Introduction to Aromatic Hydrocarbons

Hydrocarbons are categorized into open-chain and closed-chain hydrocarbons. In the closed-chain hydrocarbons, there is another sub-divison of homonuclear and heteronuclear hydrocarbons. An aromatic hydrocarbon is a closed-chain hydrocarbon consiting of carbon atoms linked together in forming a closed ring-like structure. In the early days, aromatic hydrocarbons were characterized by their sweet smell - termed as aroma. An aromatic hydrocarbon can be homonuclear or heteronuclear both.

In modern days, aromatic hydrocarbons are also defined as benzene and derivatives of benzene. Examples are: Benzene, Pyridine, Napthalene, etc. 

Figure given alongside is of Benzene (C6H6). The empirical formula of Benzene is (CH). 

Characteristics of Aromatic Hydrocarbons

Following are the characteristics of aromatic hydrocarbons:

• They have closed ring-like structure and a conjugated system.
• As discussed in the introduction section, they are closed-chain hydrocarbons. Also, they have a conjugated system. It means that there exists alternate double bond or delocalized pi-electrons in the system of carbon atoms.
• They have high resistance towards addition reaction.
• No compound in nature would like to loose its property after getting invovled in a chemical reaction. If an aromatic hydrocarbon gets involved in addition reaction, it looses its aromaticity.
• They are highly unsaturated hydrocarbons. They contain multiple double bonds in their carbon structure.
• Aromatic hydrocarbons produce a dark and sooty flame when burnt completely.
• These compounds exhibit electrophilic substitution reaction.

Following are the criteria:

• The carbon structure should be cyclic and planar. All the carbon atoms should lie on the same plane.
  
• It should have a conjugated system.
• It should follow Huckel's rule of Aromaticity (4n+2) pi electrons where n = 0,1,2,3,... .

• Rules for Aromaticity: The 4 Key Factors – Master Organic Chemistry
• Benzene-2D-flat.png (958×1100) (wikimedia.org)