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16 -- Part 5: AROMATIC COMPOUNDS
The Alkenes are given with air to give it's shape.
The carbocation is not immediately attacked by floride ion.
If benzene is present, a substitution occurs.
The more stable carbocation, which alkylates the aromatic ring, is formed after the protonsation step.
Alcohols are used in Friedel-Crafts alkylations.
Lewis acids such as Boron trifluoride are used to treat alcohols.
substitution may occur if benzene is present.
Predict the products for each reaction by showing the generation of the electrophile.
Friedel-Crafts alkylation has three major limitations that make it hard to use.
Friedel-Crafts reactions only work with benzene, activated benzene derivatives, and halobenzenes.
They fail with systems that are strongly deactivated.
Friedel-Crafts reactions fail with deactivated systems.
The correct product is given by nitration.
The butyl group would fail.
The Friedel-Crafts alkylation is susceptible to carbocation rearrangements.
The Friedel-Crafts alkylation is not prone to rearrangement and can be used to synthesise butylbenzene, isopropylben Alkyl and ethylbenzene.
Consider what happens.
Friedel-Crafts alkylations are hard to avoid.
This limitation can be very serious.
We need to make ethyl to multiple alkylations.
It is activated even faster than benzene when it is formed.
There is a mixture of some diethylbenzenes, some triethylbenzenes, and a small amount of ethylbenzene.
A large excess of benzene can be used to minimize the problem of overalkylation.
The concentration of ethylbenzene is always low if 1 mole of ethyl chloride is used with 50 moles of benzene.
The product is separated from excess benzene.
The unreacted benzene can be recycled with a continuous distillation.
In the laboratory, alkylate aromatic compounds are more expensive than benzene.
A moreselective method is needed because we can't afford a large amount of starting material.
The Friedel-Crafts acylation introduces just one group without the risk of polyalkylation or rearrangement.
You may think that aluminum chloride is added as a catalyst.
Predict the major products for the reactions that won't give a good yield.
Acyl chlorides are made by reacting carboxylic acids with thionyl chloride.
When we study acid derivatives in Chapter 21, we consider acyl chlorides.
The acylium ion reacts with benzene to form acylbenzene.
Friedel-Crafts acylation is an aromatic substitution with an acylium ion.
A sigma complex is formed by anphilic attack and the loss of a protons.
The free acylbenzene must be released from the product complex.
A ketone is the product of acylation.
The ketone's carbonyl group requires a full equivalent of Lewis acid in the acylation.
The aluminum chloride complex is the initial product.
The free acylbenzene comes from the addition of water.
Para substitution usually occurs when the aromatic substrate has an ortho, para-directing group.
One of the most attractive features of the Friedel-Crafts acylation is the de 888-282-0476 888-282-0476 888-282-0476 888-282-0476.
A deactivating group is bonded to the aromatic ring of the acylbenzene.
The acylation stops after one substitution because Friedel-Crafts reactions do not occur on strongly deactivated rings.
The acylium ion is resonance-stabilized, so that no rearrangements occur, and the acylbenzene product is deactivated, so that no further reaction occurs.
The acylation fails with aromatic rings.
Only benzene, halobenzenes, and activated derivatives can be used with the alkylation.
The alkylation may have some changes.
The acylium is not prone to rearrangement.
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