Non-classical ion
Encyclopedia
Non-classical ions in organic chemistry
are a special type of carbonium ion
s displaying delocalization of sigma bond
s in 3-center-2-electron bonds of bridged systems. The term non-classical ion was first used by John D. Roberts
in 1951 in relation to the properties of cyclobutyl
cations but the actual ions were first described by Saul Winstein
in 1949 in order to explain the reactivity of certain norbornyl
compounds.
The compounds in question are exo-norbornyl brosylate 1 and its endo isomer 3 in scheme 1 and the reaction is solvolysis
or acylation
with the potassium
salt
of acetic acid
in acetic acid. A key observation is that in this nucleophilic displacement both isomer
s give the same reaction product an exo-acetate 2. Also the reaction rate
for the exo-reaction is 350 times the reaction rate for the endo reaction or a cyclohexyl
control reaction
In a related experiment both enantiomer
s 1 and 2 of the exo-brosylate on solvolysis give the same racemic
reaction product (scheme 2). The optical activity of the reaction disappears at the same reaction rate
as that of the solvolysis.
These observations are explained by invoking a non-classical ion 3 in scheme 3 as a reactive intermediate
as the initial reaction product of both endo 1 and exo isomer 2. This ion is formed when sigma electrons in the C1-C6 bond assist by neighbouring group participation
with the expulsion of the leaving group
and now the positive charge residing on C1 is delocalized on C2 as well. The formation of the carbocation
is the slow rate determining step. In this reaction step the exo leaving group is better positioned in relation to C1 than the endo leaving group and this explains the markedly difference in reactivity. The C2 carbon atom in the intermediate is pentavalent and therefore a carbonium ion
. The ion is also symmetrical which is more obvious in the equivalent structure 3b. This symmetry explains the observed racemization. In classical resonance treatment the ion 3 can be regarded as a hybrid of resonance structures 3.1 through 3.3 with a full positive charge on C2, C6, and C7.
Further evidence for this model was obtained by tracer studies in which two carbon atoms in the norbornyl skeleton were replaced by the radioactive carbon 14 isotope
s (Scheme 4) . The primary solvolyis reaction product was derivatized by reduction with lithium aluminium hydride
and oxidation with sodium permanganate
to the dicarboxylic acid
followed by a Curtius rearrangement
expelling carbon dioxide
. When the acetyl anion is indeed able to attack both C1 and C2 positions of the symmetrical norbornyl cation 2 then 50% of the generated CO2 will contain 14C. The experimental value in this experiment was 40% and in order to account for the presence of 14C in other positions (a scrambling process) the nortricyclonium cation was postulated which is basically a face-capped norbornyl cation allowing hydride shifts.
In 1964 George A Olah
began to produce direct evidence for the norbornyl cation when he subjected the norbornyl antimony chloropentafluoride salt obtained by reaction of exo-2-chloro-norbornane with antimony pentafluoride
to NMR
analysis. The room temperature NMR spectrum was a single broad peak due to the presence of hydride shifts but these could be partially frozen when cooled to −60 °C in liquid sulfur dioxide
. It was also found that the norbornyl cation could also be generated from norbornanes with chlorine substituent
at the bridging position or bridgehead positions and from reaction of norbornene
with HSbF6 thereby confirming the presence of the rearrangements taking place in the 14C scrambling process.
Solid-state NMR analysis was possible at temperatures as low as 5 kelvin
s at which temperature all positions are assumed to be frozen. One of the two signals visible in the spectrum corresponded to the identical C1 and C2 carbon atoms.
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...
are a special type of carbonium ion
Carbonium ion
A carbonium ion is a carbocation of the penta- or tetracoordinated nonclassical type such as an ion of the type R5C+.- Methanium:The parent compound methanium or CH5+ is protonated methane and a superacid. This ion exists as a reactive intermediate in the interstellar medium and can be produced in...
s displaying delocalization of sigma bond
Sigma bond
In chemistry, sigma bonds are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is...
s in 3-center-2-electron bonds of bridged systems. The term non-classical ion was first used by John D. Roberts
John D. Roberts
John Dombrowski Roberts is an American chemist. He has made contributions to the integration of physical chemistry, spectroscopy and organic chemistry for the understanding of chemical reaction rates....
in 1951 in relation to the properties of cyclobutyl
Cyclobutane
Cyclobutane is an organic compound with the formula 4. Cyclobutane is a colourless gas and commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes...
cations but the actual ions were first described by Saul Winstein
Saul Winstein
Saul Winstein was the Canadian chemist who discovered the Winstein reaction, in which he argued a non-classical cation was needed to explain the stability of the norbornyl cation. This fueled a debate with Herbert C. Brown over the existence of delocalized cations such as this. Richard F...
in 1949 in order to explain the reactivity of certain norbornyl
Norbornane
Norbornane is an organic compound and a saturated hydrocarbon with chemical formula C7H12. It is a crystalline compound with melting point 88 °C. The carbon skeleton is a cyclohexane ring bridged by a methylene group in the 1,4- position, and is a bridged bicyclic compound...
compounds.
The compounds in question are exo-norbornyl brosylate 1 and its endo isomer 3 in scheme 1 and the reaction is solvolysis
Solvolysis
Solvolysis is a special type of nucleophilic substitution or elimination where the nucleophile is a solvent molecule. For certain nucleophiles, there are specific terms for the type of solvolysis reaction...
or acylation
Acylation
In chemistry, acylation is the process of adding an acyl group to a compound. The compound providing the acyl group is called the acylating agent....
with the potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
of acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
in acetic acid. A key observation is that in this nucleophilic displacement both isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...
s give the same reaction product an exo-acetate 2. Also the reaction rate
Reaction rate
The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
for the exo-reaction is 350 times the reaction rate for the endo reaction or a cyclohexyl
Cyclohexane
Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon...
control reaction
In a related experiment both enantiomer
Enantiomer
In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are non-superposable , much as one's left and right hands are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without...
s 1 and 2 of the exo-brosylate on solvolysis give the same racemic
Racemic
In chemistry, a racemic mixture, or racemate , is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. The first known racemic mixture was "racemic acid", which Louis Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid.- Nomenclature :A...
reaction product (scheme 2). The optical activity of the reaction disappears at the same reaction rate
Reaction rate
The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
as that of the solvolysis.
These observations are explained by invoking a non-classical ion 3 in scheme 3 as a reactive intermediate
Reactive intermediate
In chemistry a reactive intermediate is a short-lived, high energy, highly reactive molecule. When generated in a chemical reaction it will quickly convert into a more stable molecule. Only in exceptional cases can these compounds be isolated and stored, e.g. low temperatures, matrix isolation...
as the initial reaction product of both endo 1 and exo isomer 2. This ion is formed when sigma electrons in the C1-C6 bond assist by neighbouring group participation
Neighbouring group participation
Neighbouring group participation or NGP in organic chemistry has been defined by IUPAC as the interaction of a reaction centre with a lone pair of electrons in an atom or the electrons present in a sigma bond or pi bond . When NGP is in operation it is normal for the reaction rate to be increased...
with the expulsion of the leaving group
Leaving group
In chemistry, a leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. Leaving groups can be anions or neutral molecules. Common anionic leaving groups are halides such as Cl−, Br−, and I−, and sulfonate esters, such as para-toluenesulfonate...
and now the positive charge residing on C1 is delocalized on C2 as well. The formation of the 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,...
is the slow rate determining step. In this reaction step the exo leaving group is better positioned in relation to C1 than the endo leaving group and this explains the markedly difference in reactivity. The C2 carbon atom in the intermediate is pentavalent and therefore a carbonium ion
Carbonium ion
A carbonium ion is a carbocation of the penta- or tetracoordinated nonclassical type such as an ion of the type R5C+.- Methanium:The parent compound methanium or CH5+ is protonated methane and a superacid. This ion exists as a reactive intermediate in the interstellar medium and can be produced in...
. The ion is also symmetrical which is more obvious in the equivalent structure 3b. This symmetry explains the observed racemization. In classical resonance treatment the ion 3 can be regarded as a hybrid of resonance structures 3.1 through 3.3 with a full positive charge on C2, C6, and C7.
Further evidence for this model was obtained by tracer studies in which two carbon atoms in the norbornyl skeleton were replaced by the radioactive carbon 14 isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s (Scheme 4) . The primary solvolyis reaction product was derivatized by reduction with lithium aluminium hydride
Lithium aluminium hydride
Lithium aluminium hydride, commonly abbreviated to LAH or known as LithAl, is an inorganic compound with the chemical formula LiAlH4. It was discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic synthesis, especially for the reduction of esters,...
and oxidation with sodium permanganate
Potassium permanganate
Potassium permanganate is an inorganic chemical compound with the formula KMnO4. It is a salt consisting of K+ and MnO4− ions. Formerly known as permanganate of potash or Condy's crystals, it is a strong oxidizing agent. It dissolves in water to give intensely purple solutions, the...
to the dicarboxylic acid
Dicarboxylic acid
Dicarboxylic acids are organic compounds that contain two carboxylic acid functional groups. In molecular formulae for dicarboxylic acids, these groups are often written as HOOC-R-COOH, where R may be an alkyl, alkenyl, alkynyl, or aryl group...
followed by a Curtius rearrangement
Curtius rearrangement
The Curtius rearrangement , as first defined by Theodor Curtius, is a chemical reaction that involves the rearrangement of an acyl azide to an isocyanate. Several reviews have been published....
expelling carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
. When the acetyl anion is indeed able to attack both C1 and C2 positions of the symmetrical norbornyl cation 2 then 50% of the generated CO2 will contain 14C. The experimental value in this experiment was 40% and in order to account for the presence of 14C in other positions (a scrambling process) the nortricyclonium cation was postulated which is basically a face-capped norbornyl cation allowing hydride shifts.
In 1964 George A Olah
George Andrew Olah
George Andrew Olah is an American chemist. His research involves the generation and reactivity of carbocations via superacids. For this research, Olah was awarded a Nobel Prize in Chemistry in 1994...
began to produce direct evidence for the norbornyl cation when he subjected the norbornyl antimony chloropentafluoride salt obtained by reaction of exo-2-chloro-norbornane with antimony pentafluoride
Antimony pentafluoride
Antimony pentafluoride is the inorganic compound with the formula SbF5. This colourless, viscous liquid is a valuable Lewis acid and a component of the superacid fluoroantimonic acid, the strongest known acid...
to NMR
NMR
NMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solid-state nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon-13 NMR...
analysis. The room temperature NMR spectrum was a single broad peak due to the presence of hydride shifts but these could be partially frozen when cooled to −60 °C in liquid sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
. It was also found that the norbornyl cation could also be generated from norbornanes with chlorine 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...
at the bridging position or bridgehead positions and from reaction of norbornene
Norbornene
Norbornene or norbornylene or norcamphene is a bridged cyclic hydrocarbon. It is a white solid with a pungent sour odor. The molecule consists of a cyclohexene ring bridged with a methylene group in the para position...
with HSbF6 thereby confirming the presence of the rearrangements taking place in the 14C scrambling process.
Solid-state NMR analysis was possible at temperatures as low as 5 kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s at which temperature all positions are assumed to be frozen. One of the two signals visible in the spectrum corresponded to the identical C1 and C2 carbon atoms.