Карбониев химичен йон

Карбониев йон, всеки член на клас органични молекули с положителни заряди, локализирани на въглероден атом. Някои карбониеви йони могат да се приготвят по такъв начин, че да са достатъчно стабилни за изследване; по-често те са само краткотрайни форми (междинни продукти), възникващи по време на химични реакции.

Въглеродните йони всъщност са един от най-често срещаните класове междинни продукти в органичните реакции и познаването на структурите и свойствата на тези вещества е от основно значение за разбирането на реакциите, в които те се появяват. Много от тези реакции са от синтетично, биохимично или промишлено значение.

Първите карбонови йони са наблюдавани през 1901 г.; Едва 21 години по-късно германският химик Ханс Меерваййн заключи, че от неутрален реагент (камфен хидрохлорид) се образува неутрален продукт (изоборнилов хлорид) чрез пренареждане, включващо междинно съединение на карбониев йон. Това беше първата концептуализация на карбониевия йон като междинен продукт в органична реакция на пренареждане. Идеята е обобщена от американския химик Франк Клифърд Уитмор от 1932 г. нататък и поставена на твърда експериментална основа от английските химици сър Кристофър Индолд и Е. Д. Хюз в началото на края на 20-те години. Въпреки че по косвени методи се предполагаше много за карбониевите йони, едва след 1960 г. се появиха общи методи за формиране на стабилни, дълговечни карбониеви йони.

Класификация.

Познати са два различни класа карбониеви йони. Първите са „класическите“ карбонови йони, които съдържат тривалентен въглероден атомен център. Въглеродният атом е в състояние на хибридизация sp ² - тоест три електрона на въглеродния атом заемат орбитали, образувани от комбинацията (хибридизация) на три обикновени орбитали, една обозначена s и две, p. И трите орбитали лежат в една равнина; по този начин, катионният център на молекулата, образуван чрез свързване на въглеродния атом с други три атома или групи, има тенденция да бъде равнинен. Родител за тези йони е метил катионът с формула СН ⁺ ₃. Схематично структурата е както е показано по-долу (плътните линии, представляващи връзки между атомите):

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.