ISOMERS.PPT organic chemistry slides by V.U
- 1. © E.V. Blackburn, 2005 Isomers • Isomers are molecules with the same molecular formula (i.e. the same number of atoms of each element), but different structural or spatial arrangements of the atoms within the molecule. • Therefore, isomers are the different compounds with the same molecular formula.
- 2. © E.V. Blackburn, 2005 Classification of Isomers
- 3. © E.V. Blackburn, 2005 Constitutional or Structural Isomers • Constitutional or structural isomers are compounds with the same molecular formula but different structural formulae. • In isomerism, constitutional isomers are molecules with different connectivity. • Therefore, constitutional isomers differ in the way the atoms are connected to each other.
- 4. © E.V. Blackburn, 2005 Properties of Constitutional isomers • Different IUPAC names. • Different bonding connectivity. • The same or different functional groups. • Different physical properties, so they are separable by physical techniques such as distillation. • Different chemical properties.
- 5. © E.V. Blackburn, 2005 Constitutional or Structural Isomers Structural isomers can be split again into three main subgroups: •Chain isomers. •Position isomers. •Functional group isomers. Chain isomers •Are molecules with the same molecular formula, but different arrangements of the carbon ‘skeleton’. •Organic molecules can be arranged differently either as one continuous chain, or as a chain with multiple side groups of carbons branching off.
- 6. © E.V. Blackburn, 2005 Constitutional or Structural Isomers • Positional isomers are constitutional isomers that have the same carbon skeleton and the same functional groups but differ from each other in the location of the functional groups on or in the carbon chain. Functional isomerism • Occurs when substances have the same molecular formula but different functional groups. • This means that functional isomers belong to different homologous series. • E.g alcohols and ethers; aldehydes and ketones; carboxylic acids and esters, etc.
- 7. © E.V. Blackburn, 2005 Stereoisomers • Stereoisomerism, or spatial isomerism, here molecules have the same molecular formula and same sequence of bonded atoms (bonding connectivity), but differ in the three-dimensional orientations of their atoms in space. • This contrasts with structural isomers, which share the same molecular formula, but the bond connections differs. • Stereoisomers have identical IUPAC names (except for a prefix like cis or trans). • Because they differ only in the three-dimensional arrangement of atoms, stereoisomers always have the same functional group(s). There are two main types of stereoisomerism- • Geometric isomerism and Optical isomerism.
- 8. © E.V. Blackburn, 2005 Geometrical Isomers • These isomers have different directional arrangements of specifically located groups in the molecule, and considered to be caused by prevention of free rotation in parts of the molecule by the double bond or a ring). • This type of isomerism is involved in compounds containing carbon- carbon double bonds with suitable substituents. • Rotation of these bonds is restricted, compared to single bonds which can rotate freely. • This means that, if there are two different atoms, or groups of atoms, attached to each carbon of the carbon-carbon double bond, they can be arranged in different ways to give different molecules
- 9. © E.V. Blackburn, 2005 Optical Isomers • Optical isomers are two compounds which contain the same number and kinds of atoms and bonds (i.e., the connectivity between atoms is the same), but different spatial arrangements of the atoms, and have non- superimposable mirror images. • Each non-superimposable mirror image structure is called Enantiomer. • Optical isomers are named due to their effect on plane-polarised light, and come in pairs. • They usually (although not always) contain a Chiral centre – this is a carbon atom, with four different atoms (or groups of atoms) attached to it. • These atoms or groups can be arranged differently around the central carbon, so that the molecule can’t be rotated to make the two arrangements align.
- 10. © E.V. Blackburn, 2005 Enantiomers-Optical Isomers • Enantiomers are optical stereoisomers that are non-superimposable on their mirror images. A A B B C C D D mirror CO2H H3C NH2 H CH3 CO2H H2N H
- 11. © E.V. Blackburn, 2005 Diastereomers-Optical Isomers Diastereomers are optical stereoisomers that are not mirror images of each other, and they arenot enantiomers. C C H CH3 H H3C C C CH3 H H H3C
- 12. © E.V. Blackburn, 2005 Chirality • Chirality means non-superimposable mirror-images of molecules, and a molecule is chiral if its mirror image is not the same as itself(non-superimposable) thus chiral centers. • Chirality is a necessary and sufficient condition for the existence of enantiomers. • Chiral compounds actively rotate plane polarized light. • Molecules that are superimposable on their mirror images are said to be achiral and optically inactive. Molecules that can exist as enantiomers are said to be chiral, and they are non-superimposable on their mirror images.
- 13. © E.V. Blackburn, 2005 Chiral Molecules and Chiral Samples • When a molecule is chiral, it must be either “right-handed” or “left- handed”. Meaning it can either rotate light clockwise or anti- clockwise. • A racemic mixture is a 50:50 mixture of two enantiomers. Because they are mirror images, each enantiomer rotates plane-polarized light in an equal but opposite direction and is optically inactive. • Any chiral macroscopic sample may be racemic or non-racemic. • Chiral and non-racemic sample: The sample is made up of molecules that all have the same sense of chirality (homochiral molecules). • Chiral but racemic sample: The sample is made up of equal (or very nearly equal) numbers of molecules of opposite sense of chirality (heterochiral molecules).
- 14. © E.V. Blackburn, 2005 Chiral Molecules and Chiral samples • Chiral, racemic means that the sample is made up of equal numbers of molecules of opposite sense of chirality. • But in a “chiral, non-racemic” sample, some molecules are of a sense of chirality opposite to that of the majority, thus the sample may not be enantiomerically pure (or enantiopure). • All compounds have mirror images. • What’s important in chemistry is whether a molecule is identical to or different from its mirror image. • Some molecules are like hands, the left and right hands are mirror images of each other, but they are non-superimposable and not identical.
- 15. © E.V. Blackburn, 2005 Chiral and Achiral Molecules • To superimpose an object on its mirror image means to align all parts of the object with its mirror image. With molecules, this means aligning all atoms and all bonds. • A molecule (or object) that is not superimposable on its mirror image is said to be chiral.
- 16. © E.V. Blackburn, 2005 Tetrahedral stereogenic centres A carbon atom bonded to four different groups is called a tetrahedral stereogenic centre, asymmetric centre, or chirality centre. O H (+)-carvone (+)-Carvone is responsible for the odour of caraway seed oil. *
- 17. © E.V. Blackburn, 2005 • The two are non-superimposable, mirror images. • Such isomers are called enantiomers. D B A C B D A C mirror
- 18. © E.V. Blackburn, 2005 Configurations • Configurations are not the same as conformations. • Conformations are interconvertible by rotation about single bond(s) whereas bonds must be broken to change one configuration into another. • The particular arrangement of atoms in space that is characteristic of a given molecule is called its configuration.
- 19. © E.V. Blackburn, 2005 Properties of enantiomers b) Chemical: They have identical chemical properties except for their reaction with reagents which are, themselves, optically active. In this case, reaction rates differ and depend on which enantiomer of the reagent is used. E.g (+)-Glucose is central to the fermentation process whereas (-)-glucose doesn’t react a) Physical: Enantiomers have identical physical properties with the exception that they rotate the plane of polarized light in opposite directions.
- 20. © E.V. Blackburn, 2005 Ordinary Light and Plane Polarised Light • An ordinary light beam consists of a group of electromagnetic waves of a range of different wavelengths that vibrate in many different planes at right angles to the direction of propagation of the light ray. It vibrates in all directions. • When such a beam strikes a polarising film or a Nicol prism, only those waves vibrating in a specific plane with respect to the axis of the film or prism may pass through and all others are blocked out. • Light beam is plane polarized once passed through the prism in one direction. • If light waves vibrate in the same light polarization plane, then light of this kind is said to be plane polarised.
- 21. © E.V. Blackburn, 2005 Ordinary Light and Plane Polarised Light • Monochromatic light: Monochromatic light, such as emitted by a sodium lamp (λ = 589 nm), is of discrete wavelength but still vibrates in an infinite number of planes.
- 22. © E.V. Blackburn, 2005 Plane-polarized light • Ordinary light is a moving wave whose vibrations take place in all directions perpendicular to the direction in which the light is travelling. • One of these components can be eliminated by passing ordinary light through a polarizer(Polaroid filter). • The resulting light is said to be polarized, and all its vibrations are parallel to a single plane.
- 23. © E.V. Blackburn, 2005 Polarimeter source 5893 Å polarizer sample tube analyzer
- 24. © E.V. Blackburn, 2005 • If from the point of the observer the rotation is in the clockwise direction, the sample is said to be dextrorotatory. The angle of rotation is considered to be positive (+). • If the rotation is in the counterclockwise direction, the sample is said to be levorotatory and the angle is then negative (-). • R chiral isomers are dextrorotatory and S chiral isomers are levorotatory • Thus (S)-2-chlorobutane is the levorotatory enantiomer. Optical activity • An optically active compound is one which rotates the plane of light polarization.
- 25. © E.V. Blackburn, 2005 Racemic mixtures • Is an equimolar mixture of two enantiomers. • Prefix the name + - • Contain enantiomers which are mirror images of each other and rotate plane polarized light in equal but opposite directions, thus optically inactive. • If enantiomers are separated in solution, then the mixture is said to have been resolved.
- 26. © E.V. Blackburn, 2005 Meso compounds • A meso compound is an achiral compound with chiral centers but with superimposable mirror images. • Meso compounds are achiral and optically inactive due to their internal plane of symmetry that makes them superimposable on their mirror images. Cl H CH3 Cl H CH3 H Cl CH3 H Cl CH3
- 27. © E.V. Blackburn, 2005 Optically active natural products CO2H HO2C H H OH HO (+)-tartaric acid H3CO H3CO O O H H H H N H (-)-brucine
- 28. © E.V. Blackburn, 2005 •END