13.3 STRUCTURAL FORMULA

Frequently more than one organic compounds are represented by the same molecular formula. However, they have different properties. They have different structural formulas. For example, ethanol and dimethyl ether have the same molecular formula C₂H₅O. But they have different structures.

In ethanol, the oxygen atom is bonded to a carbon atom and a hydrogen atom. Whereas in dimethyl ether, the oxygen atom is bonded to two carbon atoms.

Similarly, two organic compounds have the molecular formula C₃H₆O₃. They have different arrangement of atoms.

A formula that describes the arrangement of atoms in a molecule is called a structural formula.

The simple alkanes are straight-chain hydrocarbons. First three members of alkanes have following structural formulas.

Methane
CH₄
Ethane
C₂H₆
Propane
C₃H₈

The condensed structural formulas of these alkanes are CH₄, CH₃CH₃, CH₃CH₂CH₃. The corresponding molecular formulas are C₁H₄, C₂H₆, C₃H₈ respectively.

A condensed formula is a structural formula that uses established abbreviation for various groups of chain. In condensed structural formula, we list the main chain carbon atoms and the hydrogen atoms attached to them in the sequence in which they appear in the naming system.

Table 13.2: Properties of different states of matter

Name Molecular Formula Condensed Formula
Butane C₄H₁₀ CH₃CH₂CH₂CH₃
Pentane C₅H₁₂ CH₃CH₂CH₂CH₂CH₃
Hexane C₆H₁₄ CH₃CH₂CH₂CH₂CH₂CH₃
Heptane C₇H₁₆ CH₃CH₂CH₂CH₂CH₂CH₂CH₃
Octane C₈H₁₈ CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₃
Nonane C₉H₂₀ CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₃
Decane C₁₀H₂₂ CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₃

Example 13.1

Give the molecular formula, the structural formula and the condensed structural formula for pentane

Problem Solving Strategy

  1. The stem pent- means five carbon atoms.
  2. The ending -ane indicates an alkane.
  3. Write a string or chain of five carbon atoms.
  4. For the condensed molecular formula, write each carbon atom's set of hydrogen atoms next to the carbon.
  5. For molecular formula, simply count the carbon and hydrogen atoms or use the general formula CₙH₂ₙ₊₂ with n=5.
  6. Attach hydrogen atoms to the carbons to give each carbon atom four bonds. This requires three hydrogen atoms on each end carbon and two each on others.

Solution:

Structural formula

H₃C-CH₂-CH₂-CH₂-CH₃
Condensed Structural formula
C₅H₁₂

Molecular formula
C₅H₁₂

CONCEPT ASSESSMENT EXERCISE 13.2

Give the molecular, structural and condensed structural formulas for

  1. Butane
  2. Hexane
  3. Octane