Do you know the difference between aromatic and aliphatic compounds?
Compounds of carbon and hydrogen form the basis of organic chemistry. Aromatic and aliphatic compounds are two broad classes of organic compounds.
Aliphatic compounds are formed when the carbon and hydrogen interact through straight chains, branched chains or non-aromatic rings. The number of bonds may vary from one (alkanes) to three (alkynes). The bonds in aliphatic compounds can be saturated (hexane) or unsaturated (hexene). Besides hydrogen and carbon, aliphatic compounds can also contain oxygen, nitrogen, sulphur and chlorine. Most aliphatic compounds are flammable gases. Methane (CH4) is the simplest aliphatic compound.
The bond between carbon and hydrogen is weak in aliphatic compounds. Therefore, aliphatic compounds are often very reactive.
The spatial distribution of carbon and hydrogen atoms is different in aromatic compounds. The atoms arrange themselves to form a flat aromatic ring. Benzene is the prototypical aromatic compound.
The aromatic ring contains many double bonds that interact with each other to form a very stable configuration. Therefore, these rings form easily and once formed cannot be easily broken. This (the stable nature of the ring) is why aromatic compounds are not as reactive as aliphatic compounds.
The stability of an aromatic ring is more than that expected from conjugated bonds alone. Single and double bonds alternate in aromatic compounds. Therefore, electrons can flow freely in a circle through the alternating single and double bonds. Therefore, the aromatic ring can be considered as a hybrid of single and double bonds.
The first described aromatic compound is the Benzene. Benzene has the following structure.
Benzene has six carbon atoms. The carbon atoms arrange themselves in a perfect hexagon. The carbon atoms are held together by alpha and pi bonds. The interaction of the pi bonds forms a pi-orbital above and below the plane of the carbon atoms. As the carbon atoms are out of the plane, these orbitals can freely interact with each other and become delocalised. Therefore, the electron is shared freely with all six carbon atoms. Thus, there are not enough electrons to form double bonds at each carbon atom, but the excess electrons strengthen every bond in the aromatic ring. Therefore, the bonds between the carbon atoms are much stronger than that would be expected from simple covalent bonds alone. Therefore, these bonds do not break easily. Hence, these (aromatic) compounds are considerably less reactive than aliphatic compounds.
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