1. CHE 240 Unit II Chemical Reactions, Infrared and Mass Spectroscopy Terrence P. Sherlock Burlington County College 2004

2. Chapter 42 Tools for Study To determine a reaction’s mechanism, look at:  Equilibrium constant  Free energy change  Enthalpy  Entropy  Bond dissociation energy  Kinetics  Activation energy =>

3. Chapter 43 Chlorination of Methane Requires heat or light for initiation. The most effective wavelength is blue, which is absorbed by chlorine gas. Lots of product formed from absorption of only one photon of light (chain reaction). =>

4. Chapter 44 Overall Reaction =>

5. Chapter 45 Termination Steps Collision of any two free radicals Combination of free radical with contaminant or collision with wall. Can you suggest others? =>

6. Chapter 46 Equilibrium constant K eq = [products] [reactants] For chlorination K eq = 1.1 x 10 19 Large value indicates reaction “goes to completion.” =>

7. Chapter 47 Free Energy Change  G = free energy of (products - reactants), amount of energy available to do work. Negative values indicate spontaneity.  G o = -RT(lnK eq ) where R = 1.987 cal/K-mol and T = temperature in kelvins Since chlorination has a large K eq, the free energy change is large and negative. =>

8. Chapter 48 Factors Determining  G  Free energy change depends on  enthalpy  entropy  H  = (enthalpy of products) - (enthalpy of reactants)  S  = (entropy of products) - (entropy of reactants)  G  =  H  - T  S  =>

9. Chapter 49 Enthalpy  H o = heat released or absorbed during a chemical reaction at standard conditions. Exothermic, (-  H), heat is released. Endothermic, (+  H), heat is absorbed. Reactions favor products with lowest enthalpy (strongest bonds). =>

10. Chapter 410 Entropy  S o = change in randomness, disorder, freedom of movement. Increasing heat, volume, or number of particles increases entropy. Spontaneous reactions maximize disorder and minimize enthalpy. In the equation  G o =  H o - T  S o the entropy value is often small. =>

11. Chapter 411 Bond Dissociation Energy Bond breaking requires energy (+BDE) Bond formation releases energy (-BDE) Table 4.2 gives BDE for homolytic cleavage of bonds in a gaseous molecule. We can use BDE to estimate  H for a reaction. =>

12. Chapter 412 Which is more likely? Estimate  H for each step using BDE. 104 103 58 84 => 104 84 58 103

13. Chapter 413 Kinetics Answers question, “How fast?” Rate is proportional to the concentration of reactants raised to a power. Rate law is experimentally determined. =>

14. Chapter 414 Reaction Order For A + B  C + D, rate = k[A] a [B] b  a is the order with respect to A  a + b is the overall order Order is the number of molecules of that reactant which is present in the rate- determining step of the mechanism. The value of k depends on temperature as given by Arrhenius: ln k = -E a + lnA RT =>

15. Chapter 415 Activation Energy Minimum energy required to reach the transition state. At higher temperatures, more molecules have the required energy. =>

16. Chapter 416 Reaction-Energy Diagrams For a one-step reaction: reactants  transition state  products A catalyst lowers the energy of the transition state. =>

17. Chapter 417 Energy Diagram for a Two-Step Reaction Reactants  transition state  intermediate Intermediate  transition state  product =>

18. Chapter 418 Rate-Determining Step Reaction intermediates are stable as long as they don’t collide with another molecule or atom, but they are very reactive. Transition states are at energy maximums. Intermediates are at energy minimums. The reaction step with highest E a will be the slowest, therefore rate-determining for the entire reaction. =>

19. Chapter 419 Bromination vs. Chlorination =>

20. Chapter 420 Endothermic and Exothermic Diagrams =>

21. Chapter 421 Hammond Postulate Related species that are similar in energy are also similar in structure. The structure of a transition state resembles the structure of the closest stable species. Transition state structure for endothermic reactions resemble the product. Transition state structure for exothermic reactions resemble the reactants. =>

22. Chapter 422 Radical Inhibitors Often added to food to retard spoilage. Without an inhibitor, each initiation step will cause a chain reaction so that many molecules will react. An inhibitor combines with the free radical to form a stable molecule. Vitamin E and vitamin C are thought to protect living cells from free radicals. =>

23. Chapter 423 Reactive Intermediates Carbocations (or carbonium ions) Free radicals Carbanions Carbene =>

24. Chapter 424 Carbocation Structure Carbon has 6 electrons, positive charge. Carbon is sp 2 hybridized with vacant p orbital. =>

25. Chapter 425 Carbocation Stability Stabilized by alkyl substituents 2 ways: (1) Inductive effect: donation of electron density along the sigma bonds. (2) Hyperconjugation: overlap of sigma bonding orbitals with empty p orbital. =>

26. Chapter 426 Free Radicals Also electron- deficient Stabilized by alkyl substituents Order of stability: 3  > 2  > 1  > methyl =>

27. Chapter 427 Carbanions Eight electrons on C: 6 bonding + lone pair Carbon has a negative charge. Destabilized by alkyl substituents. Methyl >1  > 2  > 3   =>

28. Chapter 428 Carbenes Carbon is neutral. Vacant p orbital, so can be electrophilic. Lone pair of electrons, so can be nucleophilic. =>

29. Chapter 429 Types of Spectroscopy Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group. Mass spectrometry (MS) fragments the molecule and measures the masses. Nuclear magnetic resonance (NMR) spectroscopy detects signals from hydrogen atoms and can be used to distinguish isomers. Ultraviolet (UV) spectroscopy uses electron transitions to determine bonding patterns. =>

30. Chapter 430 The Spectrum and Molecular Effects =>

31. Chapter 431 Molecular Vibrations Covalent bonds vibrate at only certain allowable frequencies. =>

32. Chapter 432 Stretching Frequencies Frequency decreases with increasing atomic weight. Frequency increases with increasing bond energy. =>

33. Chapter 433 Carbon-Carbon Bond Stretching Stronger bonds absorb at higher frequencies:  C-C 1200 cm -1  C=C 1660 cm -1  C  C 2200 cm -1 (weak or absent if internal) Conjugation lowers the frequency:  isolated C=C 1640-1680 cm -1  conjugated C=C 1620-1640 cm -1  aromatic C=C approx. 1600 cm -1 =>

34. Chapter 434 Carbon-Hydrogen Stretching Bonds with more s character absorb at a higher frequency.  sp 3 C-H, just below 3000 cm -1 (to the right)  sp 2 C-H, just above 3000 cm -1 (to the left)  sp C-H, at 3300 cm -1 =>

35. Chapter 435 An Alkane IR Spectrum =>

36. Chapter 436 An Alkene IR Spectrum =>

37. Chapter 437 An Alkyne IR Spectrum =>

38. Chapter 438 O-H and N-H Stretching Both of these occur around 3300 cm -1, but they look different.  Alcohol O-H, broad with rounded tip.  Secondary amine (R 2 NH), broad with one sharp spike.  Primary amine (RNH 2 ), broad with two sharp spikes.  No signal for a tertiary amine (R 3 N) =>

39. Chapter 439 An Alcohol IR Spectrum =>

40. Chapter 440 An Amine IR Spectrum =>

41. Chapter 441 Carbonyl Stretching The C=O bond of simple ketones, aldehydes, and carboxylic acids absorb around 1710 cm -1. Usually, it’s the strongest IR signal. Carboxylic acids will have O-H also. Aldehydes have two C-H signals around 2700 and 2800 cm -1. =>

42. Chapter 442 A Ketone IR Spectrum =>

43. Chapter 443 An Aldehyde IR Spectrum =>

44. Chapter 444 O-H Stretch of a Carboxylic Acid This O-H absorbs broadly, 2500-3500 cm -1, due to strong hydrogen bonding. =>

45. Chapter 445 Summary of IR Absorptions =>

46. Chapter 446 Strengths and Limitations IR alone cannot determine a structure. Some signals may be ambiguous. The functional group is usually indicated. The absence of a signal is definite proof that the functional group is absent. Correspondence with a known sample’s IR spectrum confirms the identity of the compound. =>

47. Chapter 447 Mass Spectrometry Molecular weight can be obtained from a very small sample. It does not involve the absorption or emission of light. A beam of high-energy electrons breaks the molecule apart. The masses of the fragments and their relative abundance reveal information about the structure of the molecule. =>

48. Chapter 448 Electron Impact Ionization A high-energy electron can dislodge an electron from a bond, creating a radical cation (a positive ion with an unpaired e - ). =>

49. Chapter 449 Separation of Ions Only the cations are deflected by the magnetic field. Amount of deflection depends on m/z. The detector signal is proportional to the number of ions hitting it. By varying the magnetic field, ions of all masses are collected and counted. =>

50. Chapter 450 Mass Spectrometer =>

51. Chapter 451 The Mass Spectrum Masses are graphed or tabulated according to their relative abundance. =>

52. Chapter 452 The GC-MS => A mixture of compounds is separated by gas chromatography, then identified by mass spectrometry.

53. Chapter 453 Molecules with Heteroatoms Isotopes: present in their usual abundance. Hydrocarbons contain 1.1% C-13, so there will be a small M+1 peak. If Br is present, M+2 is equal to M +. If Cl is present, M+2 is one-third of M +. If iodine is present, peak at 127, large gap. If N is present, M + will be an odd number. If S is present, M+2 will be 4% of M +. =>

54. Chapter 454 Isotopic Abundance => 81 Br

55. Chapter 455 Mass Spectrum with Sulfur =>

56. Chapter 456 Mass Spectrum with Chlorine =>

57. Chapter 457 Mass Spectrum with Bromine =>

58. Chapter 458 Mass Spectra of Alkanes More stable carbocations will be more abundant. =>

59. Chapter 459 Mass Spectra of Alkenes Resonance-stabilized cations favored. =>

60. Chapter 460 Mass Spectra of Alcohols Alcohols usually lose a water molecule. M + may not be visible. =>

61. Chapter 461

62. Chapter 462 POWER POINT IMAGES FROM “ORGANIC CHEMISTRY, 5 TH EDITION” L.G. WADE ALL MATERIALS USED WITH PERMISSION OF AUTHOR PRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGE ORGANIC CHEMISTRY COURSE BY: ANNALICIA POEHLER STEFANIE LAYMAN CALY MARTIN