GOODLUCK TUITION CENTER FOR CHEMISTRY 655 A 48TH STREET 9 TH SECTOR CHENNAI - 78 Ph: 2366 3848 Cell : 9444357037 ISOMERISM 1. Mesotartaric acid is an optically inactive compound with asymmetric carbon atoms. Justify. In one isomer of tartaric acid, one asymmetric carbon atom is dextro rotatory and other is laevo rotatory - both rotating plane polarised light to the same extent in opposite directions. The net result is that this isomer becomes optically inactive and is called the Meso isomer. Though the meso isomer has two asymmetric carbon atoms, the configuration of one carbon is the mirror image of the other, the net result being the molecule as a whole becomes symmetric. This molecule is said to have a symmetric plane, which divides the molecule into two equal halves. The molecule becomes achiral. The optical inactivity of the meso isomer is due to internal compensation. This form is a single substance and not a mixture. 2. Trans isomer is more stable than cis isomer why? In the case of cis isomer similar groups are very near to each other, Vander Waals repulsion and steric hindrance make the molecule much unstable. In the trans isomer, similar groups are diagonally opposite to each other, Hence there is no such steric interaction. Hence trans isomer is more stable than cis isomer. PH: 23663848, Cell : 9444357037 Pg. 1
3. Distinguish enantiomers and diastereomers. Enantiomers Optical isomers having the same magnitude but different sign of optical rotation They have configuration with non- super imposable object mirror image relationship. They are identical in all properties except the sign of optical rotation. Separation of enantiomers is a tedious process d - tartaric acid and l - tartaric acid Diastereomer Optical isomers differ in the magnitude of optical rotation. They are never mirror images They differ in all physical properties. Separation from the other pairs of enantiomers is easy meso - tartaric acid It is a diastereomer of d tartaric acid (or) l tartaric acid 4. Distinguish between racemic and meso forms. Racemic mixture It is a mixture that contains equal amount of d-isomer and l-isomer. It is optically inactive due to external compensation. Molecules of isomers present are chiral Meso form It is a single compound It is optically inactive due to internal compensation Molecules are achiral Example : 50 % d and 50% l Tartaric acid Meso tartaric acid 5. What are the conditions for a compound to be optically active? The compounds should contain asymmetric (or) chiral carbon atom. The compounds should have non-superimposable object and mirror image configuration. PH: 23663848, Cell : 9444357037 Pg. 2
6. Label the following as E, Z isomers : E - isomers E - isomers Z isomers E - isomers 7. What is racemic mixture? Give an example. When equal amounts of d-isomer and l-isomer are mixed racemic mixture is obtained and this process is called racemisation. The mixture is optically inactive. Because in this mixture, rotation towards clockwise direction by the dextro isomer is compensated by the rotation towards the anticlockwise direction by the laevo isomers. e.g., 50% d-lactic acid and 50% l - lactic acid are mixed togetherto get dl or + lactic acid which is known as racemic mixture of lactic acid. The optical inactivity of a racemic mixture is said to be due to external compensation. 8. What are optical isomers? Give example. Isomerism in which the compounds having the same molecular formula but different spatial arrangement of atoms or groups and differ in the rotation of plane polarised light. eg. Lactic acid d,l pairs of lactic acid. PH: 23663848, Cell : 9444357037 Pg. 3
9. Give the structure of cis-trans isomers of 2-pentene. 10. Justify that boat form is less stable than chair form of cyclohexane. In chair form every pair of carbon atoms ( Two carbon atoms directly bonded to each other ) exists in skew form. That is C- H bonds are lie away from each other (dihedral angle is 60 o ) bond pair repulsion is minimum hence chair form is more stable. In Boat form every pair of carbon atoms with eclipising bonds. This eclipsing interaction destabilies boat form.that is dihedral angle is 0 o bond pair electrons repelsion is maximum which make the molecule is unstable. 11. Write a note on external compensation When equal amounts of d isomer and l isomer are mixed one gets a racemic mixture and this process is called racemisation. A racemic mixture becomes optically inactive. Because in this mixture rotation towards clockwise direction by the dextroisomer is compensated by the rotatin towards the anticlockwise direction by the leavo isomer. The optical inactivity of a racemic mixture is said to be due to external compensation.. PH: 23663848, Cell : 9444357037 Pg. 4
12. Why equatorial cyclohexanol is more stable than axial cyclohexanol? In one form axial cyclohexanol OH group is axially oriented. In the equatorial cyclohexanol OH group is equatorially oriented. The energy of the axial conformer is little higher than that of the equatorial conformer. Because the axial substituent experiences steric interaction with axial H- atom present at the thrid carbon atom. This decreases the stability of the axial conformer. This is called 1 : 3 diaxial interaction. This interaction is absent in the equatorial conformer. Hence equatorial cyclohexanol is more stable than axial cyclohexanol. 13. Draw the chair and boat forms showing different C H bonds PH: 23663848, Cell : 9444357037 Pg. 5
14. Write a note on internal compensation Meso isomer has two asymmetric carbon atom.the molecule have a symmetric plane. Which divides the molecule into two equal halves. The molecule becomes achiral. The configuration of one carbon is the mirror image of the other(super impossible). The net result being, the molecule as a whole become symmetric one asymmetric carbon atom is dextro rotatory and other is laevo rotatory - both rotating plane polarised light to the same extent in opposite directions. isomer becomes optically inactive The optical inactivity of the meso isomer is due to internal compensation 15. What is Geometrical isomerism? Give an example. Isomerism that arises out of difference in the spatial arrangement of atoms or groups about the doubly bonded carbon atoms is called geometrical isomerism. The isomer in which similar groups lie on the same side is called Cis isomer, and the similar groups lie in opposite direction is called Trans isomer. PH: 23663848, Cell : 9444357037 Pg. 6