Chemical Reviews is peer-reviewed scientific journal published twice per month by the American Chemical Society. It publishes review articles on all aspects of chemistry, it was established in 1924 by William Albert Noyes. As of 1 January 2015 the editor-in-chief is Sharon Hammes-Schiffer; the journal is abstracted and indexed in Chemical Abstracts Service, CAB International, EBSCOhost, ProQuest, PubMed and the Science Citation Index. According to the Journal Citation Reports, the journal has a 2017 impact factor of 52.613. Official website
Hydrolysis is a term used for both an electro-chemical process and a biological one. The hydrolysis of water is the separation of water molecules into hydrogen and oxygen atoms using electricity. Biological hydrolysis is the cleavage of biomolecules where a water molecule is consumed to effect the separation of a larger molecule into component parts; when a carbohydrate is broken into its component sugar molecules by hydrolysis, this is termed saccharification. Hydrolysis or saccharification is a step in the degradation of a substance. Hydrolysis can be the reverse of a condensation reaction in which two molecules join together into a larger one and eject a water molecule, thus hydrolysis adds water to break down, whereas condensation builds up by removing water and any other solvents. Some hydration reactions are hydrolysis. Hydrolysis is a chemical process in which a molecule of water is added to a substance. Sometimes this addition causes both water molecule to split into two parts. In such reactions, one fragment of the target molecule gains a hydrogen ion.
It breaks a chemical bond in the compound. A common kind of hydrolysis occurs when a salt of weak base is dissolved in water. Water spontaneously ionizes into hydroxide anions and hydronium cations; the salt dissociates into its constituent anions and cations. For example, sodium acetate dissociates in water into acetate ions. Sodium ions react little with the hydroxide ions whereas the acetate ions combine with hydronium ions to produce acetic acid. In this case the net result is a relative excess of hydroxide ions. Strong acids undergo hydrolysis. For example, dissolving sulfuric acid in water is accompanied by hydrolysis to give hydronium and bisulfate, the sulfuric acid's conjugate base. For a more technical discussion of what occurs during such a hydrolysis, see Brønsted–Lowry acid–base theory. Acid–base-catalysed hydrolyses are common, their hydrolysis occurs when the nucleophile attacks the carbon of the carbonyl group of the ester or amide. In an aqueous base, hydroxyl ions are better nucleophiles than polar molecules such as water.
In acids, the carbonyl group becomes protonated, this leads to a much easier nucleophilic attack. The products for both hydrolyses are compounds with carboxylic acid groups; the oldest commercially practiced example of ester hydrolysis is saponification. It is the hydrolysis of a triglyceride with an aqueous base such as sodium hydroxide. During the process, glycerol is formed, the fatty acids react with the base, converting them to salts; these salts are called soaps used in households. In addition, in living systems, most biochemical reactions take place during the catalysis of enzymes; the catalytic action of enzymes allows the hydrolysis of proteins, fats and carbohydrates. As an example, one may consider proteases, they catalyse the hydrolysis of interior peptide bonds in peptide chains, as opposed to exopeptidases. However, proteases do not catalyse the hydrolysis of all kinds of proteins, their action is stereo-selective: Only proteins with a certain tertiary structure are targeted as some kind of orienting force is needed to place the amide group in the proper position for catalysis.
The necessary contacts between an enzyme and its substrates are created because the enzyme folds in such a way as to form a crevice into which the substrate fits. Therefore, proteins that do not fit into the crevice will not undergo hydrolysis; this specificity preserves the integrity of other proteins such as hormones, therefore the biological system continues to function normally. Upon hydrolysis, an amide converts into an amine or ammonia. One of the two oxygen groups on the carboxylic acid are derived from a water molecule and the amine gains the hydrogen ion; the hydrolysis of peptides gives amino acids. Many polyamide polymers such as nylon 6,6 hydrolyse in the presence of strong acids; the process leads to depolymerization. For this reason nylon products fail by fracturing. Polyesters are susceptible to similar polymer degradation reactions; the problem is known as environmental stress cracking. Hydrolysis is related to energy storage. All living cells require a continual supply of energy for two main purposes: the biosynthesis of micro and macromolecules, the active transport of ions and molecules across cell membranes.
The energy derived from the oxidation of nutrients is not used directly but, by means of a complex and long sequence of reactions, it is channelled into a special energy-storage molecule, adenosine triphosphate. The ATP molecule contains pyrophosphate linkages. ATP can undergo hydrolysis in two ways: the removal of terminal phosphate to form adenosine diphosphate and inorganic phosphate, or the removal of a terminal diphosphate to yield adenosine monophosphate and pyrophosphate; the latter undergoes further cleavage in
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one methine group replaced by a nitrogen atom, it is a flammable, weakly alkaline, water-soluble liquid with a distinctive, unpleasant fish-like smell. Pyridine is colorless; the pyridine ring occurs in many important compounds, including agrochemicals and vitamins. Pyridine was produced from coal tar. Today it is synthesized on the scale of about 20,000 tonnes per year worldwide; the molecular electric dipole moment is 2.2 debyes. Pyridine is diamagnetic and has a diamagnetic susceptibility of −48.7 × 10−6 cm3·mol−1. The standard enthalpy of formation is 100.2 kJ·mol−1 in the liquid phase and 140.4 kJ·mol−1 in the gas phase. At 25 °C pyridine has a viscosity of 0.88 mPa/s and thermal conductivity of 0.166 W·m−1·K−1. The enthalpy of vaporization is 35.09 kJ · mol − 1 at normal pressure. The enthalpy of fusion is 8.28 kJ·mol−1 at the melting point. The critical parameters of pyridine are pressure 6.70 MPa, temperature 620 K and volume 229 cm3·mol−1.
In the temperature range 340–426 °C its vapor pressure p can be described with the Antoine equation log 10 p = A − B C + T where T is temperature, A = 4.16272, B = 1371.358 K and C = −58.496 K. Akin to benzene, pyridine ring forms a C5N hexagon. Electron localization in pyridine is reflected in the shorter C–N ring bond, whereas the carbon–carbon bonds in the pyridine ring have the same 139 pm length as in benzene; these bond lengths lie between the values for the single and double bonds and are typical of aromatic compounds. Pyridine crystallizes in an orthorhombic crystal system with space group Pna21 and lattice parameters a = 1752 pm, b = 897 pm, c = 1135 pm, 16 formula units per unit cell. For comparison, crystalline benzene is orthorhombic, with space group Pbca, a = 729.2 pm, b = 947.1 pm, c = 674.2 pm, but the number of molecules per cell is only 4. This difference is related to the lower symmetry of the individual pyridine molecule. A trihydrate is known; the optical absorption spectrum of pyridine in hexane contains three bands at the wavelengths of 195 nm, 251 nm and 270 nm.
The 1H nuclear magnetic resonance spectrum of pyridine contains three signals with the integral intensity ratio of 2:1:2 that correspond to the three chemically different protons in the molecule. These signals originate from γ-proton and β-protons; the carbon analog of pyridine, has only one proton signal at 7.27 ppm. The larger chemical shifts of the α- and γ-protons in comparison to benzene result from the lower electron density in the α- and γ-positions, which can be derived from the resonance structures; the situation is rather similar for the 13C NMR spectra of pyridine and benzene: pyridine shows a triplet at δ = 150 ppm, δ = 124 ppm and δ = 136 ppm, whereas benzene has a single line at 129 ppm. All shifts are quoted for the solvent-free substances. Pyridine is conventionally detected by mass spectrometry methods; because of the electronegative nitrogen in the pyridine ring, the molecule is electron deficient. It, enters less into electrophilic aromatic substitution reactions than benzene derivatives.
Correspondingly pyridine is more prone to nucleophilic substitution, as evidenced by the ease of metalation by strong organometallic bases. The reactivity of pyridine can be distinguished for three chemical groups. With electrophiles, electrophilic substitution takes place where pyridine expresses aromatic properties. With nucleophiles, pyridine reacts at positions 2 and 4 and thus behaves similar to imines and carbonyls; the reaction with many Lewis acids results in the addition to the nitrogen atom of pyridine, similar to the reactivity of tertiary amines. The ability of pyridine and its derivatives to oxidize, forming amine oxides, is a feature of tertiary amines; the nitrogen center of pyridine features a basic lone pair of electrons. This lone pair does not overlap with the aromatic π-system ring pyridine is a basic, having chemical properties similar to those of tertiary amines. Protonation gives pyridinium, C5H5NH+; the pKa of the conjugate acid is 5.25. The structures of pyridine and pyridinium are identical.
The pyridinium cation is isoelectronic with benzene. Pyridinium p-toluenesulfonate is an illustrative pyridinium salt. In addition to protonation, pyridine undergoes N-centered alkylation, N-oxidation. Pyridine has a conjugated system of six π electrons; the molecule is planar and, follows the Hückel criteria for aromatic systems. In contrast to benzene, the electron density is not evenly distributed over the ring, reflecting the negative inductive effect of the nitrogen atom. For this reason, pyridine has a dipole moment and a weaker resonant stabilization than benzene (re
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei, can be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur; the substance involved in a chemical reaction are called reactants or reagents. Chemical reactions are characterized by a chemical change, they yield one or more products, which have properties different from the reactants. Reactions consist of a sequence of individual sub-steps, the so-called elementary reactions, the information on the precise course of action is part of the reaction mechanism. Chemical reactions are described with chemical equations, which symbolically present the starting materials, end products, sometimes intermediate products and reaction conditions.
Chemical reactions happen at a characteristic reaction rate at a given temperature and chemical concentration. Reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms. Reactions may proceed in the forward or reverse direction until they go to completion or reach equilibrium. Reactions that proceed in the forward direction to approach equilibrium are described as spontaneous, requiring no input of free energy to go forward. Non-spontaneous reactions require input of free energy to go forward. Different chemical reactions are used in combinations during chemical synthesis in order to obtain a desired product. In biochemistry, a consecutive series of chemical reactions form metabolic pathways; these reactions are catalyzed by protein enzymes. Enzymes increase the rates of biochemical reactions, so that metabolic syntheses and decompositions impossible under ordinary conditions can occur at the temperatures and concentrations present within a cell.
The general concept of a chemical reaction has been extended to reactions between entities smaller than atoms, including nuclear reactions, radioactive decays, reactions between elementary particles, as described by quantum field theory. Chemical reactions such as combustion in fire and the reduction of ores to metals were known since antiquity. Initial theories of transformation of materials were developed by Greek philosophers, such as the Four-Element Theory of Empedocles stating that any substance is composed of the four basic elements – fire, water and earth. In the Middle Ages, chemical transformations were studied by Alchemists, they attempted, in particular, to convert lead into gold, for which purpose they used reactions of lead and lead-copper alloys with sulfur. The production of chemical substances that do not occur in nature has long been tried, such as the synthesis of sulfuric and nitric acids attributed to the controversial alchemist Jābir ibn Hayyān; the process involved heating of sulfate and nitrate minerals such as copper sulfate and saltpeter.
In the 17th century, Johann Rudolph Glauber produced hydrochloric acid and sodium sulfate by reacting sulfuric acid and sodium chloride. With the development of the lead chamber process in 1746 and the Leblanc process, allowing large-scale production of sulfuric acid and sodium carbonate chemical reactions became implemented into the industry. Further optimization of sulfuric acid technology resulted in the contact process in the 1880s, the Haber process was developed in 1909–1910 for ammonia synthesis. From the 16th century, researchers including Jan Baptist van Helmont, Robert Boyle, Isaac Newton tried to establish theories of the experimentally observed chemical transformations; the phlogiston theory was proposed in 1667 by Johann Joachim Becher. It postulated the existence of a fire-like element called "phlogiston", contained within combustible bodies and released during combustion; this proved to be false in 1785 by Antoine Lavoisier who found the correct explanation of the combustion as reaction with oxygen from the air.
Joseph Louis Gay-Lussac recognized in 1808 that gases always react in a certain relationship with each other. Based on this idea and the atomic theory of John Dalton, Joseph Proust had developed the law of definite proportions, which resulted in the concepts of stoichiometry and chemical equations. Regarding the organic chemistry, it was long believed that compounds obtained from living organisms were too complex to be obtained synthetically. According to the concept of vitalism, organic matter was endowed with a "vital force" and distinguished from inorganic materials; this separation was ended however by the synthesis of urea from inorganic precursors by Friedrich Wöhler in 1828. Other chemists who brought major contributions to organic chemistry include Alexander William Williamson with his synthesis of ethers and Christopher Kelk Ingold, among many discoveries, established the mechanisms of substitution reactions. Chemical equations are used to graphically illustrate chemical reactions, they consist of chemical or structural formulas of the reactants on the left and those of the products on the right.
They are separated by an arrow which indicates the type of the reaction.