Lecture Notes: Drawing Constitutional Isomers

Key Concepts

• Constitutional Isomers:
  • Have the same chemical formula but different connectivity.
  • They are also called structural isomers.

Examples of Constitutional Isomers

Butane (C4H10)

• Straight Chain: All four carbons in a line.
• Branched: Three carbons in a row with a methyl group on the second carbon.

Pentane (C5H12)

• Straight Chain: Five carbons in a single line.
• Branched:
  • Four carbons with a methyl group on the second carbon.
  • Three carbons with two methyl groups on the second carbon.

Hexane (C6H14)

• Straight Chain: Six carbons in a row.
• Branched:
  • Five carbons with a methyl group on the second or third carbon.
  • Four carbons with two methyl groups on carbons 2, 3, or 4.
• Naming Convention: Isomers must have different names.

Heptane (C7H16)

• Variety of Chains:
  • Seven carbons in a straight chain.
  • Six carbons with a methyl group on carbons 2 or 3.
  • Five carbons with two methyl groups on various carbons.
  • Four carbons with three methyl groups or an ethyl group.

Octane (C8H18)

• Eighteen Constitutional Isomers:
  • Start with an 8-carbon straight chain.
  • Shorten chain and add various groups like methyl or ethyl.

Special Cases

C2H6O

• Two Isomers:
  • Ether: Dimethyl ether.
  • Alcohol: Ethanol.

Four Carbons, C4H8

• Possible Structures:
  • Alkenes with double bonds (cis, trans isomers).
  • Cycloalkanes, e.g., cyclobutane.

Stereoisomers

• Cis-Trans Isomers:
  • Not constitutional isomers but stereoisomers.
  • Different spatial arrangement despite having the same connectivity.

Conclusion

• Constitutional isomers have the same molecular formula but differ in connectivity.
• Naming and spatial arrangement are crucial for differentiating isomers.
• Stereoisomers are a different category where atoms are connected in the same order but differ in spatial orientation.

By understanding these basic principles, you can draw and identify constitutional isomers effectively.