1. Isomerism Dean Martin City and Islington College

2. Topic Chemistry – Isomerism Aims  To help introduce Isomerism  Aimed at 16-19 Year olds Level Level 3 Method PowerPoint slides, ALL hand-outs (apart from slide 2) Equipment  Projector  Laptop  Hand-Outs  Pens Duration >30 Minutes

3. Learning Outcomes What are isomers? Types of isomerism Explain each type of isomerism

4. What are Isomers? Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. That excludes any different arrangements which are simply due to the molecule rotating as a whole, or rotating about particular bonds

5. Types of Isomerism

6. Occurs due to the restricted rotation of C=C double bonds... two forms… E and Z (CIS and TRANS) STRUCTURAL ISOMERISM STEREOISOMERISM GEOMETRICAL ISOMERISM OPTICAL ISOMERISM CHAIN ISOMERISM Same molecular formula but different structural formulae Occurs when molecules have a chiral centre. Get two non- superimposable mirror images. Same molecular formula but atoms occupy different positions in space. POSITION ISOMERISM FUNCTIONAL GROUP ISOMERISM

7. What are Structural Isomers? Structural isomers have different structural formulae because their atoms are linked together in different ways.

8. Structural Isomerism Introduction Chaindifferent arrangements of the carbon skeleton similar chemical properties slightly different physical properties more branching = lower boiling point Positionalsame carbon skeleton same functional group functional group is in a different position similar chemical properties - slightly different physical properties Functional Groupdifferent functional group different chemical properties different physical properties Sometimes more than one type of isomerism occurs in the same molecule. The more carbon atoms there are, the greater the number of possible isomers

9. Chain Isomerism These isomers arise because of the possibility of branching in carbon chains. butane, C 4 H 10 pentane, C 5 H 12

10. Structural Isomerism - Chain DIFFERENCES BETWEEN CHAIN ISOMERS Chemical Isomers show similar chemical properties because the same functional group is present. PhysicalProperties such as density and boiling point show trends according to the of the degree of branching Boiling Point “straight” chain isomers have higher values than branched ones. The greater the degree of branching the lower the boiling point branching decreases the effectiveness of intermolecular forces less energy has to be put in to separate the molecules - 0.5°C straight chain - 11.7°C branched greater branching = lower boiling point

11. Position Isomerism In position isomerism, the basic carbon skeleton remains unchanged, but important groups are moved around on that skeleton.

12. Functional Group Isomerism In this variety of structural isomerism, the isomers contain different functional groups – that is, they belong to different families of compounds (different homologous series). molecular formula C 3 H 6 O molecular formula C 3 H 6 O 2

13. molecules have same molecular formula molecules have different functional groups molecules have different chemical properties molecules have different physical properties ALCOHOLS and ETHERS ALDEHYDES and KETONES ACIDS and ESTERS Functional Group Isomerism

14. ALCOHOLS and ETHERS NameETHANOLMETHOXYMETHANE ClassificationALCOHOLETHER Functional GroupR-OHR-O-R Physical propertiespolar O-H bond gives riseNo hydrogen bonding to hydrogen bonding.low boiling point get higher boiling pointinsoluble in water and solubility in water Chemical propertiesLewis baseInert Wide range of reactions Functional Group Isomerism

15. ALDEHYDES and KETONES NamePROPANALPROPANONE ClassificationALDEHYDEKETONE Functional GroupR-CHOR-CO-R Physical propertiespolar C=O bond givespolar C=O bond gives dipole-dipole interactiondipole-dipole interaction Chemical propertieseasily oxidised to acids ofundergo oxidation under same number of carbonsextreme conditions only reduced to 1° alcoholsreduced to 2° alcohols Functional group Isomerism

16. CARBOXYLIC ACIDS and ESTERS NamePROPANOIC ACIDMETHYL ETHANOATE ClassificationCARBOXYLIC ACIDESTER Functional GroupR-COOH R-COOR Physical propertiesO-H bond gives rise No hydrogen bonding to hydrogen bonding.insoluble in water get higher boiling point and solubility in water Chemical propertiesacidicfairly unreactive react with alcohols hydrolysed to acids Functional group Isomerism

17. What are Stereoisomers? Stereoisomers have the same structure and bond order but their atoms and groups of atoms are arranged differently in space. They have different spatial arrangements and their molecules are not super imposable.

18. Geometric Isomerism Involves a double bond, usually C=C, that does not allow free rotation about the double bond (unlike a C Csingle bond). They are not superimposable. e.g. cis-but-2-ene and trans-but-2-ene Stereoisomers may possess quite different physical properties, such as melting point, density and solubility in water. Ring structures and other steric factors also result in geometric isomerism.

19. Geometric Isomerism To get geometric isomers you must have: 1.Restricted rotation (often involving a carbon double bond for introductory purposes); 2. Two different groups on the left-hand end of the bond and two different groups on the right hand end. It doesn't matter whether the left hand groups are the same as the right-hand ones or not.

20. an example of stereoisomerism found in some, but not all, alkenes occurs due to the RESTRICTED ROTATION OF C=C bonds get two forms... CIS (Z) Groups/atoms are on the SAME SIDE of the double bond TRANS (E) Groups/atoms are on OPPOSITE SIDES across the double bond Geometric Isomerism

21. RESTRICTED ROTATION OF C=C BONDS Single covalent bonds can easily rotate. What appears to be a different structure is not. It looks like it but, due to the way structures are written out, they are the same. ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE ‘FREE’ ROTATION Geometric Isomerism

22. RESTRICTED ROTATION OF C=C BONDS C=C bonds have restricted rotation so the groups on either end of the bond are ‘frozen’ in one position; it isn’t easy to flip between the two. This produces two possibilities. The two structures cannot interchange easily so the atoms in the two molecules occupy different positions in space. Geometric Isomerism

23. E/Z or CIS-TRANS E / Z Z (zusammen)higher priority groups / atoms on the SAME side of C=C bond E (entgegen)higher priority groups / atoms on OPPOSITE sides of C=C bond To determine priority, the Cahn, Ingold and Prelog convention is used. eg C 2 H 5 > CH 3 > H and I > Br > Cl > F > C > H E Z Z E Geometric Isomerism

24. E/Z or CIS-TRANS CIS / TRANS Should only be used when there are two H’s and two non-hydrogen groups attached to each carbon. cisnon-hydrogen groups / atoms on the SAME side of C=C bond trans non-hydrogen groups / atoms on OPPOSITE sides of C=C bond cis transcis trans Geometric Isomerism

25. Isomerism in Butene There are 3 structural isomers of C 4 H 8 that are alkenes. Of these ONLY ONE exhibits geometrical isomerism. but-1-ene 2- methylpropene trans but-2-ene (E) but-2-ene cis but-2- ene (Z) but-2- ene Geometric Isomerism

26. How to tell if it exists   Two different atoms/groups attached Two similar atoms/groups attached Two different atoms/groups attached GEOMETRICAL ISOMERISM Once you get two similar atoms/groups attached to one end of a C=C, you cannot have geometrical isomerism Geometric Isomerism

27. The effect of Geometric Isomerism on Physical Properties cis and trans isomers of 1,2-dichloroethene You will notice that: the trans isomer has the higher melting point; the cis isomer has the higher boiling point.

28. Why is the boiling point of the cis Isomers higher? There must be stronger intermolecular forces between the molecules of the cis isomers than between trans isomers. The difference between the two is that the cis isomer is a polar molecule whereas the trans isomer is non-polar. Both molecules contain polar chlorine-carbon bonds, but in the cis isomer they are both on the same side of the molecule. That means that one side of the molecule will have a slight negative charge while the other is slightly positive. The molecule is therefore polar. Because of this, there will be dipole-dipole interactions as well as dispersion forces - needing extra energy to break. That will raise the boiling point.

29. Why is the melting point of the cis Isomers lower? In order for the intermolecular forces to work well, the molecules must be able to pack together efficiently in the solid. Trans isomers pack better than cis isomers. The "U" shape of the cis isomer doesn't pack as well as the straighter shape of the trans isomer. The poorer packing in the cis isomers means that the intermolecular forces aren't as effective as they should be and so less energy is needed to melt the molecule - a lower melting point. trans cis

30. Optical Isomerism Involves an atom, usually carbon, bonded to four different atoms or groups of atoms. They exist in pairs, in which one isomer is the mirror image of the other. These isomers are referred to as enantiomers. The central carbon atom to which four different atoms or groups are attached, is called an asymmetrical carbon atom. Enantiomers have identical physical constants, such as melting points and boiling points, but are said to be optically active since they can be distinguished from each other by their ability to rotate the plane of polarised light in opposite directions. A mixture of enantiomers in equal proportions is optically inactive, and is called a racemic mixture.

31. Chiral & Achiral Molecules A molecule which has no plane of symmetry is described as chiral. The carbon atom with the four different groups attached which causes this lack of symmetry is described as a chiral centre or as an asymmetric carbon atom. The molecule on the left above (with a plane of symmetry) is described as achiral. Only chiral molecules have optical isomers.

32. Detecting Stereoisomers http://www.youtube.com/watch?v=HuHphmJw-fA&feature=related

33. Some real examples of Optical Isomers The asymmetric carbon atom in a compound (the one with four different groups attached) is often shown by a star. The two enantiomers are:

34. Some real examples of Optical Isomers 2-hydroxypropanoic acid (lactic acid) The two enantiomers are:

35. 2-aminopropanoic acid (alanine) The two enantiomers are: Naturally occurring alanine Some real examples of Optical Isomers

37. Glycine

38. Optical Isomers of Sugars

39. Fischer Projections A Fischer projection is used to represent carbohydrates places the most oxidized group at the top shows chiral carbons as the intersection of vertical and horizontal lines

40. D and L Notations In a Fischer projection, the  OH group on the chiral carbon farthest from the carbonyl group determines an L or D isomer. left is assigned the letter L for the L form right is assigned the letter D for the D form

41. Examples of D and L Isomers of Monosaccharides

42. Difference between Starch & Cellulose 1.Cellulose is a polymer of glucose whose units can be rotated around the axis of a backbone of polymer chains of glucose units while starch is a polymer of glucose wherein all the repeat units are directed in one direction. 2.The glucose units in starch are connected by alpha linkages while the glucose units of cellulose are connected by beta linkages. 3.Starch is fit for human consumption while cellulose is not. 4.Starch is soluble in water while cellulose cannot be dissolved in water. 5.Cellulose is stronger than starch. 6.Cellulose is more crystalline than starch. 7.The main function of starch is as food and supplying the body with energy and helps in its proper metabolism while cellulose has a more significant use in the clothing industry and in the production of important materials like cellophane and rayon.

43. Difference between Starch & Cellulose

44. Chiral molecules often react differently with other chiral molecules. This is like the idea that a right hand does not fit a left handed glove – the molecule must be the correct shape to fit the molecule it is reacting with. Many natural molecules are chiral and most natural reactions are affected by optical isomerism. Chiral

45. For example, most amino acids (and so proteins) are chiral, along with many other molecules. In nature, only one optical isomer occurs (e.g. all natural amino acids are rotate polarised right to the left). Many drugs are optically active, with one enantiomer only having the beneficial effect. In the case of some drugs, the other enantiomer can even be harmful, e.g. Thalidomide. Continue…..

46. S thalidomide (effective drug) The body racemises each enantiomer, so even pure S is dangerous as it converts to R in the body. R thalidomide (dangerous drug) Continue…..

47. Thalidomide was banned worldwide when the effects were discovered. However, it is starting to be used again to treat leprosy and HIV. Its use is restricted though and patients have to have a pregnancy test first (women!) and use two forms of contraception (if sexually active). Continue…..

48. S carvone (caraway seed) R carvone (spearmint) Caraway Seed has a warm, pungent, slightly bitter flavour with aniseed overtones. Continue…..

49. S limonene (lemons)R limonene (oranges) Continue…..

50. Why is Isomerism important? The reason that isomers are so important in drug design is that normally, only one particular isomer is effective in treating the condition. Other isomers are less effective or even harmful! The classic example is thalidomide, where one enantiomer helped women overcome morning sickness associated with the early weeks of pregnancy, and the opposite enantiomer caused the horrendous birth defects (lack of arms and legs) seen in what are now known as the "thalidomide babies." It was developed by a German pharmaceutical company and sold between 1957 to 1961 in 50 countries.

51. Importance of Isomers in Fuels A common fuel used today is octane, which has the formula C 8 H 18 or CH 3 (CH 2 ) 6 CH 3. It is the primary component of gasoline. This particular fuel has 18 isomers, each of which is useful for different applications. For most applications, stable burning is the goal in creating fuels. Certain isomers of octane are more useful in achieving this result

52. Importance of Isomers in Fuels Useful Isomers In fuel design, the most useful octane isomers are those which have "branched" structures. These isomers are distinguished from those that have more linear structures. A linear isomer has all its elements connected in a line, whereas a branched may resemble a tree in structure, with each element not necessarily touching other branches. This branched structure causes the octane to burn more slowly and evenly, therefore, the most commonly used octane isomers in fuels are those with branched structures. Octane Rating When gassing your car, you may notice the "octane rating" of the fuel you are using. This rating is a measure of how fast the fuel will burn. The rating is determined both by the isomers used in the octane, as well as the percentage of fuel which is not octane due to the addition of an additive such as ethanol. The octane rating is largely a measure of the isomers in a given fuel.

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