Neighbouring group participation

Neighbouring Group Participation

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Neighbouring group participation

Neighbouring group participation or NGP in organic chemistry

Organic chemistry

Organic chemistry is a discipline within chemistry which involves the science study of the structure, properties, composition, chemical reaction, and preparation of chemical compounds that contain carbon....
has been defined by IUPAC as the interaction of a reaction centre with a lone pair

Lone pair

A lone pair is a valence electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons....
of electrons in an atom or the electrons present in a sigma bond

Sigma bond

In chemistry, sigma bonds are the strongest type of covalent bond chemical bond. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups....
or pi bond

Pi bond

In chemistry, pi bonds are covalent bond chemical bonds where two lobes of one involved electron atomic orbital overlap two lobes of the other involved electron orbital....
. When NGP is in operation it is normal for the reaction rate

Reaction rate

The reaction rate or rate of reaction for a reactant or product in a particular chemical reaction is intuitively defined as how fast a reaction takes place....
to be increased. It is also possible for the stereochemistry

Stereochemistry

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Neighbouring group participation or NGP in organic chemistry

Organic chemistry

Lone pair

Sigma bond

Pi bond

Reaction rate

Stereochemistry

Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chirality molecules ....
of the reaction to be abnormal (or unexpected) when compared with a normal reaction. While it is possible for neighbouring groups to influence many reactions in organic chemistry (For instance the reaction of a diene

Diene

Dienes or diolefins are hydrocarbons which contain two covalent bond. Dienes are intermediate between alkenes and polyenes....
such as cyclohex-1,3-diene with maleic anhydride

Maleic anhydride

Maleic anhydride is an organic compound with the formula C4H2O3. In its pure state it is a colourless or white solid with an acrid odour....
normally gives the exo isomer because of a secondary effect this page is limited to neighbouring group effects seen with 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 Electron shell#Valence shell instead of the eight valence electrons that ensures maximum stability ....
s and S_N2 reactions.

NGP by heteroatom lone pairs

A classic example of NGP is the reaction of a sulfur

Sulfur

Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant Valence non-metal....
or nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674?. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere....
mustard

Sulfur mustard

The sulfur mustards, of which mustard gas is a member, are a class of related cytotoxic, vesicant chemical warfare agents with the ability to form large blisters on exposed skin....
with a nucleophile

Nucleophile

In chemistry, a nucleophile is a reagent that forms a chemical bond to its reaction partner by donating both bonding electrons. Because nucleophiles donate electrons, they are by definition Lewis bases ....
, the rate of reaction is much higher for the sulfur mustard

Sulfur mustard

Alkyl

An alkyl is a univalent Radical consisting of carbon and hydrogen atoms, arranged in a chain. The Alkyls form homologous series with the general formula CnH2n+1....
chloride

Chloride

The chloride ion is formed when the chemical element chlorine picks up one electron to form an anion Cl−....
without a heteroatom

Heteroatom

In the organic nomenclature of organic chemistry, a heteroatom is any atom that is not carbon or hydrogen. It is typically, but not exclusively, nitrogen, oxygen, sulfur, phosphorus, boron, chlorine, bromine, or iodine....
.

In sugar

Sugar

Sugar is a class of edible crystalline substances, mainly sucrose, lactose, and fructose. Human taste buds interpret its flavor as sweet. Sugar as a basic food carbohydrate primarily comes from sugar cane and from sugar beet, but also appears in fruit, honey, sorghum, sugar maple , and in many other sources....
chemistry anchimeric assistance is an example of NGP.

NGP by an alkene

The p orbitals of an alkene

Alkene

In organic chemistry, an alkene, olefin, or olefine is an Saturation chemical compound containing at least one carbon-to-carbon double bond....
can stabilize a 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....
by helping to delocalize the positive charge of the carbocation

Carbocation

Saturation

Saturation or saturated may mean:* Dew point, which is a temperature that occurs when atmospheric humidity reaches 100% and the air is saturated with moisture...
tosylate will react more quickly with a nucleophile than the saturated tosylate.

The carbocationic intermediate will be stabilized by resonance where the positive charge is spread over several atoms, in the diagram below this is shown.

Here is a different view of the same intermediates.

Even if the alkene is more remote from the reacting center the alkene can still act in this way. For instance in the following alkyl

Alkyl

An alkyl is a univalent Radical consisting of carbon and hydrogen atoms, arranged in a chain. The Alkyls form homologous series with the general formula CnH2n+1....
benzenesulfonate the alkene is able to delocalise the carbocation.

Also the increase in the rate of the SN2 reaction

SN2 reaction

The SN2 reaction is a type of nucleophilic substitution, where a lone pair from a nucleophile attacks an electron deficient electrophile center and covalent bond to it, expelling another group called a leaving group....
of allyl

Allyl

An allyl group is an alkene hydrocarbon group with the formula H2C=CH-CH2-. It is made up of a vinyl group, CH2=CH-, attached to a methylene -CH2....
bromide with a nucleophile compared with the reaction of n-propyl bromide is because the orbital

Orbital

The term orbital has several meanings:In chemistry and physics:* Atomic orbital* Molecular orbitalIn astronomy and space flight:...
s of the p bond overlap with those of the 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....
. In the allyl system the alkene orbitals overlap with the orbitals of a S_N2 transition state.

NGP by a cyclopropane, cyclobutane or a homoallyl group

If Cyclopropylmethyl chloride is reacted with ethanol

Ethanol

Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatility , flammable, colorless liquid....
and water

Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
then a mixture of 48% cyclopropylmethyl

Cyclopropane

Cyclopropane is a cycloalkane molecule with the molecular formula C3H6, consisting of three carbon atoms linked to each other to form a ring, with each carbon atom bearing two hydrogen atoms....
alcohol, 47% cyclobutanol

Cyclobutane

Cyclobutane, C4H8, with a molecular mass of 56.107g/mol, is a four carbon alkane in which all the carbon atoms are arranged cyclically, hence cyclobutane....
and 5% homoallyl alcohol (but-3-enol) is obtained. This is because the carbocationic intermidate is delocalised onto many different carbons through a reversible ring opening.

NGP by an aromatic ring

In the case of a benzyl

Benzyl

In organic chemistry, benzyl is the term used to describe the substituent or molecular fragment possessing the structure C6H5CH2-....
halide the reactivity is higher for the because the S_N2 transition state enjoys a similar overlap effect to that in the allyl system.

An aromatic ring can assist in the formation of a carbocation

Carbocation

When the following tosyl

Tosyl

A tosyl group is CH3C6H4SO2. This group is usually derived from the compound 4-Toluenesulfonyl chloride, CH3C6H4SO2Cl, which forms esters and amides of toluenesulfonic acid....
ate reacts with acetic acid

Acetic acid

Acetic acid, CH3COOH, also known as ethanoic acid, is an organic acid which gives vinegar its sour taste and pungent smell. Pure, water-free acetic acid is a colourless liquid that absorbs water from the environment , and freezes at 16.7 Celsius to a colourless crystalline solid....
in solvolysis

Solvolysis

Solvolysis is a special type of nucleophilic substitution or elimination reaction where the nucleophile is a solvent molecule. For certain nucleophiles, there are specific terms for the type of solvolysis reaction....
then rather than a simple S_N2 reaction forming B, a 48:48:4 mixture of A, B and (C+D) was obtained .

The mechanism which forms A and B is shown below.

External links

A PhD
PHD

PHD may refer to:* Parisada Hindu Dharma, an Indonesian reform organization* PHD, a track on The Crystal Method album Tweekend* PHD finger, a protein sequence...
thesis
Thesis

A dissertation is a document that presents the author's research and findings and is submitted in support of candidature for a degree or professional qualification....
written on this subject is at .