02/07/2017. Isomerism. Structural isomerism. 1. Structural isomerism different linkages of atoms. Same molecular formula Different structural formulae

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1 hain isomerism Position isomerism Metamerism Tautomerism Functional group isomerism Geometrical isomerism Optical isomerism 02/07/2017 Isomerism The presence of two or more compounds which has the same molecular formula is known as Isomerism. Isomers ompounds having the same molecular formula but different linkages or spatial arrangements of atoms. Two main types of isomerism: 1. Structural isomerism 2. Stereoisomerism 1. Structural isomerism different linkages of atoms. Same molecular formula Different structural formulae 2. Stereoisomerism different spatial arrangements of atoms Same molecular formula & structural formulae Structural isomers Isomerism Differ in the way the atoms are connected to each other. Structural isomers having: o different IUPA names. o The same or different functional groups. o Different physical properties, so they are separable by physical techniques such as distillation. o Different chemical properties.they behave differently or give different products in chemical reaction. Structural isomerism Stereoisomerism 1

2 hain isomerism It occurs when carbon atoms are linked to the main chain in different ways hain isomerism Different physical properties b.p.: Straight-chain > branched-chain Reason: larger surface area and thus stronger Van der Waals Forces > hain isomers have similar chemical properties because they have the same functional groups. Position isomerism Same carbon skeleton Different positions of functional groups Butan-1-ol and butan-2-ol (molecular formula: 4 10 O) Functional group isomerism Isomerism where the compounds are different due to different arrangements of atoms leading to different functional groups. As functional groups are usually the reactive centre of a molecule this leads to entirely different properties. 2 6 O O O Alcohol Ether 2

3 Metamerism O 3 6 O O This form of isomerism is rare and is limited to molecules having a divalent atom like O or S and alkyl groups around it. The main examples come from ethers and thioethers. Methoxypropane and ethoxyethane ( 4 10 O) Aldehyde Ketone Tautomerism Occurs when functional group isomers are in dynamic equilibrium with each other. Structural requirements: 1. ompounds should have an electronegative atom which is bonded with multiple bond (i.e. O and N). 2. ompounds should have at least one acidic hydrogen present on -carbon of the molecule. O O ethenol ethanal enol keto tautomerism 3

4 Order of enolic contents O But-2-en-1-ol Not an enol Does not exhibit tautomerism Difference between Tautomerism and Resonance Tautomerism and resonance are two different phenomena and the differences are as follows: (i) In tautomerism, an atom changes its place but resonance involves a change of position of pi electrons or unshared electrons. (ii) Tautomers are different compounds and they can be separated by suitable methods but resonating structures cannot be separated as they are imaginary structures of the same compounds. (iii) Tautomers are in dynamic equilibrium but in resonance only one compound exists. (iv) Resonance in a molecule lowers the energy and thus stabilizes a compound and decreases its reactivity but no such effects occur in tautomerism. Ring-hain isomerism ere one isomer is an open chain molecule and the other a cyclic molecule. Propene is an alkene and cyclopropane an alkane, two different classes of compounds. 4

5 Stereoisomerism Occurs when compounds having the same structural formula show different spatial arrangements of atoms. Geometrical Isomerism Arises from restricted rotation about a = double bond. Geometrical isomerism - criteria a c b d where a b and c d cannot be inter-converted at lower temperatures E/Z notation If there are three or four different groups attached to the carbons of = double bond E/Z notation rather than the cis/trans notation is used to name the stereoisomers of a molecule. E : in opposition to trans Z : together cis 1 Optical isomers Optical isomers are molecules that differ three-dimensionally by the placement of substituents around one or more atoms in a molecule. butan-2-ol (O) Z 1 E 5

6 hiral ompound A compound containing a carbon atom with four different groups attached to it. It s mirror image will be a different compound (enantiomer). Stereocenter Stereocenter (or stereogenic atom): any atom at which the interchange of two groups gives a stereoisomer. hirality center (chiral carbon). Double-bonded carbon atoms in cis-trans isomers. The carbon is called chirality or chiral center or asymmetric center or stereocenter or stereogenic center ompounds with one chiral center will show optical activity. ompounds without chiral centers do not normally show optical activity. ompounds with more than one chiral center may or may not show optical activity depending on whether or not they are nonsuperimposable on their mirror image (chiral) or superimposable (achiral). Achiral ompound If any symmetry is present in the compound and it s mirror image can be superimposed on it then the compound is achiral or symmetric. 6

7 Symmetric, Asymmetric and Dissymmetric molecules Symmetric molecules : If any symmetry is present in the molecule then molecule will be symmetric molecule. Dissymmetric molecules: is the property of non superimposability on its mirror image, a dissymmetric molecule may have a simple axis of symmetry yet it will be optically active and exist as a pair of enantiomers. Polarized Light Ordinary (nonpolarized) light consists of many beams vibrating in different planes. Asymmetric molecules : Asymmetric is defined as the complete absence of symmetry and having at least one asymmetric carbon. Thus, while all asymmetric molecules are chiral, not all chiral molecules are asymmetric. Note: Both asymmetric and dissymmetric molecules are optically active. Plane-polarized light is composed of waves that vibrate in only one plane. Polarized Light Plane-polarized light is composed of waves that vibrate in only one plane. Optical activity Substances which can rotate the plane of polarised light are said to be optically active. Dextrorotatory (Latin: dextre means right) and is indicated by (+) sign. Dextrorotatory new older (+) d Rotates the plane of plane-polarized light to the right. 7

8 Optical activity Laevorotatory (Latin: laeves mean left) and is indicated by (-) sign. Levorotatory new (-) l older Rotates the plane of plane-polarized light to the left. Those substance which do not rotate the plane of polarised light are called optically inactive. lockwise Dextrorotatory (+) Polarimeter Polarimeter: Device that measures the optical rotation of the optically active substance. Not related to (R) and (S) ounterclockwise Levorotatory (-) Specific Rotation [ ] D It is defined as the number of degrees of rotation caused by a solution of 1.0 g of compound per ml of solution taken in a polarimeter tube 1.0 dm (10 cm) long at a specific temperature and wavelength. The specific rotation is calculated from observed angle of rotation, as below: Specific rotation calculated in this way is a physical property of an optically active substance. = observed rotation You always get the same value of c = concentration ( g/ml ) l = length of cell ( dm ) D = yellow light from sodium lamp t = temperature ( elsius ) Angle of rotation ( ) is the angle (degrees) by which the analyser is rotated to get maximum intensity of light. It depends upon: (i) (ii) Nature of the substance; oncentration of the solution in g/ml; (iii) Length of the polarimeter tube; (iv) of the incident monochromatic light (598nm). (v) Temperature of the sample. 8

9 hirality and symmetry A necessary and sufficient criterion for chirality is an absence of S n axes; the existence of any S n axis renders an object achiral. Simple axis of Symmetry ( n ): A molecule has a n fold simple axis of symmetry if it is rotated by an angle of /n around the axis, you obtain a same looking molecule o 2 entre of symmetry or inversion (i) or ( i ) A centre of symmetry (centre of inversion) is defined as a point within the molecule such that if an atom is joined to it by a straight line which if extrapolated to an equal distance beyond it in opposite direction meets an equivalent atom. OO 3 3 OO entre of symmetry 2,4-Dimethylcyclobutane -1,3- dicarboxylic acid has i Plane of symmetry (σ ): An imaginary mirror plane dividing the molecule into two halves such that one half is the mirror image of the other half. Alternating axis of symmetry (S n ) A molecule rotated around an axis by /n an image taken through a mirror placed perpendicular to the axis the mirror image looks identical with the original same

10 Enantiomers Enantiomers: stereoisomers that are nonsuperimposible mirror images; only properties that differ are direction (+ or -) of optical rotation O O O O Enantiomers (+) or (-)butan-2-ol Mirror images of each other Non-superimposable with each other Rotate plane-polarized light to the same extent but in opposite directions Enantiomers The direction of optical rotation cannot be predicted from the structural formulae. It can only be determined experimentally. Fischer mirror images Easy to draw, easy to find enantiomers 3 3 l l l 3 l 3 Enantiomers Diastereomers Diastereomers: stereoisomers that are not mirror images; different compounds with different physical properties Geometric isomers (cis-trans) Molecules with 2 or more chiral carbons. 3 3 cis-2-butene 3 3 trans-2-butene is-trans isomers are not mirror images, so these are diastereomers. 10

11 RULE - a compound can have a maximum of 2 n stereoisomers, where n = the number of asymmetric centers (not counting cis-trans isomeric centers) 2,3-dichloropentane 3 (l)(l) * * 2 3 n = = 4 stereoisomers I II III IV I & II are enantiomers; III & IV are enantiomers; I & III; I & IV are diastereomers; II & III; II & IV are diastereomers Properties of enantiomers and diastereomers Diastereomers have different physical properties: m.p., b.p. They can be separated easily. Enantiomers differ only in reaction with other chiral molecules and the direction in which polarized light is rotated. Enantiomers are difficult to separate. Racemic modification/mixtures A mixture containing equal amounts of a pair of enantiomers is called a racemic mixture. Equal quantities of d- and l- enantiomers. Notation: (d,l) or ( ) No optical activity. The mixture may have different b.p. and m.p. from the enantiomers! The clockwise rotation caused by the (+)isomer is cancelled by the anti-clockwise rotation caused by the (-)isomer external cancellation Racemic mixture (racemate) The (+)isomer and (-)isomer are different structures One serves as the impurity of the other in a racemic mixture. A racemic mixture has a lower m.p. than its components. (+)butan-2-ol (-)butan-2-ol ( )butan-2-ol m.p <

12 Meso compounds A compound with 2 or more asymmetric centers, and a plane of symmetry - cuts molecule in half so that one half of the molecule is the mirror image of the other Symmetry plane not drawn through asymmetric centers Note this plane of symmetry does not eliminate the presence of stereocenters (you still have sp 3 carbon atoms with four different atoms attached - you just have multiple stereocenters with the same four atoms!) onfiguration Relative configuration compares the arrangement of atoms in space of one compound with those of another (glyceraldehyde, amino acid). Until the 1950s, all configurations were relative. Absolute configuration is the precise arrangement of atoms in space. The configuration of carbohydrates, amino acids and some other compounds is commonly designated by the D,L system proposed by Emil Fischer in At that time, it was known that one enantiomer of glyceraldehyde has a specific rotation of ; the other has a specific rotation of Fischer proposed that these enantiomers be designated D and L (for dextro and levorotatory) but he had no experimental way to determine which enantiomer has which specific rotation. Fischer, assigned the dextrorotatory enantiomer an arbitrary configuration and named it D-glyceraldehyde. e named its enantiomer L-glyceraldehyde. Fischer could have been wrong, but by a stroke of good fortune he was correct, as proven in 1952 by a special application of X-ray crystallography. D- and L-glyceraldehyde serve as reference points for the assignment of relative configuration to all other aldoses and ketoses. Fischer-Rosanoff convention Before 1951, only relative configurations could be known. Sugars and amino acids with same relative configuration as (+)-glyceraldehyde were assigned D and same as (-)-glyceraldehyde were assigned L. With X-ray crystallography, now known absolute configurations: D is (R) and L is (S). No relationship to dextro- or levorotatory. 12

13 D and L assignments for carbohydrates In carbohydrates, in general, the O group attached to the penultimate carbon atom in the chain determines the assignment of D or L. O group attached to the penultimate carbon is on the RS, in which the O group appears at the top, the configuration is defined as D If O group attached to LS, it is defined as L O * O 2 O D-(+)-glyceraldehyde R-(+)-glyceraldehyde O * O O O O 2 O D-(+)-glucose D and L assignments for amino acids In case of amino acids, the N 2 group is on the LS of the Fischer projection in which the carboxyl group appears at the top, the configuration is L onversely, If N 2 group is on the RS it is defined as D 2 N * OO 2 O L-(-) serine S-(-) serine 2 N * OO 2 2 OO L-(+)-glutamic acid Amino acid with two asymmetric carbons No relationship between D/L and dextrorotatory (+)/levorotatory (-) onfiguration of the chiral carbon is unchanged, but direction of the rotation is changed 13

14 D, L system Mainly used for amino acids and carbohydrates Fischer projection orizontal lines: bonds coming out of the plane of the paper Vertical lines: bonds projecting behind the plane of the paper The most oxidized group: top 2 O (carbohydrates) or R (amino acids): bottom D: dextro, right L: levo, left No direct relationship between the R/S and D/L and the sign of optical rotation of the molecule. 14