Electrophilic substitution
Encyclopedia
Electrophilic substitution reactions are chemical reactions in which an electrophile
displaces a group in a compound, typically but not always hydrogen
. Electrophilic aromatic substitution is characteristic of aromatic compounds and is an important way of introducing functional groups onto benzene
rings. The other main reaction type is electrophilic aliphatic substitution.
The most important reactions of this type that take place are aromatic nitration, aromatic halogenation, aromatic sulfonation
and acylation and alkylating Friedel-Crafts reaction
s.
displaces a functional group
. This reaction is similar to nucleophilic aliphatic substitution where the reactant is a nucleophile
rather than an electrophile
. The two electrophilic reaction mechanisms, SE1 and SE2 (Substitution Electrophilic), are also similar to the nucleophile counterparts SN1 and SN2. In the SE1 course of action the substrate first ionizes into a carbanion
and a positively charged organic residue. The carbanion then quickly recombines with the electrophile
. The SE2 reaction mechanism has a single transition state
in which the old bond and the newly formed bond are both present.
Electrophilic aliphatic substitution reactions are:
Electrophile
In general electrophiles are positively charged species that are attracted to an electron rich centre. In chemistry, an electrophile is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile...
displaces a group in a compound, typically but not always hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
. Electrophilic aromatic substitution is characteristic of aromatic compounds and is an important way of introducing functional groups onto benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
rings. The other main reaction type is electrophilic aliphatic substitution.
Electrophilic aromatic substitution
In electrophilic substitution in aromatic compounds, an atom appended to the aromatic ring, usually hydrogen, is replaced by an electrophile.The most important reactions of this type that take place are aromatic nitration, aromatic halogenation, aromatic sulfonation
Aromatic sulfonation
Aromatic sulfonation is an organic reaction in which a hydrogen atom on an arene is replaced by a sulfonic acid functional group in an electrophilic aromatic substitution.-Mechanism:...
and acylation and alkylating Friedel-Crafts reaction
Friedel-Crafts reaction
The Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877. There are two main types of Friedel–Crafts reactions: alkylation reactions and acylation reactions. This reaction type is a form of electrophilic aromatic substitution...
s.
Electrophilic aliphatic substitution
In electrophilic substitution in aliphatic compounds, an electrophileElectrophile
In general electrophiles are positively charged species that are attracted to an electron rich centre. In chemistry, an electrophile is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile...
displaces a functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...
. This reaction is similar to nucleophilic aliphatic substitution where the reactant is a nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
rather than an electrophile
Electrophile
In general electrophiles are positively charged species that are attracted to an electron rich centre. In chemistry, an electrophile is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile...
. The two electrophilic reaction mechanisms, SE1 and SE2 (Substitution Electrophilic), are also similar to the nucleophile counterparts SN1 and SN2. In the SE1 course of action the substrate first ionizes into a carbanion
Carbanion
A carbanion is an anion in which carbon has an unshared pair of electrons and bears a negative charge usually with three substituents for a total of eight valence electrons. The carbanion exists in a trigonal pyramidal geometry. Formally a carbanion is the conjugate base of a carbon acid.where B...
and a positively charged organic residue. The carbanion then quickly recombines with the electrophile
Electrophile
In general electrophiles are positively charged species that are attracted to an electron rich centre. In chemistry, an electrophile is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile...
. The SE2 reaction mechanism has a single transition state
Transition state
The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules will always...
in which the old bond and the newly formed bond are both present.
Electrophilic aliphatic substitution reactions are:
- NitrosationNitrosationNitrosation is a process of converting organic compounds into nitroso derivatives, i.e. compounds containing the R-NO functionality.-C-Nitroso compounds:C-Nitroso compounds, such as nitrosobenzene, are typically prepared by oxidation of hydroxylamines:...
- Ketone halogenationKetone halogenationIn organic chemistry ketone halogenation is a special type of halogenation.The position alpha to the carbonyl group in a ketone is easily halogenated, due to the ability to form an enolate in basic solution, or an enol in acidic solution...
- Keto-enol tautomerismKeto-enol tautomerismIn organic chemistry, keto-enol tautomerism refers to a chemical equilibrium between a keto form and an enol . The enol and keto forms are said to be tautomers of each other...
- aliphatic diazonium coupling
- carbeneCarbeneIn chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is RR'C:, but the carbon can instead be double-bonded to one group. The term "carbene" may also merely refer to the compound H2C:, also called...
insertion into C-H bonds