Intermolecular Force

1. Intermolecular Force Enzymes

2. Before knowing more about enzymes, we should have some basic concepts on intermolecular force first. Broadly speaking, there are two types of intermolecular forces, namely van der Waal’s force and hydrogen bond. To begin with, we’ll introduce the basic concepts of van der Waal’s force first.

3. Van der Waals’ force Van der Waals’ force is an attractive force which exists between ALL molecules. Cl Cl -------- Cl Cl Generally, there are altogether 3 types of dipoles: permanent dipole, instantaneousdipole and induced dipole. Covalent bond Van der Waals’ force

4. Permanent dipole Permanent dipole exists in all POLAR molecules. Because of the difference in electronegativity of bonded atoms, the electron density is not evenly distributed between the 2 nuclei. A partial negative charge will form on a more electronegative atom while a partial positive charge will form on a less electronegative atom. Thus, a permanent dipole is formed. e.g. HCl,HF

5. Instantaneous dipole It is a temporary dipole. It exists in NON-POLAR molecules. It occurs due to the chance when electrons happen to be more concentrated in one place than the other in one molecule. e.g. H2,He

6. Induced dipole It occurs when a molecule having a dipole comes close to a non-polar molecule. The non-polar molecule will be induced to form a dipole. The induction is caused by the partial charges on a molecule with a dipole.

7. Van der waals’ forces include 3types of intermolecular attractions, namelypermanent dipole-permanentdipole attraction(e.g. HCl----HCl),Instantaneousdipole-induced dipole attractions(e.g. Ne---Ne) andpermanent dipole-induced dipole attractions(e.g. CHCl3-----CCl4)

8. Permanent dipole-permanent dipole attraction It is also known as dipole-dipole attraction. It is the forces that occur between two molecules with permanent dipoles. Dipole-dipole attraction requires the presence of polar bonds and a unsymmetric molecule. The intermolecular force is weak when compared to a covalent bond. However, this dipole-dipole interaction is one of the stronger intermolecular attractions.

9. It should be noted that at high temperature, the vigorous molecular movement may overcome the dipole-dipole attractions so that the molecules arrange randomly. However, at low temperature, the molecules align regularly in a head-to-tail fashion.

10. Strength of dipole-dipole attraction The strength of dipole-dipole attractions increase when there is a large difference in electronegativities of the atoms forming the bond. Moreover, a large charge separation (long bond length) can increase the strength. Meanwhile, the strength increases if the molecule is not totallysymmetrical so that the bond dipole moment cannot be cancelled out. When the strength of dipole-dipole attraction increase, the boiling point always increases. However, melting point may not increase. It’s because melting point also depends on the symmetry of the molecule. The more symmetrical, the higher is the melting point.

11. Instantaneous dipole-induced dipole attraction It is also known as dispersion force or London force. Instantaneous dipole-induced dipole attractions occur in ALL molecules. It’s because electrons in a molecule move randomly and continuously. At a particular moment, the electron density will be unevenly distributed and become more concentrated on one side of the molecule. This results in forming a temporarily instantaneous dipole. This dipole will immediately induce the neighbouring molecules to form an induced dipole. Hence the instantaneous dipole and the induced dipole will attract each other which result in a weak instantaneous dipole-induced dipole attraction.

13. Strength of dispersion force Dispersion forces between molecules are much weaker than the covalent bonds within molecules. The ease with which an external electric field can induce a dipole (alter the electron distribution) with a molecule is referred to as the "polarizability" of that molecule. * The greaterthe polarizability of a molecule the easier it is to induce a momentary dipole and the stronger the dispersion forces. * Larger molecules tend to have greater polarizability. Their electrons are further away from the nucleus (any asymmetric distribution produces a larger dipole due to larger charge separation) .The number of electrons is greater (higher probability of asymmetric distribution) ,thus, dispersion forces tend to increase with increasing molecular mass.

14. Permanent dipole-induced dipole attraction When a polar molecule approaches a non-polar molecule, the permanent dipole on the polar molecule can distort the electron cloud of the non-polar molecule. This results in the formation of an induced dipole. Hence the permanent dipole and the induced dipole will attract each other. This is known as permanent dipole-induced dipole attraction. e.g. CHCl3---CCl4 It should be noted that dipole-induced dipole attractions are usually weaker the dispersion forces.

15. Summary

16. Hydrogen Bonding

17. What is hydrogen bonding? Hydrogen bond is a kind of electrostatic attraction, a kind of permanent dipole-permanent dipole interaction. The electrostatic force of attraction existing between polar hydrogenδ+ and electronegative atom of dipoles δ- . A hydrogen bond is weaker than a covalent bond but much stronger than van der Waal’s forces. There are 2 types of hydrogen bodings. One is intermolecular hydrogen bonding which is formed between two molecules. The other is intramolecular hydrogen bonding which is formed between 2 different atoms in the same molecule.

18. How can a hydrogen bond form? • Forming a hydrogen bond requires both the following condition : • A hydrogen atom must be directly bonded to a highly • electronegative atom (fluorine, oxygen and nitrogen) or a highly • electronegative entity. In molecules such as HCl, the • intermolecular forces are said to be dipole-dipole interactions • but not hydrogen bonding. • An unshared pair of electrons (lone pair electrons) is present on • the electronegative atom.

19. Anomalous Properties of the Second Period Hydrides A hydride is a compound formed by hydrogen and another element. The boiling points of the Group IVA hydrides increase with increasing molecular masses from carbon to tin. This is in agreement with the change of van der Waal’s forces of the molecules. However, for the hydrides of Groups VA, VIA and VIIA, the general trend is broken by an abnormally high boiling point of the first member of each group, i.e. NH3 and H2O and HF respectively. Why is it so ?

20. Actually, this is due to the high electronegativities of F(4.0), N (3.0) and O (3.5), which lead tothe formation of intermolecular hydrogen bonds in their hydrides. As hydrogen bonds are much stronger than van der Waal’s forces, more energy is required to break the hydrogen bonds in HF, NH and HO. As a result, the boiling points of these three hydrides are relatively higher.

21. The picture shows the boiling points of the simple hydrides of group 4A and 6A elements. In general, the boiling point increases with increasing molecular weight, owing to increased dispersion forces. The compounds NH3 and HF also have many other characteristics that distinguish them from other substances of similar molecular weight and polarity. For example, water has a high melting point, a high specific heat, and a high heat of vaporization. Each of these properties indicates that the intermolecular forces between H2O molecules are abnormally strong.

22. Dimerization of Organic Acids When ethanoic acid is dissolved in non-polar solvents or exists in the vapour state, the molecular mass is found to be 120 instead of 60. This is because the ethanoic acid molecules form dimers (i.e. two molecules bonded together) through the formation of hydrogen bonds. Therefore, the boiling point of ethanoic acids are higher than compounds with similar relative molecular masses. Besides, carboxylic acids of low molecular masses are soluble in water, as hydrogen bonds can also be formed between water molecules and the carboxylic acid groups.

23. The viscosity of liquid alcohols When liquids flow, the intermolecular bond between them are broken. The more hydrogen bonding between comparable molecules, the more hydrogen bonding between comparable molecules, the more viscous the liquid will be. Propan-1-ol can form one hydrogen bond per molecule, as only The hydrogen bonded to the oxygen can form hydrogen bond. (dimers) Propan-1,2-diol is more viscous as it can form two hydrogen bonds per molecule. (chains) Propane-1,2,3-triol forms three hydrogen bonds per molecule and can therefore form 3-D hydrogen bonded networks of molecules. (cross-links)

24. Solubility WATER Hydrogen bond inside water Polarity is an uneven charge distribution within a molecule. In water, one part, or pole, of the molecule is slightly positive and the other slightly negative. This is known as a dipole. It occurs because the oxygen atom has greater electron-attracting power than the hydrogen atoms. As a result,the oxygen atom tends to attract the single electrons of the hydrogen atoms. Electrons are negatively charged, so giving the oxygen atom a slightly negative charge relative to the hydrogen atom. Therefore, water have hydrogen bond. They are constantly being formed, broken and reformed in water. Although individually weak, their collective effect is responsible for many of the unusual physical properties of water.

26. Solubility Water is an excellent solvent for polar substances. These include ionic substances like salts, which contain charged particles(ions), and some non-ionic substances like sugars that contain polar groups (slightly charged) such as the slightly negative hydroxyl group(-OH). On contact with water, the ions and the polar groups are surrounded by water molecules which separate(dissociate) the ions or molecules form each other. This is what happens when a substance dissolves in water.

27. Example 1 Solubilities of Alcohols In an alcohol molecule, there is a hydroxyl group. This hydrogen atom can form a hydrogen bond with the lone pair of electrons on the oxygen atom of a neighboring alcohol molecule. Organic compounds are usually insoluble in water. However, alcohols of low molecular masses like ethanol is soluble in water as the alcohol molecules can form hydrogen bonds with the water molecules. Haloalkanes such as chloroethane do not form hydrogen bonds with water molecules and thus they are insoluble in water.

28. Example 2 When NaCl is added into water…

29. An example of Ionic solute and Polar solvent

30. Enzymes

31. Structure of enzymes As we all know, enzymes are protein in nature. Knowing the structure about protein, we can know what the structure enzyme is. And we can also have deeper understanding about enzyme. The primary structure of a protein is a polypeptide which is a polymer of amino acids. Polypeptide chains form a helical structure owing to the hydrogen bonds formed between the N-H and C＝O groups. This creates the secondary structure of proteins. In many proteins, hydrogen bonding causes the polypeptide chains to become twisted into tightly coiled helices.

32. The most common secondary structure is an extended spiral spring, The αhelix, whose structure is maintained by many hydrogen bonds which are formed between neighboring CO and NH groups. The H atom of the NH group of one amino acid is bonded to the O atom of the CO group four amino acids away. Thus amino acid 1 would be bonded to amino acid5, no. 2 to no.6, and so on. Theoretically, all CO and NH groups can participate in hydrogen bonding as described, so the αhelix is a very stable, and hence a common structure.

33. Figures of protein structure Here are some pictures of protein structure. We can have a better understanding about protein. Amino acid 1 2 3 4 5

35. The structure of an enzyme

36. Effect of temperature and pH on enzyme structure Enzyme is composed of protein. Extreme temperature and pH will cause denaturation. Denaturation is the loss of the specific 3-dimensional conformation of a protein molecule. The change may be temporary or permanent, but the amino acid sequence of the protein remains unaffected. If denaturation occurs, the protein molecule unfold and can no longer perform its normal biological function.

37. How does temperature affect the enzyme structure? When temperature increases, kinetic energy of enzyme and substrate increase. So, they move faster. As a result, this increase the enzyme activity. However, if the temperature continue to increase, more atoms which make up the enzyme molecule will vibratevigorously. This breaks the hydrogenbonds and other forces which hold the molecules in their precise shape. Thus, the 3-dimensional shape of the enzyme molecules is altered so that their active sites can no longer fit the substrate. The enzyme is said to be denatured.

38. As a result, enzyme works best at their optimum temperature. However, the optimum temperature for an enzyme varies considerably. For example, many artic and alpine plants have enzymes which function efficiently at temperatures around 10°C, whereas those in algae inhabiting some hot springs continue to function at temperatures around 80°C.

40. How does pH affect the structure of enzyme? The precise 3-dimensional molecular shape which is vital to the functioning of enzymes is partly the result of hydrogen bonding. These bond may be broken by the concentration of hydrogen ions present.

41. pH is a measure of hydrogen ion concentration.

42. Every enzyme has its own optimum pH. When the pH is altered above or below the optimum pH, the rate of enzymatic activities diminishes. These hydrogen bonds may be broken by the concentration of hydrogen ions present. Thus, any change in pH can effectively denature enzyme.

43. enzyme substrate Normal condition + + + Acidiccondition H+ H+ H+ + + H+ + + H+ H+ H+ H+

44. The important uses of enzymes in daily lives Extension material about enzyme technology

45. Introduction Enzyme technology Enzymes are the biological catalysts which make possible the organized chemical activity of cells. Although we have made use of them for thousands of years, we did not begin to understand how they work until the late nineteenth century. We now know that they are complex protein molecules with specific three-dimensional shapes and that their structure is coded for by DNA. The number of possible overall shapes is infinite.

46. To the industrialist, enzymes have 2 major attractions. Firstly, because of their variety, they have the potential to catalyse a vast range of industrially important chemical reactions. Secondly, they are much more efficient and specific than the inorganic catalysts commonly used. As a result, they may achieve at normal temperatures and pressures what might otherwise require extremely high temperatures and pressures.

47. For example, one of the world’s largest industrial processes is the Haber process in which ammonia (NH3) is produces from nitrogen\ and hydrogen gases at a temperature of 500℃ and at high pressures. Nitrogen-fixing bacteria, however, can make ammonia from atmospheric nitrogen and hydrogen at room temperature and normal atmospheric pressure using enzymes, with ATP as an energy source. If the technology could be devised to do this with enzymes, great energy savings would be made. Another advantage is that, because of their specificity, enzymes also give purer products, which is important in the pharmaceutical, food and agricultural industries.

48. The uses in our daily lives • Enzymes can be used in many useful ways. Some common uses • are : • Fruit juice production • Meat tenderization • Biological washing powders • We’ll introduce something about the Fruit juice production on • the next page.

49. Fruit juice production During the manufacture of fruit juices the fruit must fist be crushed. Like any other part of the plant, fruits are made up of cells with cell walls. These contain cellulose and other complex polysaccharides called hemicelluloses. Cell walls are very tough and difficult to break open. In order to improve the yields and the quality of the product, cellulases and hemicellulases are added during the crushing stage to digest the cellulose and hemicelluloses in the walls, making them more ‘soluble’ and ensuring more complete disintegration of the tissues. The Enzymes are selected to work at low pH because fruit juice is acidic. Their pH optimum is about 4-5. Plant cells are held together by other complex polysaccharides called pectins. These are the sticky compounds which cause jam to set. They tend to be converted into water-soluble pectins during the storage and processing of fruits, and are therefore present in fruit juices in solution even if the ‘bits’, the unbroken cells and cell wall debris, are removed. At low temperatures, the pectins start to come out of solution and form a colloidal suspension. This gives the drink a cloudy appearance. This is an unattractive feature to some consumers. If enzymes called pectinases are added to the juice they partially digest the pectins to smaller Polysaccharides and sugars which remain in solution even at low temperatures. The pectinases therefore clarify the drink. The drink is then described as ‘chill-proofed’.The source of the pectinases is bacteria.

50. The End This project is done by : GROUP 4 Pang Dzit Wai (23) Lee Yu Ying (17)