Nomenclature of Alkenes, Alkynes, and Aromatic Compounds

Exam Tip: When comparing isomers, remember that polarity plays a role. For instance, cis-isomers often have higher boiling points than trans-isomers because their molecular polarity can lead to stronger intermolecular forces.

Alkynes: The Triple Bond

Alkynes are characterized by the presence of a carbon-carbon triple bond. They follow the general molecular formula $C_nH_{2n-2}$.

Naming alkynes follows a similar logic to alkenes: take the parent alkane name and change the suffix from -ane to -yne. The position of the triple bond is indicated by a number representing the first carbon atom involved in the bond.

Examples of Alkyne Naming:

• $HC \equiv CH$: Ethyne
• $HC \equiv C-CH_3$: Prop-1-yne
• $CH_3-C \equiv C-CH_3$: But-2-yne

Aromatic Compounds and Benzene

Aromatic compounds are a special class of cyclic molecules that possess unique stability. The most common aromatic structure is benzene ($C_6H_6$).

The Structure of Benzene

Unlike simple cyclic alkanes (like cyclohexane) or cyclic alkenes (like cyclohexa-1,3-diene), benzene does not behave like a typical alkene. In benzene:

• Each carbon atom is trigonal planar with bond angles of $120^\circ$.
• The molecule is entirely planar.
• The carbon-carbon bonds are all of equal length.

Resonance and Delocalization

In many textbooks, benzene is drawn as a hexagon with alternating single and double bonds. However, these are actually resonance structures. Neither the structure with alternating bonds nor a structure with only single bonds is the "true" molecule.

Instead, benzene is a resonance hybrid. In this hybrid state, the $\pi$ electrons are not fixed between specific carbons but are delocalized throughout the entire six-membered ring. This delocalization of electron density provides benzene with significant extra stability (lower energy) compared to other unsaturated hydrocarbons, making aromatic compounds much less reactive than alkenes.

To represent this delocalized nature simply, chemists frequently draw benzene as a hexagon with a circle inside.

Summary Table of Hydrocarbon Suffixes

To keep your nomenclature straight, refer to this hierarchy of functional group suffixes: