1 / 29

Noncovalent Molecular Forces - Part 1 Lecture Supplement page 200

d -. d -. d +. d +. F -. d +. d -. d +. d -. Noncovalent Molecular Forces - Part 1 Lecture Supplement page 200. Biological properties Drug effects etc. Chemical properties Reactions Reactivity. Physical properties Boiling point Solubility etc. Chemistry 14C Part 3.

vernon
Download Presentation

Noncovalent Molecular Forces - Part 1 Lecture Supplement page 200

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. d- d- d+ d+ F- d+ d- d+ d- Noncovalent Molecular Forces - Part 1Lecture Supplement page 200

  2. Biological properties Drug effects etc. Chemical properties Reactions Reactivity Physical properties Boiling point Solubility etc. Chemistry 14C Part 3 Structure Controls Everything  + Example: Penicillin G (an antibiotic) Ring strain allows irreversible enzyme inhibition Influence antibiotic behavior Salt enhances water solubility

  3. which causes Noncovalent Molecular Forces • Definition: Attractive forces (other than covalent bonding) between atoms or molecules • Why should I study this? Noncovalent forces control association of molecules Physical properties: Melting point, boiling point, solubility, etc. Molecular organization into larger structures: Membranes, etc. Molecular recognition: Substrate/enzyme docking, etc.

  4. Evaporation How Do We Measure Noncovalent Forces? Consider evaporation of water... • Stronger attraction = more energy required to disrupt attraction • = more energy needed for evaporation • = higher boiling point (bp) • Boiling point easily measured • Therefore bp a useful approximation of attractive forces add energy Water vapor Liquid water • Attractive force disrupted • Attractive force keeps molecules close • Covalent bonds still intact

  5. } Therefore mp is ambiguous measure of attractive forces How Do We Measure Noncovalent Forces? • Boiling point (bp): Temperature at which vapor pressure of substance = ambient pressure • Melting point (mp): Influenced by... • Attractive forces • Crystal packing What Kinds of Noncovalent Forces Occur? Consider these substances: NaCl bp 1413 oC H2O bp 100 oC BrF bp 22 oC Ar bp -186 oC Increasing boiling point indicates increasing magnitude of attractive forces

  6. = Na+ = Cl- NaClbp 1413 oC • What attractive force is operating? • What is the nature of association between Na and Cl? Cl Na 3.0 EN 0.9 • DEN = 3.0 - 0.9 = 2.1 • Attractive force = ionic bond = anion-cation • = electrostatic (opposite charges attract) • Evaporation or melting = separating opposite charges • NaCl evaporates as Na+ and Cl- not Na-Cl • NaCl heat of vaporization = 188 kcal mol-1 • High bp and mp typical of ionic compounds

  7. Ionic versus Covalent Bonds Chemical bond = sharing of electron pair • Ionic bond: Highly unequal sharing of electron pair • Covalent bond: Approximately equal sharing of electron pair Increasing DEN causes Increasing ionic (polar) character • Reducing bond length reduces polarity Example: C–H DEN = 0.4; short bond  polarity

  8. BrFbp 22 oC (liquid at room temperature) • What attractive force is operating? • What is the nature of association between Br and F? DEN = 4.0 - 2.8 = 1.2 = not ionic = polar covalent F Br 4.0 EN 2.8 d+ d- d+ d- • Attractive force = dipole-dipole • = electrostatic (d+/d-) • Bp suggests dipole-dipole attraction weaker than cation-anion

  9. H O EN d+ d+ d+ d- d+ H2Obp 100 oC (liquid at room temperature) • What attractive force is operating? • What is the nature of association between H and O? DEN = 3.5 - 2.1 = 1.4 = not ionic = polar covalent 2.1 3.5 d- Attractive force = electrostatic = dipole-dipole = hydrogen bonding

  10. Hydrogen bond donor X H A d+ d- d+ Hydrogen Bonding d- d+ d+ In general... Hydrogen bonding requires a donor and an acceptor • Hydrogen bond acceptor • Has high electron density to attract d+ • Must have lone pair • Can be small, neutral atom: O or N • Or can be any anion • Examples: H2O, (CH3)3N, I- Must have large d+ X = high EN = F, O, N (rarely anything else)

  11. http://en.wikipedia.org/wiki/File:Hex_ice.GIF

  12. d- d- d+ d+ F- d+ Adenine Thymine d- d+ d- Hydrogen Bonding • Common attractive force • Important in biology • Not always dipole-dipole • Example: F- in CH3CH2OH • Many O-H, N-H, H2O in organisms • Example: DNA base pairs • Also influences protein structure • Hydrogen bond strongest when linear Dynamic: H-O-H OH2 ~3 x 10-12 s lifetime ||||||

  13. Adenine Thymine Guanine Cytosine Fig 26-11 Vollhardt

  14. Arbp -186 oC (monoatomic gas at room temperature) • What attractive force is operating? • Ionic? No electronegativity difference  no ions • Dipole-dipole? No covalent bonds  no bond dipoles • Hydrogen bonding? No hydrogens • No attractive force? = no energy required for vaporization? • Bp -186 oC > -273 oC (absolute zero) • Therefore some attractive force must be present • Bp is very low so attractive force must be weak • Student homework: Figure out what attractive force exists between two Ar atoms

  15. Noncovalent Molecular Forces - Part 2Lecture Supplement page 206

  16. Summary of Part 1 • Physical properties such (boiling point, solubility, etc.) controlled by noncovalent association • Stronger attractive force = more energy required for vaporization = higher boiling point • Noncovalent attractive forces caused by electrostatic attractions Examples NaCl bp 1413 oC Noncovalent attractive force = anion-cation BrF bp 22 oC Noncovalent attractive force = dipole-dipole H2O bp 100 oC Noncovalent attractive force = hydrogen bonding • H-bond donor usually O-H or N-H bond • H-bond acceptor = neutral atom with lone pair and high d- = O or N or any anion with lone pair H-bonds of wide biological importance: Protein and DNA structure etc. Ar bp -186 oCNoncovalent attractive force = ?

  17. e- d+ e- e- d- e- e- e- e- e- e- e- e- Ar Ar Ar Ar e- e- ball of electrons e- e- } e- Weak force Arbp -186 oC (monoatomic gas at room temperature) What attractive force is operating? • Ionic? Dipole-dipole? Hydrogen bonding? • No attractive force? = no energy required for vaporization? • bp -186 oC > -273 oC (absolute zero) so some weak attractive force must be present d+ d- • Induced charges • Momentary electrostatic attraction • Called London force • All molecules have electrons so all molecules influenced by this force

  18. Increasing ease of electron cloud distortion Increasing attraction Strength of London Forces What influences strength of London forces? Boiling point -269 oC -246 oC -186 oC -152 oC -107 oC -62 oC Atomic radius 0.32 Å 0.69 Å 0.97 Å 1.10 Å 1.30 Å 1.45 Å • Polarizability: Ability to distort electron cloud • Distortion easy = soft Example: Rn • Distortion difficult = hard Example: He What controls polarizability? • Larger atomic radius = softer • Larger electronegativity = harder • Surface area effect?

  19. CH4 CH3CH3 CH3CH2CH3 CH3CH2CH2CH3 Strength of London Forces Surface area effect? • Compare molecules with same polarizability but different surface areas • Hydrogens = small = hard Boiling point: -162 oC -88 oC -42 oC -0.5 oC Surface area: 56.6 Å2 80.1 Å2 102.7 Å2 125.2 Å2 MW: 16 30 44 58 • Conclusion • Increasing surface area = increasing London force • Maybe just a molecular weight effect?

  20. Pentane 2-Methylbutane 2,2-Dimethylpropane Strength of London Forces Molecular weight effect? • Compare molecules with same polarizability and different surface areas, but same molecular weight, such as isomers of C5H12 Boiling point: 36 oC 30 oC 9.5 oC Shape: Most elongated Most spherical Highest surface area Lowest surface area Conclusion Higher surface area = _______________ attraction = ________________ London forces stronger stronger

  21. Cl- Na+ K+ Other Noncovalent Interactions • Cation-pi • Cation attracted to pi electron cloud Ion-dipole • Bond dipole attracted to anion or cation • Example: Na+ and Cl- in water d+ d- H O H d+ Aromatic ring pi cloud • Explains water solubility of NaCl • Important in some enzyme-substrate binding

  22. Aromatic rings Other Noncovalent Interactions Pi stacking Important in DNA • Also called aromatic stacking

  23. Relative Strength of Noncovalent Forces • Approximate ranking of noncovalent force strengths useful Dipole-dipole Hydrogen bonding Ion-dipole Cation-pi Pi stacking Cation-anion (ionic bonds) Covalent bonds > > London forces Weakest force Strongest force • When more than one force operates, strongest force dominates • Example: In CH3OH evaporation, H-bonding harder to overcome than London forces

  24. Water + Acetic acid + Oil (glycerol tristearate) Application of Noncovalent Interactions: Solubility Oil layer Vinegar layer (water + acetic acid) • Questions • Why acetic acid dissolves in water? • Why oil does not dissolve in water?

  25. AAAAAAAAAAAAAAAAAA BBBBBBBBBBBBBBBBBB Application of Noncovalent Interactions: Solubility What causes one substance to dissolve in another? • Solubility is an equilibrium issue... ABABABA ABABABA BABABAB ABABABA Two layers A and B immiscible Homogeneous A and B dissolve • Dissolving: A interrupts attractive forces in B • Soluble: A/B attractions better than A/A and B/B attractions • Insoluble: A/B attractions not better than A/A and B/B attractions • “Better” = stronger attractions and/or more attractions

  26. Water + acetic acid: Polar bonds Hydrogen bond donor Hydrogen bond acceptor Water + oil: Application of Noncovalent Interactions: Solubility • Many attractive interactions • Water + CH3COOH soluble • Poor attraction between oil and water • Strong attraction between water and water • Water + oil insoluble Like dissolves like

  27. Melamine- Cyanuric Acid Solubility in water: 2.7 g/L Melamine Solubility in water: 3.2 g/L

  28. Melamine cyanurate Insoluble in water…

More Related