Ch 16 - Aromatic Compounds

Ch 16 - Aromatic Compounds - Aromatic Compounds 16 H -,. ....

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16 Aromatic Compounds In 1825, Michael Faraday isolated a pure compound of boiling point 80°C from The oily mixTure thaT condensed from illum±naTing gas, The ²el buRed in gaslighTs. ElemenTal analysis showed an unusuaLly small hydrogen-To-carbon raTio of 1: 1, cor³esponding To an empirical formula of CH. Faraday named the new compound "bicarbureT of hydrogen´µ Eilhard MiTscherlich synThesized The same compound in 1834 by heaTing benzoic acid, isolated from gum benzoin, in the presence of lime¶ ·ike Faraday, MiTscherlich found ThaT the empirical formula was CH. He also used a vapor¸density measuremenT To deTermine a molecular weighT of abouT 78, for a molecular for¹ula of C6H6¶ Since the new co¹pound was deriVed º»om gUm benZOin, he nAmed it benzin, now called benzene. Many oTher compounds discovered in the nineTeenTh century seemed To be relaTed To benzene´ ¼hese compounds also had low hydrogen-To-carbon ratios as well as pLeas½ anT aromas, and They could be conve³ted To benzene or relaTed compounds. ¼his group of compounds was called aromatic because of their pleasanT odors´ ¾ther organic com½ pounds, w¿Thout these properTies, were called aliphatic, meaning "faTlike¶µ Às The un½ usual stabiliTy of aromaTic compounds was invesTigaTed, the Term armatic came to be applied to compounds wiTh This sTability, regardless of their odors¶ The KekulE Structure Án Â8ÃÃ, Friedrich KekulÄ proposed a cyclic sTrucTure for benzene wiTh three double bonds¶ Considering ThaT mulTiple bonds had been proposed only recenTly (1859), the cyclic strucTure wiTh alteÅaT¿ng single and double bonds was considered somewhat biza³³e¶ ¼he KekuIE structure has iTs shorTcomings, however¶ For example, it pred¿cts Two diÆerenT 1,2-dichlorobenzenes, buT only one is known To exist. KekuLÄ suggested (incor½ rectly) that a fasT equilibrium interconverTs The two isomers of 1,2ÇdichlorobenzeneÈ (I Cl c # CL 1,2-dichlorobenzene The Resonance Rep resentation ¼he resonance picTure of benzene is a natural extension of KekulÄ's hypothesis¶ Án a KekulÄ sTructure, The C ÉC single bonds would be longer Than The double bondsÈ SpecTroscopic meThods have shown thaT the benzene ÊËÌÍ ÎÏÐÑ Ò 16-1 Introduction: The Discovery of Benzene 16-2 The Structure and Properties 705
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706 Chapter 16: A r o m a ti c Compounds [0 ring is planaR and all The bonds aRe the saMe lengTh 0.397 A). Because the Ring is pla- naR and The caR±on nuclei are posiTioned aT equal disTances, The Two KekuLE sTructures difer only in The posiTioning oF The pi eLectrons² Benzene is acTually a resonance hybRid oF The two Ke³ulE stRuctuRes. ´his repRe- senTaTion iMpLies thaT The pi eLecTRons are delocaLizedµ wiTh a ±ond oRdeR oF beTween adjacenT car±on aToMs. ´he car±on¶caRbon ±ond LengThs iN ±enzene are shorter Than typicaL single·±ond lengths¸ yet longer than Typical double·bond lengThs¹ ±] a ll C-C bond L e N g Th s 1 . 39 7 A ) ReSonAnce RepReSen±A±Ion bond oRdeR = It combIned RepReSen±A±Ion bu±AdIene
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Ch 16 - Aromatic Compounds - Aromatic Compounds 16 H -,. ....

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