Electrophilic substitution reactions are chemical reactions in which an electrophile displaces another group, typically but not always hydrogen. Electrophilic substitution is characteristic of aromatic compounds. It is an important way of introducing functional groups on benzene rings. In chemistry, an electrophile (literally electron-lover) is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a substance. ... General Name, Symbol, Number Hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1 (IA), 1, s Density, Hardness 0. ... In chemistry, an aromatic molecule is one in which electrons are free to cycle around circular arrangements of atoms, which are alternately singly and doubly bonded to one another. ... Benzene, C6H6, PhH, or benzol is a colorless and flammable liquid with a pleasant, sweet smell. ...

General mechanism

General Mechanism of Electrophilic Aromatic Substitution

Key Steps: Image File history File links File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...

• Benzene, a nucleophile, reacts with an electrophile (Y⁺)
• Addition of an electrophile to the ring forms a carbocation with loss of aromaticity
• The carbocation loses a proton from the site of the electrophilic attack and the electrons are added to the ring again
• Aromaticity is reformed

Notes: In chemistry, a nucleophile (literally nucleus lover) is a reagent which is attracted to centres of positive charge. ... In chemistry, an electrophile (literally electron-lover) is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a substance. ... A carbocation is an ion with a positively-charged carbon atom. ... A carbocation is an ion with a positively-charged carbon atom. ...

• B and HB⁺ represent a base present in the mixture (water, solvent, conjugate base)
• An electrophilic addition reaction does not occur because this would result in the permanent loss of aromaticity, an energetically very unfavourable condition. The ΔG° is close to zero for an electrophilic substitution reaction with benzene.
• The cyclohexadienyl cation intermediate goes by the name of Wheland intermediate.

An electrophilic substitution reaction on benzene does not always result in monosubstitution. While electrophilic substituents usually withdraw electrons from the aromatic ring and thus deactivate it against further reaction, a sufficiently strong electrophile can perform a second or even a third substitution. In the case of weak electrophiles, such as halogens, the loss of aromaticity can be so small that achieving only monosubstitution is virtually impossible. The use of catalysts might also result in polysubstitution. The common (Arrhenius) definition of a base is a chemical compound that either donates hydroxide ions or absorbs hydrogen ions when dissolved in water. ... A solvent is a liquid that dissolves a solid, liquid, or gaseous solute, resulting in a solution. ... In the field of chemistry, in the Brønsted-Lowry (protonic) theory of acids and bases, a conjugate base is the basic member, X-, of a pair of compounds that differ only by the presence or absence of a proton at a certain position. ... In organic chemistry, an electrophilic addition reaction is an addition reaction where in chemical compound a pi bond is removed by the creation of two new covalent bonds. ... A cation is an ion with positive charge. ... The halogens are a chemical series. ... A catalyst (Greek: καταλύτης) is a substance that accelerates the rate of a chemical reaction, at some temperature, but without itself being transformed or consumed by the reaction (see also catalysis). ...

Performing an electrophilic substitution on an already substituted benzene compound raises the problem of regioselectivity. In case of a monosubstituted benzene, the electrophile can react on a position next the already existing substituent (ortho) or once removed (meta) or twice removed (para)?. It turns out that substituents can be grouped into two classes, those that activate the aromatic ring towards further electrophilic substitution - the activating groups - and those that deactivate the compound. Each group also drives the direction to a specific position on the ring. For example the orientation in toluene nitration is towards the aromatic ortho position and aromatic para position. In chemistry, regioselectivity is the preference of one direction of chemical bond making or breaking over all other possible directions. ... In organic chemistry, a substituent is an atom or group of atoms subsituted in place of a hydrogen atom on the parent chain of a hydrocarbon. ... In organic chemistry, a functional group is called an activating group if a benzene molecule to which it is attached more readily participates in electrophilic substitution reactions. ... Toluene, also known as methylbenzene or phenylmethane is a clear water-insoluble liquid with the typical smell of paint thinners, reminiscent of the sweet smell of the related compound benzene. ... Nitration occurs with aromatic organic compounds via an electrophilic substitution mechanism involving the attack of the electron-rich benzene ring by the nitryl (nitronium) cation. ... IUPAC nomenclature is a systematic way of naming organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). ... IUPAC nomenclature is a systematic way of naming organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). ...

Common electrophilic substitution reactions

All of the following reactions follow the same general mechanism as above. If present, catalysts are used to form electrophiles and do not react with benzene. Ar = aryl (an aromatic; benzene) A catalyst (Greek: καταλύτης, catalytis) is a substance that accelerates the rate of a chemical reaction, at some temperature, but without itself being transformed or consumed by the reaction (see also catalysis). ...

Nitration

Aromatic nitrations to form nitro compounds take place by generating a nitronium ion from nitric acid and sulfuric acid. Nitration occurs with aromatic organic compounds via an electrophilic substitution mechanism involving the attack of the electron-rich benzene ring by the (nitryl) nitronium ion. ... Nitro compounds are organic compounds that contain one or more nitro functional groups (NO2). ... The chemical compound nitric acid (HNO3), otherwise known as aqua fortis, is a colorless, corrosive liquid, a toxic acid which can cause severe burns. ... Sulfuric acid (British English: sulphuric acid), H2SO4, is a strong mineral acid. ...

Sulfonation

Reaction:

Sulfonation of Benzene

The formation of sulfur trioxide (the electrophile) from concentrated sulfuric acid when heated is shown below: Image File history File links File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Sulfur trioxide (British spelling: Sulphur trioxide) has the chemical formula SO3. ...

Reaction of heated concentrated sulfuric acid

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Halogenation

Typical reaction of halogenation of benzene: In chemistry, halogenation is a chemical reaction that replaces a hydrogen atom with a halogen atom. ...

Halogenation of Benzene

Where X is the halogen, [catalyst] represents the catalyst (if needed) and HX represents the protonated base. Image File history File links File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...

For Bromination / Chlorination
X = Br₂ / Cl₂
[catalyst] = FeBr₃ / FeCl₃
HX = HBr / HCl

The mechanism for bromination of benzene:

Bromination of Benzene

The mechanism for chlorination of benzene is the same as bromination of benzene, except as noted above. Ferric Bromide and ferric chloride become inactivated if they react with water, including moisture in the air. Therefore, they are generated in situ by adding iron fillings and bromine or chlorine. Image File history File links File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...

The mechanism for iodination is slightly different: iodine (I₂) is treated with an oxidizing agent (such as nitric acid) to obtain the electrophilic iodine (2 I⁺). Unlike the other halogens, iodine does not serve as a base since it is positive.

Halogenation of aromatic compounds differs from the halogenation of alkenes, which do not require a Lewis Acid catalyst. The formation of a carbocation in benzene results in the loss of aromaticiy, which has a higher activation energy compared to carbocation formation in alkenes. In other words, alkenes are more reactive and don't need to have the Br-Br or Cl-Cl bond weakened. An alkene is one of the three classes of unsaturated hydrocarbons that contain at least one carbon-carbon double bond and have the general molecular formula of CnH2n (the other two being alkynes and arenes). ...

If the ring contains a a strongly activating substituent such as -OH, -OR or amines, a catalyst is not necessary. However, if a catalyst is used with excess bromine, then a tribromide will be formed. Ammonia Amines are organic compounds containing nitrogen as the key atom in the amine functional group. ...

Friedel-Crafts acylation

Reaction:

Friedel-Crafts Acylation with acyl chloride

The formation of an acylium ion (the electrophile) from an acyl chloride with aluminium trichloride is shown below: Image File history File links File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...

Friedel-Crafts Acylation with acyl chloride

Benzene reacts with the resonance contributor with a positive charge on the carbon. Friedel-Crafts acylation must be carried out with more than one equivalent of aluminium trichloride since it will complex with the carbonyl group. Once the reaction is over, water is added to free the product from the complex. Image File history File links File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... In chemistry, acylation is the process of adding an acyl group to a compound. ... In chemistry, a carbonyl group is a functional group composed of an atom of carbon double-bonded to an atom of oxygen. ...

Friedel-Crafts alkylation

Reaction:

Friedel-Crafts Alkylation

The formation of the carbocation (the electrophile) from an alkyl halide with aluminium trichloride is shown below: Image File history File links File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...

Reaction of alkyl halide with AlCl₃

Alkyl fluorides, alkyl chlorides, alkyl bromides, and alkyl iodides can all be used with aluminium trichloride. Since an alkyl substituted benzene is more reactive, large amounts of benzene are added so it is more likely that aluminium trichloride will encounter unsubstituted benzene. Carbocation rearrangement can occur if it leads to a more stable form. In this case, the major product will contain the more stable carbocation. Image File history File links File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...

Both the Friedel-Crafts acylation and alkylation are named after Charles Friedel and James Crafts. In chemistry, acylation is the process of adding an acyl group to a compound. ... Alkylation is the transfer of an alkyl group from one molecule to another. ... Charles Friedel (March 12, 1832 – April 20, 1899) was a French chemist and mineralogist. ... James Crafts (March 8, 1839 - 20 June 1917) was an American chemist, most famous for developing the Friedel-Crafts alkylation and acylation reactions with Charles Friedel in 1877. ...