![Mesomer: The Compound consisting of 2 chiral C atoms but do not show
optical activity. Such molecule have plane of symmetry.
Example: Meso-Tartaric Acid. This molecule is symmetric along horizontal
plane of symmetry (see fig). C-2 and C-3 are the chiral C atoms. The
configuration at C-2 and C-3 is opposite to each other [C-2 R and C-3 S].
Hence optical rotation at these centers is opposite to each other. Thus optical
rotation by one C is exactly cancelled by other C atom and result is no optical
rotation.
C
COOH
C
OHH
1
2
3
COOH
OHH
4 Plane of symmetry
Racemic Mixture: The mixture of enantiomers in equal concentration is optically
inactive.
Such mixture is called as racemic mixture.
Optical rotation by two enantiomers is opposite to each other. Thus optical rotation
by one enantiomer is exactly cancelled by other and result is no optical rotation.
Imp Note: Optical rotation is
additive and vector property](https://image.slidesharecdn.com/steriochemistrysybsc-180929114354/85/Steriochemistry-syb-sc-32-320.jpg)
![• According to Bayer’s strain theory due to angle strain
cycloalanes are unstable.
• But actually it has been observed that except cyclopropane
and cyclobutane other cycloalkanes are highly stable. This
clear from the combustion energies per -CH2- group.
• Combustion energies per -CH2- group for acyclic (open
structure) alkanes compounds 158.4 K cal per mole
-CH2-[ ]n + 3/2nO2 nCO2 + nH2O + Heat
• They are, 166.4 for cyclopropane, 164 for cyclobutane, and
very close to 157.4 K cal per mole for other cycloalkanes.
(see tablw on next slide)](https://image.slidesharecdn.com/steriochemistrysybsc-180929114354/85/Steriochemistry-syb-sc-36-320.jpg)
Steriochemistry syb sc
- 1. Stereochemistry S. Y. B. Sc. Chemistry paper-II Sec-I Dr. D. R. Shinde P.D.E.As. Prof Ramkrishna More Arts, Commerce and Science College, Akurdi, Pune-44
- 2. • Stereochemistry refers to the 3-dimensional three dimensional properties of molecules. • Stereochemistry has its own language and terms that need to be learned in order to fully communicate and understand the concepts. • Def. Isomers: The compound having same molecular formula but different structures. • e.g. Molecular formula: C3H6O, Different isomers possible with this formula are H3C C CH3 O H3C CH2 C O H Aceton Propanealdehyde H2C CH CH2OH prop-2-en-1-ol
- 3. Do the compounds have the same molecular formula No Not isomers Yes Isomers Do the compound have same connectivity Constitutional isomers No Yes Stereoisomer's Configurational isomers Conformational isomers Does C in compound is SP3 hybrid Geometric isomers Optical Isomers
- 4. Representation of molecule Molecules possess three dimensional structure. However, on paper, we have represent them as two dimensional. To view three dimensional structure on paper some methodologies are used. 1) Wedge and Dotted line presentation: In this method the atoms in the plane are presented with simple line. The atom pointing behind the plane is shown by dotted line. The atom pointing above the plane is shown by wedge line. C H H H H In plane of paper Pointing forward - out of plane of paper Pointing backward out of plane of paper e.g. In CH4 molecule, two H and central C atom lie in a plane of paper. From remaining two H, one point slanting behind the plane while another point slanting above the plane.
- 6. 2) Newman’s Projection Formula • In Newman’s Projection Formula molecule is viewed end-on, i.e. along the C-C bond. • The two carbon atoms joined by the bond are represented as two superimposed circles; however for representation only one circle is drawn. • The centre of the circle represents the front carbon atom C whilst the circle represents the back carbon atom. The line joining the two carbon atoms is not visible. • Bonds attached to the front C carbon atom are drawn as lines from the centre and the bonds attached to back C atom drawn from the circumference of the circle. The projected angle between each bond is kept as 120o. • The ethane molecule, with the help of Newman’s Projection Formula, can be drawn as below. Front C atom Back C atom H H H H HH
- 7. CH3 H H CH3 HH n-butane, staggered conformation
- 8. 3) Sawhorse Projection Formula: • In Sawhorse Projection Formula a side view of the molecule is taken. • The two carbon atoms are joined by a larger diagonal line which is taken to be in the plane of the paper. The remaining bonds are projected on paper by small lines drawn out from the terminal ends of the diagonal line. • a) The heavy wedged bond projects the bond towards the viewer, b) the dashed bond projects the bond away form the viewer and c) the normal bonds lie in the plane of the page. • The eclipsed and the staggered conformational form of the Ethane molecule can be represented with the help of Sawhorse Projection Formula of staggered and eclipsed ethane as shown in Fig. C CH H H H H H Eclipsed Ethane C C H H HH H H Staggered ethane
- 9. Optical Isomerism: Optical isomerism is observed in organic compounds consisting of Chiral C atom. The chiral c atom is – i) sp3 hybrid or tetrahedral ii) It consists of four different groups of atoms or atoms bonded to it. It is also called as asymmetric C atom. C H I Cl Br C H CH3 HO C2H5 The chiral C atom shown by * on its head
- 11. Optical activity is related to Plane-Polarized Light (PPL). PPL is different from normal light. Photons of normal light vibrate in all possible planes passing through a point i.e. in infinite number of planes while all photon in a beam of PPL vibrate only in one plane. From ordinary light, PPL is obtained by passing ordinary light through a prism called as Nicol Prism or polarizer.
- 12. When Plane-Polarized Light is passed through an Achiral Compound plane of PPL do not changes. When Plane-Polarized Light is passed through a chiral Compound then plane of PPL rotate by certain degree.
- 13. • The compound which is able to rotate plane of polarized light by certain degree is called as optically active substance. • Optical isomers: The compounds having same molecular formula but different optical rotations are called as optical isomers. Optical Isomers Disteriomers Optical isomers which are not mirror images of each other Enantiomers Optical isomers which are non super- imposible mirror images of each other CHO C C CH2OH OHH HOH CHO C C CH2OH OHH OHH C COOH CH3 NH2H C COOH CH3 HNH2 mirror plane
- 14. Enantiomers are the non superimposible mirror images of each other Disteriomers are not the mirror images of each other
- 15. Configuration at Chiral C atom: • The orientation of groups or atoms in space around chiral C atom is called as configuration • Scientist Cahn, Ingold and Prelog prescribed the method for assigning configuration at Chiral C atom called as absolute configuration. Rules: i) Represent the molecule in Fscher representation.
- 16. 16 16 Fischer Projections - representation of a three-dimensional molecule as a flat structure. A tetrahedral carbon is represented by two crossed lines: Vertical line is going back behind the plane of the Paper i.e. away from you Horizontal line is coming out of the plane of the paper i.e. toward you lactic acid CO2H CH3 HO H H3C CO2H OH H CO2H CH3 H OH H3C CO2H H OH OHH CO2H CH3 HHO CO2H CH3
- 17. ii) Assign priority sequence to the four groups attached to chiral C atom. a) Consider directly bonded atoms to the chiral C atoms. Assign their At. No. The atom with highest Atomic Number has 1 priority or highest priority. Assign priority sequence 1 to 4 depending on their At. No. in decreasing order. eg: 1-chloroethanol, CH3CHCl(OH).C CH3 Cl OHH The atomic numbers of the atoms are; Cl (17), O (8), C (6) of CH3, and H(1) Therefore, the priority order given to the four groups would be -Cl > –OH > -CH3 > -H. 1 2 3 4 C CH3 Cl OHH 1 2 3 4
- 18. • In case of alkyl group, the group having large size has highest priority. • For example in 2-butanol, the four different groups attached to chiral C aoms are –CH3, -C2H5, -OH and H. The highest priority group would be –OH and the lowest priority group would be H. • Between the groups –CH3 and –C2H5, the –C2H5 group is larger in size than –CH3. Therefore –C2H5 highest priority than –CH3. C C2H5 CH3 HOH The priority sequence is -OH > –C2H5 > -CH3 > -H 1 2 3 4
- 19. In order to decide priority for groups having double or triple bonds; Each bond is duplicated in case of a double bond and Triplicated for a triple bond. Thus, the consideration is as follows C is equivalent to C C C C C C C C O C C O O is equivalent to is equivalent to
- 20. Group priority orders for some groups are C CHO COOH OHCH3 2 1 3 4 C NH2 COOH CH3 H 1 2 3 4 C CHO OH CH2OH H 1 2 3 4 C C CH3 HC H 1 2 3 N CH2 4
- 21. v. If it is not then, perform the allowed manipulations of the Fischer projection to place the lowest priority group at the bottom. 2. If one group of a Fischer projection is held steady, the other three groups can be rotated clockwise or counterclockwise. CO2H CH3 H OH CO2H H HO CH3 hold steady (R) (R) iv. To assign absolute configuration, in the Fischer projection the lowest priority group should be present at the bottom.
- 22. • Manipulation of Fischer Projections Fischer projections can be rotated by 180° only! But not by 90˚ CO2H CH3 H OH (R) 90 ° H OH CH3HO2C (S) ° • A 90° rotation inverts the stereochemistry and is illegal! CO2H CH3 H OH CO2H CH3 HHO (R) (R) CO2H CH3 OHH CO2H CH3 HO H (S) (S) 180 ° 180 °
- 23. OH C CH3 H C2H5 1 2 3 4 Rotate other three group clockwise direction CH3 C H OH C2H5 Step-1: CH3 C H OH C2H5 1 2 3 4 Step-2: Hold C2H5 Group Steady Assign Priority orders to the groups CH3 C H OH C2H5 1 2 3 4 Number 1 to 4 indicate priority order
- 24. C CH3 COOH H OH C CH3 COOH H OH 1 2 3 4 C H COOH HO CH33 2 4 1 Step-2: Rotate other three group anti-clockwise direction Step-1: Assign Priority orders to the groups C CH3 COOH H OH 1 2 3 4 Hold -COOH Group Steady
- 25. After Manipulation of the formula assign R or S onfiguration at Chiral C atom: If the priority of the groups 123 are clockwise then assign the center as R configuration If 123 are counterclockwise then assign the center as S configuration OH C CH3 H C2H5 1 2 3 4 The priority of the groups 123 is anti- clockwise. Therefore configuration is S
- 26. C H COOH HO CH33 2 4 1 The priority of the groups 123 is clockwise Therefore configuration is R. C H COOH H3C NH21 2 4 3 The priority of the groups 123 is Anti-clockwise Therefore configuration is S.
- 28. Three stereoisomers of 2,3-butanediol 2R,3R 2S,3S 2R,3S Optically inactive It is Mesomer CH3 CH3 OHH HHOH OH CH3 CH3 HHO H CH3 CH3 OH OHH Optically active Optically active
- 29. Number of optical isomers to the molecule with chiral C atom = 2n where, n = number of chiral C atoms in molecule Ex: i) D-glucose has 4 chiral C atoms. Therefore number of optical isomers = 24 = 16 ii) 2-3 butane diol has 2 chiral C atoms, hence number of optical isomers = 22 = 4 CHO C OHH C C C CH2OH H OHH HHO OH 1 2 3 4 5 6 D-Glucose: Carbon 2, 3, 4, 5 are chiral CH3 C OHH C CH3 HHO 2-3 butane diol : Carbon 2, 3, are chiral
- 30. Relative Configuration In a compound consists of large number of chiral C atoms then configuration is assigned at each chiral C atom. Such process is tome consuming and inconvenient. Hence the another method called as relative configuration was prescribed. It is mostly used to describe the isomers of carbohydraates. Rules: i) Represent the molecule in Fscher representation. ii) Number the C atoms in the vertical chain according to rules of IUPAC. iii) Select a C atom in vertical chain which at longest distance from carbonyl carbon. iv) If at this Chiral C atom position of –OH group is right hand side of vertical chain of C atoms then configuration D. v) If at this Chiral C atom position of –OH group is left hand side of vertical chain of C atoms then configuration L. Capital letter D and L do not same as that of small letter d and l used for dextrorotatory and laevorotatory.
- 31. Examples: Glyceraldehyde C CHO CH2OH OHH 1 2 3 -OH group on right hand side of vertical chain of C atoms hence configuration D C CHO CH2OH HOH 1 2 3 -OH group on left hand side of vertical chain of C atoms hence configuration L Carbon 2 to 5 are chiral. C-5 is at longest distance from >C=O carbon. Hence configuration is assigned at C-5. At this C atom -OH group on right hand side of vertical chain of C atoms hence configuration D CHO C OHH C C C CH2OH H OHH HHO OH 1 2 3 4 5 6
- 32. Mesomer: The Compound consisting of 2 chiral C atoms but do not show optical activity. Such molecule have plane of symmetry. Example: Meso-Tartaric Acid. This molecule is symmetric along horizontal plane of symmetry (see fig). C-2 and C-3 are the chiral C atoms. The configuration at C-2 and C-3 is opposite to each other [C-2 R and C-3 S]. Hence optical rotation at these centers is opposite to each other. Thus optical rotation by one C is exactly cancelled by other C atom and result is no optical rotation. C COOH C OHH 1 2 3 COOH OHH 4 Plane of symmetry Racemic Mixture: The mixture of enantiomers in equal concentration is optically inactive. Such mixture is called as racemic mixture. Optical rotation by two enantiomers is opposite to each other. Thus optical rotation by one enantiomer is exactly cancelled by other and result is no optical rotation. Imp Note: Optical rotation is additive and vector property
- 33. Stereochemistry of Cyclohexane •Cycloalkanes are saturated cyclic hydrocarbons • Have the general formula (CnH2n) •Some of the cycloalkanes are represented below
- 34. Bayer’s Strain Theory • Cycloalkanes are less flexible than open chain alkanes • Bayer believed that cycloalkanes, including cyclohexane, have planar structures. • Carbon in cycloalakanes is sp3 hybrid. Thus expected C-C-C bond angele is 109° 28’. • The expected C-C-C bond angles in different cycloalkanes are as follows: these angles are presented by considering that these molecules are planar.
- 35. • All cycloalkanes, except cyclopentane, become unstable because of angle strain in the molecule. According to Bayer, angle strain arises in molecule since C atom is SP3 hybrid and around it bond should have 109˚28′, but it is either less or greater than this value. • Angle Strain = 109˚28′ - bond angle in cycloalkane • The angle strain are 49 for cyclopropane, 19 for cyclobutane, 1 for cyclopentane and 11 for cyclohexane. • Except cyclopentane, in all other cyclic alkanes the bond angle is either very less or very greater that sp3 hybrid bond angle. 109 28’ 60 49 109 28’ 9019 108 109 28’ 1 120 109 28’ 11
- 36. • According to Bayer’s strain theory due to angle strain cycloalanes are unstable. • But actually it has been observed that except cyclopropane and cyclobutane other cycloalkanes are highly stable. This clear from the combustion energies per -CH2- group. • Combustion energies per -CH2- group for acyclic (open structure) alkanes compounds 158.4 K cal per mole -CH2-[ ]n + 3/2nO2 nCO2 + nH2O + Heat • They are, 166.4 for cyclopropane, 164 for cyclobutane, and very close to 157.4 K cal per mole for other cycloalkanes. (see tablw on next slide)
- 37. 4-37 Ring Strain in Cycloalkanes • The relative stabilites of cycloalkanes are determined by measuring their heats of combustion. Ring Size Heat per CH2 kcal/mol Ring Strain per CH2, kcal/mol Total Ring Strain, kcal/mol Long-Chain Alkane 157.4 0.0 0.0 3 166.6 9.2 27.6 4 164.0 6.6 26.4 5 158.7 1.3 6.5 6 157.4 0.0 0.0 7 158.3 0.9 6.3 8 158.6 1.2 9.6 • The more heat per CH2, the less stable the alkane.
- 38. • Interpretation of Bayer is incorrect since except cyclopropane and cyclobutane all other molecules are nonplanar. As planarity of molecules is loosed bond angle become equal to 109˚28’. Thus other than cyclopropane and cyclobutane all other cycloalkane do not consists of angle strain. cyclopropane cyclobutane cyclopentane cyclohexane
- 40. Steriochemistry of Cyclohexane Molecular Formula: C6H12 It has hexagonal but nonplanar structure. The hexagonal ring in two forms can be represented as follows: 1 2 3 4 56 1 2 3 4 56 or • This structure has chair like shape hence it is called as chair conformation of cyclohexane. • In these structure C-2, C-3, C-5 and C-6 are in horizontal plane while C-1 and C-4 are one is to above side of horizontal plane while another lie lower side of the horizontal plane • With hydrogen atoms the structure of cyclohexane can be shown as follows:
- 41. C C C C C C H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H or • With hydrogen atoms the structure of cyclohexane can be shown as follows:
- 42. • In the structure presented here, one can easily observe that on each C atom one H atom (shown by blue colour) is orientated along vertical axis, either in upward or downward direction. They are called as axial H atoms. H H H H H H H H H H H H H H H H H HAxial H atoms
- 43. H H H H H HH H H H H H H H H H H H Equitorial H atoms • In the structure presented here, one can easily observe that on each C atom one H atom (shown by red colour) is orientated in horizontal plane, either slanting upward or downward direction. It is called as equatorial H atoms.
- 44. • As seen earlier Cyclohexane exists in two Chair conformations which are interconvertible with each other. H H H H H H H H H H H H H H H H H H H H H H H H • When one conformation of cyclohexane changes to another, axial H atoms in one chair conformation becomes equatorial in another conformation. Likely equatorial H atoms in one chair conformation becomes axial in another conformation (see above diagrams carefully).
- 46. • As seen earlier Cyclohexane exists in two Chair conformations which are interconvertible with each other. When one conformation changes to another conformation, changes take place in series, and other less stable conformations of cyclohexane are formed. This is represented below. Chair Conformation Half-Chair Conformation Boat Conformation Twist Boat Conformation Chair Conformation Half-Chair Conformation Boat Conformation Two chair forms exists in equilibrium with each other
- 47. 1) Chair Conformation: • It is most stable conformation of cyclohexane. • In chair conformation H atoms on adjutant C are in staggered position (see Newmans projection) hence minimum steric interactions exists between them. This gives rise minimum energy and highest stability to this conformation. • Angle strain is absent in chair conformation. H H H CH H C H h H HH H H Newmans projection of chair conformation 2) Half-Chair Conformation: • It is least stable conformation of cyclohexane. • In this conformation there is angle strain in molecule at C4 carbon atom. • Angle strain increases energy of molecule and hence it is least stable.
- 48. 3) Boat Conformation: • In boat conformation H atoms on adjutant C are in ecliped position (see Newmans projection) hence maximum steric interactions exists between them. • The flag pole interactions between H atoms also increases the energy of conformation. • Both of these interactions increases the energy of this conformation and make it less stable. • Angle strain is absent in boat conformation. • It is less stable than chair conformation of cyclohexane but more stable than half chair conformation. H H C H H H H CH H H HH H H H Flag pole Hatoms
- 49. 4) Twist Boat Conformation: •It formed twisting of boat conformation due to steric interaction between flag pole H atoms. •In twist boat conformation H atoms on adjutant C are in ecliped position but moves slightly away from each other. Hence steric interactions exists between them but less than boat conformation. • The flag pole interactions between H atoms decreases in this conformation. • Angle strain is absent in boat conformation. •It is less stable than chair conformation of cyclohexane but more stable than boat conformation. Stability order: Chair conformation> Twist boat conformation > boat conformation > Half Chair conformation
- 50. Substituted Cyclohexane: 1) Monosubstituted Cyclohexane a) Methyl cyclohexane: • It can exists in axial and equatorial conformation. • Axial Conformation is less stable than equitorial conformation • In axial conformation methyl H are close to axial H at C3 and C5. hence there exists steric interaction between then called as 1,3 diaxial interaction. • 1,3 diaxial interaction decreases the stability of axial methyl cyclohexane. Hence, it less stable. • In equitorial conformation methyl H are away from H atoms of cyclohexane. Hence there exists minimum steric interaction between them, which increases the stability of equitorial methyl cyclohexane. • Hence most of the methyl cyclohexane exists in equitorial conformation. H3C Equitorial - Methyl cyclohexane C H HH H H Axial - Methyl cyclohexane 1 2 3
- 51. Substituted Cyclohexane: 1) Di-substituted Cyclohexane • In di-substituted we may have 1-2, 1-3 and 1-4 di-substitued cyclohexane. 1 2 1,2 Axial i.e. a, a conformation 1 2 1 equitorial,2 Axial i.e. e, a conformation 1 2 1Axial 2 equitoriali.e. a, e conformation 1,2 equitorial i.e. e,e conformation • According to the positions of groups in space, in 1-2, di- substituted we have following 4-conformations.
- 52. • 1-2 di-sustituted Conformations can be clsssified as cis and trans. • In Cis conformation both substituents lie in same plane i.e. either in upper plane or lower plane. • IF one substituent is axial and another is equatorial then it lie in same plane, hence it cis conformation. • IF both substituent are axial or equatorial then they lie in different plane, hence it trans conformation. 1 2 1,2 Axial i.e. a, a conformation 1,2 equitorial i.e. e,e conformation Trans Conformations 1 2 1 equitorial,2 Axial i.e. e, a conformation 1 2 1Axial 2 equitoriali.e. a, e conformation Cis- Conformations