A carbocation , sometimes called ion carbonium , is a Ion derived from a Composé organics, which has a electric Charge positive on one or more Atome S of Carbone. The carbocations are intermediate of reaction.
History
Discovered - Halochromie
The study of the carbocations starts in 1902 when the chemists James Flack Norris and Kehrman discover independently that when alcohol S of triphénylméthyl, colorless, are put in solution in sulphuric Acid concentrated, the solution takes a major yellow color. In the same way the Chloride of triphénylméthyl form of the complex oranges with Chloride tin or aluminum. Johann Friedrich Wilhelm Adolf von Baeyer discovers into 1902 that the compounds formed are salts.-
Ph3COH + H2SO4 → Ph3C+HSO4- + H2O (pH means that there is a substituent phenyl)
Adolf von Baeyer calls the relation between the color and the formation of the salt Halochromie, whose malachite Vert is an good example.
Intermediary of reaction
The carbocations are intermediate of reaction, i.e. molecules of short durations of life, unstable, which appear during the chemical reactions. This idea is initially developed by Hans Meerwein in her study of the Réarrangement of Wagner-Meerwein. The carbocations are also implied in reactions of the type SN1 (nucleophilic Substitution in two stages) and E1 (Réaction of elimination in two stages) and in reactions of rearrangement, like the 1,2-rearrangement of Whitmore. The Chimiste S were initially reticent to accept the concept of carbocation, and it was necessary to wait a long time before the Newspaper of the American company of Chemistry ( the Journal off the American Chemical Society ) accepts the articles which evoked it. In 1962, George Andrew Olah dissolves Fluorure of tert-butyl in a Superacide and observes by nuclear Magnetic resonance that the tert-butyl carbocation obtained is stable.
Structure
Model of Lewis
The Atome of Carbone has four electron S single people in his Couche of valence. These four electrons enable him to form four doublets, therefore to have eight electrons in its layer of valence and thus to respect the Règle of the byte and to have a maximum stability. However, in a carbocation, it misses a doublet with the carbon charged which has nothing any more but six electrons in its layer of valence. This carbon thus has a quantum Case vacuum, a deficiency in electrons which explains its positive electric charge. This deficiency in electrons makes the carbocation very unstable and reactive.
Because of this deficiency in electrons the carbocation behaves like a reagent électrophile, or like a Acide of Lewis. It will react with compounds Nucléophile S which have an excess of electrons or bases of Lewis.
Space structure
On the level of carbon charged the Molécule has a plane structure with Angle S of valence of 120 °. Carbon charged is related to the remainder of the molecule by three connections , and thus has a atomic Orbitale 2pz vacuum, it is thus hybrid sp ² (see the diagram).Stability
The carbocation is unstable because it is charged positively. It will be thus more stable if this load is partially compensated.Inductive effect
When two different atoms are dependant the electrons are overall closer to the electronegative atom more (which attracts more the electrons), it is the inductive Effet due to polarization of the connection. The most electronegative atom will be thus charged a little negatively, the other atom a little positively.Thus the carbocation can compensate for its positive load partially, therefore to stabilize itself. If carbon charged profits from an inductive effect donor, if it is connected to one or more atoms (or groups of atoms) less electronegative, it will be stabilized. And conversely, if it is connected to one or more atoms (or groups of atoms) more electronegative than him, it will be less stable.
The groupings Alkyle S are less electronegative than carbon. Thus more carbon charged is related to a big number of groupings alkyls, plus the carbocation will be stable. A tertiary carbocation (carbon charged is related to three alkyls groups) is more stable than a secondary, him even more stable than a primary education.
Contrary, carbon charged will be strongly destabilized if it is related to a grouping Nitrile (- CN), very electronegative.
Mésomère effect
When in a chemical compound of the electrons are delocalized one speaks about Mésomérie. In this case one finds several structures electronic possible, several possible diagrams. None of these diagrams describes with him only reality, it is " the somme" of these diagrams which approaches the most reality.The carbocations can be stabilized by mésomères effect. Indeed a carbocation has orbital a 2p vacuum, and it is between orbital 2p close that the electrons can be delocalized. And if the positive load of the carbocation benefits from a mésomère effect, it will be it also delocalized and to some extent distributed on all the compound, which will be then more stable.
The mésomère effect is more important than the inductive effect. The compound taken in example is, if one is worried only inductive effect, very unstable: it is a primary carbocation. But in this compound the electrons are delocalized because of double connection, the compound thus exists in two forms mésomères and is thus relatively stable.
Rearrangement
The carbocations can rearrange them-even to pass from an unstable form to a more stable form.This rearrangement takes place by transfer of Hydrogène, or by transfer of alkyl group. If carbon charged is primary or secondary, therefore not very stable, and that an adjacent carbon is tertiary or quaternary, there can be transfer of hydrogen or grouping alkyl of carbon noncharged (tertiary sector or quaternary) towards carbon charged (primary or secondary). Carbon initially noncharged will become positive, while carbon charged will become neutral. Thus the positive load will be located on a tertiary carbon (in all the cases), the carbocation will be more stable since the load will be partially compensated for by the inductive effects donors of the alkyls groups.
This rearrangement can also occur if carbon charged is destabilized by an electronegative grouping (attractile inductive effect). The positive load will seek to some extent with " fuir" electronegative grouping.
This possibility of rearrangement explains the fact that a reaction which is carried out via a carbocation can in certain cases lead to two different products, one, majority, corresponding to the most stable carbocation, and the other with the least stable carbocation. The Règle of Markovnikov results from this.
See too
Internal bond
External bonds