The Friedel-Craft reaction is widely used in organic synthesis; e. Friedel—Crafts reactions have uses in the synthesis of triarylmethane and xanthene dyes and the Friedel—Crafts test for aromatic hydrocarbons where reaction of chloroform with aromatic compounds using an aluminium chloride catalyst gives brightly coloured triarylmethanes is a bench test for aromatic compounds.
Despite being over years since it was discovered this reaction and its associated reactions continue to play a key role in organic synthesis e. Keep your current shopping and add the saved Cart? Remove your current shopping cart and replace with the saved cart? Add the items to your existing shopping cart? Replace your shopping cart with these items?
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Once you have done so, you may check suggested answers by clicking on the question mark for each. Compounds in which two or more benzene rings are fused together were described in an earlier chapter , and they present interesting insights into aromaticity and reactivity. The smallest such hydrocarbon is naphthalene. Naphthalene is stabilized by resonance. Three canonical resonance contributors may be drawn, and are displayed in the following diagram.
The two structures on the left have one discrete benzene ring each, but may also be viewed as pi-electron annulenes having a bridging single bond.
The structure on the right has two benzene rings which share a common double bond. As expected from an average of the three resonance contributors, the carbon-carbon bonds in naphthalene show variation in length, suggesting some localization of the double bonds.
The C1—C2 bond is 1. This contrasts with the structure of benzene, in which all the C—C bonds have a common length, 1. Naphthalene is more reactive than benzene, both in substitution and addition reactions, and these reactions tend to proceed in a manner that maintains one intact benzene ring. The following diagram shows three oxidation and reduction reactions that illustrate this feature. Electrophilic substitution reactions take place more rapidly at C1, although the C2 product is more stable and predominates at equilibrium.
Examples of these reactions will be displayed by clicking on the diagram. The kinetically favored C1 orientation reflects a preference for generating a cationic intermediate that maintains one intact benzene ring. By clicking on the diagram a second time , the two naphthenonium intermediates created by attack at C1 and C2 will be displayed.
The structure and chemistry of more highly fused benzene ring compounds, such as anthracene and phenanthrene show many of the same characteristics described above. The chief products are phenol and diphenyl ether see below. This apparent nucleophilic substitution reaction is surprising, since aryl halides are generally incapable of reacting by either an S N 1 or S N 2 pathway.
The presence of electron-withdrawing groups such as nitro ortho and para to the chlorine substancially enhance the rate of substitution, as shown in the set of equations presented on the left below. To explain this, a third mechanism for nucleophilic substitution has been proposed. This two-step mechanism is characterized by initial addition of the nucleophile hydroxide ion or water to the aromatic ring, followed by loss of a halide anion from the negatively charged intermediate.
This is illustrated by clicking the "Show Mechanism" button next to the diagram. The sites over which the negative charge is delocalized are colored blue, and the ability of nitro, and other electron withdrawing, groups to stabilize adjacent negative charge accounts for their rate enhancing influence at the ortho and para locations. Three additional examples of aryl halide nucleophilic substitution are presented on the right.
Only the 2- and 4-chloropyridine isomers undergo rapid substitution, the 3-chloro isomer is relatively unreactive. Nitrogen nucleophiles will also react, as evidenced by the use of Sanger's reagent for the derivatization of amino acids.
The resulting N-2,4-dinitrophenyl derivatives are bright yellow crystalline compounds that facilitated analysis of peptides and proteins, a subject for which Frederick Sanger received one of his two Nobel Prizes in chemistry. Such addition-elimination processes generally occur at sp 2 or sp hybridized carbon atoms, in contrast to S N 1 and S N 2 reactions. When applied to aromatic halides, as in the present discussion, this mechanism is called S N Ar. Some distinguishing features of the three common nucleophilic substitution mechanisms are summarized in the following table.
Elimination There is good evidence that the synthesis of phenol from chlorobenzene does not proceed by the addition-elimination mechanism S N Ar described above. However, ortho-chloroanisole gave exclusively meta-methoxyaniline under the same conditions. These reactions are described by the following equations. The explanation for this curious repositioning of the substituent group lies in a different two-step mechanism we can refer to as an elimination-addition process.
The intermediate in this mechanism is an unstable benzyne species, as displayed in the above illustration by clicking the "Show Mechanism" button. In contrast to the parallel overlap of p-orbitals in a stable alkyne triple bond, the p-orbitals of a benzyne are tilted ca.
In the absence of steric hindrance top example equal amounts of meta- and para-cresols are obtained. The steric bulk of the methoxy group and the ability of its ether oxygen to stabilize an adjacent anion result in a substantial bias in the addition of amide anion or ammonia.
For additional information about benzyne and related species , Click Here. Addition Although it does so less readily than simple alkenes or dienes, benzene adds hydrogen at high pressure in the presence of Pt, Pd or Ni catalysts.
The product is cyclohexane and the heat of reaction provides evidence of benzene's thermodynamic stability. Substituted benzene rings may also be deduced in this fashion, and hydroxy-substituted compounds, such as phenol, catechol and resorcinol, give carbonyl products resulting from the fast ketonization of intermediate enols. Nickel catalysts are often used for this purpose, as noted in the following equations.
Benzene is more susceptible to radical addition reactions than to electrophilic addition. We have already noted that benzene does not react with chlorine or bromine in the absence of a catalyst and heat.
In strong sunlight or with radical initiators benzene adds these halogens to give hexahalocyclohexanes. It is worth noting that these same conditions effect radical substitution of cyclohexane, the key factors in this change of behavior are the pi-bonds array in benzene, which permit addition, and the weaker C-H bonds in cyclohexane. The addition of chlorine is shown below; two of the seven meso-stereoisomers will appear if the "Show Isomer" button is clicked. The Birch Reduction Another way of adding hydrogen to the benzene ring is by treatment with the electron rich solution of alkali metals, usually lithium or sodium, in liquid ammonia.
To see examples of this reaction, which is called the Birch Reduction , Click Here. Practice Problems The following problems review various aspects of aromatic chemistry. The first two questions review some simple concepts. The next two questions require you to analyze the directing influence of substituents.
The fifth question asks you to draw the products of some aromatic substitution reactions. The sixth question takes you through a mutistep synthesis. The last selection leads to a large number of multiple choice questions.
Compounds in which a hydroxyl group is bonded to an aromatic ring are called phenols. The chemical behavior of phenols is different in some respects from that of the alcohols, so it is sensible to treat them as a similar but characteristically distinct group.
A corresponding difference in reactivity was observed in comparing aryl halides, such as bromobenzene, with alkyl halides, such as butyl bromide and tert-butyl chloride. Thus, nucleophilic substitution and elimination reactions were common for alkyl halides, but rare with aryl halides.
Acidity of Phenols On the other hand, substitution of the hydroxyl hydrogen atom is even more facile with phenols, which are roughly a million times more acidic than equivalent alcohols. This phenolic acidity is further enhanced by electron-withdrawing substituents ortho and para to the hydroxyl group, as displayed in the following diagram. The alcohol cyclohexanol is shown for reference at the top left. It is noteworthy that the influence of a nitro substituent is over ten times stronger in the para-location than it is meta, despite the fact that the latter position is closer to the hydroxyl group.
Furthermore additional nitro groups have an additive influence if they are positioned in ortho or para locations. The trinitro compound shown at the lower right is a very strong acid called picric acid. Why is phenol a much stronger acid than cyclohexanol? To answer this question we must evaluate the manner in which an oxygen substituent interacts with the benzene ring.
Development of methods for the synthesis of pentacyclic triterpenes based on a mechanistic interpretation of the stereochemical outcome of the Friedel-Crafts cyclialkylation reaction Robert E.
Ireland, Steven W. Baldwin, and Steven C. A systematic study of benzyl cation initiated cyclization reactions Steven R. Angle and Michael S. Coote, Stephen G. Chirality can be induced through an unusual but advanced methodology involving organometallics — reversibly forming an arene-Cr CO 3 complex.
Intramolecular acylation: The Synthesis of 2-Hydroxyequilenone E. Bachmann and W. This is not used all that much anymore, since it is a pain to handle — very corrosive, and extremely viscous. Methanesulfonic acid. A useful cyclizing acidic reagent Alberto A.
Leon, Guido Daub, and I. It is cheap, readily available, and is a easily handled liquid, comparable in acidity to PPA. Schubert, W. Sweeney, and H. Efficient synthesis of selected indenones Brawner. Floyd and George Rodger. Tandem reactions with both acylation and alkylation are also possible; these are termed cycli-acyalkylations. Unsaturated acids or lactones can be used. Olson, Alfred R. Bader, and G. Dana Johnson Org. Bader, uses a lactone to efficiently do a tandem F-C cycli-acylalkylation.
But as I said, these aren't super stable. This guy really, really, really wants to react. And now this is where benzene comes into the mix. And actually, let me draw a little dividing line here, just so we know that this was a separate stage of our reaction.
So that was the first stage. Then we go over here and now benzene comes into the mix. The benzene was floating around. So we have our benzene floating around, just like that. And then I'm going to draw one of the hydrogens on one of the carbons. All of these carbons have hydrogens on them. I just won't draw them all. It just make things complicated.
But this guy we said is a really good electrophile, and you have to be a really good electrophile to attract electrons from a benzene ring, to break it's aromaticity.
But if it bumps into this guy in just the right way, at just the right angle, you could imagine that this electron on this carbon right here gets swiped by the acyl group. So then what do we have? So now I will go back in this direction.
So you have what was a benzene ring. We can draw the double bonds here and here. And we, of course, have this hydrogen. But now this bond, which was a double bond there, is now bonded to the acyl group.
So it has that blue electron that the acyl group nabbed. And let me draw the acyl group. And I'll flip it over so that we have the methyl on the right-hand side.
So it's carbonyl bonded to a CH3. It was positive. It now gained an electron. It is now neutral. This carbon over here lost an electron, so it is now positive, so it is now positively charged. Now, we mentioned the aluminum chloride is a catalyst, so it won't just sit around there as the anion. It has to go back to being aluminum chloride, so let's bring the aluminum chloride back into the scene.
So we have our aluminum chloride. Let me copy and paste it. So we have our aluminum chloride here, and so you can imagine that the benzene ring wants to go back to being aromatic, so this electron right here on the hydrogen might really want to go back to this carbon right over here, this carbocation. At the same time, if this anion now passes the hydrogen at just the right angle at the right time while this guy's attracted to this carbon, this chlorine can give this green electron to the hydrogen nucleus, which is really just a proton.
And then the hydrogen's electron can be taken up by what was this carbocation. And then what do we have?
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