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Calixarene
calixarenes: Originally macrocyclic compounds capable of assuming a basket (or 'calix') shaped conformation. They are formed from p-hydrocarbyl phenols and formaldehyde. The term now applies to a variety of derivatives by substitution of the hydrocarbon cyclo{oligo(1,3-phenylene)methylene}.
A calixarene is a macrocycle or cyclic oligomer based on a methylene-linked phenols. With hydrophobic cavities that can hold smaller molecules or ions, calixarenes belong to the class of cavitands known in host–guest chemistry.
Calixarene nomenclature is straightforward and involves counting the number of repeating units in the ring and including it in the name. A calix[4]arene has 4 units in the ring and a calix[6]arene has 6. A substituent in the meso position Rb is added to the name with a prefix C- as in C-methylcalix[6]arene The word calixarene is derived from the Greek calix or chalice because this type of molecule resembles a vase (or cup) and from the word arene that refers to the aromatic building block.
Calixarenes are generally produced by condensation of two components: an electron-rich aromatic compound, classically a 4-substituted phenol, and an aldehyde, classically formaldehyde.
Calixarenes can be challenging to synthesize, producing instead complex mixtures of linear and cyclic oligomers. With finely tuned starting materials and reaction conditions, synthesis can also be surprisingly efficient. Calixarenes are sparingly soluble as parent compounds and have high melting points.
Calixarenes are characterised by a three-dimensional basket, cup or bucket shape. In calix[4]arenes the internal volume is around 10 cubic angstroms. Calixarenes are characterised by a wide upper rim and a narrow lower rim and a central annulus. With phenol as a starting material the 4 hydroxyl groups are intrannular on the lower rim. In a resorcin[4]arene 8 hydroxyl groups are placed extraannular on the upper ring. Calixarenes exist in different chemical conformations because rotation around the methylene bridge is not difficult. In calix[4]arene 4 up–down conformations exist: cone (point group C2v,C4v), partial cone Cs, 1,2 alternate C2h and 1,3 alternate D2d. The 4 hydroxyl groups interact by hydrogen bonding and stabilize the cone conformation. This conformation is in dynamic equilibrium with the other conformations. Conformations can be locked in place with proper substituents replacing the hydroxyl groups which increase the rotational barrier. Alternatively placing a bulky substituent on the upper rim also locks a conformation. The calixarene based on p-tert-butyl phenol is also a cone. Calixarenes are structurally related to the pillararenes.
In 1872 Adolf von Baeyer mixed various aldehydes, including formaldehyde, with phenols in a strongly acidic solution. The resultant tars defied characterization; but represented the typical products of a phenol/formaldehyde polymerization. Leo Baekeland discovered that these tars could be cured into a brittle substance which he marketed as "Bakelite". This polymer was the first commercial synthetic plastic.
The success of Bakelite spurred scientific investigations into the chemistry of the phenol/formaldehyde reaction. One result was the discovery made in 1942 by Alois Zinke, that p-alkyl phenols and formaldehyde in a strongly basic solution yield mixtures containing cyclic tetramers. Concomitantly, Joseph Niederl and H. J. Vogel obtained similar cyclic tetramers from the acid-catalyzed reaction of resorcinol and aldehydes such as benzaldehyde. A number of years later, John Cornforth showed that the product from p-tert-butylphenol and formaldehyde is a mixture of the cyclic tetramer and another ambiguous cyclomer. His interest in these compounds was in the tuberculostatic properties of their oxyethylated derivatives.
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Calixarene
calixarenes: Originally macrocyclic compounds capable of assuming a basket (or 'calix') shaped conformation. They are formed from p-hydrocarbyl phenols and formaldehyde. The term now applies to a variety of derivatives by substitution of the hydrocarbon cyclo{oligo(1,3-phenylene)methylene}.
A calixarene is a macrocycle or cyclic oligomer based on a methylene-linked phenols. With hydrophobic cavities that can hold smaller molecules or ions, calixarenes belong to the class of cavitands known in host–guest chemistry.
Calixarene nomenclature is straightforward and involves counting the number of repeating units in the ring and including it in the name. A calix[4]arene has 4 units in the ring and a calix[6]arene has 6. A substituent in the meso position Rb is added to the name with a prefix C- as in C-methylcalix[6]arene The word calixarene is derived from the Greek calix or chalice because this type of molecule resembles a vase (or cup) and from the word arene that refers to the aromatic building block.
Calixarenes are generally produced by condensation of two components: an electron-rich aromatic compound, classically a 4-substituted phenol, and an aldehyde, classically formaldehyde.
Calixarenes can be challenging to synthesize, producing instead complex mixtures of linear and cyclic oligomers. With finely tuned starting materials and reaction conditions, synthesis can also be surprisingly efficient. Calixarenes are sparingly soluble as parent compounds and have high melting points.
Calixarenes are characterised by a three-dimensional basket, cup or bucket shape. In calix[4]arenes the internal volume is around 10 cubic angstroms. Calixarenes are characterised by a wide upper rim and a narrow lower rim and a central annulus. With phenol as a starting material the 4 hydroxyl groups are intrannular on the lower rim. In a resorcin[4]arene 8 hydroxyl groups are placed extraannular on the upper ring. Calixarenes exist in different chemical conformations because rotation around the methylene bridge is not difficult. In calix[4]arene 4 up–down conformations exist: cone (point group C2v,C4v), partial cone Cs, 1,2 alternate C2h and 1,3 alternate D2d. The 4 hydroxyl groups interact by hydrogen bonding and stabilize the cone conformation. This conformation is in dynamic equilibrium with the other conformations. Conformations can be locked in place with proper substituents replacing the hydroxyl groups which increase the rotational barrier. Alternatively placing a bulky substituent on the upper rim also locks a conformation. The calixarene based on p-tert-butyl phenol is also a cone. Calixarenes are structurally related to the pillararenes.
In 1872 Adolf von Baeyer mixed various aldehydes, including formaldehyde, with phenols in a strongly acidic solution. The resultant tars defied characterization; but represented the typical products of a phenol/formaldehyde polymerization. Leo Baekeland discovered that these tars could be cured into a brittle substance which he marketed as "Bakelite". This polymer was the first commercial synthetic plastic.
The success of Bakelite spurred scientific investigations into the chemistry of the phenol/formaldehyde reaction. One result was the discovery made in 1942 by Alois Zinke, that p-alkyl phenols and formaldehyde in a strongly basic solution yield mixtures containing cyclic tetramers. Concomitantly, Joseph Niederl and H. J. Vogel obtained similar cyclic tetramers from the acid-catalyzed reaction of resorcinol and aldehydes such as benzaldehyde. A number of years later, John Cornforth showed that the product from p-tert-butylphenol and formaldehyde is a mixture of the cyclic tetramer and another ambiguous cyclomer. His interest in these compounds was in the tuberculostatic properties of their oxyethylated derivatives.