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8.4: Alkylation and Acylation of Aromatic Rings - The Friedel-Crafts Reaction

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    Friedel-Crafts Alkylation

    Friedel-Crafts Alkylation was first discovered by French scientist Charles Friedel and his partner, American scientist James Crafts, in 1877. This reaction allowed for the formation of alkyl benzenes from alkyl halides, but was plagued with unwanted supplemental activity that reduced its effectiveness.Edit section

    Benzene reacts with chloroethane and catalyst aluminum chloride to produce ethyl benzene.

    The mechanism takes place as follows:

    Step 1:

    mechpart1_(1).png

    Step one creates a carbocation that acts as the electrophile in the reaction. This steps activates the haloalkane. Secondary and tertiary halides only form the free carbocation in this step.

    Steps 2 and 3:

    mechpart2a.png

    Step 2 has an electron pair from the aromatic ring attack the carbocation forming a new C-C bond. The arenium ion intermediate results with stabilization from multiple resonance forms. The loss of a proton then gives the neutral alkylated substitution product.

    Final Products

    ch 18 sect 5 final products.png

    The reactivity of haloalkanes increases as you move up the periodic table and increase polarity. This means that an RF haloalkane is most reactive followed by RCl then RBr and finally RI. This means that the Lewis acids used as catalysts in Friedel-Crafts Alkylation reactions tend have similar halogen combinations such as BF3, SbCl5, AlCl3, SbCl5, and AlBr3, all of which are commonly used in these reactions.

    Some limitations of Friedel-Crafts Alkylation

    There are possibilities of carbocation rearrangements when you are trying to add a carbon chain greater than two carbons. The rearrangements occur due to hydride shifts and methyl shifts. For example, the product of a Friedel-Crafts Alkylation will show an iso rearrangement when adding a three carbon chain as a substituent. One way to resolve these problems is through Friedel-Crafts Acylation.

    Benzene reacts with chloropropane and aluminum chloride to produce (propan-2-yl)benzene.

    Also, the reaction will only work if the ring you are adding a substituent to is not deactivated. Friedel-Crafts fails when used with compounds such as nitrobenzene and other strong deactivating systems.

    There is no reaction when nitrobenzene reacts with aluminum chloride and RCl.

    Friedel-Crafts reactions cannot be preformed then the aromatic ring contains a NH2, NHR, or NR2 substituent. The lone pair electrons on the amines react with the Lewis acid AlCl3. This places a positive charge next to the benzene ring, which is so strongly activating that the Friedel-Crafts reaction cannot occur.

    The positive charge strongly deactivates the benzene ring.

    Lastly, Friedel-Crafts alkylation can undergo polyalkylation. The reaction adds an electron donating alkyl group, which activates the benzene ring to further alkylation.

    There is an activated ring as an intermediate ring.

    This problem does not occur during Friedel-Crafts Acylation because an acyl group is deactivating, thus prevents further acylations.

    Benzene reacts with an acyl group and aluminum chloride to produce a deactivated ring.

    Friedel-Crafts Acylation

    The goal of the reaction is the following:

    Attaching the acyl group to the benzene.

    The very first step involves the formation of the acylium ion which will later react with benzene:

    Friedel-Crafts_Part_1_(1).jpg

    The second step involves the attack of the acylium ion on benzene as a new electrophile to form one complex:

    Friedel-Crafts_Part_2_(1).jpg

    The third step involves the departure of the proton to reform aromaticity:

    Friedel-Crafts_Part_3_(2).jpg

    During the third step, AlCl4 returns to remove a proton from the benzene ring, which enables the ring to return to aromaticity. In doing so, the original AlCl3 is regenerated for use again, along with HCl. Most importantly, we have the first part of the final product of the reaction, which is a ketone. Thie first part of the product is the complex with aluminum chloride as shown:

    Friedel-Crafts_Part_4_(2).jpg

    The final step involves the addition of water to liberate the final product as the acylbenzene:

    Friedel-Crafts_Part_5_(1).jpg

    Because the acylium ion (as was shown in step one) is stabilized by resonance, no rearrangement occurs (unlike in Friedel-Crafts Alkylation reactions - see Limitation 1 above). Also, because of of the deactivation of the product, it is no longer susceptible to electrophilic attack and hence, no longer goes into further reactions (Limitation 3 above from Friedel-Crafts Alkylation reactions). However, as not all is perfect, Limitation 2 still prevails where Friedel-Crafts Acylation fails with strong deactivating rings.


    8.4: Alkylation and Acylation of Aromatic Rings - The Friedel-Crafts Reaction is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Steven Farmer, Dietmar Kennepohl, William Reusch, James Kabrhel, Lauren Reutenauer, & Lauren Reutenauer.