The Polar effect or electronic effect in chemistry

Chemistry

Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

 is the effect exerted by a substituent

Substituent

In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...

 on modifying electrostatic forces operating on a nearby reaction

Chemical reaction

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

 center. The main contributors to the polar effect are the inductive effect

Inductive effect

In chemistry and physics, the inductive effect is an experimentally observable effect of the transmission of charge through a chain of atoms in a molecule by electrostatic induction...

, mesomeric effect

Mesomeric effect

The mesomeric effect or resonance effect in chemistry is a property of substituents or functional groups in a chemical compound. The effect is used in a qualitative way and describes the electron withdrawing or releasing properties of substituents based on relevant resonance structures and is...

 and the through-space electronic field effect.

An electron withdrawing group or EWG draws electrons away from a reaction center. When this center is an electron rich 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...

 or an alkoxide

Alkoxide

An alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They can be written as RO−, where R is the organic substituent. Alkoxides are strong bases and, when R is not bulky, good nucleophiles and good ligands...

 anion with the presence of the substituent that has a stabilizing effect.

Examples of electron withdrawing groups are

• halogen
  Halogen
  The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
  
  s (F, Cl);
• nitrile
  Nitrile
  A nitrile is any organic compound that has a -C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, one example being super glue .Inorganic compounds containing the -C≡N group are not called...
  
  s CN;
• carbonyl
  Carbonyl
  In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
  
  s CO;
• nitro groups
  Nitro compound
  Nitro compounds are organic compounds that contain one or more nitro functional groups . They are often highly explosive, especially when the compound contains more than one nitro group and is impure. The nitro group is one of the most common explosophores used globally...
  
   NO2.

An electron releasing group or ERG (otherwise called electron donating groups or EDG) releases electrons into a reaction center and as such stabilizes electron deficient carbocation

Carbocation

A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability . Therefore carbocations are often reactive,...

s.

Examples of electron releasing groups are

• alkyl groups;
• alcohol
  Alcohol
  In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
  
   groups;
• amino
  Amine
  Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
  
   groups.

The total substituent effect is the combination of the polar effect and the combined steric effects.

In electrophilic aromatic substitution

Electrophilic aromatic substitution

Electrophilic aromatic substitution EAS is an organic reaction in which an atom, usually hydrogen, appended to an aromatic system is replaced by an electrophile...

 and nucleophilic aromatic substitution

Nucleophilic aromatic substitution

right|300px|Aromatic nucleophilic substitutionA nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring...

 substituents are divided into activating group

Activating group

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...

s and deactivating groups where the direction of activation or deactivation is also taken into account.

External links

• Polar effect definition by the IUPAC Gold Book
  Gold Book
  The Compendium of Chemical Terminology is a book published by the International Union of Pure and Applied Chemistry containing internationally accepted definitions for terms in chemistry...
  
  Link