Some of the early researchers in organic chemistry became intrigued by fragrant oils that could be extracted from certain plants. The compounds responsible for the aromas, had similar chemical properties. As a result, they were grouped together and called AROMATICS COMPOUNDS.
As more and more aromatic compounds were isolated and studied, chemist gradually realized that aromatic contained six carbon atoms and had low hydrogen – to – carbon ratios (relative to other hydrocarbon). They also realized that the term aromatic was not always accurate. Because there are many fragrant compounds do not have aromatic properties.
Kekule was to say later that he must have dozed off at this point. In his dream the black balls of carbon turned into black imps with forked tails that began racing around the room and would soon be upsetting the apparatus of the laboratory. He was ready to run the rascals out. Then, almost suddenly, the confusion died away as each imp grabbed the tail of the one ahead of him, the six forming a whirling circle. One hand of each imp held a tail, the other a white handkerchief--and they waved to him as the group whirled by. He said that he came awake with a start, realizing that the imps were acting out the formula for benzene . As his hand grabbed the sketching pencil, the imps were back to black balls again and the handkerchiefs had changed to hydrogen atoms. How simple the arrangement turned out to be. “ The carbon atoms of benzene form a ring."
<ul><li>Compounds formed by replacing a hydrogen of benzene with more complex hydrocarbon group can be named by designating the benzene ring as substituent . </li></ul><ul><li>We called them phenyl group. </li></ul><ul><li>benzyl </li></ul>
CH 3 CHCH ═ CH 2 3 – phenyl – 1 – butene CH 3 — C — C — C — CH 3 ا ا ا ا ا ا Br Br 3 HC 3 HC 2,2 – dibromo – 4,4 – dimethyl – 3,3 – diphenylpentane
<ul><li>When two or more groups are attached to a benzene ring, their positions can be indicated by numbering the carbon atoms of the ring so as to obtain the lowest possible numbers for the attachment positions. </li></ul><ul><li>Group that comes first in alphabetical order is given the lower number. </li></ul>
1 – bromo – 2 – chloro – 4 – iodobenzene 2,4,6 – trinitrophenol 3 – bromo – 5 - nitrotoluene Br Cl I O 2 N NO 2 OH NO 2 CH 3 NO 2 Br
1,3,5 – trinitrobenzene 2,4,6 - trinitrotoluene TNT NO 2 NO 2 NO 2 CH 3 NO 2 2 ON NO 2
Preparation <ul><li>Major sources of aromatic compounds are petroleum and coal tar – (a sticky, dark – colored material derived from coal. </li></ul><ul><li>Natural sources </li></ul><ul><li>Can be synthesize synthetically </li></ul>
Physical Properties <ul><li>Similar to those of ALKANES & ALKENES </li></ul><ul><li>Non – Polar </li></ul><ul><li>Insoluble in water (hydrophobic) </li></ul><ul><li>Flammable </li></ul><ul><li>Low density in water </li></ul>