2. 1 Organic Chemistry MDL233 Chapter 2 BY Mahwash Hafeez

3. 2 1.London attractive forces (van der Waals forces) 2.Dipole – dipole attractive forces 3.Hydrogen bond 4.Ionic attractive forces 5.Competing intermolecular forces and solubility Five concepts for predicting physical properties Physical properties such as melting point, boiling point and solubility are largely determined by intermolecular attractive forces. Using five simple concepts, you should be able to look at structures of group of different organic molecules and predict which might be gases, liquids, or solids and which might be soluble in water. You can predict the boiling point, melting point, or solubility

4. 3 Intermolecular forces: Interactions between molecules. Intramolecular forces: Within the same molecule. Intramolecular forces are also of three types: 1. Ion-ion interactions (ionic bond) 2. Covalent interactions ( Covalent bond) 3. Co-ordinate covalent interactions (co-ordinate covalent bond) Intermolecular forces are of three types: 1) vander Waals forces ( London dispersion forces) 2) dipole-dipole interactions 3) H-bonding Intermolecular Forces

5. 4 Ion – Ion interactions are strong intramolecular forces between oppositely charged ions. e.g. Na + Cl - (oppositely charged particles or ions): The attractive forces between these oppositely charged ions or particles are extremely strong. These are Ion – Ion interactions or electrostatic interactions. Ion – Ion interactions are much stronger than the intermolecular forces in covalent molecules. Intramolecular Forces 1 - Ion – Ion Interactions

6. 5 Sodium acetate mp ˃ 300 potassium acetate mp 306 Acetic acid mp 17

7. Sodium phenolate mp 382 phenol mp 41 C Isopropyl amine mp -95 Isopropyl amine hydrochloride mp -162

8. Covalent Interactions (Covalent Bond) These are intramolecular forces between non metallic atoms formed by mutual sharing of electrons. It can be between two similar atoms or two different atoms having different electronegativities. Examples; O 2, CO 2, C 2 H 6, H 2 O, SiC. It can be of two types: a)Non – polar covalent bond. b)Polar covalent bond. 7

9. Non – Polar covalent bond: Two same atoms having same electronegativities share one or more pairs of outer-shell electrons to form non polar covalent bond. Oxygen Atom Oxygen Molecule (O 2 ) Oxygen Molecule (O 2 )

10. Polar covalent bond: Two different atoms having different electronegativities share one or more pairs of outer-shell electrons to form polar covalent bond. More electronegative Less electronegative Polar covalent Bond element (atom) Element (atom) 9

11. CO- Ordinate Covalent bond Bond formed by sharing of electron but both the electrons come from the same elements. i.e. The sharing is not mutual. This bond is also called dative bond. This bond is represented by an arrow (  )from the donor to the acceptor atom.

12. bond covalent ordinate-Example Of Co

13. 12 Intermolecular Forces 1. Van Der Waals Forces (London forces) They are also called Temporary dipole-temporary dipole interactions. A Temporary dipole is formed when there are momentary changes in electron density in a molecule. These are the only attractive forces(Van Der Waals forces) present in non-polar compounds. Example: CH 4 (no net dipole)

14. Temporary dipole-temporary dipole interaction Forces Cl-Cl e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- non-polar INDUCED DIPOLE   TEMPORARY DIPOLE non-polar Cl-Cl e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e-   Dispersion (weakest and very short-lived)

15. 14 1) All compounds exhibit van der Waals forces. 2) The larger the surface area of a mol, the stronger the van der Waals forces. van der Waals Forces and Surface Area

16. Tokay Gecko: Dispersion Forces!

17. Organic compound that contain only carbon and hydrogen (hydrocarbon) are weekly attracted to each other by Van Der Waals forces These attractive forces increase as molecular size increases. CH4 Mp -180 bp -162 hexane Mp -95 bp +69

18. 17 1)The more polarizable, the stronger the van der Waals Forces. 2)the Polarizability: (1) a measure of how the e - cloud around an atom responds to changes in its electronic environment. (2) Larger atoms (more loosely held valence e - ) are more polarizable than smaller atoms (more tightly held e - ). van der Waals Forces and Polarizability

19. 18 1)Dipole – dipole interactions: the permanent dipole-dipole interactions between polar molecules. 2) much stronger than the van der Waals forces. 2. Dipole – Dipole Interactions

20. Example H Cl ++  Ar e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- A DIPOLE (it’s polar) non-polar INDUCED DIPOLE   Dipole – Induced Dipole (weak and short-lived) b/w a polar & a non-polar molecule

21. 20 The attractive forces between molecules increases when functional groups containing electronegative atoms such as chlorine, oxygen and nitrogen are present. mp -137C bp 0C mp -117C bp +35C mp -94C bp +56C

22. 21 1)Very strong dipole-dipole interactions. 2)Between H bonded to electronegative elements (O, N, X) and a lone pair (2e - ) on an electronegative element. 3)Hydrogen bond occurs among organic molecules containing OH group (alcohol and carboxlic acid) and N-H group 3. Hydrogen Bonding

23. 22 Which of the following have high boiling point Ethane bp. -88 C mp - 172 gas at room temp. ethanol bp. +65 C mp - 97 liquid at room temp. Hydrogen bond is an even stronger intermolecular attractive force

24. 23 As the polarity of a mol increases, the strength of its intermolecular forces increases. Intermolecular Forces—Summary

25. 24 Physical Properties—bp & mp Boiling point (bp): liquid mol.  gas. Melting point (mp): solid  liquid. The stronger the intermolecular forces, the higher the bp & mp.

26. 25 For compounds with similar functional groups: The larger the surface area, the higher the bp. The more polarizable the atoms, the higher the bp. Other Factors Affecting Boiling Points

27. 26 The more compact and symmetrical the shape (a crystalline lactice), the higher the mp. Ex) Neopentane has a much higher mp than isopentane. Effect of Symmetry on Melting Points

28. 27 Solubility: the extent of solute dissolution in a solvent. Hydrophobic and Hydrophilic Hydrophobic: Nonpolar part of a mol, water-insoluble (not attracted to H 2 O). Hydrophilic: Polar part of a mol, water-soluble (H-bond to H 2 O). Solubility

29. 28 “Like dissolves like.” 1) Ionic compounds & polar compounds dissolve in polar solvents. 2) Nonpolar or weakly polar compounds dissolve in: - nonpolar solvents (e.g., CCl 4, hexane). -weakly polar solvents (e.g., diethyl ether) 3) Solubility of organic molecules - Relative size of non-polar portion to a polar portion - Water soluble: ≤ 5 C for one functional group Solubility Trends

30. 29 Competing intermolecular forces and solubility The sum of attractive forces tend to work together to raise melting and boiling point s as the size of molecule’s hydrocarbon skeleton increases polar, incorporated into molecule hydrogen bonded, incorporated into molecule ionic function group is incorporated into molecule Solubility involves the interaction of two different molecules, which may have different types of attractive forces Ex: Adding hydrocarbon to water (oil and water do not mix) Water have hydrogen bond Hydrocarbon is nonpolar and no hydrogen bond

31. 30 Dissolve in water by many ion–dipole interactions. Figure 3.4 Solubility of Ionic Compounds

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34. 33 Relative size of non-polar portion to a polar portion Water soluble when; ≤ 5 C for one functional group For example, Solubility of Organic Molecules

35. 34 Solubility Properties of Representative Compounds