Deactivating groups
Encyclopedia
In organic chemistry
, a deactivating group (or electron withdrawing group) is a functional group
attached to a benzene
molecule
that removes electron density
from the benzene ring, making electrophilic aromatic substitution
reactions slower and more complex relative to benzene. Depending on their relative strengths, deactivating groups also determine the positions (relative to themselves) on the benzene ring where substitutions must take place; this property is therefore important in processes of organic synthesis
.
s to attack the benzene molecule at the ortho-
and para-
positions, while strongly and moderately deactivating groups direct attacks to the meta-
position. This is not a case of favouring the meta- position like para- and ortho- directing functional groups, but rather disfavouring the para- and ortho- positions more than they disfavour the meta- position.
-NR3+ Quaternary amine /Quaternary ammonium base
-CF3, CCl3 trihalide
s
-SO3H Sulfonates
-COOH Carboxylic acid
-COOR Esters
-CHO, -COR Aldehydes and Ketones
Halides are ortho- para- directing groups due to the lone pair activating the benzene ring. F directs strongly to the para position (86%) while I directs to ortho and para (45% and 54% respectively).
s (specifically the ease through which mesomeric effects travel through such systems) to create regions of positive charge within the resonance contributors.
For example, in nitrobenzene
the resonance structures have positive charges around the ring system:
The resulting resonance hybrid, now possessing δ+ charges in the ortho- and para- positions repels approaching electrophiles increasing the relative success of attack in the meta position.
The selectivities observed with activating groups and deactivating groups were first described in 1892 and have been known as the Crum Brown-Gibson Rule.
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
, a deactivating group (or electron withdrawing group) is 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...
attached to a 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....
molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
that removes electron density
Electron density
Electron density is the measure of the probability of an electron being present at a specific location.In molecules, regions of electron density are usually found around the atom, and its bonds...
from the benzene ring, making 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...
reactions slower and more complex relative to benzene. Depending on their relative strengths, deactivating groups also determine the positions (relative to themselves) on the benzene ring where substitutions must take place; this property is therefore important in processes of organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
.
Categories
Deactivating groups are generally sorted into three categories. Weakly deactivating groups direct 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...
s to attack the benzene molecule at the ortho-
Arene substitution patterns
Arene substitution patterns are part of organic chemistry IUPAC nomenclature and pinpoint the position of substituents other than hydrogen in relation to each other on an aromatic hydrocarbon.- Ortho, meta, and para substitution :...
and para-
Arene substitution patterns
Arene substitution patterns are part of organic chemistry IUPAC nomenclature and pinpoint the position of substituents other than hydrogen in relation to each other on an aromatic hydrocarbon.- Ortho, meta, and para substitution :...
positions, while strongly and moderately deactivating groups direct attacks to the meta-
Arene substitution patterns
Arene substitution patterns are part of organic chemistry IUPAC nomenclature and pinpoint the position of substituents other than hydrogen in relation to each other on an aromatic hydrocarbon.- Ortho, meta, and para substitution :...
position. This is not a case of favouring the meta- position like para- and ortho- directing functional groups, but rather disfavouring the para- and ortho- positions more than they disfavour the meta- position.
Strongly Deactivating Groups
-NO2, nitro groupsNitro 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...
-NR3+ Quaternary amine /Quaternary ammonium base
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,...
-CF3, CCl3 trihalide
Trihalide
A Trihalide in chemistry is an organohalide consisting of three halide atoms bonded to a single carbon atom. An example of a trihalide is chloroform....
s
Moderately Deactivating Groups
-CN Cyano Groups-SO3H Sulfonates
Sulfonic acid
Sulfonic acid usually refers to a member of the class of organosulfur compounds with the general formula RS2–OH, where R is an alkyl or aryl. The formal part of acid, HS2–OH, are formally derivatives of the "parent" inorganic compound with the formula HSO2.-Preparation:Sulfonic acid is...
-COOH Carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
-COOR Esters
-CHO, -COR Aldehydes and Ketones
Halides are ortho- para- directing groups due to the lone pair activating the benzene ring. F directs strongly to the para position (86%) while I directs to ortho and para (45% and 54% respectively).
Mechanism
While steric effects are a consideration, the major contribution of deactivating groups is achieved by utilizing the nature of conjugated systemConjugated system
In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...
s (specifically the ease through which mesomeric effects travel through such systems) to create regions of positive charge within the resonance contributors.
For example, in nitrobenzene
Nitrobenzene
Nitrobenzene is an organic compound with the chemical formula C6H5NO2. It is a water-insoluble pale yellow oil with an almond-like odor. It freezes to give greenish-yellow crystals. It is produced on a large scale as a precursor to aniline. Although occasionally used as a flavoring or perfume...
the resonance structures have positive charges around the ring system:
The resulting resonance hybrid, now possessing δ+ charges in the ortho- and para- positions repels approaching electrophiles increasing the relative success of attack in the meta position.
The selectivities observed with activating groups and deactivating groups were first described in 1892 and have been known as the Crum Brown-Gibson Rule.