Isomerism Class Notes for JEE 2025 PDF

Organic Chemistry ISOMERISM In One Shot Om Pandey, IIT Delhi Topics 1 Isomer 2 Types of isomerism 3 Structural Isomerism & No of isomers 4 Stereoisomerism & No of isomers 5 Confirmational Isomerism 12 BOARD TH Degree of carbon Degree of unsaturation Isomers Compounds...

Organic Chemistry ISOMERISM In One Shot Om Pandey, IIT Delhi Topics 1 Isomer 2 Types of isomerism 3 Structural Isomerism & No of isomers 4 Stereoisomerism & No of isomers 5 Confirmational Isomerism 12 BOARD TH Degree of carbon Degree of unsaturation Isomers Compounds with same molecular formula but different physical and chemical property.  Isomers Structural isomers Stereoisomers Different arrangement of atoms or Different arrangement of atoms or groups within the molecule groups in 3D space 1) Chain isomers 1) Configurational isomers 2) Position isomers a) Geometrical isomers 3) Functional isomers b) Conformational isomers 4) Metamers 2) Conformational isomers 5) Tautomers Structural Isomers Skeletal isomers , Linkage isomers & Constitutional isomers Compounds having same molecular formula but different structure  Chain Isomers Isomers having different size of main carbon chain / side alkyl chain Same nature of F.G. , M.B. , Substituents C4H10 C5H12 3 isomers C6H14 5 isomers QUESTION (04 April, 2024 / Shift-I) The number of different chain isomers for C7H16 is ______________. Positional Isomers Compounds having same MF but different position of FG / MB / Substituents OH OH Cl Cl Cl IIT-JEE 2014 The correct combination of name for isomeric alcohols with molecular formula C4H10O is/are A tert-butanol and 2-methylpropane-2-ol B tert-butanol and 1, 1-dimethylethan-1-ol C n-butanol and butan-1-ol D isobutyl alcohol and 2-methylpropane-1-ol Find no of isomers with MF C8H10 ?  Functional Isomers Compounds having same molecular formula but different functional group. Acid & Ester 1°, 2°, 3° Amines Alcohol & Ether Ketone & Aldehyde 1°, 2°, 3° Amides Alkene & Alkyne Enol and Alcohol Ring – Chain Isomerism Cyclic Alkane & Alkene Cyclic Alkene & Alkyne  Compounds having same nature of functional groups but different Metamers nature of alkyl groups along polyvalent functional group Ether 2o Amide Thio-Ether 3o Amide 2o Amine Ester 3o Amine Ketone QUESTION (05 April, 2024 / Shift-II) Match List-I with List-II List-I List-II (A) 1-propanol and isopropanol (I) Metamerism (B) Methoxypropane and ethoxyethane (II) Chain Isomerism (C) Propanone and propanal (III) Position isomerism (D) Neopentane and Isopentane (IV) Functional Isomerism A (A) (II), (B) (I), (C)-(IV), (D)-(III) B (A)-(III), (B)-(I), (C)-(II), (D)-(IV) C (A)-(I), (B)-(III), (C)-(IV), (D)-(II) D (A)-(III), (B)-(I), (C)-(IV), (D)-(II) JEE Mains-2021 Which one of the following pairs of isomers is an example of metamerism? CH3 A CH3CH2CH2CH2CH3 , H3C – C – CH3 O CH3 B C6H5 , H5C6 H O OH OH C H5C6 , H5C6 O O D ,  No of Structural Isomers No of carbon DU = 0 DU = 1 ( Cyclic ) C3 C4 C5 How many structural isomers are possible for following MF ? C4H9Cl Different type of carbon with atleast 1 H C4H9OH Alcohol C4H10O Ether How many structural isomers are possible in cyclic form for following MF ? C4H7Cl C4H8O Cyclic Alcohol C4H8O Cyclic Ether Ketones & Aldehydes C4H8O C5H8O Cyclic Ketones JEE Mains - 2021 Compound with molecular formula C3H6O can show : A Positional isomerism B Both positional isomerism and metamerism. C Metamerism. D Functional group isomerism.  Wedge Dash Projection F Wedge : Towards Observer H Dash : Away from observer Br HO Fischer Projection OH F Br Vertical line : Away from observer H Horizontal line : Towards observer  Stereoisomerism Same structural formula but different orientation in space CH3 CH3 CH3 H C C C C H H H CH3 Restricted Rotation :  Geometrical Isomerism Conditions Restricted Rotation Two different groups at each restricted point The groups / atoms showing G.I. must be in same plane. ( 1  2 ) 1. In Double bond system 2. In Ring system D D D Cl C C C N H H H H Cl N N H H Which will show Geometrical Isomerism ? H H F (1) C C H H (4) Cl D D (2) H C C H I F (3) C C Br Cl JEE Mains 2020 / 5 Sept/Shift-II Among the following compounds, geometrical isomerism is exhibited by : CHCl CHCl A B CH3 CH2 CHCl C D H3C CH3 Cl  Cis – Trans Nomenclature a a a b C C C C b b a b D D C C H H Cl F C C F Cl  Syn — Anti Nomenclature R R R C N C N C N H G H OH H NH2 R G C N H R N N N N R R R Note : If n is the no. of carbon atoms present in a ring then (i) n < 8 (ii) n  8 JEE Mains 2020 / 6 Sept / Shift-I Which of the following compounds shows geometrical isomerism ? A pent-2-ene B 4-methylpent-1-ene C 4-methylpent-1-ene D 2-methylpent-1-ene E / Z Nomenclature a b a d C C C C c d c b  CIP Rule [ Sequence Rule ] 1) Group having first atom with higher atomic no. is senior. Cl Br C C F I 2) The higher mass isotope is senior. Cl D C C F H 3) If first atom of group is identical then second atom is observed for seniority. Cl CH2 — CH3 CH3 CH2 — OH C C C C F CH2 — H I CH2 — NH2 4) Group having D.B. / T.B. are treated as following. F C C Cl JEE(Main) 2017 Online In the following structure, the double bonds are marked as I, II, III and IV II III I IV Geometrical isomerism is not possible at site (s) : A I B III C I and III D I and IV  JEE(Main) 2015 Which of the following compound will exhibit geometrical isomerism? A 1-Phenyl-2-butene B 3-Phenyl-1-butene C 2-Phenyl-1-butene D 1, 1-Diphenyl-1-propene  Physical Properties of G.I. Dipole moment Stability Boiling point Melting point Solubility in water Br H Br Br C C C C H Br H H AIEEE 2002 ( H.W. ) Which of the following does not show geometrical isomerism? A 1, 2-Dichloro-1-pentene B 1, 3-Dichloro-2-pentene C 1, 1-Dichloro-1-pentene D 1, 4-Dichloro-2-pentene Polarimeter Experiment +15°0 –15° +30° –30° +45° –45°  → 0o Optically Inactive Polarizer Plane Sample cell Analyzer Polarised Light  → (+) , Clockwise Dextro-rotatory compound Optical Rotation :  → (-) , Anti-clockwise Leavo-rotatory compound  Specific Rotation [  ] l → dm C → gm/ml  → degree Specific Rotation is the degree of rotation observed if a 1 dm tube is used and compound has concn 1gm / ml. Chiral Centre / Chiral Atom / Asymmetric Atom 1. sp3 Hybridisation 2. 4 different groups/atoms How many chiral atoms are present in following compounds ? H H Cl + N (a) (b) N CH3 (c) C2H5 O N Br O H H5C2 D (d) Si (e) CH3 S C2H5 H3C H JEE Mains 2022 /29 June/Shift-I Observe structure of the following compounds Cl NH2 OH OH H2N O O OH The total number of structures compounds which possess asymmetric atoms is ______________. Chiral Compounds A compound which is non-superimposable on its mirror image Asymmetric compounds F H HO Br OH F Br H A compound having only one chiral carbon is Chiral / Optically Active.  AIEEE 2004 Amongst the following compounds, the optically active alkane having lowest molecular mass is A CH3 – CH2 – CH2 – CH3 B CH3 – CH2 – CH2 – CH3 H C CH3 – C D CH3 – CH2 – C  CH C2H5 AIEEE 2004 Among the following four structures I to IV. CH3 O CH3 H CH3 C2H5 – CH – C3H7 CH3 – C – CH – C2H5 H – C C2H5 – CH – C2H5 (I) (II) (IV) H (III) It is true that : A All four are chiral compounds B Only I and II are chiral compounds C Only III is a chiral compound D Only II and IV are chiral compounds  Rule for Correct Fischer Projection On the vertical line , main chain is taken with first carbon at top. CH2OH CH CHO OH CH3 CH COOH NH2  Nomenclature Absolute Configuration Relative Configuration R – S Nomenclature D – L Nomenclature Standard : Glyceraldehyde CHO CHO H OH HO H CH3 CH3 Note : Applied only in Correct Fischer Projection  R - S Nomenclature - Fischer Projection Apply CIP Rule and find priority order. a c b d Cl CHO I Br HO CH3 F H If lowest priority group is at horizontal line then O a CHO C H c d H Cl HO H b H3C H OH CH2OH Odd & Even Exchange CHO H Cl H3C JEE (Main) 2016 CO2H H OH The absolute configuration of is H Cl CH3 A (2S, 3R) B (2S, 3S) C (2R, 3R) D (2R, 3S) R - S Nomenclature in Wedge Dash Projection CH3 H F Cl C2H5 C2H5 OH H H2N H H2N OH  Wedge – Dash Projection Fischer Projection D H F I D I F H D F H I  Find D – L Nomenclature for following ? CH3 HO H CHO COOH H H2N CH3  AIEEE 2008 HO2C CO2H The absolute configuration of is H H OH OH A R, R B R, S C S, R D S, S  Elements of Symmetry Plane of Symmetry Centre of Symmetry Imaginary plane which bisects the molecule in COS is a point from which two same two equal halves, in such a way that each half atom/groups are at same distance. of the molecule is a mirror image of other half. H CH3 CH3 HO H CH3 CH3 HO H H H OH H H H OH  Plane of symmetry & Centre of symmetry H D Optically Active F C Cl C Acyclic Compounds POS H H Cl F Cyclic Compounds POS COS CH3 CH3 CH3 CH3 H OH HO H H OH H OH CH3 H D CH3  AIEEE 2007 Which of the following molecules is expected to rotate the plane of polarized light? NH2 H2N COOH CHO H (1) H (2) H2N H (3) HO H (4) Ph Ph H CH2OH SH  IIT-JEE-2008 The correct statement(s) about the compound given below is (are). H Cl CH3 H3C Cl H A The compound is optically active B The compound possesses centre of symmetry C The compound possesses plane of symmetry D The compound possesses axis of symmetry Enantiomers Br Br 1 or 2 chiral carbon Cl I l CI Mirror Images F F Non-superimposable Diastereomers CH3 CH3 HO H H OH 2 chiral carbon H OH H OH Neither mirror images CD3 CD3 Nor-superimposable  Physical Properties of Enantiomer & Diastereomer CH3 CH3 CH3 CH3 HO H H OH D I D I H OH HO H Br Cl Cl Br CH3 CH3 F F CH3 CH3 CH3 H C C C C H H H CH3 Enantiomers have similar physical properties except the opposite sign of specific rotation. Diastereomers have different physical properties ( melting points, boiling points, densities, solubilities, refractive indices, dielectric constants and specific rotations ). JEE Mains 2022 / 29 July/Shift-II CH3 Statement I : The compound NO2 is optically active. (A) H CH3 CH3 O2N Statement II : The compound is mirror image of above compound A. H CH3 A Both statement-I and Statement-II are correct B Both statement-I and Statement-II are incorrect C Statement-I is correct but Statement-II is incorrect D Statement-I is incorrect but statement-II is correct  Meso Compound Min. 2 chiral carbon Opposite configuration Same connectivity CH3 H C2H5 H C2H5 CH3 Properties of Meso compound (i) Meso compounds are optically inactive due to internal compensation of rotation of PPL. (ii) Meso compound and its mirror image are identical. AIEEE 2004 Which of the following will have a meso-isomer also ? A 2-Chlorobutane B 2, 3-Dichlorobutane C 2, 3-Dichloropentane D 2-Hydroxypropanoic acid  Racemic Mixture Equi-molar mixture of enantiomers Br Br Cl I l CI F F Note : Racemic mixture is optically inactive due to external compensation of rotation of PPL.  Asymmetric Nitrogen : Amine Inversion Two enantiomers of amines always exist as racemic mixture hence optically inactive. H Cl Me  N N — Me Me C2H 5 CH3 Nitrogen salts and cyclic amines will not undergo amine inversion. Geometrical & Optical Isomerism Allenes / Spirans / Cycloalkylidenes Cl H Br F H H C C C C H Cl I D D D Cl l F Br JEE Mains 2023/01 Feb/Shift-I The total number of chiral compound/s from the following is _____________. Ph OH O O Cl COOH CH2 OH COOH HO OH OH OH HO2C COOH HO OH COOH OH Biphenyls O2N F I I I I I Cl I l CI Br I F Br IIT - JEE 2007 Statement-1 : Molecules that are not superimposable on their mirror images are chiral. Statement-2 : All chiral molecules have chiral centres. A Statement-1 is true, statement-2 is true; statement-2 is a correct explanation for statement-1. B Statement-1 is true, statement-2 is true; statement-2 is NOT a correct explanation for statement-1. C Statement-1 is true, statement-2 is false. D Statement-1 is false, statement-2 is true. No of stereoisomers 1 Stereo centre : D – CH – CH2 – CH3 CH3 – CH = CH – CH3 CH3 2 Stereo centre : CH3 – CH – CH – CH3 CH3 – CH – CH – C2H5 CH3 – CH = CH – CH – OH OH OH OH OH CH3 2 Stereo centre : CH3 – CH = CH – CH = CH –CH3 CH3 – CH = CH – CH = CH –C2H5 3 Stereo centre : H3C(HO)HC – CH = CH – CH(OH)CH3 H5C2(HO)HC – CH = CH – CH(OH)CH3 AIEEE 2009 The number of stereoisomers possible for a compound of the molecular formula CH3 – CH = CH – CH(OH) – Me is : A 2 B 4 C 6 D 3  IIT-JEE 2009 The correct statement(s) about the compound H3C(HO)HC – CH = CH – CH(OH)CH3 (X) is (are): A The total number of stereoisomers possible for X is 6. B The total number of diastereomers possible for X is 3. C If the stereochemistry about the double bond in X is trans, the number of enantiomers possible for X is 4. D If the stereochemistry about the double bond in X is cis, the number of enantiomers possible for X is 2. JEE (Advanced) - 2018 For the given compound X, the total number of optically active stereoisomers is ………… HO HO This type of bond indicates that the configuration at the specific carbon and the geometry of the double bond is fixed This type of bond indicates that the configuration at the HO HO specific carbon and the geometry of the double bond is NOT X fixed  Conformational isomerism Free rotation Sawhorse Projection Newman Projection Conformations : Different arrangements of atoms that can be converted into one another by rotation about single bonds are called conformations.  Dihedral Angle The angle between C – X and C – Y in X – CH2 – CH2 – Y X X Y Y Staggered conformation Eclipsed conformation Skew conformation : Other than Staggered & Eclipsed  Factors affecting stability of conformations Torsional Strain : Any deviation from the X staggered conformation X Y Y Pitzer strain or Eclipsing strain Vander Waals Strain : It is due to steric-crowding.  Conformational Analysis of Ethane Energy Profile of Ethane POTENTIAL ENERGY 12.5 kJ/mol 0° 60° 120° 180° 240° 300° 360° DIHEDRAL ANGLE JEE Mains-2022 Staggered and eclipsed conformers of ethane are : A Polymers B Rotamers C Enantiomers D Mirror images Conformational Analysis of Butane  Conformational Analysis of Butane POTENTIAL ENERGY 19 kJ 16 kJ /mol /mol 3.8 kJ/mol 0° 60° 120° 180° 240° 300° 360° DIHEDRAL ANGLE n-Butane exists as 3 conformers one anti and two gauche. JEE Mains-2022 Arrange the following conformational isomers of n-butane in order of their increasing potential energy CH3 CH3 CH3 HH H CH3 H H CH3 I II III IV H H H H H H H H H H HC CH 3 3 CH3 H A II < IV < III < I B I < III < IV < II C II < III < IV < I D I < IV < III < II  IIT – JEE 2010 In the Newman projection for 2, 2-Dimethylbutane X H3C CH3 H H Y X and Y can respectively be: A H&H B H & C2H5 C C2H5 & H D CH3 & CH3  IIT-JEE 2004 Newman projection of Butane is given, C-2 is rotated by 120° along C-2 & C-3 bond in anticlockwise direction the conformation formed is : CH 4 3 H H 3 2 H H 1 CH3 A Anti B Fully eclipsed C Gauche D Partially eclipsed Conformational Enantiomers CH3 CH3 H CH3 CH3 H The gauche conformers of butane are mirror images of each other. H H H H H H Conformational Diastereomers CH3 CH3 H CH3 H Gauche conformations and anti conformation H of butane are not mirror images of each other. H H H H H CH3  Case of Intramolecular Hydrogen Bonding G – CH2 – CH2 – OH where G = – OH , – NH2 , – F , – NR2 , – NO2 , – COOH ,–CHO 2-Flouroethanol Gauche form is more stable than the anti-form due to intra molecular hydrogen bonding. AIEEE 2006 Increasing order of stability among the three main conformations (i.e. eclipse, anti, gauche) of 2-fluoroethanol is A eclipse, gauche, anti B gauche, eclipse, anti C eclipse, anti, gauche D anti, gauche, esclipse AIEEE / JEE AD Which of the following conformers for ethylene glycol is most stable : OH OH OH H A B H H HO H H H H H OH OH H OH H H C D H H H H H OH IIT-JEE 2012 Which of the given statement(s) about N, O, P and Q with respect to M is (are) correct ? Cl CH3 CH3 HO H HO H H H OH HO H OH HO Cl H CH3 HO H OH H HO H HO H CH3 Cl CH3 Cl Cl A M and N are non-mirror image stereoisomers. B M and O are identical C M and P are enantiomers D M and Q are identical JEE ( Advanced ) - 2014 The total number(s) of stable conformers with non-zero dipole moment for the following compound is (are) : Cl Br CH3 Br Cl CH3 JEE Adv. 2019 Total number of isomers considering both structural and stereoisomers, of cyclic ethers with the molecular formula C4H8O is ____________ O O O O O (S) CH3 * CH3 CH3 (R) CH3 (1 isomer) (1 isomer) (2 isomers) O O O O CH3 * (S) (R) CH3 CH3 CH3 CH3 (1 isomer) (2 isomers) O O O O (R) (R) (S) (S) (R) (S) H3C * * CH3 H3C CH3 H3C CH3 H3C CH3 (3 isomers) JEE Mains 2022 / 27 June / Shift-I Total number of possible stereoisomers of dimethyl cyclopentane is ________________. CH3 CH3 CH3 R R S CH3 CH3 CH3 S R S CH3 CH3 CH3 R R S S R S CH3 CH3 CH3 Out of six isomers, two are meso compounds and are optically inactive Therefore, only 4 isomers will be optically active. QUESTION (30 Jan, 2024 / Shift-II) Number of geometrical isomers possible for the given structure is/ are _______________. D H H D