space, where atoms are not perfectly aligned in a flat, linear structure.
  3. Conformation of the Molecule: Butane can adopt different conformations (e.g., staggered or eclipsed). The condensed formula does not describe these conformations, which involve the relative rotation of the carbon-carbon bonds and the resulting 3D orientation of the hydrogen atoms around each carbon. In reality, the molecule oscillates between these conformations, influencing its physical properties, but these details are not captured in the condensed formula.

In conclusion, while CH3CH2CH2CH3 is a convenient shorthand for representing butane’s connectivity, it simplifies the actual geometry by omitting crucial 3D structural details. These details include the tetrahedral bond angles and conformational variability, which are essential for a complete understanding of the molecule’s structure and behavior.

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