Carbonium ion chemical ion

Carbonium ion, ang anumang miyembro ng isang klase ng mga organikong molekula na may positibong singil na naisalokal sa isang carbon atom. Ang ilang mga ion ng karbono ay maaaring ihanda sa paraang sila ay sapat na matatag para sa pag-aaral; mas madalas ang mga ito ay mga maikling porma lamang (mga tagapamagitan) na nagaganap sa mga reaksyon ng kemikal.

Ang mga ion ng Carbonium ay, sa katunayan, ang isa sa mga pinaka-karaniwang klase ng mga tagapamagitan sa mga organikong reaksyon, at ang kaalaman sa mga istruktura at pag-aari ng mga sangkap na ito ay mahalaga sa pag-unawa sa mga reaksyon kung saan nangyari ito. Marami sa mga reaksyon na ito ay ng sintetiko, biochemical, o kahalagahan sa industriya.

Ang mga unang ion ng karbono ay na-obserbahan noong 1901; ito ay hindi hanggang 21 taon mamaya, gayunpaman, na ang kemikal na Aleman na si Hans Meerwein ay nagtapos na ang isang neyutral na produkto (isobornyl chloride) ay nabuo mula sa isang neutral na reaktor (camphene hydrochloride) sa pamamagitan ng pagbabagong muli na kinasasangkutan ng isang kargamento ng ion ng karagatan. Ito ang kauna-unahan na pag-konsepto ng isang karotulang ion bilang isang intermediate sa isang organikong reaksyon ng muling pagsasaayos. Ang ideyang ito ay isinagawa ng chemist ng Amerikanong si Frank Clifford Whitmore mula pa noong 1932 at inilagay sa isang matatag na batayan ng eksperimento ng mga chemist ng Ingles na si Sir Christopher Ingold at ED Hughes, na nagsisimula sa huling bahagi ng 1920s. Kahit na ang isang mahusay na deal ay na-surmised tungkol sa mga ion ng carbon sa pamamagitan ng hindi direktang mga pamamaraan, pagkatapos lamang ng 1960 na ang mga pangkalahatang pamamaraan para sa pagbuo ng matatag, matagal nang nabuong mga ion ng karbilonya ay magagamit.

Pag-uuri.

Dalawang natatanging mga klase ng mga karbid na ion ang nakilala. Ang una ay ang "klasiko" na mga carbon ion, na naglalaman ng isang trivalent na sentro ng atom ng carbon. Ang carbon atom ay nasa isang sp ² estado ng hybridization - iyon ay, tatlong elektron ng carbon atom na sinakop ang mga orbital na nabuo sa pamamagitan ng pagsasama (hybridization) ng tatlong ordinaryong orbital, isa na nagsasaad ng s at dalawa, p. Ang lahat ng tatlong mga orbit ay namamalagi sa isang eroplano; sa gayon, ang sentro ng cationic ng molekula na nabuo sa pamamagitan ng pagbubuklod ng carbon atom na may tatlong iba pang mga atom o grupo ay may posibilidad na maging planar. Ang magulang para sa mga ion na ito ay ang methyl cation, na may formula na CH ⁺ ₃. Sa eskematiko, ang istraktura ay tulad ng ipinapakita sa ibaba (ang mga solidong linya na kumakatawan sa mga bono sa pagitan ng mga atomo):

The second class of carbonium ions includes the pentacoordinated, or “nonclassical,” carbonium ions, which have three single bonds, each joining the carbon atom to one other atom, and a two-electron bond that connects three atoms, rather than the usual two, with a single electron pair. The parent structure for these ions is that of the methonium ion, CH⁺₅ , in which the dotted lines represent a three-centre bond:

It is frequently possible to distinguish between these two types of carbonium ions experimentally, as, for example, by the use of certain instrumental methods. These methods include nuclear magnetic-resonance spectroscopy, which gives information about atomic nuclei; infrared and Raman spectroscopy, which are based on light absorption; and, more recently, X-ray-induced electron-emission spectroscopy, which gives information about bond energies.

Preparation and stability.

Several methods are known for the generation of carbonium ions. They may all, however, be classified in one of the following categories: (1) heterolytic (unsymmetrical) cleavage of the two-electron bond between a carbon atom and an attached group; (2) electron removal from a neutral organic compound; (3) addition of a proton, or other cation, to an unsaturated system; and (4) protonation, or alkylation (addition of an alkyl, or hydrocarbon, group), of a carbon–carbon or carbon–hydrogen single bond. Since carbonium ions are positively charged species, they are most readily formed in relatively polar solvents (solvents consisting of molecules with unsymmetrical distribution of electrons), which help disperse their charges or the charges on the accompanying negative ions throughout the medium. Commonly used solvents include methanol, aqueous acetone, acetic acid, and trifluoroacetic acid.

The fate of a carbonium ion produced by one of these methods is determined essentially by two factors: (1) the nature of the medium in which the ion is generated and (2) the inherent stability of the ion itself. Carbonium ions react rapidly with the solvent or with any available substance attracted to positively charged entities. Therefore carbonium ions have only a fleeting existence, and indirect methods must be used for their study. The common methods are kinetics (measurements of rates of reaction), chemical analysis of the product formed by reaction of the carbonium ion (particularly, determination of spatial arrangements of atoms in a molecule), and isotopic labelling (that is, the use of radioactive isotopes to identify particular atoms).

Solvents have been found that do not react with many carbocations. These solvents are hydrogen fluoride–antimony pentafluoride and fluorosulfuric acid–antimony pentafluoride with sulfur dioxide or sulfuryl chloride fluoride also present. In these solvents, the lifetime of many carbonium ions is sufficient to allow direct observation.

Tertiary carbonium ions are generally more stable than secondary carbonium ions, which, in turn, are more stable than primary ones. In tertiary carbonium ions, the sp² carbon is bonded to three alkyl groups; in secondary carbonium ions, the sp² carbon atom is bonded to two alkyl groups and one hydrogen atom; in primary carbonium ions, the sp² carbon is bonded to either one alkyl group and two hydrogen atoms or, in the case of the methyl cation, three hydrogen atoms. Examples of each are shown below.

This order of relative stability is explained on the basis of the ability of an alkyl group to disperse the charge on the sp² carbon atom.

Benzyl cations are more stable than most primary cations because in the benzyl ions the positive charge can become distributed among the carbon atoms of the aromatic ring so the cation can exist in many forms, all of which contribute to the overall structure. Such forms of the benzyl cation are shown below:

In these structures the benzene ring is indicated by a hexagon, each corner of which is considered to be a carbon atom (the attached hydrogens not being shown). The form with a circle in the hexagon represents structures with alternating single and double bonds in the ring; the other forms are those in which charges appear at various locations in the ring.

Reactions.

Since carbonium ions are electron-deficient entities, they react with any electron-donor molecules, which are also referred to as nucleophiles. There are three types of nucleophiles: n-bases, pi bases, and sigma bases, in which n, pi, and sigma refer to the bonding state of the donor-electron pair in the nucleophile—that is, nonbonded, pi-bonded, and sigma-bonded, respectively. (Sigma bonds are ordinary covalent bonds between atoms, and pi bonds are the special bonds that occur in unsaturated and aromatic systems.) The nucleophile may be either external or internal (that is, constituting a portion of the cation itself). In the latter case, rearrangement may occur. Examples of the various possible reaction types are shown below:

1. Reaction with external n-base: acid-catalyzed hydration (addition of water) of isobutylene. In this reaction, there is an unshared (nonbonded) electron pair on the oxygen atom of the water molecule:

2. Reaction with external base: alkylation of benzene using isopropyl chloride (Friedel–Crafts reaction). Benzene acts as the donor molecule, with the donated electrons coming from the pi-bonded system of the benzene ring:

In the above equation, the partial circle with the plus charge in the hexagon stands for those forms of the cation in which the positive charge is distributed around the ring (as in the benzyl cation, pictured above).

3. Reaction with external sigma base: hydride transfer reaction in which the donor electron pair comes from the carbon–hydrogen sigma bond in isobutane:

4. Reaction with internal n-base: cyclization reaction, with nonbonded electron pair on an oxygen atom serving as donor:

5. Reaction with internal pi base: acid-catalyzed cyclization to form β-ionone, with the donor electrons coming from the pi electrons of the unsaturated system:

6. Reaction with internal sigma base: acid-catalyzed rearrangement of neopentyl alcohol, the electron pair coming from an internal carbon–carbon sigma bond:

Each of these reaction types is widely employed in synthetic organic reactions, and the many acid-catalyzed hydrocarbon transformation reactions are fundamental in petroleum chemistry and in vital bio-organic processes. An important process in the manufacture of high-octane gasoline, for example, consists of the acid-catalyzed isomerization of straight-chain hydrocarbons to branched-chain hydrocarbons. One example of the significance of carbonium ions in bio-organic processes may be found in the biological synthesis of the important material cholesterol from a precursor, squalene, by way of another compound, lanosterol. In this transformation, acid-catalyzed rearrangements—reaction type 6, described earlier—occur repeatedly.