Unit One Part 5: intermolecular forces

Transcription

1 Unit ne Part 5: intermolecular forces 1 Bond polarisation and molecular dipoles Intermolecular forces (attraction / repulsion between molecules Relative strengths of these forces Briefly look at each of the different intermolecular forces dr gareth rowlands; g.j.rowlands@massey.ac.nz; science tower a4.12

2 Intermolecular forces 2 pentane bp methylbutane bp 28 2,2-dimethylpropane bp 9.6 Bonding within a molecule obviously has a great effect on its properties But as important is the forces between molecules - intermolecular forces The 3 isomers above have different bp due to different intermolecular forces 2-methylpropan-2-ol tert-butanol 4 10 mp 26 ethoxyethane diethyl ether 4 10 mp 116

3 Dipoles and dipole moments 3 = Bond dipole charge separation We have seen that different atoms attract electrons by varying amounts The greater the difference the larger the bond dipole Molecular dipole is the sum of the bond dipoles molecular dipole molecular dipole no molecular dipole A molecule with polar bonds and a molecular dipole is polar A molecule that has no overall molecular dipole is nonpolar

4 Inductive effects or 3 > 2 > or 3 2 Bond dipoles can induce a small dipole into neighbouring bonds ere chlorine is an electron-withdrawing group Alkyl groups are electron-donating groups 3 > Effect on chemistry is shown with the strength of carboxylic acids 3 3 Lower the pka value the easier it is to remove 3 pk a = 4.75 pk a = 2.85 pk a = 1.48 pk a = 0.70 < pk a = 2.85 pk a = 4.05 pk a = 4.50 «

5 Intermolecular forces 5 covalent bond (strong) intermolecular attraction (weak) intramolecular forces intermolecular forces interaction typical energy (kjmol 1 ) ionic-ionic (ionic bond) 250 carbon-containing covalent bond 350 oxygen-hydrogen covalent bond 460 hydrogen (-) bond 20 ion-dipole 15 dipole-dipole 2 London (dispersion) 2 Intermolecular forces - affect physical properties (mp, bp etc.) Intramolecular forces - govern chemical reactions

6 Ion-Dipole Forces 6 δ-- a δ-- harged ion (+ / ) interacts with oppositely charged part of molecule with a permanent dipole ( / δ +) Ion-dipole interactions are a strong intermolecular force (15 kjmol 1 ) Explains how water dissolves salts (ionic compounds) rganic molecules can interact in the same way a

7 Dipole-Dipole forces 7 Molecules with permanent dipoles orient themselves to match charges These interactions are quite weak (>> 2kJmol 1 ) Thus polar molecules can interact with one another & mix (miscible) 3 3 an have profound effect on boiling points propanone acetone Mol Wt. 58; bp 56 permanent dipole butane 2 3 Mol Wt. 58; bp 0.6 no dipole

8 ydrogen Bonding 8 hydrogen bond Special example of dipole-dipole attraction Requires in highly polar bond (joined to, etc.) - -bond donor Requires lone-pair on electronegative atom (, & F) - -bond acceptor Strongest intermolecular force (>> 20kJmol 1 ) Explains water's abnormally high mp & bp 2 (MW=18), bp 100 2S (MW=34): bp 60 4 (MW=16): bp 162

9 ydrogen bonding II Methanol has only one attached to It can only form one ydrogen bond Its bp is lower than water (62 ) arboxylic acids form extensive -bond arrays Acetic acid has two polarised bond and forms dimers readily bonding explains why acetic acid is so soluble in water 3

10 ydrogen bonding in biology 10 R 1 R 2 R 3 R 4 -bonding occurs between two different heteroatoms Interaction between = & important in protein secondary structure Whilst & are important -bond acceptors in DA base pairs 3 adenine thymine

11 London (dispersion or van der Waals) forces 11 momentary dipole induced dipole London forces are weak forces found in all molecules Electrons move to create small, temporary dipole This dipole can induce a dipole into nearby molecules & thus create attraction Bigger the molecules surface area the greater the London force / dipole

12 London forces II 12 methane 4 (MW=16) mp 182 ; bp 164 gas at rt hexane 6 14 (MW=86) mp 95 ; bp 69 liquid at rt eicosane (MW=282) mp 36 ; bp 343 solid at rt Longer the chain the bigger the possible dipole & greater the attraction More branching in an alkane decreases relative surface area & reduces London forces pentane bp 36 2,2-dimethylpropane bp 9.5

13 Solvation - interaction between molecule and the solvent molecules ydrophobic - molecules/motifs are insoluble in water (e.g. greasy chains) ydrophilic - molecules / functional groups are soluble in water (e.g. -) Polar materials dissolve in polar solvents Solvation compound bp ( ) solubility (g/100ml 2) 13 propanol infinite butanol 3(2) propanoic acid infinite butanoic acid 3(2) infinite hexanoic acic 3(2) decanoic acid 3(2)82 insoluble

14 Solvation II 14 Does glucose dissolve in water or hexanes? ydroxyl groups are polar & -bond to water - so soluble in water What about DEET, an insect repellant? Large nonpolar aromatic ring, polar amide but no -bonding capability So better in ethanol than water 3 3 3

15 verview 15 What have we learnt? That molecules are attracted to each other Many different forms of attraction These forces have profound effects on physical properties What's next? Looking at chemical reactions (yippee) Looking at reaction profiles (boo) Shapes of molecules (the return of...) dr gareth rowlands; science tower a4.12