|
Calixarene
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. Nomenclature Calixarene nomenclature is straightforward and involves counting the number of repeating units in the ring and including it in the name. A calix rene has 4 units in the ring and a calix rene has 6. A substituent in the meso position Rb is added to the name with a prefix C- as in C-methylcalix rene 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. Synthesis Calixarenes are generally produced by condensation of two components: an electron-rich aromatic compound, classically a 4-substituted phenol, and an aldehyde, classically formaldehyde. *The scope for the aromatic component is broad diverse. The key at ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Macrocycle
Macrocycles are often described as molecules and ions containing a ring of twelve or more atoms. Classical examples include the crown ethers, calixarenes, porphyrins, and cyclodextrins. Macrocycles describe a large, mature area of chemistry. Synthesis The formation of macrocycles by ring-closure is called macrocylization. Pioneering work was reported for studies on terpenoid macrocycles. The central challenge to macrocyclization is that ring-closing reactions do not favor the formation of large rings. Instead, small rings or polymers tend to form. This kinetic problem can be addressed by using high-dilution reactions, whereby intramolecular processes are favored relative to polymerizations. Some macrocyclizations are favored using template reactions. Templates are ions, molecules, surfaces etc. that bind and pre-organize compounds, guiding them toward formation of a particular ring size. The crown ethers are often generated in the presence of an alkali metal cation, whic ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Meso Position
Arene substitution patterns are part of organic chemistry IUPAC nomenclature and pinpoint the position of substituents other than hydrogen in relation to each other on an aromatic hydrocarbon. ''Ortho'', ''meta'', and ''para'' substitution * In ''ortho''-substitution, two substituents occupy positions next to each other, which may be numbered 1 and 2. In the diagram, these positions are marked R and ''ortho''. * In ''meta''-substitution the substituents occupy positions 1 and 3 (corresponding to R and ''meta'' in the diagram). * In ''para''-substitution, the substituents occupy the opposite ends (positions 1 and 4, corresponding to R and ''para'' in the diagram). The toluidines serve as an example for these three types of substitution. Synthesis Electron donating groups, for example amino, hydroxyl, alkyl, and phenyl groups tend to be ''ortho''/''para''-directors, and electron withdrawing groups such as nitro, nitrile, and ketone groups, tend to be ''meta''-directors. Propert ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Formaldehyde
Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section Forms below), hence it is stored as an aqueous solution (formalin), which is also used to store animal specimens. It is the simplest of the aldehydes (). The common name of this substance comes from its similarity and relation to formic acid. Formaldehyde is an important precursor to many other materials and chemical compounds. In 1996, the installed capacity for the production of formaldehyde was estimated at 8.7 million tons per year. It is mainly used in the production of industrial resins, e.g., for particle board and coatings. Forms Formaldehyde is more complicated than many simple carbon compounds in that it adopts several diverse forms. These compounds can often be used interchangeably and can be interconverted. *Molecular formald ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Pyrogallolarenes
A pyrogallolarene (also calix yrogallolarene) is a macrocycle, or a cyclic oligomer, based on the condensation of pyrogallol (1,2,3-trihydroxybenzene) and an aldehyde. Pyrogallolarenes are a type of calixarene, and a subset of resorcinarenes that are substituted with a hydroxyl at the 2-position. Pyrogallolarenes, like all resorcinarenes, form inclusion complexes with other molecules forming a host–guest complex. Pyrogallolarenes (like resorcinarenes) self-assemble into larger supramolecular structures forming a hydrogen-bonded hexamer. The pyrogallolarene hexamer is unique from those formed from resorcinarene, in that it does not incorporate solvent molecules into the structure. Both in the crystalline state and in organic solvents, six molecules will form an assembly with an internal volume of around one cubic nanometer (nanocapsules) and shapes similar to the Archimedean solids. A number of solvent or other molecules may reside in the capsule interior. The pyrogallolare ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hydroxymethylation
Hydroxymethylation is a chemical reaction that installs the CH2OH group. The transformation can be implemented in many ways and applies to both industrial and biochemical processes. Hydroxymethylation with formaldehyde A common method for hydroxymethylation involves the reaction of formaldehyde with active C-H and N-H bonds: :R3C-H + CH2O → R3C-CH2OH :R2N-H + CH2O → R2N-CH2OH A typical active C-H bond is provided by a terminal acetylene or the alpha protons of an aldehyde. In industry, hydroxymethylation of acetaldehyde with formaldehyde is used in the production of pentaerythritol: P-H bonds are also prone to reaction with formaldehyde. Tetrakis(hydroxymethyl)phosphonium chloride ( (CH2OH)4l) is produced in this way from phosphine (PH3). Hydroxymethylation in demethylation 5-Methylcytosine is a common epigenetic marker. The methyl group is modified by oxidation of the methyl group in a process called hydroxymethylation: :RCH3 + O → RCH2OH This oxidation is t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Resorcinarene
In chemistry, a resorcinarene (also resorcarene or calix esorcinarene) is a macrocycle, or a cyclic oligomer, based on the condensation of resorcinol (1,3-dihydroxybenzene) and an aldehyde. Resorcinarenes are a type of calixarene. Other types of resorcinarenes include the related pyrogallolarenes and octahydroxypyridines, derived from pyrogallol and 2,6-dihydroxypyridine, respectively. Resorcinarenes interact with other molecules forming a host–guest complex. Resorcinarenes and pyrogallolarenes self-assemble into larger supramolecular structures. Both in the crystalline state and in organic solvents, six resorcinarene molecules are known to form hexamers with an internal volume of around one cubic nanometer (nanocapsules) and shapes similar to the Archimedean solids. Hydrogen bonds appear to hold the assembly together. A number of solvent or other molecules reside inside. The resorcinarene is also the basic structural unit for other molecular recognition scaffolds, typ ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cavitand
In chemistry, a cavitand is a container-shaped molecule. The cavity of the cavitand allows it to engage in host–guest chemistry with guest molecules of a complementary shape and size. The original definition proposed by Cram includes many classes of molecules: cyclodextrins, calixarenes, pillararenes and cucurbiturils. However, modern usage in the field of supramolecular chemistry specifically refers to cavitands formed on a resorcinarene scaffold by bridging adjacent phenolic units. The simplest bridging unit is methylene (), although dimethylene (), trimethylene (), benzal, xylyl, pyridal, 2,3-disubstituted-quinoxaline, ''o''-dinitrobenzyl, dialkylsilydine, and phosphonates are known. Cavitands that have an extended aromatic bridging unit, or an extended cavity containing 3 rows of aromatic rings are referred to as deep-cavity cavitands and have broad applications in host-guest chemistry. These types of cavitands were extensively investigated by Rebek, and Gibb, am ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Oligomer
In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relative molecular mass, the structure of which essentially comprises a small plurality of units derived, actually or conceptually, from molecules of lower relative molecular mass.'' The name is composed of Greek elements '' oligo-'', "a few" and '' -mer'', "parts". An adjective form is ''oligomeric''. The oligomer concept is contrasted to that of a polymer, which is usually understood to have a large number of units, possibly thousands or millions. However, there is no sharp distinction between these two concepts. One proposed criterion is whether the molecule's properties vary significantly with the removal of one or a few of the units. An oligomer with a specific number of units is referred to by the Greek prefix denoting that number, wi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Rotational Barrier
In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted just by rotations about formally single bonds (refer to figure on single bond rotation). While any two arrangements of atoms in a molecule that differ by rotation about single bonds can be referred to as different conformations, conformations that correspond to local minima on the potential energy surface are specifically called conformational isomers or conformers. Conformations that correspond to local maxima on the energy surface are the transition states between the local-minimum conformational isomers. Rotations about single bonds involve overcoming a rotational energy barrier to interconvert one conformer to another. If the energy barrier is low, there is free rotation and a sample of the compound exists as a rapidly equilibrating mixture of multiple conformers; if the energy barrier is high enough then there is restricted rotation, a molecule may exist for a rel ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hydrogen Bond
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a lone pair of electrons—the hydrogen bond acceptor (Ac). Such an interacting system is generally denoted , where the solid line denotes a polar covalent bond, and the dotted or dashed line indicates the hydrogen bond. The most frequent donor and acceptor atoms are the second-row elements nitrogen (N), oxygen (O), and fluorine (F). Hydrogen bonds can be intermolecular (occurring between separate molecules) or intramolecular (occurring among parts of the same molecule). The energy of a hydrogen bond depends on the geometry, the environment, and the nature of the specific donor and acceptor atoms and can vary between 1 and 40 kcal/mol. This makes them somewhat stronger than a van der Waals interaction, and weaker than fully covalent ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Point Group
In geometry, a point group is a mathematical group of symmetry operations (isometries in a Euclidean space) that have a fixed point in common. The coordinate origin of the Euclidean space is conventionally taken to be a fixed point, and every point group in dimension ''d'' is then a subgroup of the orthogonal group O(''d''). Point groups are used to describe the symmetries of geometric figures and physical objects such as molecules. Each point group can be represented as sets of orthogonal matrices ''M'' that transform point ''x'' into point ''y'' according to Each element of a point group is either a rotation (determinant of ''M'' = 1), or it is a reflection or improper rotation (determinant of ''M'' = −1). The geometric symmetries of crystals are described by space groups, which allow translations and contain point groups as subgroups. Discrete point groups in more than one dimension come in infinite families, but from the crystallographic restriction theorem and ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chemical Conformation
In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted just by rotations about formally single bonds (refer to figure on single bond rotation). While any two arrangements of atoms in a molecule that differ by rotation about single bonds can be referred to as different conformations, conformations that correspond to local minima on the potential energy surface are specifically called conformational isomers or conformers. Conformations that correspond to local maxima on the energy surface are the transition states between the local-minimum conformational isomers. Rotations about single bonds involve overcoming a rotational energy barrier to interconvert one conformer to another. If the energy barrier is low, there is free rotation and a sample of the compound exists as a rapidly equilibrating mixture of multiple conformers; if the energy barrier is high enough then there is restricted rotation, a molecule may exist for a rel ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
