Intermolecular Forces & Refraction Student Notes PDF

Intermolecular Forces & Refraction Dr Lynda Storey Room: N548 X2528 [email protected]  Summary of Last Lecture A polar covalent bond results from the uneven share of electrons dictated by electronegativity. Polarity is denoted by the symbols d+ and...

Intermolecular Forces & Refraction Dr Lynda Storey Room: N548 X2528 [email protected]  Summary of Last Lecture A polar covalent bond results from the uneven share of electrons dictated by electronegativity. Polarity is denoted by the symbols d+ and d-, partial positive and partial negative charge. The higher the dielectric constant, the more polar the compound is. Polar compounds dissolve in polar solvents; Nonpolar compounds dissolve in nonpolar solvents; “Like” dissolves “like”!  Homework from Last Lecture 1. Place partial charges on atoms indicated in red: H O Li Cl O N 2. On the above structures draw bond dipoles. http://www.chemguide.co.uk/atoms/bonding/electroneg.html#top  Learning Objectives Be able to describe the intermolecular forces which may operate between polar and non-polar molecules. Understand what is meant by the term refractive index. Explain why the refractive index can vary. Be able to explain how the refractive index may be measured.  Intermolecular Forces Intermolecular forces are the forces between molecules. Intermolecular forces influence chemistry in many ways: – melting point, boiling point, the energy required to convert a solid to a liquid or a liquid to a vapour. – solubility in determining solubility of gases, liquids, and solids in various solvents – crucial in determining the structures of biologically important molecules such as DNA and proteins.  Intermolecular Forces Involving Polar Molecules Interactions Between Ions and Ions Ions (e.g. Na+ Cl-) are held together by very strong Coulombic attractions. These interactions are in the order of 500 kJ/mol. In the solid state each Na+ is surrounded by 6 Cl- ions, and each Cl- is surrounded by 6 Na+ ions.  Intermolecular Forces Involving Polar Molecules Ion / Dipole Interactions Polar molecules, such as H2O, have a positive & a negative end. The positive end will be attracted to a negative anion. The negative end will be attracted to a positive cation. Forces of attraction between positive or negative ions and polar molecules are called ion / dipole forces. Intermolecular Forces Involving Polar Molecules Examples of hydrated salts H H O O H H H H O O H H Br- Na+ H H H O O H H H H H O O O Water surrounding an anion H H Water surrounding a cation  Intermolecular Forces Involving Polar Molecules Ion / Dipole Interactions The closer the ion and dipole, the stronger the attraction The higher the ion charge, the stronger the attraction. The greater the magnitude of the dipole, the stronger the attraction.  Intermolecular Forces van der Waals Forces These are non-ionic interactions between molecules. They involve no charge-charge interactions. They may determine: – organic reactions between molecules (e.g. SN1, SN2, etc) – how drugs may be formulated – interactions between drugs and targets  Intermolecular Forces van der Waals Forces There are three main types: Dipole / Dipole (Keesom Forces) Dipole / Induced-Dipole (Debye Forces) Induced-Dipole / Induced Dipole (London Dispersion Forces) (although most people refer to these as van der Waals)  Intermolecular Forces Involving Polar Molecules Dipole / Dipole (Keesom Forces) When a polar molecule encounters another (or the same) polar molecule, the positive end of one is attracted to the negative end of the other. This is known as dipole / dipole attraction. H H H C O H H H O  Intermolecular Forces Involving Polar Molecules Hydrogen Bonding Hydrogen bonds are an extreme form of dipole / dipole interaction. One atom is always a hydrogen and the other atom is usually highly electronegative, e.g. O, N, F. Hydrogen bonding is extremely important and gives rise to the unusual properties of H2O. The base pairs in DNA form a helix due to Hydrogen bonding. Intermolecular Forces Involving Polar Molecules Why do snow flakes have 6 sides? Intermolecular Forces Involving Polar Molecules Hydrogen Bonding O H O H O O O H O O O H H O O  Intermolecular Forces Involving Non-Polar Molecules Dipole / Induced Dipole (Debye Forces) Polar molecules can create, or induce, a dipole in a molecule that does not have a permanent dipole. H O O O O O H Induced dipole Neutral (non-polar) compound  Intermolecular Forces Involving Non-Polar Molecules Dipole / Induced-Dipole (Debye Forces) The O2 molecule is symmetrical, there is no dipole. As the negative end of the polar H2O molecule approaches, however, the O2 electron cloud distorts. In this way, the O2 molecule becomes polarized. A dipole is induced, or created. The result is that the H2O and O2 molecules are now attracted to one another. This is why O2 can dissolve in water. This is referred to as dipole / induced-dipole interactions.  Intermolecular Forces Involving Non-Polar Molecules Dipole / Induced-Dipole (Debye Forces) The higher the molecular mass, the larger the electron cloud and the greater the polarizability of the molecule. The solubility of some gases in water Gas Molar Mass Solubility at 20C (g/mol) (g gas /100 g water) H2 2.01 0.000160 N2 28.0 0.000190 O2 32.0 0.000434  Intermolecular Forces Involving Non-Polar Molecules Induced Dipole / Induced Dipole (London Dispersion Forces) As two non-polar molecules approach each other, their electron clouds become distorted, thereby producing momentary dipoles, leading to intermolecular attraction. O O No dipole O O O O O O Induced dipole Induced dipole  Intermolecular Forces Bond Energy Type of Bond Bond Energy (kJ/mole) Ion / ion ~ 500 Ion / dipole 40 – 600 Dipole / dipole 20 – 30 Hydrogen Bonding 5 – 30 Dipole / induced dipole 2 – 10 Induced-dipole / induced-dipole 0.05 – 40  Intermolecular Forces What type of Intermolecular Force? Carbamazapine Pyridoxine N N OH O NH2 O O OH H OH H R O O R' H H O  Refractive Index Light passes slower through substances than it does through a vacuum. Light Vacuum Light Atoms The atoms absorb some of the energy from the light causing the light waves to shorten and the speed to reduce.  Refractive Index If light strikes a surface at an angle, the light becomes bent towards the perpendicular line to the surface. Less Dense Substance (1) e.g. Air i Surface of substance More Dense Substance (2) r This is called refraction  Refractive Index If light enters a less dense substance, the light bends away from the interface More Dense Substance (1) i Surface of substance r Less Dense Substance (2)  Refractive Index Refraction is measured by the refractive index (n): velocity of light in substance 1 C1 n= = = velocity of light in substance 2 C2 Substance 1 is usually air. Therefore the refractive index is usually greater than 1.  Refractive Index The refractive index varies with: – Wavelength of light – temperature These two variables are therefore identified for a refractive index: 20 nD 20 = 20C D = D-line emission of sodium (589 nm)  Refractive Index A refractive index can: – identify a substance – be a measure of purity – determine concentration of one substance dissolved in another Important pharmaceutically for compounds which do not exhibit extensive UV-Vis absorption (i.e. compounds without a chromophore).  Refractive Index A refractometer is used to determine the refractive index.  Summary Intermolecular forces influence many physicochemical properties (e.g. Mpt, solubility, shape & crystal structure). Intermolecular forces are to do with positive & negative charges, partial charges & induced charges. Intermolecular forces determine drug-target interactions. The refractive index is a measure of the speed of light through a substance and can be used to help identify that substance.  Homework A pure substance has an angle of incidence of 48.41 and an angle of refraction of 34.13 at 20 C using a wavelength of 589 nm. The substance is 20 n D A Methanol 1.329 B Water 1.333 C Ethanol 1.362 D Benzene 1.501  Homework What intermolecular forces are at play between these two molecules: O O O O H O H O