Isomerism

Isomerism: the phenomenon in which different compounds have the same molecular formula but different structural formulas, and therefore different physical and chemical properties.

Classic example, C₂H₆O:

• Ethanol CH₃CH₂OH — b.p. 78.5 °C, m.p. −117 °C; contains an –OH group; reacts with sodium giving hydrogen.
• Dimethyl ether CH₃–O–CH₃ — b.p. −24 °C (a gas at room temperature); no –OH, does not react with sodium.

(The much higher boiling point of ethanol is due to hydrogen bonds between its molecules.)

• Further example, C₄H₁₀ has two isomers: butane CH₃CH₂CH₂CH₃ and methylpropane (isobutane) (CH₃)₂CHCH₃.
• C₅H₁₂ has three: pentane, 2-methylbutane and 2,2-dimethylpropane.
• Practice: draw all isomers of C₅H₁₂ (or C₆H₁₄ – there are five).

• Structural formulas look flat, but real molecules are three-dimensional (for example the four bonds of carbon in methane point toward the corners of a tetrahedron, H–C–H angle 109.5°).
• Molecular models (coloured plastic balls and sticks) are used to picture the true shape.