Acyl Cation
An acyl cation is a positively charged species derived from carboxylic acids, formed when the hydroxyl group (−OH) is lost as a hydroxide ion (OH−). This results in a carbonyl functional group with an unstable positive charge on the carbon atom. Acyl cations play a significant role in organic synthesis, particularly in Friedel-Crafts acylation reactions, where they serve as key reagents that facilitate the addition of acyl groups to aromatic compounds. The formation of an acyl cation typically involves an acid chloride reacting with a Lewis acid, which stabilizes the cation and allows it to bond with a substrate.
Acyl cations can also be referred to as acylium ions, and their nomenclature is based on the parent carboxylic acid they derive from. The positive charge on the carbon atom is stabilized by adjacent electronegative oxygen atoms, exemplifying the principles of charge separation in carbonyl compounds. In biochemical contexts, acyl cations are involved in processes such as the formation of plant metabolites and play an essential role in cellular respiration through compounds like acetyl coenzyme A. Overall, understanding acyl cations is crucial for grasping their applications in both synthetic and metabolic reactions.
Acyl Cation
FIELDS OF STUDY: Organic Chemistry; Biochemistry
ABSTRACT
The basic structure of acyl cations is defined, and their importance in organic chemical synthesis is described. An acyl cation is formed when the hydroxyl (−OH) group of the carboxylic acid functional group is lost as the hydroxide ion. They are important reagents in Friedel-Crafts acylation reactions.
The Nature of the Acyl Cations
An acyl cation (CO+) is a carbonyl functional group that has a positive charge due to separation from a hydroxide ion (OH−), usually in a carboxylic acid (a hydrocarbon containing the −RCOOH group). The loss of the hydroxide portion leaves the acyl cation. The term "acyl" originates from acetic acid, a two-carbon carboxylic acid, with which different naturally occurring compounds share a common structural feature. Specifically, the hydroxyl portion (−OH) of the carboxylic acid function is replaced by a bond to another carbon atom. The corresponding structure can be identified for most other carboxylic acids, and the generic term "acyl group" was coined to indicate this structure in a molecule. In acylation, an acyl group is bonded to a substrate molecule in a biochemical system. Studies of its reaction mechanism demonstrate that the process is enzyme-mediated in metabolic processes. A useful laboratory method of modifying a molecular structure is the electrophilic addition reaction known as "Friedel-Crafts acylation," after Charles Friedel (1832–99) and James Mason Crafts (1839–1917).
Electronic Structure of Acyl Cations
An acyl cation is an electron-poor species due to the positive charge on the carbon atom of the carbonyl group. The positive charge is stabilized by the adjacent oxygen atom through overlap of the p orbitals of the oxygen and carbon atoms, and equivalent molecular structures can be drawn for an acyl cation in which one structure effectively has a triple bond to the carbonyl oxygen atom, which also bears the positive charge, as
The reactive center of an acyl cation is accordingly the carbon atom of the carbonyl group. The charge on the carbonyl carbon in an acyl cation can be described as an extreme case of the charge separation that normally exists in a carbonyl group as a result of the oxygen atom being more electronegative than the carbon. In an acyl cation, the carbon atom bears a full positive charge rather than just the enhanced positive charge character.
Electrophilic Addition and Friedel-Crafts Acylation
As the name suggests, electrophilic addition is the addition of an electrophile (a chemical species attracted to electrons) to an electron-rich target molecule, or "substrate." The substrate, which is by definition a nucleophile (an electron donor), typically has a chemical bond system that is able to reorganize electrons to form new bonds. In some cases, additional bonds are formed as the electrons in the molecular orbitals are reorganized. In other systems, the overall effect is a substitution reaction, rather than new bond formation. In the Friedel-Crafts acylation reaction, an acid chloride provides the corresponding acyl cation to replace one of the −H substituents on an aryl system (derived from an aromatic ring). A good Lewis acid, such as aluminum trichloride (AlCl3) or ferric chloride (FeCl3), catalyzes the reaction. In the reaction mechanism, the acyl group and the substrate molecule are coordinated to the metal atom of the Lewis acid, forming an intermediate. This allows the acyl cation to bond to one of the carbon atoms of the aryl group, and recovery of the electrons from the C−H bond reforms the pi-bond system, leaving H+. The reaction is charge neutral, since the positive charge that was on the acyl cation becomes the positive charge on the H+ ion.
Acylation is an important feature of biochemical systems. Plant metabolites called "acetogenins" are formed by repeated acylation reactions, and the presence of acetyl coenzyme A (acetyl CoA) is vital to cellular respiration.
Nomenclature
Acyl cations are also called "acylium ions," indicating their positive charge. This is the preferred method of naming acyl cations, and the name is generated by adding -ium to the name of the basic structure of the corresponding carboxylic acid. The acyl cation of propanoic acid and benzoic acid, for example, would be known as the propanoylium and benzoylium ions. The names can become unwieldy, however, and it is much more common to refer to the acyl group as the substituent group, as in propanoyl and benzoyl.
PRINCIPAL TERMS
- acylation: a reaction process by which an acyl group is added to a compound.
- cation: any chemical species bearing a net positive electrical charge, which causes it to be drawn toward the negative pole, or cathode, of an electrochemical cell.
- electrophilic addition: an addition reaction in which an electrophile, typically a positively charged chemical species, is bonded to a nucleophile, a molecule with a free pair of electrons that can be easily donated; results in the breaking of a multiple bond to form two single bonds.
- octet rule: the tendency of atoms when bonding to either accept or donate electrons so as to arrive at eight electrons in the outermost electron shell.
- reaction mechanism: the sequence of electron and orbital interactions that occurs during a chemical reaction as chemical bonds are broken, made, and rearranged.
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