Chemistry 240Summer 2001

Benzene Structure and the fragrant Ring
Symbols for the Benzene RingKekule StructuresResonanceHuckel RuleElectrophilic fragrant Substitution -- Mechanism
Today we"ll discover that resonance is really important in knowledge boththe structure and also the reactions of aromatic compounds. First, let"s takea look at the structural representations which distinguish aromatic compoundsfrom those the aren"t aromatic.The most generally encountered fragrant compound is benzene. The usualstructural representation for benzene is a 6 carbon ring (representedby a hexagon) which includes three twin bonds. Each of the carbons representedby a edge is additionally bonded come one other atom. In benzene itself, theseatoms room hydrogens. The dual bonds are separated by solitary bonds sowe acknowledge the plan as including conjugated double bonds. One alternativesymbol provides a circle inside the hexagon to stand for the 6 pi electrons.Each of these signs has great and negative features. We"ll use the 3 doublebond price simply since it is additionally routinely used in the text.

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Keep in mind the if the hexagon contains neither the three twin bondsnor the circle, the compound is no aromatic. The is merely cyclohexaneand there room two hydrogens on each carbon atom. This is basic to mistakewhen hurrying, therefore be careful when you space intepreting any type of structural formulaswhich include hexagons.The structure with three twin bonds was proposedby Kekule as an effort to describe how a molecule whose molecular formulawas C6H6 might be built out of carbons i beg your pardon makefour bonds. The ring and the three twin bonds fit the molecule formula,but the framework doesn"t explain the chemical habits of benzene in ~ allwell. Each of the twin bonds would certainly be meant to present the characteristicbehavior of one alkene and also undergo addition reactions, however this is no howbenzene reacts.In particular, us would suppose a carbon-carbon twin bond to reactquickly through bromine to make a dibromo compound. This is what alkenes dovery readily, and also in truth it is a helpful test because that alkenes in the laboratory.Benzene does not react with bromine uneven a an extremely bright light or a strongcatalyst is used, and also then the reaction is no an enhancement reaction. Weconclude that there is something rather unusual around the dual bonds inbenzene.Kekule (thinking about this problem prior to bonds were understood aspairs the electrons) argued that there are two develops of benzene whichdiffer in the locations of the twin bonds. His idea was the these werein fast equilibrium, so rapid that over there was never ever a fixed ar forthe double bond. One could say the an pull close bromine molecule couldnot "find" a double bond to reaction with.There were several other structures propose forbenzene, yet a much an ext satisfactory approach became possible when webegan to understand that covalent bonds consist of bag of electrons sharedbetween atoms. The difference between the two structures Kekule envisioned(called Kekule structures) is only the difference between the locationsof 3 pairs the electrons. This is precisely the type of situation whereresonance have to be involved. The hybrid or "average" that the 2 Kekule structureshas one sigma bond and also one-half that a pi bond in between each two carbon atoms.Thus every carbon is joined to each of its next-door neighbors by a one-and-half bond.Each bond in the benzene ring has the same variety of electrons and also is thesame length. This snapshot is in complete accord through experiments whichshow that all carbon-carbon bonds in benzene room the exact same length, withno hints of shorter (double) or longer (single) bonds. It also helps explainwhy benzene does no undergo enhancement reactions: there are no an easy pibonds.Recall that resonance has another important feature: as soon as resonanceis involved, the actual structure is an ext stable 보다 we would suppose fromany of the structures we write utilizing the one line = two electrons symbolism.This extra lowering the energy, which for benzene is about one-third asmuch as making a common covalent bond, is quite vital in the reactionsof benzene and also other fragrant compounds. As we will certainly see, reactions of thebenzene ring nearly always result in products which in i m sorry the benzenering stubborn -- an outcome of its stability.When resonance concept was very first applied to understandingthe framework of benzene, the vital feature appeared to be a resonance hybridof ring structures containing alternating solitary and twin bonds. Thisimmediately brought about attempts come make and study compounds prefer cyclooctatetraeneand cyclobutane. These compounds also have ring frameworks with alternatingsingle and double bonds.Cyclooctatetraene has been made, however it does not posess the propertiesof extra stability and also resistance to enhancement reactions i m sorry distinquisharomatic compounds. It easily adds bromine, because that example. Cyclobutadieneis incredibly unstable -- one cyclobutadiene molecule reacts v anothercyclobutadiene molecule instantaneously also at an extremely low temperatures --so it certainly does no act like an fragrant molecule and it has actually beencalled "antiaromatic" as a result.It seems that there is an ext to being fragrant than simply a ring withalternating solitary and double bonds. ~ considerable advance ofthe underlying theory, the pattern which has arised is that aromatic characteristicsare only expected once there is a ring the pi electrons in which the numberof pi electron is equal to 4n + 2 (where n is an integer,0, 1, 2, etc.). (This is recognized as the Huckel preeminence after that discoverer.)We can check this versus the compounds we have considered so far: Benzenehas 6 pi electron (two because that each pi bond) i m sorry is the number we obtain from4n + 2 if n = 1. Cyclooctatetraene has actually 8 pi electrons, and there is nointeger "n" which will certainly make 4n + 2 = 8. Cyclobutadiene has 4 pi electronsand also doesn"t to the right 4n + 2. Over there are many other examples which supportHuckel"s rule.It is crucial to be certain that the ring of alternating solitary and doublebonds is complete. If there is an sp3 hybridized carbon in thering, the conditions for aromatic character are not present, and we donot worry around checking because that 4n + 2. Here"s an example:Another way to see this is to look in ~ the p orbitals which combine tomake the pi bonds. If these p orbitals integrate to form an uninterruptedring as is the situation in benzene, then we have the right to go ahead to usage Huckel"s ruleto check for the proper variety of pi electron for fragrant character.If the ring of ns orbitals is damaged by a CH2 (group or anothertetrahedral carbon) v no p orbital, climate the link cannot be aromaticand we require not try to use Huckel"s rule.The p orbitals which make up the unbroken ns orbital ring can be associatedwith other atoms 보다 carbon. Two examples are furan and also pyrrole, in whichtwo the the 6 electrons required come formally from unshared electron pairson oxygen.Such an unshared pair can likewise come native a carbon atom, which will haveto have a negative charge. An example of this is the cyclopentadienideion which can be made by dealing with cyclopentadiene with a middle strongbase. Cyclopentadienide ion is sufficiently stabilized by its aromaticcharacter that cyclopentadiene (its conjugate acid) has a pKaof 16, close to that of water. Cyclopentadiene is a remarkably strong acidfor a hydrocarbon because its conjugate base has the extra stability ofan aromatic compound.Extraordinarily secure cations can likewise be make if your structuresare aromatic. Below are two:Notice that right here the formally positively charged carbon atoms space sp2hybridized and also have an empty ns orbital i m sorry completes the cyclic arrangementof ns orbitals.Let"s end up up this day by looking in ~ the generalmechanism for the characteristic reaction of aromatic compounds -- electrophilicaromatic substitution. The most important features of this reactionsfollow directly from the stability of the aromatic ring. First, these reactionsare typically catalyzed by strong electrophilic (Lewis acidic) catalystslike H2SO4, AlCl3, and also FeCl3which are compelled to get over the stability of the aromatic ring. Second,these are substitution reaction since addition reactions would certainly interruptthe ns orbital ring and also destroy the fragrant stability.Even though the result of the strike of electrophiles top top benzene issubstitution rather than addition, the first step is the same as in electrophilicaddition to alkenes -- strike of the electrophile ~ above a pi bond and theformation that a new sigma bond in between a carbon atom and the electrophile.The carbocation i m sorry is created undergoes ns of the H+ fromthe carbon which was attacked. The electron from the C-H bond are returnedto the fragrant pi electron ring and also aromatic security is restored.

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Notice the the intermediate below is a carbocation, yet it is not aromatic.The carbon bearing the hydrogen and the electrophile is sp3hybridized and has no p orbital to add to a cyclic ns orbital system.The carbocation intermediary is rather resonance stabilized, though,by a resonance setup which is very comparable to the one we saw in theaddition that electrophiles come conjugated dienes.This intermediary is a carbocation, and also as we will see next time, itsstability is essential in identify how quick the reaction goes and (inbenzene rings which be affected by each other substituents at one of the carbons) whereby theelectrophile attacks. The an essential thing to recognize currently is that the positivecharge and also the corresponding carbocation characteristics only appearat location ortho and para family member to the point at whichthe electrophile attack. (Nomenclature is cure in Sec 6.3 the Atkins& Carey.) This will turn out to be quite important, therefore verify thisfor yourself.Back come the Course summary