Arsenic is a chemical element with symbol As and atomic number 33. Arsenic occurs in many minerals in combination with sulfur and metals, but as a pure elemental crystal. Arsenic is a metalloid, it has various allotropes, but only the gray form, which has a metallic appearance, is important to industry. The primary use of arsenic is in alloys of lead. Arsenic is a common n-type dopant in semiconductor electronic devices, the optoelectronic compound gallium arsenide is the second most used semiconductor after doped silicon. Arsenic and its compounds the trioxide, are used in the production of pesticides, treated wood products and insecticides; these applications are declining due to the toxicity of its compounds. A few species of bacteria are able to use arsenic compounds as respiratory metabolites. Trace quantities of arsenic are an essential dietary element in rats, goats and other species. A role in human metabolism is not known. However, arsenic poisoning occurs in multicellular life. Arsenic contamination of groundwater is a problem.
The United States' Environmental Protection Agency states that all forms of arsenic are a serious risk to human health. The United States' Agency for Toxic Substances and Disease Registry ranked arsenic as number 1 in its 2001 Priority List of Hazardous Substances at Superfund sites. Arsenic is classified as a Group-A carcinogen; the three most common arsenic allotropes are gray and black arsenic, with gray being the most common. Gray arsenic adopts a double-layered structure consisting of many interlocked, six-membered rings; because of weak bonding between the layers, gray arsenic is brittle and has a low Mohs hardness of 3.5. Nearest and next-nearest neighbors form a distorted octahedral complex, with the three atoms in the same double-layer being closer than the three atoms in the next; this close packing leads to a high density of 5.73 g/cm3. Gray arsenic becomes a semiconductor with a bandgap of 1.2 -- 1.4 eV if amorphized. Gray arsenic is the most stable form. Yellow arsenic is soft and waxy, somewhat similar to tetraphosphorus.
Both have four atoms arranged in a tetrahedral structure in which each atom is bound to each of the other three atoms by a single bond. This unstable allotrope, being molecular, is the most volatile, least dense, most toxic. Solid yellow arsenic is produced by rapid cooling of arsenic vapor, As4, it is transformed into gray arsenic by light. The yellow form has a density of 1.97 g/cm3. Black arsenic is similar in structure to black phosphorus. Black arsenic can be formed by cooling vapor at around 100–220 °C, it is brittle. It is a poor electrical conductor. Arsenic occurs in nature as a monoisotopic element, composed of 75As; as of 2003, at least 33 radioisotopes have been synthesized, ranging in atomic mass from 60 to 92. The most stable of these is 73As with a half-life of 80.30 days. All other isotopes have half-lives of under one day, with the exception of 71As, 72As, 74As, 76As, 77As. Isotopes that are lighter than the stable 75As tend to decay by β+ decay, those that are heavier tend to decay by β− decay, with some exceptions.
At least 10 nuclear isomers have been described, ranging in atomic mass from 66 to 84. The most stable of arsenic's isomers is 68mAs with a half-life of 111 seconds. Arsenic has a similar electronegativity and ionization energies to its lighter congener phosphorus and as such forms covalent molecules with most of the nonmetals. Though stable in dry air, arsenic forms a golden-bronze tarnish upon exposure to humidity which becomes a black surface layer; when heated in air, arsenic oxidizes to arsenic trioxide. This odor can be detected on striking arsenide minerals such as arsenopyrite with a hammer, it burns in oxygen to form arsenic trioxide and arsenic pentoxide, which have the same structure as the more well-known phosphorus compounds, in fluorine to give arsenic pentafluoride. Arsenic sublimes upon heating at atmospheric pressure, converting directly to a gaseous form without an intervening liquid state at 887 K; the triple point is 3.63 MPa and 1,090 K. Arsenic makes arsenic acid with concentrated nitric acid, arsenous acid with dilute nitric acid, arsenic trioxide with concentrated sulfuric acid.
Arsenic reacts with metals to form arsenides, though these are not ionic compounds containing the As3− ion as the formation of such an anion would be endothermic and the group 1 arsenides have properties of intermetallic compounds. Like germanium and bromine, which like arsenic succeed the 3d transition series, arsenic is much less stable in the group oxidation state of +5 than its vertical neighbors phosphorus and antimony, hence arsenic pentoxide and arsenic acid are potent oxidizers. Compounds of arsenic resemble in some respects those of phosphorus which occupies the same group of the periodic table; the most common oxidation states for arsenic are: −3 in the arsenides, which are alloy-like intermetallic compounds, +3 in the arsenites, +5 in the arsenates and most organoarsenic compounds. Arsenic bonds to itself as seen in the square As3−4 ions in the mineral skutterudite. In the +3 oxidation state, arsenic is pyramidal owing to the i
A weed is a plant considered undesirable in a particular situation, "a plant in the wrong place". Examples are plants unwanted in human-controlled settings, such as farm fields, gardens and parks. Taxonomically, the term "weed" has no botanical significance, because a plant, a weed in one context is not a weed when growing in a situation where it is in fact wanted, where one species of plant is a valuable crop plant, another species in the same genus might be a serious weed, such as a wild bramble growing among cultivated loganberries. In the same way, volunteer crops are regarded as weeds in a subsequent crop. Many plants that people regard as weeds are intentionally grown in gardens and other cultivated settings, in which case they are sometimes called beneficial weeds; the term weed is applied to any plant that grows or reproduces aggressively, or is invasive outside its native habitat. More broadly "weed" is applied pejoratively to species outside the plant kingdom, species that can survive in diverse environments and reproduce quickly.
Weed control is important in agriculture. Methods include hand cultivation with hoes, powered cultivation with cultivators, smothering with mulch, lethal wilting with high heat, burning, or chemical attack with herbicides. Certain classes of weeds share adaptations to ruderal environments; that is to say: disturbed environments where soil or natural vegetative cover has been damaged or gets damaged, disturbances that give the weeds advantages over desirable crops, pastures, or ornamental plants. The nature of the habitat and its disturbances will affect or determine which types of weed communities become dominant. Examples of such ruderal or pioneer species include plants that are adapted to occurring disturbed environments such as dunes and other windswept areas with shifting soils, alluvial flood plains, river banks and deltas, areas that are burned repeatedly. Since human agricultural practices mimic these natural environments where weedy species have evolved, some weeds are preadapted to grow and proliferate in human-disturbed areas such as agricultural fields, lawns and construction sites.
The weedy nature of these species gives them an advantage over more desirable crop species because they grow and reproduce they have seeds that persist in the soil seed bank for many years, or they may have short lifespans with multiple generations in the same growing season. In contrast, perennial weeds have underground stems that spread under the soil surface or, like ground ivy, have creeping stems that root and spread out over the ground; some plants become dominant when introduced into new environments because the animals in their original environment, that compete with them or feed on them are absent. An example is Klamath weed, that threatened millions of hectares of prime grain and grazing land in North America after it was accidentally introduced, but was reduced to a rare roadside weed within several years after some of its natural enemies were imported during World War II. In locations where predation and mutually competitive relationships are absent, weeds have increased resources available for growth and reproduction.
The weediness of some species that are introduced into new environments may be caused by their production of allelopathic chemicals which indigenous plants are not yet adapted to, a scenario sometimes called the "novel weapons hypothesis". These chemicals may limit the growth of established plants or the germination and growth of seeds and seedlings. Another of the ways in which the ecological role of a plant can make it a weed if it is in itself inoffensive, is if it harbours a pest, dependent on it for survival. A number of native or non-native plants are unwanted in a specific location for a number of reasons. An important one is that they interfere with food and fiber production in agriculture, wherein they must be controlled in order to prevent lost or diminished crop yields. Other important reasons are that they interfere with other cosmetic, decorative, or recreational goals, such as in lawns, landscape architecture, playing fields, golf courses, they can be of concern for environmental reasons whereby introduced species out-compete for resources or space with desired endemic plants.
For all these reasons. In weed ecology some authorities speak of the relationship between "the three Ps": plant, perception; these have been variously defined, but the weed traits listed by H. G. Ba
Aniline is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine, its main use is in the manufacture of precursors to polyurethane and other industrial chemicals. Like most volatile amines, it has the odor of rotten fish, it ignites burning with a smoky flame characteristic of aromatic compounds. Aniline is a pyramidalized molecule, with hybridization of the nitrogen somewhere between sp3 and sp2; the amine is flatter than an aliphatic amine, owing to conjugation of the lone pair with the aryl substituent. Thus, the experimental geometry reflects a balance between the stabilization of lone pairs in orbitals with higher s character and better stabilization via conjugation with the aryl ring for an orbital of pure p character; the pyramidalization angle between the C–N bond and the bisector of the H–N–H angle is 142.5°. The C−N distance is correspondingly shorter. In aniline, the C−N and C−C distances are close to 1.39 Å, indicating the π-bonding between N and C.
Industrial aniline production involves two steps. First, benzene is nitrated with a concentrated mixture of nitric acid and sulfuric acid at 50 to 60 °C to yield nitrobenzene; the nitrobenzene is hydrogenated in the presence of metal catalysts: The reduction of nitrobenzene to aniline was first performed by Nikolay Zinin in 1842, using inorganic sulfide as a reductant. Aniline can alternatively be prepared from phenol derived from the cumene process. In commerce, three brands of aniline are distinguished: aniline oil for blue, pure aniline. Many analogues of aniline are known; these include toluidines, chloroanilines, aminobenzoic acids and many others. They are prepared by nitration of the substituted aromatic compounds followed by reduction. For example, this approach is used to convert toluene into toluidines and chlorobenzene into 4-chloroaniline. Alternatively, using Buchwald-Hartwig coupling or Ullmann reaction approaches, aryl halides can be aminated with aqueous or gaseous ammonia The chemistry of aniline is rich because the compound has been cheaply available for many years.
Below are some classes of its reactions. The oxidation of aniline has been investigated, can result in reactions localized at nitrogen or more results in the formation of new C-N bonds. In alkaline solution, azobenzene results, whereas arsenic acid produces the violet-coloring matter violaniline. Chromic acid converts it into quinone, whereas chlorates, in the presence of certain metallic salts, give aniline black. Hydrochloric acid and potassium chlorate give chloranil. Potassium permanganate in neutral solution oxidizes it to nitrobenzene, in alkaline solution to azobenzene and oxalic acid, in acid solution to aniline black. Hypochlorous acid gives para-amino diphenylamine. Oxidation with persulfate affords a variety of polyanilines compounds; these polymers exhibit rich acid-base properties. Like phenols, aniline derivatives are susceptible to electrophilic substitution reactions, its high reactivity reflects that it is an enamine, which enhances the electron-donating ability of the ring. For example, reaction of aniline with sulfuric acid at 180 °C produces sulfanilic acid, H2NC6H4SO3H.
If bromine water is added to aniline, the bromine water is decolourised and a white precipitate of 2,4,6-tribromoaniline is formed. To generate the mono-substituted product, a protection with acetyl chloride is required: The reaction to form 4-bromoaniline is to protect the amine with acetyl chloride hydrolyse back to reform aniline; the largest scale industrial reaction of aniline involves its alkylation with formaldehyde. An idealized equation is shown: 2 C6H5NH2 + CH2O → CH22 + H2OThe resulting diamine is the precursor to 4,4'-MDI and related diisocyanates. Aniline is a weak base. Aromatic amines such as aniline are, in general, much weaker bases than aliphatic amines. Aniline reacts with strong acids to form anilinium ion. Traditionally, the weak basicity of aniline is attributed to a combination of inductive effect from the more electronegative sp2 carbon and resonance effects, as the lone pair on the nitrogen is delocalized into the pi system of the benzene ring.: Missing in such analysis is consideration of solvation.
Aniline is, for example, more basic than ammonia in the gas phase, but ten thousand times less so in aqueous solution. Aniline reacts with acyl chlorides such as acetyl chloride to give amides; the amides formed from aniline are sometimes called anilides, for example CH3-CO-NH-C6H5 is acetanilide. At high temperatures aniline and carboxylic acids react to give the anilides. N-Methylation of aniline with methanol at elevated temperatures over acid catalysts gives N-methylaniline and dimethylaniline: C6H5NH2 + 2 CH3OH → C6H5N2 + 2H2ON-Methylaniline and dimethylaniline are colorless liquids with boiling points of 193–195 °C and 192 °C, respectively; these derivatives are of importance in the color industry. Aniline combines directly with alkyl iodides to form tertiary amines. Boiled with carbon disulfide, it gives sulfocarbanilide, which may be decomposed into phen
Pest control is the regulation or management of a species defined as a pest, a member of the animal kingdom that impacts adversely on human activities. The human response depends on the importance of the damage done, will range from tolerance, through deterrence and management, to attempts to eradicate the pest. Pest control measures may be performed as part of an integrated pest management strategy. In agriculture, pests are kept at bay by cultural and biological means. Ploughing and cultivation of the soil before sowing reduces the pest burden and there is a modern trend to limit the use of pesticides as far as possible; this can be achieved by monitoring the crop, only applying insecticides when necessary, by growing varieties and crops which are resistant to pests. Where possible, biological means are used, encouraging the natural enemies of the pests and introducing suitable predators or parasites. In homes and urban environments, the pests are the rodents, birds and other organisms that share the habitat with humans, that feed on and spoil possessions.
Control of these pests is attempted through exclusion, physical removal or chemical means. Alternatively, various methods of biological control can be used including sterilisation programmes. Pest control is at least as old as agriculture, as there has always been a need to keep crops free from pests; as long ago as 3000 BC in Egypt, cats were used to control pests of grain stores such as rodents. Ferrets were domesticated by 500 AD in Europe for use as mousers. Mongooses were introduced into homes to control rodents and snakes by the ancient Egyptians; the conventional approach was the first to be employed, since it is comparatively easy to destroy weeds by burning them or ploughing them under, to kill larger competing herbivores. Techniques such as crop rotation, companion planting, the selective breeding of pest-resistant cultivars have a long history. Chemical pesticides were first used around 2500 BC, when the Sumerians used sulphur compounds as insecticides. Modern pest control was stimulated by the spread across the United States of the Colorado potato beetle.
After much discussion, arsenical compounds were used to control the beetle and the predicted poisoning of the human population did not occur. This led the way to a widespread acceptance of insecticides across the continent. With the industrialisation and mechanization of agriculture in the 18th and 19th centuries, the introduction of the insecticides pyrethrum and derris, chemical pest control became widespread. In the 20th century, the discovery of several synthetic insecticides, such as DDT, herbicides boosted this development. Biological control is first recorded around 300 AD in China, when colonies of weaver ants, Oecophylla smaragdina, were intentionally placed in citrus plantations to control beetles and caterpillars. In China, ducks were used in paddy fields to consume pests, as illustrated in ancient cave art. In 1762, an Indian mynah was brought to Mauritius to control locusts, about the same time, citrus trees in Burma were connected by bamboos to allow ants to pass between them and help control caterpillars.
In the 1880s, ladybirds were used in citrus plantations in California to control scale insects, other biological control experiments followed. The introduction of DDT, a cheap and effective compound, put an effective stop to biological control experiments. By the 1960s, problems of resistance to chemicals and damage to the environment began to emerge, biological control had a renaissance. Chemical pest control is still the predominant type of pest control today, although a renewed interest in traditional and biological pest control developed towards the end of the 20th century and continues to this day. Biological pest control is a method of controlling pests such as insects and mites by using other organisms, it relies on predation, herbivory or other natural mechanisms, but also involves an active human management role. Classical biological control involves the introduction of natural enemies of the pest that are bred in the laboratory and released into the environment. An alternative approach is to augment the natural enemies that occur in a particular area by releasing more, either in small, repeated batches, or in a single large-scale release.
Ideally, the released organism will breed and survive, provide long-term control. Biological control can be an important component of an integrated pest management programme. For example: mosquitoes are controlled by putting Bt Bacillus thuringiensis ssp. israelensis, a bacterium that infects and kills mosquito larvae, in local water sources. Mechanical pest control is the use of hands-on techniques as well as simple equipment and devices, that provides a protective barrier between plants and insects; this is referred to as tillage and is one of the oldest methods of weed control as well as being useful for pest control. Crop rotation can help to control pests by depriving them of their host plants, it is a major tactic in the control of corn rootworm, has reduced early season incidence of Colorado potato beetle by as much as 95%. A trap crop is a crop of a plant. Pests aggregated on the trap crop can be more controlled using pesticides or other methods. However, trap-cropping, on its own, has failed to cost reduce pest densities on large commercial scales, without the use of pesticides due to the pests' ability to disperse back into the main field
The Jmol applet, among other abilities, offers an alternative to the Chime plug-in, no longer under active development. While Jmol has many features that Chime lacks, it does not claim to reproduce all Chime functions, most notably, the Sculpt mode. Chime requires plug-in installation and Internet Explorer 6.0 or Firefox 2.0 on Microsoft Windows, or Netscape Communicator 4.8 on Mac OS 9. Jmol operates on a wide variety of platforms. For example, Jmol is functional in Mozilla Firefox, Internet Explorer, Google Chrome, Safari. Chemistry Development Kit Comparison of software for molecular mechanics modeling Jmol extension for MediaWiki List of molecular graphics systems Molecular graphics Molecule editor Proteopedia PyMOL SAMSON Official website Wiki with listings of websites and moodles Willighagen, Egon. "Fast and Scriptable Molecular Graphics in Web Browsers without Java3D". Doi:10.1038/npre.2007.50.1
Glufosinate is a occurring broad-spectrum systemic herbicide produced by several species of Streptomyces soil bacteria. Plants may metabolize bialaphos, another occurring herbicide, directly into glufosinate; the compound irreversibly inhibits glutamine synthetase, an enzyme necessary for the production of glutamine and for ammonia detoxification, giving it antibacterial and herbicidal properties. Application of glufosinate to plants leads to reduced glutamine and elevated ammonia levels in tissues, halting photosynthesis, resulting in plant death. In the 1960s and early 1970s, scientists at University of Tübingen and at the Meiji Seika Kaisha Company independently discovered that species of Streptomyces bacteria produce a tripeptide they called bialaphos that inhibits bacteria, they determined. Phosphinothricin was first synthesized by scientists at Hoechst in the 1970s as a racemic mixture. In the late 1980s scientists discovered enzymes in these Streptomyces species that selectively inactivate free phosphinothricin.
The two genes and their proteins have 80% homology on the DNA level and 86% amino acid homology, are each 158 amino acids long. Glufosinate is a broad-spectrum herbicide, used to control important weeds such as morning glories, hemp sesbania, Pennsylvania smartweed and yellow nutsedge similar to glyphosate, it is applied to young plants during early development for full effectiveness. It is sold in formulations under brands including Basta, Finale and Liberty. Glufosinate is used in three situations as an herbicide: directed sprays for weed control, including in genetically modified crops use as a crop desiccation to facilitate harvesting Glufosinate has shown to provide some protection against various plant diseases, as it acts to kill fungi and bacteria on contact. Genetically modified crops resistant to glufosinate were created by genetically engineering the bar or pat genes from streptomyces into the relevant crop seeds. In 1995 the first glufosinate-resistant crop, was brought to market, it was followed by corn in 1997, cotton in 2004, soybeans in 2011.
Phosphinothricin is a glutamine synthetase inhibitor. Glufosinate-treated plants die due to a buildup of ammonia in the thylakoid lumen, leading to the uncoupling of photophosphorylation; the uncoupling of photophosphorylation causes the production of reactive oxygen species, lipid peroxidation, membrane destruction. Elevated levels of ammonia are detectable within one hour after application of Phosphinothricin; as glufosinate is used as a pre-harvest desiccant, residues can be found in foods that humans ingest. Such foods include potatoes, beans, corn and barley. In addition, the chemical can be passed to humans through animals. Flour processed from wheat grain that contained traces of glufosinate was found to retain 10-100% of the chemicals' residues; the herbicide is persistent. Its persistent nature can be observed by its half-life which varies from 3 to 70 days depending on the soil type and organic matter content. Residues can remain in frozen food for up to two years and the chemical is not destroyed by cooking the food item in boiling water.
The EPA classifies the chemical as'persistent' and'mobile' based on its lack of degradation and ease of transport through soil. A study to revealed the presence of circulating PAGMF in women with and without pregnancy, paving the way for a new field in reproductive toxicology including nutrition and utero-placental toxicities There are no exposure limits established by the Occupational Safety & Health Administration or the American Conference of Governmental Industrial Hygienists; the WHO/FAO recommended. The European Food Safety Authority has set an ADI of 0.021 mg/kg. The Acute reference dose for child-bearing women is 0.021 mg/kg. Glufosinate is a United States Environmental Protection Agency EPA registered chemical, it is a California registered chemical. It is not banned in the country and it is not a PIC pesticide. There are no exposure limits established by OSHA or the American Conference of Governmental Industrial Hygienists. Glufosinate is registered for use as an herbicide in Europe, it has been withdrawn from the French market since October 24, 2017 by the Agence nationale de sécurité sanitaire de l'alimentation, de l'environnement et du travail due to its classification as a possible reprotoxic chemical.
Bayer's site of LibertyLink crops Basta technical guide Glufosinate in the Pesticide Properties DataBase Glufosinate-ammonium in the Pesticide Properties DataBase Glufosinate-P in the Pesticide Properties DataBase
European Chemicals Agency
The European Chemicals Agency is an agency of the European Union which manages the technical and administrative aspects of the implementation of the European Union regulation called Registration, Evaluation and Restriction of Chemicals. ECHA is the driving force among regulatory authorities in implementing the EU's chemicals legislation. ECHA helps companies to comply with the legislation, advances the safe use of chemicals, provides information on chemicals and addresses chemicals of concern, it is located in Finland. The agency headed by Executive Director Bjorn Hansen, started working on 1 June 2007; the REACH Regulation requires companies to provide information on the hazards and safe use of chemical substances that they manufacture or import. Companies register this information with ECHA and it is freely available on their website. So far, thousands of the most hazardous and the most used substances have been registered; the information is technical but gives detail on the impact of each chemical on people and the environment.
This gives European consumers the right to ask retailers whether the goods they buy contain dangerous substances. The Classification and Packaging Regulation introduces a globally harmonised system for classifying and labelling chemicals into the EU; this worldwide system makes it easier for workers and consumers to know the effects of chemicals and how to use products safely because the labels on products are now the same throughout the world. Companies need to notify ECHA of the labelling of their chemicals. So far, ECHA has received over 5 million notifications for more than 100 000 substances; the information is available on their website. Consumers can check chemicals in the products. Biocidal products include, for example, insect disinfectants used in hospitals; the Biocidal Products Regulation ensures that there is enough information about these products so that consumers can use them safely. ECHA is responsible for implementing the regulation; the law on Prior Informed Consent sets guidelines for the import of hazardous chemicals.
Through this mechanism, countries due to receive hazardous chemicals are informed in advance and have the possibility of rejecting their import. Substances that may have serious effects on human health and the environment are identified as Substances of Very High Concern 1; these are substances which cause cancer, mutation or are toxic to reproduction as well as substances which persist in the body or the environment and do not break down. Other substances considered. Companies manufacturing or importing articles containing these substances in a concentration above 0,1% weight of the article, have legal obligations, they are required to inform users about the presence of the substance and therefore how to use it safely. Consumers have the right to ask the retailer whether these substances are present in the products they buy. Once a substance has been identified in the EU as being of high concern, it will be added to a list; this list is available on ECHA's website and shows consumers and industry which chemicals are identified as SVHCs.
Substances placed on the Candidate List can move to another list. This means that, after a given date, companies will not be allowed to place the substance on the market or to use it, unless they have been given prior authorisation to do so by ECHA. One of the main aims of this listing process is to phase out SVHCs where possible. In its 2018 substance evaluation progress report, ECHA said chemical companies failed to provide “important safety information” in nearly three quarters of cases checked that year. "The numbers show a similar picture to previous years" the report said. The agency noted that member states need to develop risk management measures to control unsafe commercial use of chemicals in 71% of the substances checked. Executive Director Bjorn Hansen called non-compliance with REACH a "worry". Industry group CEFIC acknowledged the problem; the European Environmental Bureau called for faster enforcement to minimise chemical exposure. European Chemicals Bureau Official website