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Isomerism
Optical ActivityOptical activity describes the phenomenon by which chiral molecules are observed to rotate polarized light in either a clockwise or counterclockwise direction. This rotation is a result of the properties inherent in the interaction between light and the individual molecules through which it passes. Material that is either achiral or equal mixtures of each chiral configuration (called a racemic mixture) do not rotate polarized light, but when a majority of a substance has a certain chiral configuration the plane can be rotated in either direction.What Is Plane Polarized Light?Polarized light consists of waves of electromagnetic energy in the visible light spectrum where all of the waves are oscillating in the same direction simultaneously. Put simply, imagine a ray of light as a water wave, with crests and peaks. All the peaks of a water wave point in the same direction (up, against gravity) pretty much at the same time. Light is not usually this way - it's peaks and troughs are often in random array, so one ray of lights peaks might point in a direction 90o opposite of another ray. When all of the rays have their peaks pointing in the same direction - like all the waves in the ocean have peaks pointing up - then those rays of light are said to be polarized to one another.Why Polarized Light Is AffectedSo why do chiral molecules affect only polarized light, and not unpolarized? Well, they do affect unpolarized light, but since the rays have no particular orientation to one another, the effect can not be observed or measured. We observe the polarized light rays being rotated because we knew their orientation before passing through the chiral substance, and so we can measure the degree of change afterwards.What happens is this; when light passes through matter, e.g. a solution containing either chiral or achiral molecules, the light is actually interacting with each molecule's electron cloud, and these very interactions can result in the rotation of the plane of oscillation for a ray of light. The direction and magnitude of rotation depends on the nature of the electron cloud, so it stands to reason that two identical molecules possessing identical electron clouds will rotate light in the exact same manner. This is why achiral molecules do not exhibit optical activity.In a chiral solution that is not a racemic mixture, however, the chiral molecules present in greater numbers are configurationally equivalent to each other, and therefore each possesses identical electron clouds to its molecular twins. As such, each interaction between light and one of these 'majority' molecule's electron clouds will result in rotations of identical magnitude and direction. When these billions of billions of interactions are summed together into one cohesive number, they do not cancel one another as racemic and achiral solutions tend to do - rather, the chiral solution as a whole is observed to rotate polarized light in one particular direction due to its molecular properties.EnantiomersIt is just such specificity that accounts for the optical isomerism of enantiomeric compounds. Enantiomers possess identical chemical structures (i.e. their atoms are the same and connected in the same order), but are mirror images of one another. Therefore, their electron clouds are also identical but actually mirror images of one another and not superimposable. For this reason, enantiomeric pairs rotate light by the same magnitude (number of degrees), but they each rotate plane polarized light in opposite directions. If one chiral version has the property of rotating polarized light to the right (clockwise), it only makes sense that the molecule's chiral mirror image would rotate light to the left (counterclockwise).Equal amounts of each enantiomer results in no rotation. Mixtures of this type are called racemic mixtures, and they behave much as achiral molecules do.HistoryVia a magneto-optic effect, the (-)-form of an optical isomer rotates the plane of polarization of a beam of polarized light that passes through a quantity of the material in solution counterclockwise , the (+)-form clockwise. It is due to this property that it was discovered and from which it derives the name optical activity. The property was first observed by J.-B. Biot in 1815, and gained considerable importance in the sugar industry, analytical chemistry, and pharmaceuticals.Louis Pasteur deduced in 1848 that the handedness of molecular structure is responsible for optical activity. He sorted the chiral crystals of tartaric acid salts into left-handed and right-handed forms, and discovered that the solutions showed equal and opposite optical activity.Artificial composite materials displaying the analog of optical activity but in the microwave regime were introduced by J.C. Bose in 1898, and gained considerable attention from the mid-1980s.
Posted by KVSSNrao at 6:24 AM 0 comments
Labels: Isomerism, organic chemistry
Saturday, December 22, 2007
Tips to Tame the IIT-JEE
The primary emphasis is to be on depth of knowledge, analytical and comprehension skills and attitude. If you are a class 11th/12th student, try to synergise your school study with the JEE study. While doing a new topic, always do it first from your school textbook followed by higher level books. Self Study Plan: School Students: A student going to the school should follow the 4/10 plan, that is self-study for at least 4 hrs. on school days and at least 10 hrs. on holidays. There must be a 7 day or 10 day revision plan as well. 12th Pass Students: A student not going to the school should follow the 10 hrs. plan, that is self-study for at least 10 hrs. everyday. There must be a 7 day or 10 day revision plan as well. It is more important to do a question completely rather than trying to do more half-done questions.Note that too much of test taking does not help. Only a deep understanding and other personal attributes can get you through JEE and not blind test taking. The frequency of test taking may be less for JEE 2007 aspirants in class 11th but needs to be higher when they reach class 12th. Do approximations in calculations keeping an eye on the error.In objective type of questions, method of elimination of options may work to your advantage in many questions. Using dimensional analysis, putting boundary conditions, putting values of variables, working backwards and many more techniques may work in such scenario. http://iitjee-aieee-cbse.blogspot.com/2007/11/12-points-to-tame-new-pattern-iit-jee.html
Posted by KVSSNrao at 6:25 PM 1 comments
Labels: Study-tips
Monday, December 17, 2007
Another good website
Chemistry lessons VideosChemistry E booksChemistry Lecture noteshttp://http://www.learnerstv.com/http://www.learnerstv.com/lecturenotes/lecturenotes.php?note=15&cat=Chemistry
Posted by KVSSNrao at 11:28 PM 0 comments
Sunday, December 16, 2007
IIT JEE Preparation Online Material
I am putting in effort to put in useful material as number of persons are visiting the blog and visiting number of pages.I hope they are getting an additional version on the topic apart from their regular institute faculty, coaching instiute faculty, the books they are referring. In my personal study, I realise that reading a variety of books is necessary to clarify certain concepts.I hope persons who are visiting the blog are getting similar benefit. Some concept is now more clear, as it is explained in a slightly different manner.I started a blog to develop chemistry glossaryhttp://www.chemgloss.blogspot.com/
Posted by KVSSNrao at 11:23 PM 0 comments
Labels: Blog-Status
IIT JEE Chemistry Ch.16A. Coordination Compounds
See for a set of questions on this topic the posthttp://iit-jee-chemistry-ps.blogspot.com/2007/10/iit-jee-chemistry-questions.html----------------------SyllabusNomenclature of mononuclear coordination compounds, XII 10.1,2,3 Cis-trans and ionisation isomerisms, 10.4Hybridization and geometries of mononuclear coordination compounds (linear, tetrahedral, square planar and octahedral).10.7The topics are covered in detail Jauhar's XII book. The section numbers are given beside the topic.-----------------------Coordination compounds are a special class of compounds in which the centgral metal atom is surrounded by ions or molecules beyond their valency.There are also referred to as coordination complexes or complexes.Haemoglobin, Chlrophyll, and vitamin B-12 are coordinatio compounds of iron, magnesium and cobalt respectively.The interesting thing of coordination compound is that these are formed from apparently saturated molecules capable of independent existence.for example, when acqueous ammonia is addedt o green solution of nickel chloride, NiCl2, the colour changes to purple. The ni^2+ ions almost diappear from the solution. The solution on evaporation yields purple crystals corresponding to the formula [Ni(NH-3)-6]Cl-2. such a compound is called coordinatin compound. When this compound is now dissolved in water, there is hardly any evidence of Ni^2+ ions or NH-3 molecules. It ionizes to give a new species [Ni(NH-3)-6]^2+. the species in the square brackets does not ionise further. It remains as a single entity as an ion. This is the unique feature of coordination compounds.---------------Nomenclature rulesFrom http://www.iupac.org/publications/books/principles/principles_of_nomenclature.pdfIUPAC booklet available for download at the above page id.A coordination entity is composed of a central atom or atoms to which are attachedother atoms or groups of atoms, which are termed ligands. A central atom occupies acentral position within the coordination entity. The ligands attached to a centralatom define a coordination polyhedron. Each ligand is assumed to be at the vertex ofan appropriate polyhedron. The usual polyhedra are shown in Table 3.3 and they arealso listed in Table 4.4. Note that these are adequate to describe most simplecoordination compounds, but that real molecules do not always fall into these simplecategories. In the presentation of a coordination polyhedron graphically, the linesdefining the polyhedron edges are not indicative of bonds.However, many ligands do not behave as donors of a single electron pair. Someligands donate two or more electron pairs to the same central atom from differentdonor atoms. Such ligands are said to be chelating ligands, and they form chelaterings, closed by the central atom. The phenomenon is termed chelation.The number of electron pairs donated by a single ligand to a specific central atomis termed the denticity. Ligands that donate one pair are monodentate, those thatdonate two are didentate, those that donate three are tridentate, and so on.Sometimes ligands with two or more potential donor sites bond to two (or more)different central atoms rather than to one, forming a bridge between central atoms. Itmay not be necessary for the ligand in such a system to be like ethane-l ,2-diamine,with two distinct potential donor atoms. A donor atom with two or more pairs ofnon-bonding electrons in its valence shell can also donate them to different centralatoms. Such ligands, of whatever type, are called bridging ligands. They bond to twoor more central atoms simultaneously. The number of central atoms in a singlecoordination entity is denoted by the nuclearity: mononuclear, dinuclear, trinuclear,etc. Atoms that can bridge include 5, 0 and Cl.The original concepts of metal—ligand bonding were essentially related to thedative covalent bond; the development of organometallic chemistry has revealed afurther way in which ligands can supply more than one electron pair to a centralatom. This is exemplified by the classical cases of bis(benzene)chromium andbis(cyclopentadienyl)iron, trivial name ferrocene. These molecules are characterisedby the bonding of a formally unsaturated system (in the organic chemistry sense, butexpanded to include aromatic systems) to a central atom, usually a metal atom.4.4.3 Mononuclear coordination compounds4.4.3.1 Formulae. The central atom is listed first. The formally anionic ligands appear next,listed in alphabetical order of the first symbols of their individual formulae. The neutral ligands follow, also in alphabetical order. Polydentate ligands are included in alphabetical order, the formula to be presented as discussed in Chapter 3. The formula for the entire coordination entity, whether charged or not, is enclosed in square brackets. For coordination formulae, the nesting order of enclosing marks is as given on p. 13. The charge on an ion is indicated in the usual way by use of a right superscript. Oxidation states of particular atoms are indicated by an appropriate roman numeral as a right superscript to the symbol of the atom in question, and not in parentheses on the line. In the formula of a salt containing coordination entities, cation always precedes anion, no charges are indicated and there is no space between the formulae for cation and anion.Examples1. [Co(NH3)6]Cl 2. [PtC14]23. [CoC1(NH3)5]C1 6. [Cr"(NCS)4(NH3)2]4. Na[PtBrC1(N02)(NH3)J 7. [Fe"(CO)4]25. [CaC12{OC(NH2)2}2]The precise form of a formula should be dictated by the needs of the user.The precise form of a formula should be dictated by the needs of the user. Forexample, it is generally recommended that a ligand formula within a coordinationformula be written so that the donor atom comes first, e.g. [TiCl3(NCMe)3], but thisis not mandatory and should not affect the recommended order ofligand citation. Itmay also be impossible to put all the donor atoms first, e.g. where two donors arepresent in a chelate complex: [Co(NH2CH2CH2NH2)3]3. Whether the ethane-l,2-diamine is displayed as shown, or simply aggregated as [Co(C2H8N2)3]3t is a matterof choice. Certainly there is a conflict between this last form and the suggestion thatthe donor atoms be written first. The aim should always be clarity, at the expense ofrigid adherence to recommendations.It is often inconvenient to represent all the ligand formulae in detail. Abbreviationsare often used and are indeed encouraged, with certain provisos. These are: theabbreviations should all be written in lower case (with minor exceptions, such as Me,Et and Ph) and preferably not more than four letters; with certain exceptions of widecurrency, abbreviations should be defined in a text when they first appear; in aformula, the abbreviation should be enclosed in parentheses, and its place in thecitation sequence should be determined by its formula, as discussed above; andparticular attention should be paid to the loss of hydrons from a ligand precursor.This last proviso is exemplified as follows. Ethylenediaminetetraacetic acidshould be rendered H4edta. The ions derived from it, which are often ligands incoordination entities, are then (H3edta), (H2edta)2, (Hedta)3 and (edta)4. Thisavoids monstrosities such as edta-H2 and edtaH_2 which arise if the parent acid isrepresented as edta. A list of recommended abbreviations is presented in Table 4.5.4.4.3.2 Names. The addition of ligands to a central atom is paralleled in name construction.The names of the ligands are added to that of the central atom. The ligands are listedin alphabetical order regardless of ligand type. Numerical prefixes are ignored in thisordering procedure, unless they are part of the ligand name. Charge number andoxidation number are used as necessary in the usual way.Of the two kinds of numerical prefix (see Table 4.2), the simple di-, tn-, tetra-, etc.are generally recommended. The prefixes bis-, tris-, tetrakis-, etc. are to be used onlywith more complex expressions and to avoid ambiguity. They normally requireparentheses around the name they qualify. The nesting order of enclosing marks is ascited on p. 13. There is normally no elision in instances such as tetraammine and thetwo adjacent letters 'a' are pronounced separately.The names of ligands recommended for general purposes are given in Table 4.6.The names for anionic ligands end in -o. If the anion name ends in -ite, -ate or -ide, the ligand name is changed to -ito, -ato or -ido. The halogenido names are, bycustom, abbreviated to halo. Note that hydrogen as a ligand is always regarded asanionic, with the name hydride. The names of neutral and cationic ligands are nevermodified. Water and ammonia molecules as ligands take the names aqua andammine, respectively. Parentheses are always placed around ligand names, whichthemselves contain multiplicative prefixes, and are also used to ensure clarity, butaqua, ammine, carbonyl (CO) and nitrosyl (NO) do not require them.The names of all cationic and neutral entities end in the name of the element,together with the charge (if appropriate) or the oxidation state (if desired). Thenames of complex anions require modification, and this is achieved by adding thetermination -ate. All these recommendations are illustrated in the following examples.----------------Examples1. Dichloro(diphenylphosphine)(thiourea)platinum(ii)2. K4[Fe(CN)6]3. [Co(NH3)6]C134. [CoCl(NH3)5}Cl25. [CoC1(N02)(NH3)4]Cl6. [PtCl(NH2CH3)(NH3)2]Cl7. [CuC12{OC(NH2)2}2]8. K2[PdC14]9. K[OsCl5N]10. Na[PtBrC1(N02)(NH3)]11. [Fe(CNCH3)6]Br212. [Ru(HSO3)2(NH3)4]13. [Co(H20)2(NH3)4]C1314. [PtC12(C5H5N)(NH3)]15. Ba[BrF4]216. K[CrF4O]17. [Ni(H20)2(NH3)4]S04potassium hexacyanoferrate(ii)potassium hexacyanoferrate(4—)tetrapotassium hexacyanoferratehexaamminecobalt(iii) chloridepentaamminechlorocobalt(2+) chloridetetraamminechloronitrito-N-cobalt(iii) chloridediamminechloro(methylamine)platinum(ii)chloridedichlorobis(urea)copper(ii)potassium tetrachloropalladate(ii)potassium pentachloronitridoosmate(2—)sodium amminebromochloronitrito-N-platinate( 1—)hexakis(methyl isocyanide)iron(ii) bromidetetraamminebis(hydrogensulfito)ruthenium(ii)tetraamminediaquacobalt(iii) chlorideamminedichloro(pyridine)platinum(ii)barium tetrafluorobromate(iii)potassium tetrafiuorooxochromate(v)tetraamminediaquanickel(ii) sulfate***Table to be reformatted-----------------------------Designation of donor atom. In some cases, it may not be evident which atom in aligand is the donor. This is exemplified by the nitrito ligand in Examples 5 and 10,p. 59. This can conceivably bind through an 0 or N atom. In simple cases, the donoratom can be indicated by italicised element symbols placed after the specific ligandname and separated from it by a hyphen, as demonstrated in those particularexamples. More complex examples will be dealt with below. With polydentateligands, this device may still be serviceable. Thus, dithiooxalate ion may be attached through S or 0, and formulations such as dithiooxalato-S,S' and dithiooxalato-0,0'should suffice. It could be necessary to use superscripts to the donor atom symbols if these need to be distinguished because there is more than one atom of the same kind to choose from.Complicated examples are more easily dealt with using the kappa convention,and this is particularly useful where a donor atom is part of a group that does notcarry a locant according to organic rules. The two oxygen atoms in a carboxylatogroup demonstrate this. The designator i is a locant placed after that portion of theligand name that denotes the particular function in which the ligating atom is found.The ligating atoms are represented by superscript numerals, letters or primes affixedto the donor element symbols, which follow i without a space. A right superscript toi denotes the number of identically bound ligating atoms.Inclusion of structural information. The names described so far detail ligands andcentral atoms, but give no information on stereochemistry. The coordination numberand shape of the coordination polyhedron may be denoted, if desired, by apolyhedral symbol. These are listed in Table 4.4. Such a symbol is used as an affix inparentheses, and immediately precedes the name, separated from it by a hyphen.This device is not often used.Geometrical descriptors, such as cis, trans, mer (from meridional) and fac (fromfacial), have found wide usage in coordination nomenclature. The meaning isunequivocal only in simple cases, particularly square planar for the first two andoctahedral for the othersMore complex devices have been developed that are capable of dealing with allcases. The reader is referred to the Nomenclature of Inorganic Chemistry, Chapter10. ------------------------Ionisation IsomersIonisation isomers: Molecular structural formula is same. But different isomers give different ions in solution.one isomer [PtBr(NH3)3]NO2 -> gives NO2- anions in solution another isomer [Pt(NH3)3(NO2)]Br -> gives Br- anions in solution Notice that both anions are necessary to balance the charge of the complex, and that they differ in that one ion is directly attached to the central metal but the other is not. Geometric Isomers or Cis-Trans isomersGeometric isomers are two or more coordination compounds which contain the same number and types of atoms, and bonds (i.e., the connectivity between atoms is the same), but which have different spatial arrangements of the atoms.Not all coordination compounds have geometric isomers.For example, in the square planar molecule, Pt(NH3)2Cl2, the two ammonia ligands (or the two chloride ligands) can be adjacent to one another or opposite one another.Note that these two structures contain the same number and kinds of atoms and bonds but are non-superimposable. The isomer in which like ligands are adjacent to one another is called the cis isomer. The isomer in which like ligands are opposite one another is called the trans isomer.For the common structures which contain two or more different ligands, geometric isomers are possible only with square planar and octahedral structures (i.e., geometric isomers cannot exist for linear and tetrahedral structures).cis-[Co(NH3)4Cl2]+Note that the two chloride ligands are adjacent to one another in this octahedral complex ion. In aqueous solution, this complex ion has a violet color.trans-[Co(NH3)4Cl2]+Note that the two chloride ligands are opposite one another in this complex ion. In aqueous solution, this complex ion has a green color.------------------Material about all isomers - In syllabus only ionic and cis-trans are specially mentionedhttp://www.chem.purdue.edu/gchelp/cchem/whatis2.htmlCoordination IsomersCoordination isomers are two or more coordination compounds in which the composition within the coordination sphere (i.e., the metal atom plus the ligands that are bonded to it) is different (i.e., the connectivity between atoms is different).Not all coordination compounds have coordination isomers.Coordination isomers have different physical and chemical properties.Example[Cr(NH3)5(OSO3)]BrNote that the sulfate group is bonded to the Cr atom (via an O atom) and is within the coordination sphere. Note also the octahedral structure. The bromide counterion is needed to maintain charge neutrality with the complex ion (i.e., [Cr(NH3)5(OSO3)]+) and is not shown in the structure. [Cr(NH3)5Br]SO4Note that the bromine atom is bonded to the Cr atom and is within the coordination sphere. Note also the octahedral structure. The sulfate counterion is not shown in the structure.Linkage IsomersLinkage isomers are two or more coordination compounds in which the donor atom of at least one of the ligands is different (i.e., the connectivity between atoms is different).This type of isomerism can only exist when the compound contains a ligand that can bond to the metal atom in two (or more) different ways. Some ligands that can form linkage isomers are shown below.Not all coordination compounds have linkage isomers.Linkage isomers have different physical and chemical properties.[Co(NH3)4(NO2)Cl]+Note that the N atom of the nitrite group is bonded to the Co atom. The nitrite group is written as "NO2" in the molecular formula (rather than "ONO") with the N atom nearest to the Co symbol to indicate that the N atom (rather than an O atom) is the donor atom. Note also the octahedral structure.[Co(NH3)4(ONO)Cl]+Note that one of the O atoms of the nitrite group is bonded to the Co atom. The nitrite group is written as "ONO" in the molecular formula (rather than "NO2") with the O atom nearest to the Co symbol to indicate that the O atom is the donor atom. Note also the octahedral structure.Geometric IsomersGeometric isomers are two or more coordination compounds which contain the same number and types of atoms, and bonds (i.e., the connectivity between atoms is the same), but which have different spatial arrangements of the atoms.Not all coordination compounds have geometric isomers.For example, in the square planar molecule, Pt(NH3)2Cl2, the two ammonia ligands (or the two chloride ligands) can be adjacent to one another or opposite one another.Note that these two structures contain the same number and kinds of atoms and bonds but are non-superimposable. The isomer in which like ligands are adjacent to one another is called the cis isomer. The isomer in which like ligands are opposite one another is called the trans isomer.For the common structures which contain two or more different ligands, geometric isomers are possible only with square planar and octahedral structures (i.e., geometric isomers cannot exist for linear and tetrahedral structures).cis-[Co(NH3)4Cl2]+Note that the two chloride ligands are adjacent to one another in this octahedral complex ion. In aqueous solution, this complex ion has a violet color.trans-[Co(NH3)4Cl2]+Note that the two chloride ligands are opposite one another in this complex ion. In aqueous solution, this complex ion has a green color.Optical IsomersOptical isomers are two compounds which contain the same number and kinds of atoms, and bonds (i.e., the connectivity between atoms is the same), and different spatial arrangements of the atoms, but which have non-superimposable mirror images. Each non-superimposable mirror image structure is called an enantiomer. Molecules or ions that exist as optical isomers are called chiral.Not all coordination compounds have optical isomers.The Two Enantiomers of CHBrClFNote that the molecule on the right is the reflection of the molecule on the left (through the mirror plane indicated by the black vertical line). These two structures are non-superimposable and are, therefore, different compounds. Pure samples of enantiomers have identical physical properties (e.g., boiling point, density, freezing point). Chiral molecules and ions have different chemical properties only when they are in chiral environments.Optical isomers get their name because the plane of plane-polarized light that is passed through a sample of a pure enantiomer is rotated. The plane is rotated in the opposite direction but with the same magnitude when plane-polarized light is passed through a pure sample containing the other enantiomer of a pair.---------------------web siteshttp://www.chem.purdue.edu/gchelp/cchem/whatis2.html
Posted by KVSSNrao at 12:44 AM 0 comments
Labels: inorganic chemistry
Thursday, December 13, 2007
Tutorials Organic Chemistry
On Prentice Hall site, there are tutorials for organic Chemistry for Wades bookhttp://wps.prenhall.com/esm_organic_wade_5/5/1361/348631.cw/index.html
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