Aromatic Chemistry PDF

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lecture notes on aromatic chemistry...

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Aromatic Chemistry Arenes   

Arenes are hydrocarbons based on benzene C6H6 which is the simplest one Benzene is an unsaturated molecule but it is very stable It has a hexagonal ring structure with a special type of bonding

Bonding and structure of benzene               

The bonding and structure of benzene was a puzzle for organic chemists for a long times This is because in spite of being unsaturated, it does not readily form addition compounds All the carbon atoms were equivalent, which implied that all the carbon-carbon bonds are the same. Kekule suggested that the molecule had a cyclic arrangement of carbon atoms joined together by alternate single and double bonds. Two equivalent hexagonal structures can be drawn however only single benzene exists. Kekule proposed that the a rapid equilibrium existed between the two equivalent structures thereby averaging out the single and double bonds. Benzene is often considered to be a resonance hybrid of the two Kekule structures, neither of which actually exists. Single crystal X ray diffraction analysis has shown that the benzene molecule is a planar, regular hexagon in which the carbon-carbon bond lengths are all the same. The identical bonding between carbon atoms in benzene is implied by the use of a circle inside the hexagon to indicate six delocalised electrons. Delocalisation means that the electrons are spread over more than two atoms and in this case six carbon atoms form the ring Each carbon has three covalent bonds, one to a hydrogen atom and the other two to carbon atoms The fourth electron of each carbon atom is in a p orbital, so there are six of these that overlap and delocalise. They form a region of electron density above and below the ring. Overall the carbon-carbon bond has a total of three electrons making it between a single and a double bond. The delocalised system makes benzene unusually stable and this is called aromatic stability.

Thermo-chemical evidence for stability 

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Thermo-chemical evidence shows that benzene is much more stable than the hypothetical cyclohexa-1,3,5 triene molecule would be. So for example, the enthalpy change on hydrogenation of benzene is less exothermic than anticipated by comparison with cyclohexene. (Hydrogenation is the addition of hydrogen to something.)

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You would expect to get an enthalpy change of -360 kJ mol -1, because there are exactly three times as many bonds being broken and made as in the cyclohexene case. In fact what you get is -208 kJ mol-1. The cyclic electron delocalisation has a marked stabilising effect so that benzene undergoes overall addition reactions with difficulty. The increase in stability associated with electron delocalisation is called the delocalisation energy or the resonance energy.

Physical properties of arenes     

Benzene is a colourless liquid at room temperature. It boils at 353K and freezes at 279K. Its boiling point is comparable with that of hexane but its melting point is much higher than hexane’s. This is because benzene’s flat hexagonal molecules pack together well in the solid state and are therefore harder to separate and this must happen for the solid to melt. Like other hydrocarbons that are non polar, arenes do no mix with water but they mix with other hydrocarbons and other non polar solvents.

Naming aromatic compounds  

Substituted arenes are generally named as derivatives of benzene so benzene forms the root of the name e.g methylbenzene or chlorobenzene. If there is more than 1 substituent, the ring is numbered e.g 1,2 dichlorobenzene

Reactivity of aromatic compounds  

The ring is an area of high electron density, because of the delocalised bonding and is therefore attacked by electrophiles. The aromatic ring is very stable and it needs energy to be put in to break the ring before the system is destroyed. This is the delocalisation energy and it means that the ring almost always remains intact in the reaction of arenes.

Combustion of arenes   

Arenes burn in air with flames that are noticeably smoky so smoky flames suggest that it is an aromatic compound. This is because they have a high carbon: hydrogen ratio. There is usually unburned carbon remaining when they burn in the air and this produces soot.

Electrophilic substitution reactions  

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Although benzene is unsaturated it does not react like an alkene. The most typical reaction is an electrophilic substitution that leaves the aromatic system unchanged, rather than addition, which would require the input of the delocalisation energy to destroy the aromatic system. The delocalised system of the aromatic ring has a high electron density that attracts electrophiles. At the same time, the electrons are attracted to towards the electrophile El+. A bond forms between one of the carbon atoms and the electrophile. But to do this, the carbon must use electrons from the delocalised system. This destroys the aromatic system. To get back the stability of the aromatic system, the carbon loses a H+ ion and the sum of these reactions is the substitution of H+ by X+

Nitration      

Nitration is the substitution of a NO2 group for one of the hydrogen atoms on an arene ring. Nitration is an important step in the production of explosives like TNT. Nitration is the first step in making amines and these are in turn used to make industrial dyes. Benzene is nitrated by concentrated nitric acid at 50 degrees in the presence of concentrated sulphuric acid which acts as a catalyst. Although some heat is required, too much may give further nitration to form 1, 3-dinitrobenzene. This is the overall balanced equation

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The electrophile NO2+ (nitronium ion/ nitryl cation) is generated in the reaction mixture of concentrated nitric acids and concentrated sulphuric acids H2SO4 is a stronger acid than HNO3 so it donates a proton to HNO3 which acts as a base. HNO3 then loses a molecule of water to give NO2+

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This is the mechanism for the nitration of benzene

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We end up with nitrobenzene The H+ formed regenerates a molecule of H2SO4: H+ + HSO4-  H2SO4, this reacts with nitric acid to form more nitronium ions.

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TNT    

TNT is short for trinitroluene. TNT is an important explosive which is used in filling shells. It is made by nitrating methyl benzene (toluene) The reaction for the explosion of TNT is strongly exothermic and there is rapid formation of a lot of a gas as well as heat.

Nitration of methylbenzene 

Nitration of methylbenzene is easier than nitration of benzene due to the positive inductive effect of the electron releasing methyl group which increases the electron density in the benzene ring

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The presence of the electron withdrawing nitro groups makes each successive substitution progressively slower In the case of benzene itself, the rate of formation of nitrobenzene is much faster than the formation of 1,3- dinitrobenzene from nitrobenzene

Friedel-Crafts acylation reactions

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These reactions are between haloalkanes and benzene and they use aluminium chloride as a catalyst. The mechanism for acylation is a substitution with RCO substitution for hydrogen on the aromatic ring. Acyl chlorides provide the RCO group. They react with AlCl3 to form AlCl4- and RCO+. RCOCl + AlCl3  RCO+ + AlCl4This reaction takes place because the aluminium atom in aluminium chloride has only six electrons in its outer main level and readily accepts a pair from the chlorine atom of the RCOCl. RCO+ is a good electrophile that attacks the benzene ring. The aluminium chloride is reformed by reaction of the AlCl4- ion with H+. AlCl4 - + H+  AlCl3 + HCl The overall reaction is:

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The mechanism for the

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reaction:...