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Nonclassical 
The reaction products in these cases always result from the migration of the neighbouring bond.
The reaction of 
Solvolysis of cyclopropylcarbinyl, cyclobutyl and homoallyl esters are also characterized by very large rates, and have been shown to occur via a common nonclassical ion structure in the form of a bicyclobutonium ion. Siehl , H. U. ''The Conundrum of the (C4H7) Cation: Bicyclobutonium and Related Carbocations'' Adv. Phys. Org. Chem. , 2018, 52, 1-4
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carbocations
A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encount ...
are stabilized by charge delocalization from contributions of neighbouring or bonds, which can form bridged intermediates or transition state
In chemistry, 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 potential energy along this reaction coordinate. It is often marked wi ...
s. Nonclassical ions have been extensively studied with the 2-norbornyl system, which as “naked” ion unambiguously exhibit such a bridged structure. The landmark of nonclassical ions are unexpectedly fast solvolysis
In chemistry, solvolysis is a type of nucleophilic substitution (S1/S2) or elimination reaction, elimination where the nucleophile is a solvent molecule. Characteristic of S1 reactions, solvolysis of a chirality (chemistry), chiral reactant affor ...
rates and large differences between epimer
In stereochemistry, an epimer is one of a pair of diastereomers. The two epimers have opposite configuration at only one stereogenic center out of at least two. All other stereogenic centers in the molecules are the same in each. Epimerization is ...
ic esters. Such behaviour is not restricted to 2-norbornyl esters, as has been shown with some cyclopentyl
Cyclopentane (also called C pentane) is a highly flammable alicyclic hydrocarbon with chemical formula C5H10 and CAS number 287-92-3, consisting of a ring of five carbon atoms each bonded with two hydrogen atoms above and below the plane. It occu ...
and steroidal esters with the tosyloxy leaving group.
Substitution reactions of secondary esters occur by SN2- or SN1-like mechanisms. Only in highly polar solvents such as hexafluoroisopropanol
Hexafluoroisopropanol, commonly abbreviated HFIP, is the organic compound with the formula (CF3)2CHOH. This fluoroalcohol finds use as solvent and synthetic intermediate. It appears as a colorless, volatile liquid that is characterized by a stron ...
(HFIP) of low nucleophilicity
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
one can expect a nearly same uniform SN1-like mechanism. The solvolysis of several cyclopentyl and steroidal esters show that large solvolysis rates and differences between epimers can occur which surpass those of the 2-norbornyl system. In these cases a vicinal C–C or C–H bond can lead to significant delocalization of the positive charge, if these bonds are close to antiperiplanar
In organic chemistry, anti-periplanar, or antiperiplanar, describes the bond angle in a molecule. In this conformer, the dihedral angle of the bond and the bond is greater than +150° or less than −150° (Figures 1 and 2). Anti-periplanar i ...
to the leaving group, and the migration leads to a more stable tertiary carbocation. The reaction products in these cases always result from the migration of the neighbouring bond.
The reaction of epimer
In stereochemistry, an epimer is one of a pair of diastereomers. The two epimers have opposite configuration at only one stereogenic center out of at least two. All other stereogenic centers in the molecules are the same in each. Epimerization is ...
ic esters can be severely slowed by steric
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...
hindrance of solvation
Solvation (or dissolution) describes the interaction of a solvent with dissolved molecules. Both ionized and uncharged molecules interact strongly with a solvent, and the strength and nature of this interaction influence many properties of the ...
.
Solvolysis of cyclopropylcarbinyl, cyclobutyl and homoallyl esters are also characterized by very large rates, and have been shown to occur via a common nonclassical ion structure in the form of a bicyclobutonium ion. Siehl , H. U. ''The Conundrum of the (C4H7) Cation: Bicyclobutonium and Related Carbocations'' Adv. Phys. Org. Chem. , 2018, 52, 1-4/ref>
See also
* 2-Norbornyl cation *Neighbouring group participation
In organic chemistry, neighbouring group participation (NGP, also known as anchimeric assistance) has been defined by the International Union of Pure and Applied Chemistry (IUPAC) as the interaction of a reaction centre with a lone pair of electro ...
*Carbocation
A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountere ...
*Steric effects
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...
*Solvation
Solvation (or dissolution) describes the interaction of a solvent with dissolved molecules. Both ionized and uncharged molecules interact strongly with a solvent, and the strength and nature of this interaction influence many properties of the ...
References
{{reflist Carbocations Ions Organic chemistry