Top Document: Sci.chem FAQ - Part 3 of 7 Previous Document: News Headers Next Document: 13. Illicit and Government Controlled Substances See reader questions & answers on this topic! - Help others by sharing your knowledge 12.1 What are CAS Registry Numbers? When chemicals are first encountered by the Chemical Abstracts Service, they are assigned a unique number when they are registered. These numbers are not related to any structure or property of the molecule, they are arbitrarily assigned. It should be remembered that occasionally a compound may be accidentally assigned two or more numbers - especially industrial products that have not been completely characterised. When this is discovered, one of the numbers is no longer used. The numbers usually take the form of [xx-yy-z to xxxxxx-yy-z] and square brackets are often used in monographs to identify the CAS Registry Number [RN]. The easiest way to locate the CAS RN for commercially-available chemicals is to look in suppliers catalogues ( eg Aldrich) or compilations ( eg Merck or Hawley ), almost all chemical texts now list the RN, and several ( eg Merck Index and Aldrich ) have a cross-reference Index. The RN is extremely useful for on-line searching of Chemical Abstracts and several other popular chemistry-related databases, but is not particularly useful for the hardcopy version, except to confirm compound identity. 12.2 What are the correct names of recently-discovered elements? The Transfermium Working Group was established in 1986 by the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Pure and Applied Physics (IUPAP). The working group published several reports, and then recommended that elements should not be named after living persons [1]. This greatly upset the USA - who wanted to name an element after G. Seaborg. After protracted negotiations, a compromise selection of names was finally approved by the IUPAC Commission on Nomenclature in Inorganic Chemistry, the IUPAC Inorganic Division, the IUPAC Bureau, and the selection was eventually ratified by the IUPAC Council meeting in Geneva during August 1997 [2]. 101 Mendelevium Md D. Mendeleev (Russia) 102 Nobelium No Nobel Institute (Sweden) 103 Lawrencium Lr E. Lawrence (USA) 104 Rutherfordium Rf E. Rutherford (NZ) 105 Dubnium Db Dubna = Russian Research Centre 106 Seaborgium Sg G. Seaborg (USA) 107 Bohrium Bh N. Bohr (Denmark) 108 Hassium Hs Latin name for German state of Hesse 109 Meitnerium Mt L. Meitner (Austria) Note that Hesse is where the German heavy-element laboratory is based. The Gesellschaft fur Schwerionenforschung (GSI) was responsible for the first man-made creation of elements 107-110. The compromise will now move attention to the naming the recently-discovered elements 110-112. 12.3 What is the nomenclature system for CFCs/HCFCs/HFCs? The CFC naming system was developed by T.Midgley,Jr. and A.L.Henne in 1929, and further refined by J.D.Park. Originally, organic molecules that contained Chlorine and Fluorine were all referred to as CFCs. Today, the group is subdivided into CFCs, HCFCs, and HFCs. The naming system consists of:- CFC-01234a where 0 = number of double bonds ( omitted if zero ) 1 = Carbon atoms - 1 ( omitted if 0 ) 2 = Hydrogen atoms + 1 3 = Fluorine atoms 4 = Chlorine atoms replaced by Bromine ("B" prefix added ) a = letter added to identify isomers, the "normal" isomer in any number has the smallest mass difference on each carbon, and a, b, or c are added as the masses diverge from normal. If the compound is cyclic, then the number is prefixed with "C". There are several other refrigerants, some of which are hydrocarbons, hydrocarbon blends, or CFC blends. Full details of the nomenclature system are specified in ANSI/ASHRAE Standard 34-1992 with additional annual supplements. Chemical names are frequently used in place of the numbers for common materials - such as trichloroethylene and chloroform. The specified ANSI/ASHRAE prefixes were FC ( FluoroCarbon ), or R ( Refrigerant ), but today most are prefixed by more specific classifications - such as CFC, HCFC, and HFC. CFC-11 CCl3F trichlorofluoromethane [75-69-4] CFC-12 CCl2F2 dichlorodifluoromethane [75-71-8] CFC-113 CCl2F-CClF2 1,1,2-trichlorotrifluoroethane [76-13-1] HCFC-22 CHClF2 chlorodifluoromethane [75-45-6] HCFC-123 CHCl2-CF3 2,2-dichloro-1,1,1-trifluoroethane [306-83-2] HCFC-123a CHClF-CClF2 1,2-dichloro-1,1,2-trifluoroethane [354-23-4] HFC-23 CHF3 trifluoromethane [75-46-7] HFC-134 CHF2-CHF2 1,1,2,2-tetrafluoroethane [359-35-3] HFC-134a CH2F-CF3 1,2,2,2-tetrafluoroethane [811-97-2] R-20 CHCl3 chloroform [67-66-3] R-22B1 CHBrF2 bromodifluoromethane [1511-62-2] R-1120 CHCl=CCl2 trichloroethylene [79-01-6] R-1150 CH2=CH2 ethylene [74-85-1] R-C316 C4Cl2F6 1,2-dichlorohexafluorocyclobutane Another technique for naming CFCs uses the addition of 90 to the CFC number to produce a "def" number which corresponds to the CHF composition. If (e + f) < (2d + 2), then additional atoms are required for saturation. This technique has been described in detail in the Journal of Chemical Education [3]. ASHRAE +90 Empirical Composition Formula C H F (+Cl) CFC-11 101 1 - 1 3 CCl3F CFC-12 102 1 - 2 2 CCl2F2 HCFC-22 112 1 1 2 1 CHClF2 HCFC-123 213 2 1 3 2 CHCl2-CF3 HFC-134a 224 2 2 4 - CH2F-CF3 Halons are numbered according to a totally different system developed by the US Army Corps of Engineers, and the prefix term is always "Halon". Hydrogen is not numbered, and terminal zeros are not expressed. Halon-0123 where 0 = number of carbon atoms 1 = number of fluorine atoms 2 = number of chlorine atoms 3 = number of bromine atoms Halon-1211 CBrClF2 bromochlorodifluoromethane [353-59-3] Halon-1301 CBrF3 bromotrifluoromethane [75-63-8] Halon-2402 CBrF2-CBrF2 1,2-dibromo-1,1,2,2-tetrafluoroethane [124-73-2] 12.4 How can I get the IUPAC chemical name from traditional names? It depends. Usually the quickest way is to look the name up in a chemical supplier's catalog, MSDS, or a standard text like Merck or Hawley. You can also often find the correct name if you refer to an old chemistry text that lists both the traditional and IUPAC naming conventions. Some traditional or common names also refer to mixtures of chemicals, eg aqua regia, piranha solution. One reason why traditional names have been replaced is because the same name could be used for different compounds. An example is the use of caprylic to describe 1-Octanol and 2-Octanol, and attempts to qualify the name with "primary" and "secondary" were less than successful. Octyl alcohol has been used to describe both 1-octanol and 2-ethylhexanol, thus explaining why the well known dioctyl phthalate (DOP) is actually bis(2-ethylhexyl) phthalate. The following examples highlight the diversity of names often encountered. Carbon Alkane Alcohol Aldehyde Acid 1 methane methanol form- formic carbinol 2 ethane ethyl acet- acetic methyl carbinol 3 n-propane n-propyl propion- propionic ethyl carbinol 4 n-butane n-butyl n-butyr- n-butyric propyl carbinol 5 n-pentane n-amyl n-valer- n-valeric butyl carbinol 6 n-hexane hexyl capro- caproic amyl carbinol caproic 7 n-heptane enanthyl enanth- enanthic enanthic hexyl carbinol 8a n-octane capryl capryl- caprylic primary caprylic caprylic heptyl carbinol 1-octanol 8b capryl secondary caprylic methyl hexyl carbinol 2-octanol 9 n-nonane pelargonic pelargonic pelargonic octyl carbinol 10 n-decane capric capr- capric nonyl carbinol capric 12 n-dodecane lauryl laur- lauric lauric lauryl 14 n-tetradecane myristyl myrist- myristic 16 n-hexadecane cetyl palmit- palmitic cetane 18 n-octadecane stearyl stearic 20 n-eicosane arachidyl arachidic Primary - alcohol R1CH2OH - amine R1NH2 eg normal straight chain normal octane n-octane normal butanol 1-butanol iso branched chain iso-butane 2-methylpropane iso-butanol 2-methyl-1-propanol iso-octane 2,2,4-trimethylpentane Secondary - alcohol R1R2CHOH - amine R1R2NH eg sec-butanol 2-butanol iso-propanol 2-propanol Tertiary - alcohol R1R2R3COH - amine R1R2R3N eg tert-butanol 2-methyl-2-propanol - substitution onto the benzene ring 1,2 = ortho ortho-xylene 1,3 = meta meta-xylene 1,4 = para para-xylene However other names get more tricky, especially historical names, where several names may be used for the same chemical and, even worse, different chemicals can be described by the same name. Examples include:- - calcium carbonate = limestone, chalk, calcite. - calcium hydroxide = slaked lime, hydrated lime, caustic lime. - calcium oxide = calx, lime, quicklime, unslaked lime, burnt lime. - hydrochloric acid = muriatic acid, spirits of salts. - nitric acid = aqua fortis. - potassium carbonate = potash, artificial alkali, vegetable alkali. - potassium hydroxide = caustic potash, lye. - sodium carbonate - any form = soda, natural alkali, mineral alkali. - anhydrous = soda ash. - dodecahydrate = sal soda, washing soda. - monohydrate = soda crystals. - sodium chloride = rock salt. - sodium hydroxide = caustic soda, lye, soda lye. - sulfuric acid = oil of vitriol Some old chemical terms are seldom encountered these days, but have very specific meanings, eg " flowers " described any product of sublimation, hence "flowers of sulfur". " specific " in front of any quantity means " divided by mass ", hence "specific gravity". " ether " described a volatile liquid, not only compounds with the Cx-O--Cy structure, and also often known today as "spirit". " aromatic " described a liquid that had an aroma, not only those derived from benzene, or which benzene ring structure. " oil " described a liquid that was not miscible with water, thus it described different products in different chemical industries :- - Essential oils = volatile and odoriferous liquid plant extracts. Essential oils can be obtained by extraction or distillation ( steam ), often contain terpenes ( based on the isoprene structure ), are usually smelly ( aromatic ), and are used for perfumes, flavours and aromas, eg lemon oil and pine oil. - Triglyceride oils = fats and oils based on the glycerol molecule that can be obtained from plant and animal material, frequently by melting or cold pressing. They are a significant, and important, component in our diet, eg soya oil, lard, fish oils, and anhydrous milk fat. - Petroleum oil = a mixture of a large number of hydrocarbons that are usually derived from 0.1 to 3 billion-year-old organic matter. Crude oil can contain hundreds of hydrocarbons with one to sixty carbon atoms, and the hydrocarbons are usually grouped and reported by type, eg alkane ( paraffin ), alkene ( olefin ), or arene ( aromatic ). Almost all old industries had easy-to-remember names for chemicals they commonly encountered, but today many of those names can cause confusion if used outside the industry. Some common examples, just from the petroleum industry alone are:- - " ether " is a volatile hydrocarbon fraction that does not contain the Cx-O-Cy structure, eg petroleum ether ( aka petroleum spirit ). - " naphthene " is a cyclic paraffin, does not contain naphthalene, and is not a major component of naphtha ( refer Section 27.5 ). - Benzene, toluene and xylene are often called benzol, toluol, and xylol, even though they do not contain an -OH group. - Benzine ( ligroin ) was a saturated hydrocarbon fraction that boiled between 20C and 135C. Gasoline/petrol fractions are still called benzine by some older people. - Diesel fuel is often called "gas oil", which is a historical term for hydrocarbon distillate fractions. Atmospheric gas oil has a boiling range between 220C - 450C, and vacuum gas oil boils from 350C to 550C. 12.5 What does "omega-3 fatty acids" mean? Chemists recognise that they should always number carbon chains from the end with the functional group, so the location of double bonds in unsaturated fatty acids are numbered from the carboxylic acid end, and are usually designated by "delta" in their abbreviated names. Biochemists are more interested in the actual role that chemicals play, consequently they will consider the position from the end that is important. In the case of natural fatty acids the double bonds are usually cis configured, and it is the distance of the first double bond from the terminal end of the carbon chain that is important. They use "omega" to signify that the double bond is cis, and they are counting from the other end. The great advantage is that chain length can be ignored, and compounds that are subjected to the same biochemical processes are grouped together. In 1967, the IUPAC/IUB commission responsible for lipid nomenclature recommended that for unsaturated fatty acids with cis double bonds, that the "omega" symbol be replaced with "n-x", where n = the length of the carbon chain, and x is the distance from the terminal end. Some examples:- Common Chemical Chemical Biochemical Name Name d = delta o = omega Oleic cis-9-octadecenoic c-C18:1d9 C18:1o9 Elaidic trans-9-octadecenoic t-C18:1d9 - Ricinoleic D-(+)-12-hydroxy-octadec-cis-9-enoic c-C18:1d9-12OH - Linoleic cis-9,12-octadecadienoic c-C18:2d9 C18:2o6 alpha Linolenic cis-9,12,15-octadecatrienoic c-C18:3d9 C18:3o3 gamma Linolenic cis-6,9,12-octadecatrienoic c-C18:3d6 C18:3o6 Arachidonic cis-5,8,11,14-eicosatetraenoic c-C20:4d5 C20:4o6 EPA cis-5,8,11,14,17-eicosapentaenoic c-C20:5d5 C20:5o3 Erucic cis-13-docosenoic c-C22:1d13 C22:1o9 DHA cis-4,7,10,13,16,19-docosahexaenoic c-C22:6d4 C22:6o3 EPA and DHA are widely known as the omega-3 fatty acids present in high concentrations in marine lipids, and are considered beneficial in diet, although research is not complete [4,5]. 12.6 What is Conjugated Linoleic Acid? Conjugated linoleic acid describes the group of positional and geometric isomers of linoleic acid ( cis-9,12-octadecadienoic acid ) that have a conjugated double bond system starting at carbon 9, 10, or 11. They can be either cis or trans, or various combinations of them. The more abundant isomers in food are believed to be the cis-9, trans-11, and the trans-10, cis-12 isomers. It's very difficult to separate the cis-9, trans-11 and trans-9, cis-11 isomers, however the cis-9, trans-11 form is usually considered the important and usually dominant isomer. They are typically produced by bacteria in the rumen of ruminants because the hydrolysis of fats in the rumen produces more unesterified linoleic acid than is available to bacteria in other digestive systems. Plants also contain conjugated linoleic acid, but there is much less of the cis-9, trans-11 isomer, which is believed to be the biologically active isomer. Foods that contain CLA are lamb, beef, turkey and dairy fat products, ranging from 2.5 - 11 mg/g of fat - of which 75% or more is the cis-9, trans-11 ( or trans-9, cis-11 ) isomer. CLA is of interest because it has displayed antimutagenic activity in animals and human cell tests [6,7]. 12.7 What are "heavy" metals? There appears to be no standard definition, however the general consensus appears to be all metals with a density greater than 4 or 5 [8,9,10]. If you also consider the conventional analytical chemistry definition of "heavy metals" ( precipitation of sulfides from acidic solutions ), you obtain quite a diverse mixture of possible candidates. Moving the density limit from 4 to 5 really only just impacts on Ti, Y and Se. Some other texts use more complex definitions that may also include accepted "light" metals with densities less than 4, eg Hawley uses "A metal of atomic weight greater than sodium (22.9) that forms soaps on reaction with fatty acids. e.g., aluminum, lead, cobalt". The term " heavy-element " is commonly used to describe the transfermium elements, - elements with an atomic number greater than 100. 12.8 What is the difference between Molarity and Normality?. A Molar solution contains one gram molecular weight ( aka mole ) of the reagent in one litre of solution, and is represented by " M ". In modern usage, "molar" is intended to only mean " divided by amount of substance", and is not supposed to be used to describe 1M solutions. There are already exceptions to the rule ( molar conductivity, molar extinction coefficient ), so I would only worry about correct usage in exams, as in the real world most chemists use Molar to describe 1M solutions. A Molal solution is one gram molecular weight of the reagent in 1 kilogram of solvent, and is usually represented by "m". This concentration unit is relatively uncommon in the real world, so it's worth checking that the "m" is not a "M" typo. A Normal solution contains one gram equivalent weight ( aka equivalent ) of the reagent in one litre of solution, and is represented by " N ". The equivalent weight of a reagent may vary according to the reaction, but if considering just acid and base moles and equivalents, then:- 1M H2SO4 + 2M NaOH -> 2H2O + Na2SO4 1N H2SO4 + 1N NaOH -> H20 + 0.5Na2SO4 1N HCl + 1N NaOH -> H2O + NaCl So you can see that the equivalent weight of an acid is that which contains 1.0078 grams of replaceable hydrogen which, in the case of sulfuric acid, would be half the mole weight, but, in the case of hydrochloric acid, would be the mole weight. The equivalent weight of a base is that which contains one replaceable hydroxyl group ( ie 17.008g of ionisable hydroxyl ). Thus the equivalent weight of sodium hydroxide ( NaOH ) and potassium hydroxide ( KOH ) would be the mole weight, but for calcium hydroxide ( Ca(OH)2 ) it would be half the mole weight. The equivalent weight of an oxidising or reducing agent is that weight of the reagent that reacts with or contains 1.008 grams of available hydrogen or 8.000 grams of available oxygen. "Available" means being able to be utilised in oxidation or reduction reactions. The equivalent weight of an oxidising agent is determined by the change in oxidation number which the reduced element experiences, eg the reduction of potassium permanganate in dilute H2SO4 gives;- K Mn O4 --> Mn S O4 (Oxidation Number) +1 +7 -8 +2 +6 -8 This results in a change of the manganese from +7 to +2, so the equivalent weight is 1/5 of a mole. However, in neutral solution the change would only be 3 because the product is MnO2, giving an equivalent weight of 1/3 of a mole. If reacted in strongly alkaline solution the product is MnO4--, giving an equivalent weight of one mole. The equivalent weight of a reducing agent is determined by the change in oxidation number that the oxidised element undergoes. For the conversion of ferrous sulfate into ferric sulfate;- 2 (Fe SO4) --> Fe2 (SO4)3 (Oxidation Number) 2x(+2 -2 ) (+3)x2 (-2)x3 The change in oxidation number per atom of iron is 1, so the equivalent weight of ferrous sulfate is 1 mole. There are wide range of rules about the determination of the oxidation number, but if you have been taught to use molarity, I would not bother too much about normality, as it is mainly used these days by analytical chemists - because it is convenient for many common titrations. Analysts assume that 1 ml of 1N reagent will react with 1 ml of 1N reagent. However, there has been a recent Journal of Chemical Education article that claims using normality and equivalent weight does help students understand chemistry, but those concepts are unlikely to become widespread again [11]. 12.9 Where can I find the composition of common named reagents?. Often the best place to start are MSDS sheets or catalogues from commercial suppliers. Some textbooks include a list of named reagents and their composition that are mentioned in the text. The very common reagents are usually also detailed in Hawley or the Merck Index. One chemistry field that has a lot of named reagents is analytical chemistry, especially in Thin Layer Chromatography, where many of the spray detection reagents have common names. Merck produces a handy guide describing the composition and production of common TLC spray reagents [12]. Some common reagents include:- - aqua regia = 1 part nitric acid and 3 or 4 parts hydrochloric acid. - piranha solution = highly dangerous ( explodes on contact with traces of organics ), warm (65C), 70:30 mixture of 100% sulfuric acid and 30% hydrogen peroxide. It is used, with comprehensive safety precautions, in the semiconductor industry, and also in some laboratories to clean glassware [13,14,15]. Many chemical laboratories prohibit it, and there are much safer, equally effective, alternatives available - refer Section 16.7. User Contributions:Top Document: Sci.chem FAQ - Part 3 of 7 Previous Document: News Headers Next Document: 13. Illicit and Government Controlled Substances Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: B.Hamilton@irl.cri.nz
Last Update March 27 2014 @ 02:12 PM
|
Comment about this article, ask questions, or add new information about this topic: