Substitution reaction
Main article: Substitution reaction, electrophilic aromatic substitution reaction
The hydrogen atoms on the benzene ring can be substituted by halogen, nitro, sulfonic acid group, hydrocarbon group, etc. under certain conditions to generate corresponding derivatives. Due to different substituents and different positions and numbers of hydrogen atoms, isomers with different numbers and structures can be generated.
The electron cloud density of the benzene ring is relatively large, so most of the substitution reactions that occur on the benzene ring are electrophilic substitution reactions. Electrophilic substitution reaction is a representative reaction of aromatic rings. When the benzene substituent undergoes electrophilic substitution, the position of the second substituent is related to the type of the original substituent.
Halogenation reaction
The general formula of the halogenation reaction of benzene can be written as:
PhH+X2——→PhX+HX
During the reaction, the halogen molecules are split under the joint action of benzene and the catalyst, X+ attacks the benzene ring, and X- combines with the catalyst.
Taking bromine as an example, mix liquid bromine and benzene. Bromine dissolves in benzene to form a reddish-brown liquid that does not react. When iron filings are added, under the catalytic action of the generated ferric bromide, bromine and Benzene reacts, and the mixture becomes slightly boiling. The reaction exotherms and produces reddish-brown bromine vapor. When the condensed gas meets air, white mist (HBr) appears. Catalytic process:
FeBr3+Br-——→FeBr4-
PhH+Br+FeBr4-——→PhBr+FeBr3+HBr
Pour the reaction mixture into cold water, and a reddish-brown oily liquid mass (dissolved bromine) will sink to the bottom of the water. After washing with dilute alkali solution, a colorless liquid bromobenzene is obtained.
In industry, chlorine and bromine substitutes are the most important among halobenzenes.
Nitrification reaction
Benzene and nitric acid can produce nitrobenzene in the presence of concentrated sulfuric acid as a catalyst
PhH+HO-NO2—–H2SO4 (concentrated)△—→PhNO2+H2O
The nitration reaction is a strong exothermic reaction and can easily generate a substituent, but the further reaction rate is slow. Among them, concentrated sulfuric acid is used as a catalyst and reacts when heated to 50 to 60 degrees Celsius. If heated to 70 to 80 degrees Celsius, benzene will undergo a sulfonation reaction with sulfuric acid.
Sulfonation reactionConcentrated sulfuric acid or fuming sulfuric acid can be used to sulfonate benzene into benzenesulfonic acid at a higher temperature (70~80 degrees Celsius).
PhH+HO-SO3H——△→PhSO3H+H2O
After the introduction of a sulfonic acid group into the benzene ring, the reaction capacity decreases and it is difficult to further sulfonate. A higher temperature is required to introduce the second and third sulfonic acid groups. This shows that the nitro group and the sulfonic acid group are passivating groups, that is, groups that prevent electrophilic substitution from proceeding again.
Fuer-Crafts reaction
Under the catalysis of AlCl3, benzene can also react with alcohols, alkenes and halogenated hydrocarbons. The hydrogen atoms on the benzene ring are replaced by alkyl groups to generate alkylbenzenes. This reaction is called alkylation reaction, also known as Friedel-Crafts alkylation reaction. For example, alkylation with ethylene produces ethylbenzene
PhH+CH2=CH2—AlCl3→Ph-CH2CH3
During the reaction process, the R group may rearrange: for example, 1-chloropropane reacts with benzene to produce cumene. This is because free radicals always tend to a stable configuration.
Under the catalysis of a strong Lewis acid, benzene reacts with acyl chloride or carboxylic acid anhydride, and the hydrogen atoms on the benzene ring are replaced by acyl groups to generate acylbenzene. The reaction conditions are similar to the alkylation reaction.
Addition reaction
Main article: Additive reaction
Although the benzene ring is very stable, double bond addition reactions can also occur under certain conditions. Usually through catalytic hydrogenation, nickel is used as a catalyst, and benzene can generate cyclohexane. But reacting is extremely difficult.
C6H6+3H2——Catalyst △—-→C6H12
In addition, the reaction of benzene to produce hexachlorocyclohexane (HCH) can be obtained by the addition of benzene and chlorine under the condition of ultraviolet irradiation.
Oxidation reaction
Burning
Benzene, like other hydrocarbons, can burn. When oxygen is sufficient, the products are carbon dioxide and water. But when burning in the air, the flame is bright and there is thick black smoke. This is due to the large mass fraction of carbon in benzene.
2C6H6+15O2——→12CO2+6H2O
Ozonation reaction
Benzene can also be oxidized by ozone under certain circumstances, and the product is glyoxal. This reaction can be regarded as an ozonation reaction of cyclic polyolefins generated after the delocalized electrons of benzene are localized.
Under normal conditions, benzene cannot be oxidized by strong oxidants. However, in the presence of a catalyst such as molybdenum oxide, benzene can be selectively oxidized to maleic anhydride by reacting with oxygen in the air. This is one of the few reactions that can destroy benzene’s six-membered carbon ring system. (Maleic anhydride is a five-membered heterocyclic ring.)
This is a strongly exothermic reaction.
Others
Benzene can undergo a condensation reaction at high temperatures using iron, copper, and nickel as catalysts to form biphenyl. With formaldehyde and hypochlorous acid in the presence of zinc chloride, chloromethylbenzene can be generated. It reacts with alkyl metal compounds such as sodium ethyl to form phenyl metal compounds. Phenyl Grignard reagent can be generated by neutralizing magnesium in tetrahydrofuran, chlorobenzene or bromobenzene.
Light isomerization
Benzene can be converted into Dewar benzene under strong light conditions:
The nature of Dewarbenzene is very active (benzene itself is a stable aromatic state with very low energy, but turning into Dewarbenzene requires a lot of light energy, so Dewarbenzene has high energy and is unstable).
Under the action of laser, it can be converted into more active prismane:
Prismatic alkanes are in a three-dimensional state, which results in greater mutual repulsion between the π bonds formed by the sp3 hybrid orbitals of carbon atoms, making them more unstable.
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