Isomerism Notes PDF

Isomerism Isomerism Definition Isomers have same molecular formula but differen...

Isomerism Isomerism Definition Isomers have same molecular formula but different structures or configurations of compounds. Isomerism is resultant Concept Ladder factor of thermodynamic system “entropy”. y Isomers have same molecular formula but different physical and chemical properties. y Compounds having same molecular formula, but different connectivity, different bonding pattern (structural isomerism) or different 3-D relative arrangement (stereoisomerism) is known Previous Year’s Questions as isomers and this phenomenon is known as isomerism. Isomers of a substance must have y Isomers are those which can be separated at the same room temperature. [NEET-1991] y Isomers always have same Degree of Unsaturation (1) structural formula (DU) (2) physical properties y Isomers are formed to satellite a system. (3) chemical properties y Connectivity change (bonding pattern) structural (4) molecular formula isomerism. Isomerism 1.  y C2H6O (DU = 0) −δ +δ Rack your Brain (1) Alcohol CH3 − CH2 − O− H (2) Ether CH3 − O − CH3 How molecular structures y Both compounds have different connectivity so affect the value of its entropy? both are structural isomers. y Que Both are stereo isomers. Isomerism 2.  Definition STRUCTURAL ISOMERISM Ex. Same molecular fomula but different connectivity of atoms is known as structural isomers. I. Chain Isomerism (1) Degree of Unsaturation Concept Ladder (DU) also known as Double Bond Equivalent (DBE). (H+ X− N) DU = (C+ 1) − 2 (2) Definition Compounds having same MF (3) and same functional group but different P.C.C. or side chain is called chain isomers. Rack your Brain (4) How many minimum C-atoms are required to give chain isomers for Alcohol? (5) Previous Year’s Questions The isomer of ethyl alcohol is : [AIPMT] (1) diethyl ether (2) dimethyl ether Isomerism (3) acetaldehyde (4) acetone 3.  NO. OF CARBON S.NO. HYDROCARBON ISOMER 1 ISOMER 2 ATOMS 1. Alkane 4 2. Alkene 4 3. Alkyne 5 4. Alkediene 5 5. Alkenyne 5 6. Alcohol 4 II. Position Isomers Compounds having same M.F., same figure same P.C.C. or side chain but different position of Definition functional group, multiple bonds & substituent. Compounds have same M.F. but difference in position of fucntion group, multiple bond and (1) substituent. Both are Position Isomers Isomerism 4. (2) (3) (A) Alkene — 4 Definition Compounds have same M.F. but (B) Alkyne — 4 difference in functional group is knwon as functional isomers. (C) Alkane — 6 (D) Cyclopropene III. Functional isomerism 1. Acid and ester 2. Cyanide and isocyanide Isomerism 5.  3. Aldehyde and ketone 4. Keto and enol form 5. Nitro and nitrile 6. Alcohol & phenolic compounds 7. Alcohol & ether CH3—CH2—OH and CH3—O—CH3 8. 1°, 2° and 3° Amines Rack your Brain Examples : What are the Functional Isomers of monocarboxylic acids? (1) Previous Year’s Questions Functional isomerism is showsn by [AIPMT] (1) o-Nitrophenol and p-nitrophenol (2) Dimethyl ether and ethanol (3) 2-Pentenoic acid and 3-pentenoic (2) acid (4) Acetaldehyde and acetone Isomerism 6. Que. How many minimum carbon atom required to give position isomer? Sol.  Definition Compounds having same M.F. but different mode of linking. IV. Ring-chain Isomerism Que. C4H6 → (DU=2) All ring chain isomers are I, II → F.g Concept Ladder C—C≡C—C I functional group isomers C=C—C=C II I, III → F.g, RCI but vice-versa not true. III I, IV → RCI, FGI, In case of ring chain and functional group isomers, ring chain isomer will be prior. IV II, IV → RCI, FGI V. Metamers Compound having same M.F. but different alkyl group attached at polyvalent functional group. Isomerism 7.  (1) (2) Metamers-X (3) Identical Homomers (Isomerism) (4) CH3—CH2—CH2—O—CH3 CH3—CH2—O—CH2—CH3 Inter-unit change   I II   Metamers (5) Rack your Brain Find out the number of Metamers of C4H10O? Previous Year’s Questions Which of the following ether show metamerism [AIPMT] (1) CH3–CO–C3H7 (2) C2H5–O–C2H5 (3) CH3–O–CH3 (4) CH3–O–C2H5 Isomerism 8. Isomers formation probability (I) Open chain hydrocarbon 1. 1C →1 [C] 2. 2C →1 [C—C] 3. 3C →1 [C—C—C] 4. 4C →2 5. 5C →3 6. 6C →5 7. 7C →9 (II) Closed chain hydrocarbon 1C →× 2C →× 3C →1 → 4C →2 → 5C →5 → Ex. How many structural isomers are possible with following molecular formula ? (1) C5H10 Sol. DU–1, Open–5, Closed–5 (2) C6H10 (No. of terminal alkynes) Sol. DU–2, Open–4 Isomerism 9.  (3) C7H8O (No. of benzoid isomer) Sol. DU–4 (4) C7H9N (No. of benzoid isomer) Sol. DU–4 (5) C6H6 (No. of monochloro product) Previous Year’s Questions Sol. The number of isomers in C4H10O will be (6) Napthlene (No. of monochloro product) [NEET-1996] (1) 7 (2) 8 (3) 5 (4) 6 Sol. (7) No. of monochloro product in anthracene Sol. (8) Rack your Brain Find out the number of Stuctural isomers of C2H2FClBrI? Isomerism 10.  Previous Year’s Questions Sol. Number of isomers of C5H6 [AIIMS-2012] (1) 2 (2) 3 (3) 4 (4) 5 (9) C4H11N → No. of 1°, 2° and 3° amine. Sol. (1°amine) (2° amine) C—C—C—NH—C, (3° amine) Rack your Brain (10) C2FClBrI (No. of structural isomerism) Sol. DU = 1, Find out the number of Stuctural isomers (Acid and Ester) of C4H8O2? (11) C3H9N (No. of 1°, 2°, 3° amine) Sol. (1° amine) C—C—C—N (2° amine) C—C—NH—C Previous Year’s Questions (3º amine) The molecular formula of diphenylmethane, is C13H12. How many structural isomers are possible when one of the hydrogen is replaced by a chlorine atom? [NEET-2004] (1) 6 (2) 4 (3) 8 (4) 7 Isomerism 11.  Important Point CH3— C2H5— C3H7— C4H9— C5H11— 1 1 2 4 8 Free valency —X (F, Cl, Br, I), —NH2, —OH etc. Rack your Brain Ex. C5H11Cl → 8 C5H13N → 8 (1° Amine) Find out the number of Stuctural C4H9Cl → 4 isomers of C6H13Cl? Stereo Isomerism 1. Conformational Isomerism y Conformers also known as rotamers, and this Definition isomerism is known as rotational isomerism. y Study of conformers based on P.E. is known as A number of stereoisomers conformational analysis at room temperature which are formed due to free that’s why conformations interconvertible rotation around a single bond to each other at room temperature and not is known as conformations and separated and called as pseudo isomers. this isomerism is known as conformational isomerism. Representation of 3-D molecules 1. Newman’s projection formula : (Front view/Back view) y Front view projection of 3D molecules is known as Newman’s projection formula. Isomerism y C—C bond does not represent in this projection formula. 12. y 2. Saw-horse projection formula : (Side view projection of 3D molecules) Stability factor of conformers Types of strain for Acyclic Concept Ladder compound : (1) Steric strain (SS) (2) Torsional strain (TS) Rack your Brain Bond angle : The angle between 2 bonds of a same atoms lying in same plane. What is the effect of Dihedral angle : The angle between 2 bonds of conformational isomers on bond adjacent atoms. angle and bond length? Conformers of ethane : Previous Year’s Questions Dihedral angle in staggered from of ethane is [AIPMT] (1) 0° (2) 120° (3) 60° (4) 180° Isomerism 13.  → Maximum steric strain and → Maximum steric strain and torsional strain. torsional strain. → Maximum inter-electronic → Minimum inter-electron repulsion. repulsion. → Maximum inter-electron energy. → Minimum inter-electron energy. → Minimum stability. → Maximum stability. NOTE : Energy order ⇒ Eclipsed > Skew > Staggered Stability order ⇒ Staggered > Eclipsed Skew form : Infinite conformations in between eclipsed and staggered (anti) form is known as skew forms. Rack your Brain Which conformation has highest Conformers of propane : potential energy? Previous Year’s Questions NOTE : Energy orders ⇒ Eclipsed > Skew > Staggered The isomers, which can be Stability order ⇒ Staggered > Skew > Eclipsed. converted into another form by Conformers of Butane : rotation of the molecules around single bond, are [AIIMS] (1) conformers (2) enantiomers (3) diastereomers Isomerism (4) geometrical isomers 14.  Conformational Analysis : (1) Torsional strain : (2) Steric strain : Energy order ⇒ Fully eclipsed > Partially eclipsed > Gauche > Anti Stability order ⇒ Anti > Gauche > Partially eclipsed > Fully eclipsed Conformers : Out of infinite conformations which are formed at minimum possible energy profile curve, are conformers. y All conformers are conformations but vice-versa not true. (Stable form) y Conformations of ethane = ∞ Rack your Brain Conformer of ethane = 1 (Staggered) How many conformers of butane Stability : are optically active? Vanderwal repulsion (S.S. and T.S.) Staggered (more stable) > Eclipsed Gauche form may be more stable than Anti/ staggered form if following factors are present in Gauche form. Due to hydrogen bonding Concept Ladder (1) Hydrogen bonding and electrostatic (2) Electrostatic attraction attraction, Gauche form of butane is more stable than Anti form/staggered. Isomerism 15.  Ex. (1) In case of eclipsed form repulsion is more (H-bonding is not eclipsed). (2) (3) (4) (5) Isomerism 16.  Conformers of cyclic ring (Cycloalkane) : Real bond angle in propane → 109° 28’ Bond angle in cyclopropane → 60° Angle strain → 49° 28’ Relationship between p-character and bond Rack your Brain angle : What are the consequences of p 1 %p ∝ and s-character on bond angle? Bond angle Types of strain for cyclic Concept Ladder compound: (1) Steric strain (SS) y (2) Torsional strain (TS) High angle strain, planar molecule. (3) Angle strain (AS) y No flexibility. y Cyclopropane does not form conformers. (1) Cyclopentane → Forms envelop at room temperature. (appears like wave). → Highly unstable. %p %s Bond angle Concept Ladder sp3 75% 25% 109° 28’ sp2 66% 33% 120° sp 50% 50% 180° Isomerism 17.  (2) Cyclohexane : Exception ⇒ >Twisted boat form Previous Year’s Questions Among the following the most stable compound is [AIIMS-2005] (1) cis-1,2-cyclohexanediol (2) trans-1,2-cycloehxanediol (3) cis-1,3-cyclohexendiol (4) trans-1,3-cyclohexanediol Isomerism 18. Chair form of cyclohexane : Most symmetric molecule. → Thermodynamically stable molecule. → Every carbon bear 2 ideal tetrahedral. → Conformer of cyclohexane = 3 (1 Chair + 2 Twisted boat) Optically active. up — up → cis  Stability of ring : up— down → trans    down — up → trans    down — down → cis Isomerism 19.  Configurational isomerism 1. Geometrical Isomerism y Geometrical isomers occur due to differential distance between terminal group. Definition The stereoisomers which have different 3-D relative around a restricted rotatory system (RRS) is known as Geometrical isomers, this phenomenon is known as Geometrical isomerism. l1 ≠ l2 (Due to different aerial distance) Both are Geometrical Isomers. →  G.I. are not interconvertible at room temperature because of π-bond dissociation energy (62 Kcal). Compound must has R.R.S./absence of free rotation/Rigid system/hindered rotation/ Frozen rotation to exhibit configurational isomerism. Isomerism 20. Type of R.R.S : Double bonded R.R.S Isomerism 21.  1. GI in Alkene : sp2 carbon must be Concept Ladder (1) attached with different groups (Terminal position). (2) (A) GI in Imine : Previous Year’s Questions 2-butene shows geometrical (1) isomerism due to : [AIPMT] (1) restricted rotation about double bond (2) free rotation about double bond (2) (3) free rotation about single bond (4) chiral carbon (3) (B) G.I. in oxime : Rack your Brain What will be the effect of a big gruop present in a molecule on geometrical isomerism? (1) Isomerism 22.  (2) (C) G.I. in Azo-compounds : Always show G.I. Rack your Brain (1) N2H2 → H show G.I. Is unsaturation required for (2) geometrical isomerism? (3) (4) Minimum 2 sp3 carbon Concept Ladder must be attached with different group to show 2. G.I. in cycloalkane : G.I. in cycloalkane. Isomerism 23.  When terminal groups Concept Ladder are different, then odd number double bonds allenes and spiro system 3. G.I. in Allene : shows G.I. due to square- planar environment. and even number double bond containing allenes and spiro system do not show G.I. due to tetrahedral →  When 2 double bonds are continuously environment. present in a system then it is called allene system. →  When more than 2 double bonds are present in a system then it is called cumulated dienes. Rack your Brain Why is it easy to separate geometrical isomers by physical methods? 5. G.I. in spiro compound : When two rings are fused at the same C-atom then it is called as spiro compound. (Both rings are perpendicular to each other). (1) Isomerism 24.  (2) 5. G.I. in Biphenyl and Triphenyl System : In Biphenyl system each Concept Ladder ring must be attached with different bulky group (1) to exhibit G.I. At R.R.S. both terminal group must be in a same plane. (2) Rack your Brain Find out the total no. of Keto oxime with the molecule formula of C5H10O. Isomerism 25.  G.I. in endo-cyclic π-bond : Minimum 8 carbon ring with double bond shows geometrical isomerism. Rack your Brain Why G.I. do not exists for 3 to 7 membered ring? NOTE : In case of endocyclic π-bond 8 to 11 membered ring. Cis form is more stable compare to trans form and after 12 or 12 membered ring trans form is more stable compare to cis form. Nomenclature system for G.I. : y Cis-trans system : If two identical group at RRS on same side = cis If two identical group at RRS on opposite side = trans y Syn-Anti system : Whenever one terminal or both terminals of RRS have lone pair then Syn-Anti system applied. (1) Oximes : (I) Aldoxime : y If H & OH group both are on same side = Syn. y If H & OH group both are on opposite side = Anti. (II) Ketoxime : y Unsymmetrical ketoxime gives G.I. y Alphabetically preferred alkyl group & OH group on same side – Syn. y Alphabetically preferred alkyl group & OH group both on opposite side – Anti. Isomerism 26.  Whenever all the groups are Concept Ladder different at both terminal of RRS then the new system Ex. Oximes are formed with the molecular E–Z applied. formula of C3H6O is E → ENTGEGAN (Opposite side) Z → Zusamann (Same side) Total = 3. (2) Azo-compounds : y Both I.P. on same side = Syn. y Both I.P. on opposite side = Anti. Rack your Brain (3) E–Z system : Find out the total no. of G.I. CIP rule : Cohn, Ingold, Prolong with the molecular formula of C2FClBrI. Priority rules : Rule-1 : Priority of atoms or groups is given as per the atomic number of atom which is directly attached to terminal of R.R.S. (1) Rule-2 : If first rule fails then atomic number of next atom will be considered. Isomerism 27.  (2) Rule–3 : If multiple bonds are present, then each will be analysed and designated as E or Z. (3) Rule–4 : If isotopes present, [Priority will be given as per the atomic mass] (4) Physical Properties 1. Dipole Moment : Degree of polarity will be measured by dipole moment. Dipole moment is a vector quantity. Isomerism 28.  When diatomic angular Concept Ladder group is present at para- (1)  position of benzene molecule, angular group forms infinite conformations around a single bond, so in this case, mnet ≠ 0 (Non-zero). (2) Stronger the Van der Concept Ladder Waals forces higher the B.P. (3) Previous Year’s Questions 2. Boiling point : Key points : The dipole moment is the highest B.P. ∝ Molecular weight (MW) for 1 [AIIMS] B.P. ∝ (1) trans-2-butene Branching (2) 1,3-dimethylbenzene B.P. ∝ Intermolecular forces of attraction (3) acetophenone (4) ethanol Isomerism 29.  Ex. (1) (2) Isomerism 30. 3. Melting Point Key points : M.P. ∝ Molecular weight M.P. ∝ Packing efficiency (interlocking) M.P. ∝ Symmetry of molecule M.P. ∝ Branching Like dissolves Like Concept Ladder Example of polar substances protic is HCl in water Example of non polar substances is hexane in benzene P.E : II > I M.P : II > I 4. Solubility G.I. having high dipole moment will be more soluble in polar solvent and geometrical isomers having less dipole moment will be more soluble in none-polar solvent. p-nitrophenol > o-nitrophenol 5. No of Geometrical Isomerism Previous Year’s Questions Types of Geometrical isomers (1) Symmetrical System Which of the following has the (2) Unsymmetrical system highest dipole moment? [AIIMS] (1) (2) (3) (4) Isomerism 31.  GEOMETRICAL SYSTEM ISOMERISM Unsymmetrical 2n Compound If n = even 2n-1 + 2(n/2)-1 Symmetrical Compound If n = odd 2­­n-1 + 2(n-1)/2 Ex. (1) CH3 − CH = CH − CH2 − CH3 G.I = 2n = 21 =2 (2) CH3 − CH = CH − CH = CH − CH2 − CH3 G.I = 2n = 22 =4 (3) CH3 − CH = CH − CH = CH − CH3 Previous Year’s Questions Symmetrical even n = 2 How many geometrical isomers G.I = 2n-1 + 2(n/2)-1 = 3 are possible in the following two alkenes? (4) CH3 − CH = CH − CH = CH − CH = CH − CH3 (i) CH3–CH=CH–CH=CH–CH3 Symmetrical Odd (ii) CH3–CH=CH–CH=CH–Cl n = 3 [AIIMS] G.I = 2n-1 + 2(n-1)/2 = 6 (1) 4 and 4 (2) 4 and 3 (3) 3 and 3 (4) 3 and 4 (5) CH3 − CH − CH = CH − CH = CH2 | CH3 G.I = 2n = 21 = 2 Isomerism 32. Optical Isomerism Optical isomers have non-superimposable mirror images of each other. Each non-superimposable mirror image structure is called Enantiomers. Ex. Definitions Compounds having same M.F., same structural formula but different behavior towards PPL are called optical isomers & this phenomena is known as optical isomerism. Polarimeter experiment Isomerism 33.  In 2-D Representation If rotation of light is anticlock wise → laevo rotatory (l-form) Previous Year’s Questions If rotation of light is clock wise → dextro rotatory substance (d-form) Which one of the following pairs If there is no rotation of light then substance represents stereoisomerism? is called optically inactive. [NEET] (1) Structural isomerism and Compound is optically inactive if : geometrical isomerism y Compound does not give optical isomerism — (2) Optical isomerism and geometrical Achiral molecule. isomerism y Compound give optical isomerism but (3) Chain isomerism and rotation optically inactive — compound. isomerism y Compound may be equimolar mixture of ‘d’ & (4) Linkage isomerism and ‘l’ isomers — Racemic mixture. geometrical isomerism Some important terms y Optically active compounds : Compounds which are capable to rotate PPL. y Optically inactive compounds : Compounds which are not capable to rotate PPL. y Chiral molecule : Unsymmetrical molecule is known as chiral molecule. Rack your Brain y Achiral molecule : Symmetrical molecule is known as achiral molecule. How to determine optical rotation ○ All achiral molecules are optically inactive of a molecule that consist of one molecule. chiral carbon? Isomerism 34. y Chiral centre (C.C) : An sp3- hybridised atom which have all 4 different groups is known as C.C (asymmetric centre). ○ Chiral centre is not a necessary condition for a compound to be optically active (optical activity). ○ Generally if compound has only one chiral centre then compound is optically active (Except – Amine flipping). An sp3 → (C, N, O, P, S) (1) Previous Year’s Questions Which of the following is a chiral compound? [AIIMS] (2) (1) Hexane (2) n-Butane (3) Methane (4) 2,3,4-Trimethylhexane (3) Isomerism 35.  (4) Chiral → unsymmetrical → optically active Concept Ladder Achiral → symmetrical → optically inactive (5)   (6) Previous Year’s Questions Which of the following alkanes is (7) optically active? [AIIMS] (1) 3-Methylhexane (2) Propane (3) 2,3,4-Trimethylpentane (4) 2-Methylbutane (8) Rack your Brain (9) Why chiral molecules are optically active? (10) Isomerism 36. (11)    Rack your Brain (12) What are the conditions for a molecule to be optically active? Conditions for Optical activity Condition 1 : If a compound has only one chiral center then chiral centre is sufficient condition for a molecule to be optically active (optical activity). Condition 2 : If a compound has more than one chiral centre then chiral centre is not a sufficient condition for a molecule to be optically active, in this case the whole molecule must be chiral or unsymmetrical for a molecule to be optically active. Ex. 1 Previous Year’s Questions Which of the following molecule is not chiral? [AIPMT] (1) 2-butanol (2) 2,3-dibromo pentane (3) 3-bromo pentane (4) 2-hydroxy propanoic acid For a molecule to be optically active compounds positions must be different. Compound and its mirror image both are non-superimposable optically active. Isomerism 37.  Symmetry elements (i) COS — Centre of symmetry (ii) POS — Plane of symmetry (iii) AOS — Axis of symmetry (i) Centre of Symmetry (COS) It is an imaginary point situated at the centre of molecule from the centre if we draw a straight line then they will meet at same atom, same distance but in opposite direction then molecule has COS. (1) (2) (3) (4) (5) (6) (7)   (8) (9) Isomerism 38. Key Points (1) COS is different in odd number of atom Rack your Brain containing ring. (2) COS is different in cis isomers. Which geometrical isomer will (3) COS is different in all the molecules which exhibit COS in case of 2-butene? can form tetrahedral environmental. (ii) Plane of Symmetry (POS) It is an imaginary plane, passing through a molecule which can bisect a molecule into two equal halves & both half are mirror image of each other. Such plane is known as POS. (1) (2) (3) Rack your Brain Find out total no. of symmetry present in cubane. (4) Optically active d or l tartaric acid Not mirror images. Isomerism 39.  (5) (6) (7) NOTE : Chiral center is not a necessary condition for a compound to be optically active. (8) (9) Isomerism 40.  (10) Molecule POS Every planar molecule will have POS along the plane in which they exist called as molecular POS. Here molecular POS is represented by flag. (1) (2) (3) (4) (5) (6) Isomerism 41.  (iii) Axis of symmetry If new images are obtained by rotation of a molecule by an angle ‘θ’ about an axis passing through a molecule thus it gets similar orientation, then molecule has AOS. 360° θ= n Here n → fold of axis (1) (2) For a molecule to be Concept Ladder (3) optically active COS & POS should be different. Polarimeter Experiment Optical activity of a molecule can be predicted at the plane of paper by difference of COS & POS, but direction of rotation & angle of rotation is experimental value & measured by polarimeter [dextro & levo]. Specific Rotation Specific rotation given information about αobserved α specific = optical strength of a molecule whenever length LC Concept Ladder of polarimeter tube is 1 dm3 & concentration of L → length of polarimeter solution is 1 gm/ml then the observed rotation tube (taken in dm) is specific rotation. C → concentration of αspecific × L.C αobserved = solution (taken gmL–1) cons tan t  αspecific = ∴ αobserved =L.C Isomerism 42. Factors affecting angle of rotation (α observed) (1) Length of polarimeter tube Rack your Brain αobserved ∝ l , l ↑, αobserved ↑ A sample of (+)-2-chloropentane (2) Concentration of solution has an optical purity of 56%. αobserved ∝ C , C ↑, αobserved ↑ What % of this sample is (–)-2-chloropentane? (3) Solvent Solvent must be chemically & optically inert. (4) Temperature Polarimeter experiment carried out at room temperature. (5) Source of light Generally Na lamp used ( λ =589 Å ) Meso Compound y Optical isomers having chiral center more than one, with symmetrical elements (COS & POS) is Meso. Definition y Meso compound give optical isomerism but optically inactive. Meso compounds are achiral y Mirror image of Meso compound represent its compounds that have multiple identical molecule. chiral center and they are y Meso compound is optically inactive due to superimposed on their mirror internal compensation or intro-molecular images. neutralization or molecular symmetry. y Meso is achiral compound. y Meso compound cannot have enantiomers. (1) Rack your Brain Why meso compound are optically inactive? Isomerism 43.  (2) (3) Previous Year’s Questions Among the following which one can have a meso form? [AIIMS] (1) CH3CH(OH)CH(Cl)C2H5 (2) CH3CH(OH)CH(OH)CH3 (3) C2H5CH(OH)CH(OH)CH3 (4) HOCH2CH(Cl)CH3 Isomerism 44.  Enantiomers y Enantiomers have different behavior toward Definitions each other. y Enantiomers have same physical property. Enantiomers are chiral molecules that are mirror images of each other. Enantiomers with optically (1) Concept Ladder active compound will be different in comparsion to optically inactive compound which are same.. Rack your Brain Why specific rotation of (2) enantiomers have same magnitude but different sign? Previous Year’s Questions Which of the following compounds may not exist as enantiomers? [AIPMT] (1) CH3CH(OH)CO2H (3) (2) CH3CH2CH(CH3)CH2OH (3) C6H5CH2CH3 (4) C6H5CHClCH3 Isomerism 45.  Racemic Mixture y Racemic mixture is a resolvable mixture, separation of d & l isomers is known as optical resolution. y A racemic modificationi s always optically inactive when enantiomers are mixed together, Definition the rotation caused by a molecule of one A mixtrue of equal amounts of enantiomer is exactly cancelled by an equal enantiomers is called a racemic and oposite rotation caused by a molecule of mixture. its enantiomers. Isomerism 46. (1) d = 50% and l = 50% R.M. = 100% Optical Purity (O.P.) = 0% (2) d = 100% and l = 0% R.M. = 0% O.P. = 100% (3) d = 80% and l = 20% R.M. = 40% Rack your Brain O.P. = 60% Net rotation = dextro Why racemic mixture is optically inactive? (4) d = 60% and l = 40% R.M. = 20% O.P. = 80% Net rotation = dextro Isomerism 47.  Diastereomers y Diastereomers have different physical property Definition and separated by functional distillation. y Diastereomers have different specific rotation. Diastereomers are y Diastereomers may have optically active or stereoisomers which are non inactive compounds. mirror images of each other. Rack your Brain (1) Why Diastereomers are easily seprated by physical method? All geometrical isomers show diasteromerism Concept Ladder but all diastereomers do (2) not show geometrical isomerism. (3) Isomerism 48.  Epimers y One of a pair of stereoisomers that differ in the Definition absolute configuration of a single stereocenter. When the molecule has only one stereocenter then Two monosaccharide different the epimers are enantiomers. When the molecule form each other in their has two or more stereocenters then the epimers configuration around a single are diastereomers. specific carbon is known as y Epimerization is the interconversion of one epimer epimer of each other. to the other epimer All epimers are Concept Ladder diasteromers but all diastereomers are not epimers. Isomerism 49.  Representation of Optical isomers 1. Wedge – Dash formula (3D) 3D Form of ethane Concept Ladder (i) Lactic Acid (ii) Tartaric Acid Isomerism 50.  2. Fischer Projection Formula (2D) y Fisher projection is a 2D plane projection formula. In FPF, vertical line groups Concept Ladder y In it one is vertical plane (only one) & other is represent away from the horizontal plane (one or more than one). viewer & Horizontal groups y In it C-chain always in vertical line & top most represents towards figure at top position of vertical line. the viewer and Fischer y Fischer is a map of molecule. Projection is always y Fischer projection represent tetravalency. formed in eclipsed form. Conversion of different projection formula 1. Wedge Dash into Fischer projection formula (i) (ii) NOTE : When an atom or group of atom is moved form vertical board’s plane. then wedge group comes at its place. 2. Fischer projection into Wedge Dash formula Statement 1 : If Fischer projection formula is rotated by 180° at the plane of paper then it represents its identical molecule. Isomerism 51.  Statement 2 : If Fischer projection formula is kept fixed at one unit, rest all 3 units rotated in clockwise or anticlockwise direction then it represents its identical molecule. Q. Find out relation in given compounds. (1) (2) (3) Isomerism 52.  (4) ≡ Identical Configuration of Optical Isomerism’s D & L have enantiomeric Concept Ladder (1) D/L configuration (Relative configuration) relation and they are (2) R/S configuration (Absolute configuration) not valid for symmetric system. (1) D/L Configuration Carbohydrate : Polyhydroxy aldehyde or ketonic compound with mini one chiral compound is known as carbohydrate. y D & L configuration is standard relative configuration of glyceraldehyde. There is no relation with d and l isomers (optical activity). y D & L configuration valid for carbohydrate and Previous Year’s Questions amino acid like structure. Among the following L-serine is: [AIIMS] (1) (2) y If at last C.C −OH / NH2 group on right side = D (3) y If at last C.C −OH / NH2 group on left side = L (4) Isomerism 53.  Rack your Brain Will cylic form of D/L (1) (2) configuration exists for a molecule? (3) Previous Year’s Questions The absolute configuration of the following compound is : [AIIMS] (2) R/S Configuration (1) 2S, 3R (2) 2S, 3S Give conformation about arrangement of unit at (3) 2R, 3S (4) 2R, 3R C.C not rotation of PPL (d & l). (a) R/S Configuration in Fisher Projection Formula y Assign the priority according to CIP rule (E/Z configuration) y [Priority of atom group ∝ Atomic Number] y In Fischer projection formula, if lower priority If the 4th priority group is group (4) present away from viewer (vertical line) present in horizontal line Concept Ladder then clockwise direction is in 'R' configuration then we move our eyes and in anticlockwise direction is 'S' configuration. from 1 → 2 → 3 , hence configuration is Isomerism 54. y If lowest priority group present towards the viewer (Hg line) then the real orientation just opposite to the observed orientation. Always go through → 1 → 2 → 3 if lowest priority (4) presents in between ignore it. If the 4th priority group is present in vertical line Concept Ladder (1) then we move our eyes from 1 → 2 → 3 , hence configuration is (2) Previous Year’s Questions Correct configuration of the (3) following is [AIIMS] (1) 2S, 3R (2) 2S, 3S (3) 1R, 2S (4) 2R, 3R Isomerism 55.  (b) R/S Configuration in Wedge-desh Formula y Rules of R/S configuration in Fisher & Wedge desh formula → same – same. y If lowest priority group 4 present at dash line then When lowest priority group is not present on wedge or Concept Ladder dash line then we fix 1 unit and move another three units in clockwise and anti-clockwise direction and bring it to wedge or dash line. (1) Previous Year’s Questions (2) The absolute configuration of the following compound is : [AIPMT] (1) E (2) R (3) (3) S (4) Z Isomerism 56. Relation between different compounds COMPOUND 1 COMPOUND 2 RELATION 1. R R Identical/Homomers 2. S S Identical/Homomers 3. R S Enantiomers 4. RR SS Enantiomers Enantiomers (If unsymmetric) 5. RS SR Meso (if symmetric) 6. RR RS Diastereomers Isomerism 57.  Threo and Erythro System (2) Rack your Brain Why amines are not chiral? Amine Flipping Amine inversion also called umbrella inversion. Previous Year’s Questions Which of the following may exist in enantiomorphs [AIPMT] (1) (1) (2) (3) (4) (2) Isomerism 58.  Previous Year’s Questions (3) Find the number of stereoisomers of 1,2-dihydroxycyclopentane. [AIIMS] (1) 1 (2) 2 (3) 3 (4) 4 No. of Optical Isomers NO. OF OPTICALLY ACTIVE MESO Unsymmetrical Compound 2n/2 = no. of stereogenic unit Zero If n = even n −1 2n – 1 22 Symmetrical Compound If n = odd n− 1 n− 1 2 2 2n− 1 −2 2 O || (1) CH3 − CH − C− H | OH unsymmetrical Isomerism 59.  No. of stereo = 2n = 21 = 2 (a) Total no. of optically active isomers = 2 (b) Total no. of optically inactive isomers = 2 (c) Total no. of meso = 0 (d) Total no. of enantiomeric pair = 1 (2) CH3 − CH − CH − CH3 | | Cl Br Unsymmetrical n=2 no. of stereo = 2n = 22 = 4 (3) Glucose Previous Year’s Questions How many stereoisomers does this molecule have CH3CH=CHCH2CHBrCH3 [AIPMT] (1) 8 (2) 2 (3) 4 (4) 6 n=4 no. of stereo = 2n = 24 = 16 (4) CH3 − CH − CH = CH − CH2 − CH3 | Cl Unsymmetrical n=2 no. of stereo = 2n = 22 = 4 n = no. of stereogenic unit (G.I. + O.I.) Isomerism 60. Summary Š Structural Isomerism is the type of isomerism in which one of two or more compounds have same number of compounds but different in their arrangement. Š Structural Isomers are classified into chain, position, functional group, metamers and tautomers. Š Stereoisomers are the compounds having same molecular formula but having unlike spatial arrangement. Š Number of stereoisomers = 2n Š Geometrical (Cis/Trans) Isomerism also known as Configurational Isomerism. Š Compounds having same M.F., same structural formula but different behavior towards PPL are called optical isomers & this phenomena is known as optical isomerism. Š A number of stereoisomers which are formed due to free rotation around a single bond is known as conformations and this isomerism is known as conformational isomerism. Š Chiral compounds are unsymmetrical compounds where carbon is bonded with Isomerism 4 different groups. Š Chiral compounds are optically active and Achiral are optically inactive. 61.