Welcome to Wikipedia



Download 190.74 Kb.
Page4/7
Date19.10.2016
Size190.74 Kb.
#3394
1   2   3   4   5   6   7

Arene synthesis[edit]


A reaction that forms an arene compound from an unsaturated or partially unsaturated cyclic precursor is simply called an aromatization. Many laboratory methods exist for the organic synthesis of arenes from non-arene precursors. Many methods rely on cycloaddition reactions. Alkyne trimerization describes the [2+2+2] cyclization of three alkynes, in the Dötz reaction an alkyne, carbon monoxide and a chromium carbene complex are the reactants.Diels-Alder reactions of alkynes with pyrone or cyclopentadienone with expulsion of carbon dioxide or carbon monoxide also form arene compounds. In Bergman cyclization the reactants are an enyne plus a hydrogen donor.

Another set of methods is the aromatization of cyclohexanes and other aliphatic rings: reagents are catalysts used in hydrogenation such as platinum, palladium and nickel (reverse hydrogenation), quinones and the elements sulfur and selenium.[6]


Arene reactions[edit]


Arenes are reactants in many organic reactions.

Aromatic substitution[edit]


In aromatic substitution one substituent on the arene ring, usually hydrogen, is replaced by another substituent. The two main types are electrophilic aromatic substitution when the active reagent is an electrophile and nucleophilic aromatic substitution when the reagent is a nucleophile. In radical-nucleophilic aromatic substitution the active reagent is a radical. An example of electrophilic aromatic substitution is the nitration of salicylic acid:[7]

nitration of salicylic acid

Coupling reactions[edit]


In coupling reactions a metal catalyses a coupling between two formal radical fragments. Common coupling reactions with arenes result in the formation of new carbon–carbon bonds e.g., alkylarenes, vinyl arenes, biraryls, new carbon–nitrogen bonds (anilines) or new carbon–oxygen bonds (aryloxy compounds). An example is the direct arylation of perfluorobenzenes [8]

coupling reaction

Hydrogenation[edit]


Hydrogenation of arenes create saturated rings. The compound 1-naphthol is completely reduced to a mixture of decalin-ol isomers.[9]

1-naphthol hydrogenation

The compound resorcinol, hydrogenated with Raney nickel in presence of aqueous sodium hydroxide forms an enolate which is alkylated with methyl iodide to 2-methyl-1,3-cyclohexandione:[10]



resorcinol hydrogenation

Cycloadditions[edit]


Cycloaddition reaction are not common. Unusual thermal Diels-Alder reactivity of arenes can be found in the Wagner-Jauregg reaction. Other photochemical cycloaddition reactions with alkenes occur through excimers.

Benzene and derivatives of benzene[edit]


Benzene derivatives have from one to six substituents attached to the central benzene core. Examples of benzene compounds with just one substituent are phenol, which carries a hydroxyl group, and toluene with a methyl group. When there is more than one substituent present on the ring, their spatial relationship becomes important for which the arene substitution patterns ortho, meta, and para are devised. For example, three isomers exist for cresol because the methyl group and the hydroxyl group can be placed next to each other (ortho), one position removed from each other (meta), or two positions removed from each other (para). Xylenol has two methyl groups in addition to the hydroxyl group, and, for this structure, 6 isomers exist.

  • Representative arene compounds

  • http://upload.wikimedia.org/wikipedia/commons/thumb/9/9c/toluene.svg/80px-toluene.svg.png

Toluene

  • http://upload.wikimedia.org/wikipedia/commons/thumb/6/60/ethylbenzene-2d-skeletal.png/120px-ethylbenzene-2d-skeletal.png

Ethylbenzene

  • http://upload.wikimedia.org/wikipedia/commons/thumb/b/bc/p-xylene.svg/120px-p-xylene.svg.png

p-Xylene

  • http://upload.wikimedia.org/wikipedia/commons/thumb/5/58/meta-xylol_-_meta-xylene_2.svg/117px-meta-xylol_-_meta-xylene_2.svg.png

m-Xylene

  • http://upload.wikimedia.org/wikipedia/commons/thumb/8/84/mesitylene-2d-skeletal.png/120px-mesitylene-2d-skeletal.png

Mesitylene

  • http://upload.wikimedia.org/wikipedia/commons/thumb/6/62/durene.png/120px-durene.png

Durene

  • http://upload.wikimedia.org/wikipedia/commons/thumb/9/9f/2-phenyl-hexane.png/120px-2-phenyl-hexane.png

2-Phenylhexane

  • http://upload.wikimedia.org/wikipedia/commons/thumb/e/ec/bifenyl.svg/120px-bifenyl.svg.png

Biphenyl

  • http://upload.wikimedia.org/wikipedia/commons/thumb/9/91/phenol-2d-skeletal.png/70px-phenol-2d-skeletal.png

Phenol

  • http://upload.wikimedia.org/wikipedia/commons/thumb/f/fe/aniline.svg/79px-aniline.svg.png

Aniline

  • http://upload.wikimedia.org/wikipedia/commons/thumb/8/83/nitrobenzol.svg/73px-nitrobenzol.svg.png

Nitrobenzene

  • http://upload.wikimedia.org/wikipedia/commons/thumb/a/a2/benzoic_acid.svg/120px-benzoic_acid.svg.png

Benzoic acid

  • http://upload.wikimedia.org/wikipedia/commons/thumb/6/67/aspirin-skeletal.svg/120px-aspirin-skeletal.svg.png

Aspirin

  • http://upload.wikimedia.org/wikipedia/commons/thumb/2/29/paracetamol-skeletal.svg/120px-paracetamol-skeletal.svg.png

Paracetamol

  • http://upload.wikimedia.org/wikipedia/commons/thumb/e/e4/pikrins%c3%a4ure.svg/120px-pikrins%c3%a4ure.svg.png

Picric acid

The arene ring has an ability to stabilize charges. This is seen in, for example, phenol (C6H5-OH), which is acidic at the hydroxyl (OH), since a charge on this oxygen (alkoxide -O) is partially delocalized into the benzene ring.




Download 190.74 Kb.

Share with your friends:
1   2   3   4   5   6   7




The database is protected by copyright ©ininet.org 2024
send message

    Main page