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CHM 456

CHM 456. Organic Chemistry 1. 1.0 Carbon Compounds and Chemical Bonds. Objectives. Understand what a bond is Why they form Lewis strictures (assigning e- in bonds) Hybridization ( mixing of orbitals) Molecular shapes Types of bonds (covalent, ionic, dative)

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CHM 456

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  1. CHM 456 Organic Chemistry 1 1.0 Carbon Compoundsand Chemical Bonds

  2. Objectives • Understand what a bond is • Why they form • Lewis strictures (assigning e- in bonds) • Hybridization (mixing of orbitals) • Molecular shapes • Types of bonds (covalent, ionic, dative) • Formal Charge (calculatione) • Resonance (spreading of charge)

  3. What is a bond? • A electrostatic attraction between electrons and the protons inside of two (or more) nuclei. • Hydrogen (the simplest case) - attracted to + http://www.middleschoolchemistry.com/multimedia/chapter4/lesson4

  4. Why do bonds form? • Due ultimately to Entropy (symbol ‘S’) • Think of entropy as the disordered distribution of energy. • A process can only occur if it leads to a overall increase in entropy. Many bond forming reactions lead to an increase the entropy.

  5. Why do bonds form? • A chemical reaction increases the amount of entropy usually by giving off heat energy (enthalpy) (exothermic reaction), which then randomly disperses throughout the surroundings (i.e. the universe). • Sometimes reactions are endothermic, so increase in entropyby the products formed is greater than the heat (enthalpy) loss of the surroundings (Heat energy, exothermic) and Entropy (Energy of ‘disorder’) increases

  6. MORSE CURVE Repulsive Energy 0 Attraction Energy http://www.webchem.net/notes/chemical_bonding/covalent_bonding.htm

  7. How quickly do bonds form • The speed (rate) at which a reaction occurs depends on a few factors, perhaps the most important is the reactions ACTIVATION ENERGY • The minimum amount of energy necessary for a chemical reaction to occur. It acts as a ‘barrier’, preventing reaction. http://www.chem.msu.su/eng/teaching/Kinetics-online/chapter6e_ad.html

  8. Reaction rate, Activation Energy. • A HIGHER the activation energy (symbol Ea) , more of a barrier, the slower the rate of reaction. • The LOWER the activation energy, less of a barrier, the quicker the reaction will occur. Ea ↑, then rate ↓

  9. Types of bonds 1) Formation of covalent bonds: A pair of e- is shared between two atoms. Each atom contributing one electron. • A subcategory of this is dative covalent bonds or coordinate bonds. A pair of e- is shared between two atoms, both e- come from just one atom The bond is due to the mutual(joint) attraction for the shared e- pair between the nuclei s responsible for the 2) Ionic bonds. Electrons are transferred from one atom to another.

  10. Types of bonds 2) Formation of Ionic bonds. Electrons are transferred from one atom to another. The bond is due to the electrostatic attraction between the resulting positive and negative ions.

  11. Dual character • In reality many substances show a degree of each type of bonding. E.g. CaI2 is ionic with partially covalent character. • AlBr3 is covalent with partially ionic character This is due to polarisation

  12. http://www.ibchem.com/IB/ibnotes/full/bon_htm/4.2.htm

  13. Dot cross diagrams http://2a2science.blogspot.com/2011_02_10_archive.html

  14. http://www.ibchem.com/IB/ibnotes/full/bon_htm/4.2.htm

  15. Electronegativity • VERY IMPORTANT • Applies to covalent bonds • Causes the polarisation of covalent bonds

  16. http://www.pta210.info/Handouts/Electronegativity%20Table.gifhttp://www.pta210.info/Handouts/Electronegativity%20Table.gif

  17. Molecular SHAPE (VSEPR) • VSEPR in Chemistry (Low).flv (relative link) • VSEPR Theory (Low).flv (video 2)

  18. http://www.masterorganicchemistry.com/2010/09/24/how-to-calculate-formal-charge/http://www.masterorganicchemistry.com/2010/09/24/how-to-calculate-formal-charge/

  19. Intermolecular forces NEED TO KNOW ABOUT ELECTRONEGATIVITY AND MOLECULAR SHAPE • These are attractions between (or ‘in between’) DIFFERENT independent species, e.g. water molecules, or Br2 molecules or CO2 molecules. • The are NOT the same as the BONDS that are responsible for the formation of molecules ( or ion pairs).

  20. Intermolecular foeces.Van der Waals forces 1/3 Hydrogen bond. A H atom is bonded to an N, O or F atom and interacts with a lone pair of electrons on a different N, O or F Found in: Water, HF, ammonia, alcohols, carboxylic acids, amines, water with propanone, water with ethanal

  21. Water molecules http://alevelnotes.com/?id=135

  22. 2/3 Dipole-dipole forces.A polarised part of a molecule attracted to a different and oppositely charged polarised part of a molecule. Need to know about ELECTRONEGATIVITY ! and molecular shape Adapted from http://flatworldknowledge.lardbucket.org/books/introductory-chemistry/section_14_01.html

  23. 3/3 Induced dipole-induced dipole forces(London dispesion forces, or temporary dipole – temporary dipole) Electrons clouds distort randomly and at one instant become polarised which can cause polarisation in adjacent molecules. http://www.dlt.ncssm.edu/tiger/chem3.htm

  24. IMF videos - Watch these videos! • http://www.youtube.com/watch?v=S8QsLUO_tgQ&NR=1&feature=endscreenDr Paul McCord RELATIVE LINK Summary Videos = http://www.youtube.com/watch?v=dQ33TVQCfyYRELATIVE LINKNote specific animation of induced-induced dipoles = from 4m:39s onwards

  25. Nice (but brief) website on molecular geometry: http://www.elmhurst.edu/~chm/vchembook/208introgeom.html

  26. spdf notation

  27. spdf notation http://titanchem1.blogspot.com/

  28. Hybridization

  29. Sigma (s) bonding sigma bonds have a plane of symmetry along an internuclear axis. http://89.97.218.226/web1/letiochem/lezioni/sigma-pigreco.htm

  30. pi (π) bonding Nodal pane http://89.97.218.226/web1/letiochem/lezioni/sigma-pigreco.htm

  31. WEAKER ATTRACTION (=weaker bond =more reactive bond) of e- in pi-bond to the nucleus as e- are further away!!! Double bonds http://chempaths.chemeddl.org/services/chempaths/?q=book/General%20Chemistry%20Textbook/Further%20Aspects%20of%20Covalent%20Bonding/1345/sigma-and-pi-bonds

  32. C≡C (triple bond) http://chemistrypractice.blogspot.com/2010/10/multiple-bond.html

  33. Other representations… http://www.wag.caltech.edu/home/jsu/Thesis/node46.html http://www.brynmawr.edu/chemistry/Chem/Chem103Lkdl/consider/consider5.htm

  34. Other representations… http://www.chem.ufl.edu/~itl/4412_aa/ccbond.html

  35. http://www.chem.ufl.edu/~itl/2045/lectures/lec_15.html

  36. Hybridization

  37. Hybridization http://iverson.cm.utexas.edu/courses/310M/Handouts/Handoutsfl05/MOVBTheory.html

  38. Hybridization • http://www.youtube.com/watch?v=g1fGXDRxS6k VIDEO: Hybridization (Low).flv (US) • http://www.youtube.com/watch?v=R-sxDXnOgnk VIDEO: Hybridisation Hybridization (sp, sp2, sp3) (Low).flv (compilation)

  39. Antibonding • When you atomic orbitals coming together (in a chemical reaction) to form molecular orbitals, you produce bonding molecular orbitals AND anti-bonding molecular orbitals. • Read:http://www.chem.ufl.edu/~itl/2045/lectures/lec_15.html for a good discussion of this. • and also

  40. Bonding and Anti-bonding

  41. http://www.chem.ufl.edu/~itl/2045/lectures/lec_15.html

  42. ATOMIC ORBITALS  MOLECULAR ORBITALS http://bouman.chem.georgetown.edu/S00/exam1/exam1.htm

  43. Formal Charge • Describes the localised charge (on atoms) in a molecule. It’s a useful tool*. • For an atom. Consider it’s electron environment. Then calculate the formal charge Formal charge = [# of valence electrons] – [electrons in lone pairs + 1/2 the number of bonding electrons] VIDEO: Formal Charges (Low).flv

  44. Formal Charge Alternatively (slight ‘simplification’ of the formula)… • Formal Charge = [# of valence electrons on atom] – [non-bonded electrons + number of bonds].

  45. Resonance • A HUMAN IDEA / TOOL / METHOD to describe/show/predict the delocalisation of change. • Delocalisation = spreading out of electrons! • Delocalisation occurs to lower a molecules (or molecular ions) energy, chiefly by distributing negative charge to positive areas and lowering repulsion. http://chemwiki.ucdavis.edu/Theoretical_Chemistry/Chemical_Bonding/Resonance

  46. Resonance http://chemwiki.ucdavis.edu/Theoretical_Chemistry/Chemical_Bonding/Resonance << GOOD SITE!

  47. Resonance contd. Resonance video: Drawing Lewis Structures Resonance Structures - Chemistry Tutorial http://www.youtube.com/watch?v=MWDL5WCZBzE (Low).flv(RELATIVE LINK) • Resonance and Predicting Chemical Reactivity (http://www.youtube.com/watch?v=oOBNJCNnFE8) Higher Level. 29mins

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