Hydrogen sulfide is the chemical compound with the formula H2S. It is a colorless chalcogen hydride gas with the characteristic foul odor of rotten eggs, it is poisonous and flammable. Hydrogen sulfide is produced from the microbial breakdown of organic matter in the absence of oxygen gas, such as in swamps and sewers. H2S occurs in volcanic gases, natural gas, in some sources of well water; the human body uses it as a signaling molecule. Swedish chemist Carl Wilhelm Scheele is credited with having discovered hydrogen sulfide in 1777; the British English spelling of this compound is hydrogen sulphide, but this spelling is not recommended by the International Union of Pure and Applied Chemistry or the Royal Society of Chemistry. Hydrogen sulfide is denser than air. Hydrogen sulfide burns in oxygen with a blue flame to form sulfur water. In general, hydrogen sulfide acts as a reducing agent in the presence of base, which forms SH−. At high temperatures or in the presence of catalysts, sulfur dioxide reacts with hydrogen sulfide to form elemental sulfur and water.
This reaction is exploited in the Claus process, an important industrial method to dispose of hydrogen sulfide. Hydrogen sulfide is soluble in water and acts as a weak acid, giving the hydrosulfide ion HS−. Hydrogen sulfide and its solutions are colorless; when exposed to air, it oxidizes to form elemental sulfur, not soluble in water. The sulfide anion S2− is not formed in aqueous solution. Hydrogen sulfide reacts with metal ions to form metal sulfides, which are insoluble dark colored solids. Lead acetate paper is used to detect hydrogen sulfide because it converts to lead sulfide, black. Treating metal sulfides with strong acid liberates hydrogen sulfide. At pressures above 90 GPa, hydrogen sulfide becomes a metallic conductor of electricity; when cooled below a critical temperature this high-pressure phase exhibits superconductivity. The critical temperature increases with pressure. If hydrogen sulfide is pressurized at higher temperatures cooled, the critical temperature reaches 203 K, the highest accepted superconducting critical temperature as of 2015.
By substituting a small part of sulfur with phosphorus and using higher pressures, it has been predicted that it may be possible to raise the critical temperature to above 0 °C and achieve room-temperature superconductivity. Hydrogen sulfide is most obtained by its separation from sour gas, natural gas with high content of H2S, it can be produced by treating hydrogen with molten elemental sulfur at about 450 °C. Hydrocarbons can serve as a source of hydrogen in this process. Sulfate-reducing bacteria generate usable energy under low-oxygen conditions by using sulfates to oxidize organic compounds or hydrogen. A standard lab preparation is to treat ferrous sulfide with a strong acid in a Kipp generator: FeS + 2 HCl → FeCl2 + H2SFor use in qualitative inorganic analysis, thioacetamide is used to generate H2S: CH3CNH2 + H2O → CH3CNH2 + H2SMany metal and nonmetal sulfides, e.g. aluminium sulfide, phosphorus pentasulfide, silicon disulfide liberate hydrogen sulfide upon exposure to water: 6 H2O + Al2S3 → 3 H2S + 2 Al3This gas is produced by heating sulfur with solid organic compounds and by reducing sulfurated organic compounds with hydrogen.
Water heaters can aid the conversion of sulfate in water to hydrogen sulfide gas. This is due to providing a warm environment sustainable for sulfur bacteria and maintaining the reaction which interacts between sulfate in the water and the water heater anode, made from magnesium metal. Hydrogen sulfide can be generated in cells via non enzymatic pathway. H2S in the body acts as a gaseous signaling molecule, known to inhibit Complex IV of the mitochondrial electron transport chain which reduces ATP generation and biochemical activity within cells. Three enzymes are known to synthesize H2S: cystathionine γ-lyase, cystathionine β-synthetase and 3-mercaptopyruvate sulfurtransferase; these enzymes have been identified in a breadth of biological cells and tissues, their activity has been observed to be induced by a number of disease states. It is becoming clear that H2S is an important mediator of a wide range of cell functions in health and in disease. CBS and CSE are the main proponents of H2S biogenesis.
These enzymes are characterized by the transfer of a sulfur atom from methionine to serine to form a cysteine molecule. 3-MST contributes to hydrogen sulfide production by way of the cysteine catabolic pathway. Dietary amino acids, such as methionine and cysteine serve as the primary substrates for the transulfuration pathways and in the production of hydrogen sulfide. Hydrogen sulfide can be synthesized by non-enzymatic pathway, derived from proteins such as ferredoxins and Rieske proteins. H2S has been shown to be involved in physiological processes like vasoconstriction in animals, increasing seed germination and stress responses in plants. Hydrogen sulfide signaling is innately intertwined with physiological processes that are known to be moderated by reactive oxygen species and reactive nitrogen species. H2S has been shown to interact with NO resulting in severa
Polymer degradation is a change in the properties—tensile strength, shape, etc.—of a polymer or polymer-based product under the influence of one or more environmental factors such as heat, light or chemicals such as acids and some salts. These changes are undesirable, such as cracking and chemical disintegration of products or, more desirable, as in biodegradation, or deliberately lowering the molecular weight of a polymer for recycling; the changes in properties are termed "aging". In a finished product such a change is to be delayed. Degradation can be useful for recycling/reusing the polymer waste to prevent or reduce environmental pollution. Degradation can be induced deliberately to assist structure determination. Polymeric molecules are large, their unique and useful properties are a result of their size. Any loss in chain length is a primary cause of premature cracking. Today there are seven commodity polymers in use: polyethylene, polyvinyl chloride, polyethylene terephthalate, polystyrene and poly.
These make up nearly 98 % of all plastics encountered in daily life. Each of these polymers has its own characteristic modes of degradation and resistances to heat and chemicals. Polyethylene and poly are sensitive to oxidation and UV radiation, while PVC may discolor at high temperatures due to loss of hydrogen chloride gas, become brittle. PET is sensitive to hydrolysis and attack by strong acids, while polycarbonate depolymerizes when exposed to strong alkalis. For example, polyethylene degrades by random scission—that is by a random breakage of the linkages that hold the atoms of the polymer together; when this polymer is heated above 450 Celsius it becomes a complex mixture of molecules of various sizes that resemble gasoline. Other polymers—like polyalphamethylstyrene—undergo'specific' chain scission with breakage occurring only at the ends. Most polymers can be degraded by photolysis to give lower molecular weight molecules. Electromagnetic waves with the energy of visible light or higher, such as ultraviolet light, X-rays and gamma rays are involved in such reactions.
Chain-growth polymers like poly can be degraded by thermolysis at high temperatures to give monomers, oils and water. The degradation takes place by: Step-growth polymers like polyesters and polycarbonates can be degraded by solvolysis and hydrolysis to give lower molecular weight molecules; the hydrolysis takes place in the presence of water containing a base as catalyst. Polyamide is sensitive to degradation by acids and polyamide mouldings will crack when attacked by strong acids. For example, the fracture surface of a fuel connector showed the progressive growth of the crack from acid attack to the final cusp of polymer; the problem is known as stress corrosion cracking, in this case was caused by hydrolysis of the polymer. It was the reverse reaction of the synthesis of the polymer: Cracks can be formed in many different elastomers by ozone attack. Tiny traces of the gas in the air will attack double bonds in rubber chains, with Natural rubber, Styrene-butadiene rubber and NBR being most sensitive to degradation.
Ozone cracks form in products under tension, but the critical strain is small. The cracks are always oriented at right angles to the strain axis, so will form around the circumference in a rubber tube bent over; such cracks are dangerous when they occur in fuel pipes because the cracks will grow from the outside exposed surfaces into the bore of the pipe, fuel leakage and fire may follow. The problem of ozone cracking can be prevented by adding anti-ozonants to the rubber before vulcanization. Ozone cracks were seen in automobile tire sidewalls, but are now seen thanks to these additives. On the other hand, the problem seals; the polymers are susceptible to attack by atmospheric oxygen at elevated temperatures encountered during processing to shape. Many process methods such as extrusion and injection moulding involve pumping molten polymer into tools, the high temperatures needed for melting may result in oxidation unless precautions are taken. For example, a forearm crutch snapped and the user was injured in the resulting fall.
The crutch had fractured across a polypropylene insert within the aluminium tube of the device, infra-red spectroscopy of the material showed that it had oxidized, possible as a result of poor moulding. Oxidation is relatively easy to detect owing to the strong absorption by the carbonyl group in the spectrum of polyolefins. Polypropylene has a simple spectrum with few peaks at the carbonyl position. Oxidation tends to start at tertiary carbon atoms because the free radicals formed here are more stable and longer lasting, making them more susceptible to attack by oxygen; the carbonyl group can be further oxidised to break the chain, this weakens the material by lowering its molecular weight, cracks start to grow in the regions affected. Polymer degradation by galvanic action was first described in the technical literature in 1990; this was the discovery that "plastics can corrode", i.e. polymer degradation may occur through galvanic action similar to that of metals under certain conditions and has been referred to as the "Faudree Effect".
In the aerospace field, this finding has contributed to aircraft safety those aircraft that use C
Plasticizers or dispersants are additives that increase the plasticity or decrease the viscosity of a material. These are the substances; these solids. They decrease the attraction between polymer chains to make them more flexible. Over the last 60 years more than 30,000 different substances have been evaluated for their plasticizing properties. Of these, only a small number – 50 – are today in commercial use; the dominant applications are for plastics polyvinyl chloride. The properties of other materials may be modified when blended with plasticizers including concrete and related products. According to 2014 data, the total global market for plasticizers was 8.4 million metric tonnes including 1.3 million metric tonnes in Europe. Plasticizers for plastics are additives, most phthalate esters in PVC applications. 90% of plasticizers are used in PVC, giving this material improved flexibility and durability. The majority is used in cables, it was thought that plasticizers work by embedding themselves between the chains of polymers, spacing them apart, or swelling them and thus lowering the glass transition temperature for the plastic and making it softer.
For plastics such as PVC, the more plasticizer added, the lower their cold flex temperature will be. Plastic items containing plasticizers can exhibit improved durability. Plasticizers can become available for exposure due to migration and abrasion of the plastic since they are not bound to the polymer matrix; the "new car smell" is attributed to plasticizers or their degradation products. However, multiple studies on the makeup of the smell do not find phthalates in appreciable amounts due to their low volatility and vapor pressure. Plasticizers make it possible to achieve improved compound processing characteristics, while providing flexibility in the end-use product. Ester plasticizers are selected based upon cost-performance evaluation; the rubber compounder must evaluate ester plasticizers for compatibility, processibility and other performance properties. The wide variety of ester chemistries that are in production include sebacates, terephthalates, gluterates, phthalates and other specialty blends.
This broad product line provides an array of performance benefits required for the many elastomer applications such as tubing and hose products, wall-coverings and gaskets, belts and cable, print rolls. Low to high polarity esters provide utility in a wide range of elastomers including nitrile, polychloroprene, EPDM, chlorinated polyethylene, epichlorohydrin. Plasticizer-elastomer interaction is governed by many factors such as solubility parameter, molecular weight, chemical structure. Compatibility and performance attributes are key factors in developing a rubber formulation for a particular application. Plasticizers function as softeners and lubricants, play a significant role in rubber manufacturing. Antiplasticizers exhibit effects that are similar, but sometimes opposite, to those of plasticizers on polymer systems; the effect of plasticizers on elastic modulus is dependent on both temperature and plasticizer concentration. Below a certain concentration, referred to as the crossover concentration, a plasticizer can increase the modulus of a material.
The material's glass transition temperature will decrease however, at all concentrations. In addition to a crossover concentration a crossover temperature exists. Below the crossover temperature the plasticizer will increase the modulus. Antiplasticizers are any small molecule or oligomer additive which increases the modulus while decreasing the glass transition temperature. Plasticizers used in PVC and other plastics are based on esters of polycarboxylic acids with linear or branched aliphatic alcohols of moderate chain length; these compounds are selected on the basis of many critieria including low toxicity, compatibility with the host material and expense. Phthalate esters of straight-chain and branched-chain alkyl alcohols meet these specifications and are common plasticizers. Ortho-phthalate esters have traditionally been the most dominant plasticizers, but regulatory concerns have led to the move away from classified substances to non-classified which includes high molecular weight ortho-phthalates and other plasticisers in Europe.
Plasticizers or water reducers, superplasticizers or high range water reducers, are chemical admixtures that can be added to concrete mixtures to improve workability. Unless the mix is "starved" of water, the strength of concrete is inversely proportional to the amount of water added or water-cement ratio. In order to produce stronger concrete, less water is added, which makes the concrete mixture less workable and difficult to mix, necessitating the use of plasticizers, water reducers, superplasticizers, or dispersants. Plasticizers are often used when pozzolanic ash is added to concrete to improve strength; this method of mix proportioning is popular when producing high-strength concrete and fiber-reinforced concrete. Adding 1-2% plasticizer per unit weight of cement is sufficient. Adding an excessive amount of plasticizer will result in excessive segregation of concrete and is not advisable. Depending on the particular chemical used, use of too much plasticizer may result in a retarding effect.
Plasticizers are manufactured from lignosulf
Natural rubber called India rubber or caoutchouc, as produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds, plus water. Thailand and Indonesia are two of the leading rubber producers. Forms of polyisoprene that are used as natural rubbers are classified as elastomers. Rubber is harvested in the form of the latex from the rubber tree or others; the latex is a sticky, milky colloid drawn off by making incisions in the bark and collecting the fluid in vessels in a process called "tapping". The latex is refined into rubber ready for commercial processing. In major areas, latex is allowed to coagulate in the collection cup; the coagulated lumps are processed into dry forms for marketing. Natural rubber is used extensively in many applications and products, either alone or in combination with other materials. In most of its useful forms, it has a large stretch ratio and high resilience, is waterproof; the major commercial source of natural rubber latex is the Pará rubber tree, a member of the spurge family, Euphorbiaceae.
This species is preferred. A properly managed tree responds to wounding by producing more latex for several years. Congo rubber a major source of rubber, came from vines in the genus Landolphia. Dandelion milk contains latex; the latex exhibits the same quality as the natural rubber from rubber trees. In the wild types of dandelion, latex content varies greatly. In Nazi Germany, research projects tried to use dandelions as a base for rubber production, but failed. In 2013, by inhibiting one key enzyme and using modern cultivation methods and optimization techniques, scientists in the Fraunhofer Institute for Molecular Biology and Applied Ecology in Germany developed a cultivar, suitable for commercial production of natural rubber. In collaboration with Continental Tires, IME began a pilot facility. Many other plants produce forms of latex rich in isoprene polymers, though not all produce usable forms of polymer as as the Pará; some of them require more elaborate processing to produce anything like usable rubber, most are more difficult to tap.
Some produce other desirable materials, for example chicle from Manilkara species. Others that have been commercially exploited, or at least showed promise as rubber sources, include the rubber fig, Panama rubber tree, various spurges, the related Scorzonera tau-saghyz, various Taraxacum species, including common dandelion and Russian dandelion, most for its hypoallergenic properties, guayule; the term gum rubber is sometimes applied to the tree-obtained version of natural rubber in order to distinguish it from the synthetic version. The first use of rubber was by the indigenous cultures of Mesoamerica; the earliest archeological evidence of the use of natural latex from the Hevea tree comes from the Olmec culture, in which rubber was first used for making balls for the Mesoamerican ballgame. Rubber was used by the Maya and Aztec cultures – in addition to making balls Aztecs used rubber for other purposes such as making containers and to make textiles waterproof by impregnating them with the latex sap.
The Pará rubber tree is indigenous to South America. Charles Marie de La Condamine is credited with introducing samples of rubber to the Académie Royale des Sciences of France in 1736. In 1751, he presented a paper by François Fresneau to the Académie that described many of rubber's properties; this has been referred to as the first scientific paper on rubber. In England, Joseph Priestley, in 1770, observed that a piece of the material was good for rubbing off pencil marks on paper, hence the name "rubber", it made its way around England. In 1764 François Fresnau discovered. Giovanni Fabbroni is credited with the discovery of naphtha as a rubber solvent in 1779. South America remained the main source of latex rubber used during much of the 19th century; the rubber trade was controlled by business interests but no laws expressly prohibited the export of seeds or plants. In 1876, Henry Wickham smuggled 70,000 Pará rubber tree seeds from Brazil and delivered them to Kew Gardens, England. Only 2,400 of these germinated.
Seedlings were sent to India, British Ceylon, Dutch East Indies and British Malaya. Malaya was to become the biggest producer of rubber. In the early 1900s, the Congo Free State in Africa was a significant source of natural rubber latex gathered by forced labor. King Leopold II's colonial state brutally enforced production quotas. Tactics to enforce the rubber quotas included removing the hands of victims to prove they had been killed. Soldiers came back from raids with baskets full of chopped-off hands. Villages that resisted were razed to encourage better compliance locally. See Atrocities in the Congo Free State for more information on the rubber trade in the Congo Free State in the late 1800s and early 1900s. Liberia and Nigeria started production. In India, commercial cultivation was introduced by British planters, although the experimental efforts to grow rubber on a commercial scale were initiated as early as 1873 at the Calcutta Botanical Gardens; the first commercial Hevea plantations were established at Thattekadu in Kerala in 1902.
In years the plantation expanded to Karnataka, Tamil Nadu and the Andaman and Nicobar Islands of India. India today is the
Stress corrosion cracking
Stress corrosion cracking is the growth of crack formation in a corrosive environment. It can lead to unexpected sudden failure of ductile metals subjected to a tensile stress at elevated temperature. SCC is chemically specific in that certain alloys are to undergo SCC only when exposed to a small number of chemical environments; the chemical environment that causes SCC for a given alloy is one, only mildly corrosive to the metal. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks; this factor makes it common for SCC to go undetected prior to failure. SCC progresses and is more common among alloys than pure metals; the specific environment is of crucial importance, only small concentrations of certain active chemicals are needed to produce catastrophic cracking leading to devastating and unexpected failure. The stresses can be the result of the crevice loads due to stress concentration, or can be caused by the type of assembly or residual stresses from fabrication.
Certain austenitic stainless steels and aluminium alloys crack in the presence of chlorides, mild steel cracks in the presence of alkali and nitrates, copper alloys crack in ammoniacal solutions. This limits the usefulness of austenitic stainless steel for containing water with higher than few ppm content of chlorides at temperatures above 50 °C. Of concern is the fact that high-tensile structural steels have been known to crack in an unexpectedly brittle manner in a whole variety of aqueous environments when chlorides are present. With the possible exception of the latter, a special example of hydrogen cracking, all the others display the phenomenon of subcritical crack growth, i.e. small surface flaws propagate under conditions where fracture mechanics predicts that failure should not occur. That is, in the presence of a corrodent, cracks develop and propagate well below KIc. In fact, the subcritical value of the stress intensity, designated as KIscc, may be less than 1% of KIc, as the following table shows: A similar process occurs in polymers, when products are exposed to specific solvents or aggressive chemicals such as acids and alkalis.
As with metals, attack is confined to particular chemicals. Thus polycarbonate is sensitive to attack by alkalis, but not by acids. On the other hand, polyesters are degraded by acids, SCC is a failure mechanism. Polymers are susceptible to environmental stress cracking where attacking agents do not degrade the materials chemically. Nylon is sensitive to degradation by acids, a process known as hydrolysis, nylon mouldings will crack when attacked by strong acids. For example, the fracture surface of a fuel connector showed the progressive growth of the crack from acid attack to the final cusp of polymer. In this case the failure was caused by hydrolysis of the polymer by contact with sulfuric acid leaking from a car battery; the degradation reaction is the reverse of the synthesis reaction of the polymer: Cracks can be formed in many different elastomers by ozone attack, another form of SCC in polymers. Tiny traces of the gas in the air will attack double bonds in rubber chains, with natural rubber, styrene-butadiene rubber, nitrile butadiene rubber being most sensitive to degradation.
Ozone cracks form in products under tension, but the critical strain is small. The cracks are always oriented at right angles to the strain axis, so will form around the circumference in a rubber tube bent over; such cracks are dangerous when they occur in fuel pipes because the cracks will grow from the outside exposed surfaces into the bore of the pipe, so fuel leakage and fire may follow. The problem of ozone cracking can be prevented by adding anti-ozonants to the rubber before vulcanization. Ozone cracks were seen in automobile tire sidewalls, but are now seen thanks to the use of these additives. On the other hand, the problem seals; this effect is less common in ceramics which are more resilient to chemical attack. Although phase changes are common in ceramics under stress these result in toughening rather than failure. Recent studies have shown that the same driving force for this toughening mechanism can enhance oxidation of reduced cerium oxide, resulting in slow crack growth and spontaneous failure of dense ceramic bodies.
Given that most glasses contain a substantial silica phase, the introduction of water can chemically weaken the bonds preventing subcritical crack propagation. Indeed, the silicon oxygen bonds present at the tip of a crack are strained, thus more susceptible to chemical attack. In the instance of chemical attack by water, silicon-oxygen bonds bridging the crack are separated into non-connected silicon hydroxide groups; the addition of external stress will serve to further weaken these bonds. Subcritical crack propagation in glasses falls into three regions. In region I, the velocity of crack propagation increases with ambient humidity due to stress-enhanced chemical reaction between the glass and water. In region II, crack propagation velocity is diffusion controlled and dependent on the rate at which chemical reactants can be transported to the tip of the crack. In region III, crack propagation is independent of its environment, having reached a critical stress intensity. Chemicals other than water, like ammonia, can induce subcritical crack propagation in silica glass, but they must have an electron do
Hydrogen embrittlement is the process by which hydride-forming metals such as titanium, zirconium and niobium become brittle and fracture due to the introduction and subsequent diffusion of hydrogen into the metal. Susceptibility to hydrogen-induced cracking is a result of the introduction of hydrogen during forming, plating and finishing operations referred to as'internal embrittlement'. Hydrogen may be introduced over time (so-called'external embrittlement' through environmental exposure, corrosion processes, cathodic protection, and/or from hydrogen generated by corrosion of a coating. To be susceptible, a combination of three factors is required: presence of hydrogen, susceptible material, stress. For susceptible materials, cracking will initiate. The'hydrogen embrittlement' phenomenon was first described in 1875. During hydrogen assisted-cracking, hydrogen is introduced to the surface of a metal and individual hydrogen atoms diffuse through the metal structure; because the solubility of hydrogen increases at higher temperatures, raising the temperature can increase the diffusion of hydrogen.
When assisted by a concentration gradient where there is more hydrogen outside the metal than inside, hydrogen diffusion can occur at lower temperatures. Adsorbed hydrogen species recombine to form hydrogen molecules, creating pressure from within the metal; this pressure can increase to levels where the metal has reduced ductility and tensile strength, up to the point where it cracks open. Although hydrogen atoms embrittle a variety of substances, including steel and titanium, however these metals are still affected in high concentrations, hydrogen embrittlement of high-strength steel is of the most importance. Austempered iron is susceptible, though austempered steel display increased resistance to hydrogen embrittlement. Steel with an ultimate tensile strength of less than 1000 MPa or hardness of less than 23 HRC is not considered susceptible to hydrogen embrittlement. In tensile tests carried out on several structural metals under high-pressure molecular hydrogen environment, it has been shown that austenitic stainless steels, copper are not susceptible to hydrogen embrittlement along with a few other metals.
As an example of severe hydrogen embrittlement, the elongation at failure of 17-4PH precipitation hardened stainless steel was measured to drop from 17% to only 1.7% when smooth specimens were exposed to high-pressure hydrogen. However, recent computational research has shown that instead of leading to a decrease in ductility, there is local enhancement of ductility in areas that are hydrogen saturated; this increase in ductility leads to areas. This, in turn, enables failure to occur at lower-than-expected stresses. Hydrogen embrittlement/hydrogen-assisted cracking can occur during various manufacturing operations or operational use - anywhere that the metal comes into contact with atomic or molecular hydrogen. Processes that can lead to this include cathodic protection, phosphating and electroplating. A special case is arc welding, in which the hydrogen is released from moisture, such as in the coating of welding electrodes. To minimize this, special low-hydrogen electrodes are used for welding high-strength steels.
Other mechanisms of introduction of hydrogen into metal are galvanic corrosion, as well as chemical reactions with acids or other chemicals. One of these chemical reactions involves hydrogen sulfide in sulfide stress cracking, a significant problem for the oil and gas industries.. As the strength of steels increases, the susceptibility to hydrogen embrittlement increases. In high-strength steels, anything above a hardness of HRC 32 may be susceptible to early hydrogen cracking after plating processes that introduce hydrogen, they may experience long-term failures anytime from weeks to decades after being placed in service due to accumulation of hydrogen over time from cathodic protection and other sources. Numerous failures have been reported in the hardness range from HRC 32-36 and more above. Hydrogen embrittlement can be prevented through several methods, all of which are centered on minimizing contact between the metal and hydrogen during fabrication and the electrolysis of water. Embrittling procedures such as acid pickling should be avoided, as should increased contact with elements such as sulfur and phosphate.
The use of proper electroplating solution and procedures can help to prevent hydrogen embrittlement. If the metal has not yet started to crack,'hydrogen embrittlement' can be reversed by removing the hydrogen source and causing the hydrogen within the metal to diffuse out through heat treatment; this de-embrittlement process, known as "baking", is used to overcome the weaknesses of methods such as electroplating which introduce hydrogen to the metal, but is not always effective because a sufficient time and temperature must be reached. Tests such as ASTM F1624 can be used to i
An acetal is a functional group with the following connectivity RHC, where the three R groups are organic fragments. The central carbon atom has four bonds to it, is therefore saturated and has tetrahedral geometry; the R' and R" groups can be equivalent to each other or not, one or both can be hydrogen atoms rather than organic fragments. Acetals are formed from and convertible to carbonyl compounds; the term ketal is sometimes used to identify structures associated with ketones rather than aldehydes and the term acetal was used for the aldehyde cases. Formation of an acetal occurs when the hydroxyl group of a hemiacetal becomes protonated and is lost as water; the carbocation, produced is rapidly attacked by a molecule of alcohol. Loss of the proton from the attached alcohol gives the acetal. Acetals are stable compared to hemiacetals but their formation is a reversible equilibrium as with esters; as a reaction to create an acetal proceeds, water must be removed from the reaction mixture, for example, with a Dean-Stark apparatus, lest it will hydrolyse the product back to the hemiacetal.
The formation of acetals reduces the total number of molecules present and therefore is not favourable with regards to entropy, unless one uses a diol rather than two discrete alcohol molecules. A way to improve this is to use an orthoester as a source of alcohol. Aldehydes and ketones undergo. Water produced along with the acetal product is used up in hydrolysing the orthoester and producing more alcohol to be used in the reaction. Acetals are used as protecting groups for carbonyl groups in organic synthesis because they are stable with respect to hydrolysis by bases and with respect to many oxidizing and reducing agents, they can either protect the carbonyl in a diol. That is, either the carbonyl, or the alcohols, or both could be part of the molecule whose reactivity is to be controlled. Various specific carbonyl compounds have special names for their acetal forms. For example, an acetal formed from formaldehyde is sometimes called a "formal" or the methylenedioxy group; the acetal formed from acetone is sometimes called an acetonide.
Acetalisation is the organic reaction. One way of acetal formation is the nucleophilic addition of an alcohol to an aldehyde. Acetalisation is used in organic synthesis to create a protecting group because it is a reversible reaction. Acetalisation is acid catalysed with elimination of water; the reaction can be driven to the acetal when water is removed from the reaction system either by azeotropic distillation or trapping water with molecular sieves or aluminium oxide. The carbonyl group in 1 takes a proton from acidic hydronium; the protonated carbonyl group 2 is activated for nucleophilic addition of the alcohol. The structures 2a and 2b are mesomers. After deprotonation of 3 by water the hemiacetal or hemiketal 4 is formed; the hydroxyl group in 4 is protonated leading to the oxonium ion 6 which accepts a second alcohol group to 7 with a final deprotonation to the acetal 8. The reverse reaction takes place by adding water in the same acidic medium. Acetals are stable towards basic media. In a transacetalisation or crossacetalisation a diol reacts with an acetal or two different acetals react with each other.
Again this is possible. Benzylidene acetal, a protecting group Dimethoxymethane, a solvent, a.k.a. methylal, a.k.a. formal Dioxolane Metaldehyde Paraldehyde 1,3,5-Trioxane Phenylsulfonylethylidene acetal is an example of arylsulfonyl acetal possessing atypical properties, like resistance to acid hydrolysis which leads to selective introduction and removal of the protective group. Most glycosidic bonds in carbohydrates and other polysaccharides are acetal linkages. Cellulose is a ubiquitous example of a polyacetal. Although many compounds contain an acetal functional group, at least two acetal compounds are called "acetal" for short: Polyoxymethylene plastic known as "acetal" or "polyacetal", is a polyacetal, a polymer of formaldehyde. 1,1-Diethoxyethane, sometimes called "acetal", is an important flavouring compound in distilled beverages. Aminal, a.k.a. aminoacetal Hemiaminal Orthoformate Thioacetal Thioketal