Hindered amine light stabilizers
Hindered amine light stabilizers are chemical compounds containing an amine functional group that are used as stabilizers in plastics and polymers. These compounds are derivatives of tetramethylpiperidine and are used to protect the polymers from the effects of photo-oxidation, they are increasingly being used as thermal stabilizers for low and moderate level of heat, however during the high temperature processing of polymers they remain less effective than traditional phenolic antioxidants. HALS do not absorb UV radiation, but act to inhibit degradation of the polymer by continuously and cyclically removing free radicals that are produced by photo-oxidation of the polymer; the overall process is sometimes referred to as the Denisov cycle, after Evguenii T. Denisov and is exceedingly complex. Broadly, HALS react with the initial polymer peroxy radical and alkyl polymer radicals formed by the reaction of the polymer and oxygen, preventing further radical oxidation. By these reactions HALS are oxidised to their corresponding aminoxyl radicals, however they are able to return to their initial amine form via a series of additional radical reactions.
HALS's high efficiency and longevity are due to this cyclic process wherein the HALS are regenerated rather than consumed during the stabilization process. The use of a hindered amine possessing no alpha-hydrogens is key, as this prevents the HALS being converted into a nitrone species; this could either react with any alkene groups in the polymer to give inactive species. Though HALS are effective in polyolefins and polyurethane, they are ineffective in polyvinyl chloride, it is thought that their ability to form nitroxyl radicals is disrupted due them being protonated by HCl released by dehydrohalogenation of PVC. Hindered amine light stabilizers Eversorb® Light Stabilizers Gas treating
Ammonium chloride is an inorganic compound with the formula NH4Cl and a white crystalline salt, soluble in water. Solutions of ammonium chloride are mildly acidic. Sal ammoniac is a name of the mineralogical form of ammonium chloride; the mineral is formed on burning coal dumps from condensation of coal-derived gases. It is found around some types of volcanic vents, it is used as fertilizer and a flavouring agent in some types of liquorice. It is the product from the reaction of hydrochloric ammonia, it is a product of the Solvay process used to produce sodium carbonate: CO2 + 2 NH3 + 2 NaCl + H2O → 2 NH4Cl + Na2CO3In addition to being the principal method for the manufacture of ammonium chloride, that method is used to minimize ammonia release in some industrial operations. Ammonium chloride is prepared commercially by combining ammonia with either hydrogen chloride or hydrochloric acid: NH3 + HCl → NH4ClAmmonium chloride occurs in volcanic regions, forming on volcanic rocks near fume-releasing vents.
The crystals deposit directly from the gaseous state and tend to be short-lived, as they dissolve in water. Ammonium chloride appears to sublime upon heating but decomposes into ammonia and hydrogen chloride gas. NH4Cl → NH3 + HClAmmonium chloride reacts with a strong base, like sodium hydroxide, to release ammonia gas: NH4Cl + NaOH → NH3 + NaCl + H2OSimilarly, ammonium chloride reacts with alkali metal carbonates at elevated temperatures, giving ammonia and alkali metal chloride: 2 NH4Cl + Na2CO3 → 2 NaCl + CO2 + H2O + 2 NH3A 5% by weight solution of ammonium chloride in water has a pH in the range 4.6 to 6.0. Some of ammonium chloride's reactions with other chemicals are endothermic like its reaction with barium hydroxide and its dissolving in water; the dominant application of ammonium chloride is as a nitrogen source in fertilizers such as chloroammonium phosphate. The main crops fertilized this way are wheat in Asia. Ammonium chloride was used in pyrotechnics in the 18th century but was superseded by safer and less hygroscopic chemicals.
Its purpose was to provide a chlorine donor to enhance the green and blue colours from copper ions in the flame. It had a secondary use to provide white smoke, but its ready double decomposition reaction with potassium chlorate producing the unstable ammonium chlorate made its use suspect. Ammonium chloride galvanized or soldered, it works as a flux by cleaning the surface of workpieces by reacting with the metal oxides at the surface to form a volatile metal chloride. For that purpose, it is sold in blocks at hardware stores for use in cleaning the tip of a soldering iron, it can be included in solder as flux. Ammonium chloride is used as an expectorant in cough medicine, its expectorant action is caused by irritative action on the bronchial mucosa, which causes the production of excess respiratory tract fluid, easier to cough up. Ammonium salts may induce nausea and vomiting. Ammonium chloride is used as a systemic acidifying agent in treatment of severe metabolic alkalosis, in oral acid loading test to diagnose distal renal tubular acidosis, to maintain the urine at an acid pH in the treatment of some urinary-tract disorders.
Ammonium chloride, under the name sal ammoniac or salmiak is used as food additive under the E number E510, working as a yeast nutrient in breadmaking and as an acidifier. It is a feed supplement for cattle and an ingredient in nutritive media for yeasts and many microorganisms. Ammonium chloride is used to spice up dark sweets called salmiak, in baking to give cookies a crisp texture, in the liquor Salmiakki Koskenkorva for flavouring. In Iran, India and Arab countries it is called "Noshader" and is used to improve the crispness of snacks such as samosas and jalebi. Ammonium chloride has been used to produce low temperatures in cooling baths. Ammonium chloride solutions with ammonia are used as buffer solutions including ACK lysis buffer. In paleontology, ammonium chloride vapor is deposited on fossils, where the substance forms a brilliant white removed and harmless and inert layer of tiny crystals; that covers up any coloration the fossil may have, if lighted at an angle enhances contrast in photographic documentation of three-dimensional specimens.
The same technique is applied in archaeology to eliminate reflection on glass and similar specimens for photography. In organic synthesis saturated NH4Cl solution is used to quench reaction mixtures. Giant squid and some other large squid species maintain neutral buoyancy in seawater through an ammonium chloride solution, found throughout their bodies and is less dense than seawater; this differs from the method of flotation used by most fish, which involves a gas-filled swim bladder. The solution tastes somewhat like salmiakki and makes giant squid unattractive for general human consumption. Ammonium chloride is used in a ~5% aqueous solution to work on oil wells with clay swelling problems, it is used as electrolyte in zinc–carbon batteries. Other uses include in hair shampoo, in the glue that bonds plywood, in cleaning products. In hair shampoo, it is used as a thickening agent in ammonium-based surfactant systems such as ammonium lauryl sulfate. Ammonium chloride is used in the textile and leather industry, in dyeing, textile printing and cotton clustering.
Around the turn of the 20th Century, Ammonium Chloride was used in aqueous solution as the electrolyte in
Column chromatography in chemistry is a chromatography method used to isolate a single chemical compound from a mixture. Chromatography is able to separate substances based on differential adsorption of compounds to the adsorbent; the technique is applicable, as many different adsorbents can be used with a wide range of solvents. The technique can be used on scales from micrograms up to kilograms; the main advantage of column chromatography is the low cost and disposability of the stationary phase used in the process. The latter prevents stationary phase degradation due to recycling. Column chromatography can be done using gravity to move the solvent, or using compressed gas to push the solvent through the column. A thin-layer chromatograph can show how a mixture of compounds will behave when purified by column chromatography; the separation is first optimised using thin-layer chromatography before performing column chromatography. A column is prepared by packing a solid absorbent into plastic tube.
The size will depend on the amount of compound being isolated. The base of the tube contains a filter, either a cotton or glass wool plug, or glass frit to hold the solid phase in place. A solvent reservoir may be attached at the top of the column. Two methods are used to prepare a column: the dry method and the wet method. For the dry method, the column is first filled with dry stationary phase powder, followed by the addition of mobile phase, flushed through the column until it is wet, from this point is never allowed to run dry. For the wet method, a slurry is prepared of the eluent with the stationary phase powder and carefully poured into the column; the top of the silica should be flat, the top of the silica can be protected by a layer of sand. Eluent is passed through the column to advance the organic material; the individual components are retained by the stationary phase differently and separate from each other while they are running at different speeds through the column with the eluent.
At the end of the column they elute one at a time. During the entire chromatography process the eluent is collected in a series of fractions. Fractions can be collected automatically by means of fraction collectors; the productivity of chromatography can be increased by running several columns at a time. In this case multi stream collectors are used; the composition of the eluent flow can be monitored and each fraction is analyzed for dissolved compounds, e.g. by analytical chromatography, UV absorption spectra, or fluorescence. Colored compounds can be seen through the glass wall as moving bands; the stationary phase or adsorbent in column chromatography is a solid. The most common stationary phase for column chromatography is silica gel, the next most common being alumina. Cellulose powder has been used in the past. A wide range of stationary phases are available in order to perform ion exchange chromatography, reversed-phase chromatography, affinity chromatography or expanded bed adsorption; the stationary phases are finely ground powders or gels and/or are microporous for an increased surface, though in EBA a fluidized bed is used.
There is an important ratio between the stationary phase weight and the dry weight of the analyte mixture that can be applied onto the column. For silica column chromatography, this ratio lies within 20:1 to 100:1, depending on how close to each other the analyte components are being eluted; the mobile phase or eluent is a solvent or a mixture of solvents used to move the compounds through the column. It is chosen so that the retention factor value of the compound of interest is around 0.2 - 0.3 in order to minimize the time and the amount of eluent to run the chromatography. The eluent has been chosen so that the different compounds can be separated effectively; the eluent is optimized in small scale pretests using thin layer chromatography with the same stationary phase. There is an optimum flow rate for each particular separation. A faster flow rate of the eluent minimizes the time required to run a column and thereby minimizes diffusion, resulting in a better separation. However, the maximum flow rate is limited because a finite time is required for the analyte to equilibrate between the stationary phase and mobile phase, see Van Deemter's equation.
A simple laboratory column runs by gravity flow. The flow rate of such a column can be increased by extending the fresh eluent filled column above the top of the stationary phase or decreased by the tap controls. Faster flow rates can be achieved by using a pump or by using compressed gas to push the solvent through the column; the particle size of the stationary phase is finer in flash column chromatography than in gravity column chromatography. For example, one of the most used silica gel grades in the former technique is mesh 230 – 400, while the latter technique requires mesh 70 – 230 silica gel. A spreadsheet that assists in the successful development of flash columns has been developed; the spreadsheet estimates the retention volume and band volume of analytes, the fraction numbers expected to contain each analyte, the resolution between adjacent peaks. This information allows users to select optimal parameters for preparative-scale separations before the flash column itself is attempted.
Column chromatography is an time consuming stage in any lab and can become the bottleneck for any process lab. Many manufacturers like
Silicon is a chemical element with symbol Si and atomic number 14. It is a brittle crystalline solid with a blue-grey metallic lustre, it is a member of group 14 in the periodic table: carbon is above it. It is unreactive; because of its high chemical affinity for oxygen, it was not until 1823 that Jöns Jakob Berzelius was first able to prepare it and characterize it in pure form. Its melting and boiling points of 1414 °C and 3265 °C are the second-highest among all the metalloids and nonmetals, being only surpassed by boron. Silicon is the eighth most common element in the universe by mass, but rarely occurs as the pure element in the Earth's crust, it is most distributed in dusts, sands and planets as various forms of silicon dioxide or silicates. More than 90% of the Earth's crust is composed of silicate minerals, making silicon the second most abundant element in the Earth's crust after oxygen. Most silicon is used commercially without being separated, with little processing of the natural minerals.
Such use includes industrial construction with clays, silica sand, stone. Silicates are used in Portland cement for mortar and stucco, mixed with silica sand and gravel to make concrete for walkways and roads, they are used in whiteware ceramics such as porcelain, in traditional quartz-based soda-lime glass and many other specialty glasses. Silicon compounds such as silicon carbide are used as abrasives and components of high-strength ceramics. Silicon is the basis of the used synthetic polymers called silicones. Elemental silicon has a large impact on the modern world economy. Most free silicon is used in the steel refining, aluminium-casting, fine chemical industries. More visibly, the small portion of highly purified elemental silicon used in semiconductor electronics is essential to integrated circuits – most computers, cell phones, modern technology depend on it. Silicon is an essential element in biology. However, various sea sponges and microorganisms, such as diatoms and radiolaria, secrete skeletal structures made of silica.
Silica is deposited in many plant tissues. In 1787 Antoine Lavoisier suspected that silica might be an oxide of a fundamental chemical element, but the chemical affinity of silicon for oxygen is high enough that he had no means to reduce the oxide and isolate the element. After an attempt to isolate silicon in 1808, Sir Humphry Davy proposed the name "silicium" for silicon, from the Latin silex, silicis for flint, adding the "-ium" ending because he believed it to be a metal. Most other languages use transliterated forms of Davy's name, sometimes adapted to local phonology. A few others use instead a calque of the Latin root. Gay-Lussac and Thénard are thought to have prepared impure amorphous silicon in 1811, through the heating of isolated potassium metal with silicon tetrafluoride, but they did not purify and characterize the product, nor identify it as a new element. Silicon was given its present name in 1817 by Scottish chemist Thomas Thomson, he retained part of Davy's name but added "-on" because he believed that silicon was a nonmetal similar to boron and carbon.
In 1823, Jöns Jacob Berzelius prepared amorphous silicon using the same method as Gay-Lussac, but purifying the product to a brown powder by washing it. As a result, he is given credit for the element's discovery; the same year, Berzelius became the first to prepare silicon tetrachloride. Silicon in its more common crystalline form was not prepared until 31 years by Deville. By electrolyzing a mixture of sodium chloride and aluminium chloride containing 10% silicon, he was able to obtain a impure allotrope of silicon in 1854. More cost-effective methods have been developed to isolate several allotrope forms, the most recent being silicene in 2010. Meanwhile, research on the chemistry of silicon continued; the first organosilicon compound, was synthesised by Charles Friedel and James Crafts in 1863, but detailed characterisation of organosilicon chemistry was only done in the early 20th century by Frederic Kipping. Starting in the 1920s, the work of William Lawrence Bragg on X-ray crystallography elucidated the compositions of the silicates, known from analytical chemistry but had not yet been understood, together with Linus Pauling's development of crystal chemistry and Victor Goldschmidt's development of geochemistry.
The middle of the 20th century saw the development of the chemistry and industrial use of siloxanes and the growing use of silicone polymers and resins. In the late 20th century, the complexity of the crystal chemistry of silicides was mapped, along with the solid-state chemistry of doped semiconductors; because silicon is an important element in high-technology semiconductor devi
Silylation is the introduction of a substituted silyl group to a molecule. The process is the basis of organosilicon chemistry. Alcohols, carboxylic acids, amines and phosphates can be silylated; the process involves the replacement of a proton with a trialkylsilyl group trimethylsilyl. The substrate is deprotonated with a suitable strong base followed by treatment with a silyl chloride. Strong bases such butyl lithium or a Grignard reagent are used, as illustrated by the synthesis of a trimethylsilyl ethers as protecting groups from an alcohol: ROH + BuLi → ROLi + BuH ROLi + Me3SiCl → ROSiMe3 + LiClBisacetamide ("BSA", Me3SiNCMe is an efficient silylation agent used for the derivatisation of compounds; the reaction of BSA with alcohols gives the corresponding trimethylsilyl ether, together with N-acetamide as a byproduct: ROH + Me3SiNCMe → Me3SiNCMe + ROSiMe3The introduction of a silyl group gives derivatives of enhanced volatility, making the derivatives suitable for analysis by gas chromatography and electron-impact mass spectrometry.
For EI-MS, the silyl derivatives give more favorable diagnostic fragmentation patterns of use in structure investigations, or characteristic ions of use in trace analyses employing selected ion monitoring and related techniques. Desilylation is the reverse of silylation: the silyl group is exchanged for a proton. Various fluoride salts are popular for this purpose. ROSiMe3 + F− + H2O → ROH + FSiMe3 + OH− Coordination complexes with silyl ligands are well known. An early example is CpFe2Si3, prepared by a salt metathesis reaction from trimethylsilyl chloride and CpFe2Na. Typical routes include oxidative addition of Si-H bonds to low-valent metals. Metal silyl complexes are intermediates in hydrosilation, a process used to make organosilicon compounds on both laboratory and commercial scales. Silyl ether Hydrosilylation Identification of Silylation Artifacts in Derivatization Reactions for Gas Chromatography Desilylation methods
Silyl ethers are a group of chemical compounds which contain a silicon atom covalently bonded to an alkoxy group. The general structure is R1R2R3Si − O − R4 where R4 is an aryl group. Silyl ethers are used as protecting groups for alcohols in organic synthesis. Since R1R2R3 can be combinations of differing groups which can be varied in order to provide a number of silyl ethers, this group of chemical compounds provides a wide spectrum of selectivity for protecting group chemistry. Common silyl ethers are: trimethylsilyl, tert-butyldiphenylsilyl, tert-butyldimethylsilyl and triisopropylsilyl, they are useful because they can be installed and removed selectively under mild conditions. Although many methods are available for forming silyl ethers, two common strategies for the silylation of alcohols are: reaction of the alcohol with a silyl chloride using an amine base at room temperature and reaction of the alcohol with a silyl triflate using a hindered amine base at low temperature. Silyl triflates are more reactive than their corresponding chlorides, so they can be used to install silyl groups onto hindered positions.
One reliable and rapid procedure is the Corey protocol in which the alcohol is reacted with a silyl chloride and imidazole at high concentration in DMF. If DMF is replaced by dichloromethane, the reaction is somewhat slower, but the purification of the compound is simplified. A common hindered base for use with silyl triflates is 2,6-lutidine. Primary alcohols can be protected in less than one hour while some hindered alcohols may require days of reaction time; when using a silyl chloride, no special precautions are required, except for the exclusion of large amounts of water. An excess of silyl chloride is not necessary. If excess reagent is used, the product will require flash chromatography to remove excess silanol and siloxane. Silyl triflates must be run under inert atmosphere conditions. Purification involves the addition of an aqueous acid such as saturated ammonium chloride solution; this quenches remaining silyl reagent and protonates amine bases, removing them from the reaction mixture.
Following extraction, the product can be purified by flash chromatography. Silyl triflate is more reactive and converts ketones to silyl enol ethers. Reaction with acids or fluorides such as tetra-n-butylammonium fluoride removes the silyl group when protection is no longer needed. Larger substituents increase resistance to hydrolysis, but make introduction of the silyl group more difficult. In acidic media, the relative resistance is: TMS < TES < TBS < TIPS < TBDPS In basic media, the relative resistance is: TMS < TES < TBS~TBDPS < TIPS It is possible to monosilylate a symmetrical diol, although this is known to be problematic occasionally. For example, the following monosilylation was reported: However, it turns out that this reaction is hard to repeat. If the reaction were controlled by thermodynamics statistically, if the dianion is of similar reactivity to the monoanion a corresponding statistical mixture of 1:2:1 disilylated:monosilylated:unsilylated diol will result. However, the reaction in THF is made selective by two factors, kinetic deprotonation of the first anion AND the insolubility of the monoanion.
At the initial addition of TBSCl, there is only a minor amount of monoanion in solution with the rest being in suspension. This small portion reacts and shifts the equilibrium of the monoanion to draw more into solution, thereby allowing for high yields of the mono-TBS compound to be obtained. Superior results in some cases may be obtained with butyllithium: A third method uses a mixture of DMF and DIPEA. Alternatively, an excess of the diol can be used. Selective deprotection of silyl groups is possible in many instances. For example, in the synthesis of taxol: Silyl ethers are differentiated on the basis of sterics or electronics. In general, acidic deprotections deprotect less hindered silyl groups faster, with the steric bulk on silicon being more significant than the steric bulk on oxygen. Fluoride-based deprotections deprotect electron-poor silyl groups faster than electron-rich silyl groups. There is some evidence; the selective deprotection of silyl ethers has been extensively reviewed.
Although selective deprotections have been achieved under many different conditions, some procedures, outlined below, are more reliable. A selective deprotection will be successful if there is a substantial difference in sterics or electronics; some optimization is required and it is necessary to run deprotections partway and recycle material. Some common acidic conditions 100 mol% 10-CSA in MeOH, room temperature. 10 mol% 10-CSA, 1:1 MeOH:DCM, −20 or 0 °C. 4:1:1 v/v/v AcOH:THF:water, room temp.. Some common basic conditions HF-pyridine, 10:1 THF:pyridine, 0 °C. TBAF, THF or 1:1 TBAF/AcOH, THF. Example deprotection TBS silyl ether E
Steric effects are nonbonding interactions that influence the shape and reactivity of ions and molecules. Steric effects complement electronic effects, which dictate shape and reactivity. Steric effects result from repulsive forces between overlapping electron clouds. Steric effects are exploited in applied and academic chemistry. Steric hindrance is a consequence of steric effects. Steric hindrance is the slowing of chemical reactions due to steric bulk, it is manifested in intermolecular reactions, whereas discussion of steric effects focus on intramolecular interactions. Steric hindrance is exploited to control selectivity, such as slowing unwanted side-reactions. Steric hindrance between adjacent groups can affect torsional bond angles. Steric hindrance is responsible for the observed shape of rotaxanes and the low rates of racemization of 2,2'-disubstituted biphenyl and binaphthyl derivatives; because steric effects have profound impact on properties, the steric properties of substituents have been assessed by numerous methods.
Relative rates of chemical reactions provide useful insights into the effects of the steric bulk of substituents. Under standard conditions methyl bromide solvolyzes 107 faster than does neopentyl bromide; the difference reflects the inhibition of attack on the compound with the sterically bulky 3C group. A values provide another measure of the bulk of substituents. A values are derived from equilibrium measurements of monosubstituted cyclohexanes; the extent that a substituent favors the equatorial position gives a measure of its bulk. Ceiling temperature is a measure of the steric properties of the monomers. T c is the temperature where the rate of depolymerization are equal. Sterically hindered monomers give polymers with low T c's, which are not useful. Ligand cone angles are measures of the size of ligands in coordination chemistry, it is defined as the solid angle formed with the metal at the vertex and the hydrogen atoms at the perimeter of the cone. Steric effects is critical to chemistry and pharmacology.
In organic chemistry, steric effects are nearly universal and affect the rates and activation energies of most chemical reactions to varying degrees. In biochemistry, steric effects are exploited in occurring molecules such as enzymes, where the catalytic site may be buried within a large protein structure. In pharmacology, steric effects determine how and at what rate a drug will interact with its target bio-molecules. Prominent Sterically Hindered Compounds Collision theory Reaction rate accelerate as result of steric hindrance in the Thorpe–Ingold effect Sterically induced reduction Intramolecular force Van der Waals strain known as steric strain Steric Effects at the Wayback Machine Steric: A Program to Calculate the Steric Size of Molecules at the Wayback Machine