Functional Groups, Intermolecular Forces, & Infrared (IR) Spectroscopy

Transcription

1 Chapter 2 Families of Carbon Compounds Functional Groups, Intermolecular Forces, & Infrared (IR) Spectroscopy Created by Professor William Tam & Dr. Phillis Chang Ch. 2-1

2 About The Authors These Powerpoint Lecture Slides were created and prepared by Professor William Tam and his wife Dr. Phillis Chang. Professor William Tam received his B.Sc. at the University of ong Kong in 1990 and his Ph.D. at the University of Toronto (Canada) in e was an NSERC postdoctoral fellow at the Imperial College (UK) and at arvard University (USA). e joined the Department of Chemistry at the University of Guelph (Ontario, Canada) in 1998 and is currently a Full Professor and Associate Chair in the department. Professor Tam has received several awards in research and teaching, and according to Essential Science Indicators, he is currently ranked as the Top 1% most cited Chemists worldwide. e has published four books and over 80 scientific papers in top international journals such as J. Am. Chem. Soc., Angew. Chem., Org. Lett., and J. Org. Chem. Dr. Phillis Chang received her B.Sc. at New York University (USA) in 1994, her M.Sc. and Ph.D. in 1997 and 2001 at the University of Guelph (Canada). She lives in Guelph with her husband, William, and their son, Matthew. Ch. 2-2

3 1. ydrocarbons ydrocarbons are compounds that contain only carbon and hydrogen atoms Alkanes hydrocarbons that do not have multiple bonds between carbon atoms e.g. pentane cyclohexane Ch. 2-3

4 Alkenes contain at least one carbon carbon double bond e.g. propene cyclohexene Ch. 2-4

5 Alkynes contain at least one carbon carbon triple bond e.g. C C ethyne 1-pentyne 2-pentyne Ch. 2-5

6 Aromatic compound contain a special type of ring, the most common example of which is a benzene ring C 3 COO e.g. benzene toluene benzoic acid Ch. 2-6

7 1A. Alkanes The primary sources of alkanes are natural gas and petroleum The smaller alkanes (methane through butane) are gases under ambient conditions Methane is the principal component of natural gas igher molecular weight alkanes are obtained largely by refining petroleum Methane Ch. 2-7

8 1B. Alkenes Ethene and propene, the two simplest alkenes, are among the most important industrial chemicals produced in the United States Ethene is used as a starting material for the synthesis of many industrial compounds, including ethanol, ethylene oxide, ethanal, and the polymer polyethylene C C Ethene Ch. 2-8

9 Propene is the important starting material for acetone, cumene and polypropylene Examples of naturally occurring alkenes β-pinene (a component of turpentine) An aphid alarm pheromone Ch. 2-9

10 1C. Alkynes The simplest alkyne is ethyne (also called acetylene) Examples of naturally occurring alkynes C C O Br Cl C C C C C C 3 Capillin (an antifungal agent) O Br Dactylyne (an inhibitor of pentobarbital metabolism) Ch. 2-10

11 1D. Benzene All C C bond lengths are the same (1.39 Å) (compare with C C single bond 1.54 Å, C=C double bond 1.34 Å) Extra stabilization due to resonance aromatic Ch. 2-11

12 3 Dimensional structure of benzene π-electrons above and below ring Planar structure All carbons sp 2 hybridized Ch. 2-12

13 The lobes of each π orbital above and below the ring overlap with the lobes of π orbitals on the atoms to either side of it the six electrons associated with these π orbitals (one electron from each orbital) are delocalized about all six carbon atoms of the ring Ch. 2-13

14 2. Polar Covalent Bonds Li F Lithium fluoride has an ionic bond C C Ethane has a covalent bond. The electrons are shared equally between the carbon atoms Ch. 2-14

15 electronegativity δ C C C O equal sharing of e (non-polar bond) unequal sharing of e (polar bond) Ch. 2-15

16 Electronegativity (EN) The intrinsic ability of an atom to attract the shared electrons in a covalent bond Electronegativities are based on an arbitrary scale, with F the most electronegative (EN = 4.0) and Cs the least (EN = 0.7) Ch. 2-16

17 element (EN) Li (1.0) Na (0.9) K (0.8) Rb (0.8) Cs (0.7) Be (1.6) Mg (1.2) (2.1) B (2.0) C (2.5) Si (1.8) N (3.0) P (2.1) O (3.5) S (2.5) F (4.0) Cl (3.0) Br (2.8) I (2.5) Increasin ng EN Increasing EN Ch. 2-17

18 δ δ C N C Cl δ δ C Si C Ch. 2-18

19 3. Polar and Nonpolar Molecules Dipole moment = distance between the charges the charge µ = r Q Dipole moments are expressed in debyes (D), where 1 D = coulomb meter (C m) in SI units Ch. 2-19

20 δ Cl C > net dipole (1.87 D) Ch. 2-20

21 Molecules containing polar bonds are not necessarily polar as a whole, for example (1) BF (µ = 0 D) (2) CCl (µ = 0 D) 3 4 Cl F 120 o C Cl B Cl F F Cl (trigonal planar) (tetrahedral) Ch. 2-21

22 Dipole moment of some compounds Compound Dipole Moment Compound Dipole Moment NaCl O 1.85 C 3 NO C 3 O 1.70 C 3 Cl 1.87 C 3 COO 1.52 C 3 Br 1.79 N C 3 I 1.64 C 4 0 CCl CCl 4 0 Ch. 2-22

23 3A. Dipole Moments in Alkenes cis- 1,2-Dichloroethene trans- 1,2-Dichloroethene Cl C C C C Cl Cl Cl resultant dipole moment (µ = 1.9 D) (µ = 0 D) Ch. 2-23

24 Physical properties of some cis-trans isomers Compound m.p. ( o C) m.p. ( o C) (µ) cis-1,2-dichloroethene trans-1,2-dichloroethene cis-1,2-dibromoethene trans-1,2-dibromoethene Ch. 2-24

25 4. Functional Groups Alkane Alkyl Group Abbrev. Bond-Line Model C 3 Methane C 3 Methyl Me- C 3 C 2 C 2 Propane C 3 C 2 C 2 propyl C 3 C 2 C 2 C 2 Butane C 3 C 2 C 2 C 2 Butyl C 3 C 2 C 3 C 2 Et- Ethane Ethyl Pr- Bu- Ch. 2-25

26 C 3 C 3 C 2 C 3 C 2 C 2 C 3 CC 3 Methyl Ethyl Propyl Isoprypyl These and others can be designated by R General formula for an alkane is R Ch. 2-26

27 4B. Phenyl and Benzyl Groups Phenyl group or or C 6 5 φ or Ph or or Ar Benzyl group C 2 or or C 6 5 C 2 or Bn Ch. 2-27

28 5. Alkyl alides or aloalkanes R X (X = F, Cl, Br, I) Examples Attached to Attached to Attached to 1 carbon atom 2 carbon atoms 3 carbon atoms C C C C Cl C Br C I a 1 o chloride a 2 o bromide a 3 o iodide Ch. 2-28

29 6. Alcohols R O Examples C 3 O, O, O, O (1 o ) (2 o ) (3 o ) (aromatic) (phenol) Ch. 2-29

30 Alcohols may be viewed structurally in two ways: As hydroxyl derivatives of alkanes As alkyl derivatives of water ethyl group C 3 C 2 C 3 C o O o Ethane Ethyl alcohol (ethanol) hydroxyl group O Water Ch. 2-30

31 7. Ethers R O R Examples ~100 o O Acyclic O Cyclic Ch. 2-31

32 8. Amines R N 2 3 C N N 3 C C 3 C 3 N 3 C C 3 (1 o ) (2 o ) (3 o ) N (cyclic) N (aromatic) Ch. 2-32

33 9. Aldehydes and Ketones O O R R R (aldehydes) (ketones) O O O O,, ketone aldehyde Ch. 2-33

34 Aldehydes and ketones have a trigonal planar arrangement of groups around the carbonyl carbon atom 121 o O 121 o 108 o Ch. 2-34

35 10. Carboxylic Acids, Esters, and Amides O O O R O (carboxylic acid) R OR (ester) R Cl (acid chloride) O O O R NR 2 (amide) R O (acid anhydride) R Ch. 2-35

36 11. Nitriles R C N 3 C 2 C 1 N C N C N Ethanenitrile (acetonitrile) Propenenitrile (acrylonitrile) Benzenecarbonitrile (benzonitrile) Ch. 2-36

37 12. Summary of Important Families of Organic Compounds Ch. 2-37

38 Ch. 2-38

39 13. Physical Properties and Molecular Structure 13A. A.Ionic Compounds: Ion-Ion Forces The melting point of a substance is the temperature at which an equilibrium exists between the wellordered crystalline state and the more random liquid state Ch. 2-39

40 If the substance is an ionic compound, the ion ion forces that hold the ions together in the crystalline state are the strong electrostatic lattice forces that act between the positive and negative ions in the orderly crystalline structure A large amount of thermal energy is required to break up the orderly structure of the crystal into the disorderly open structure of a liquid Ch. 2-40

41 The boiling points of ionic compounds are higher still, so high that most ionic organic compounds decompose before they boil Ch. 2-41

42 Physical properties of selected compounds Compound Structure mp ( o C) bp ( o C) (1 atm) Ethane C 3 C Chloroethane C 3 C 2 Cl Ethyl alcohol C 3 C 2 O Acetaldehyde C 3 CO Acetic acid C 3 CO Sodium acetate C 3 CO 2 Na 324 dec Ch. 2-42

43 13B. Intermolecular Forces (van der Waals Forces) The forces that act between molecules are not as strong as those between ions These intermolecular forces, van der Waals forces, are all electrical in nature Dipole-dipole forces ydrogen bonds Dispersion forces Ch. 2-43

44 Dipole-dipole forces Dipole-dipole attractions between polar molecules O δ O δ dipole-dipole attraction C Cl δ C Cl δ Ch. 2-44

45 ydrogen bonds Dipole-dipole attractions between hydrogen atoms bonded to small, strongly electronegative atoms (O, N, or F) and nonbonding electron pairs on other such electronegative atoms ydrogen bonds (bond dissociation energies of about 4 38 kj mol -1 ) are weaker than ordinary covalent bonds but much stronger than the dipole dipole interactions Ch. 2-45

46 ydrogen bonds δ O δ O hydrogen bond N δ N δ Ch. 2-46

47 ydrogen bonds ydrogen bonding explains why water, ammonia, and hydrogen fluoride all have far higher boiling points than methane (bp C), even though all four compounds have similar molecular weights One of the most important consequences of hydrogen bonding is that it causes water to be a liquid rather than a gas at 25 C Ch. 2-47

48 ydrogen bonds Calculations indicate that in the absence of hydrogen bonding, water would have a bp near 80 C and would not exist as a liquid unless the temperature were lower than that temperature Ch. 2-48

49 Dispersion forces (London forces) The average distribution of charge in a nonpolar molecule over a period of time is uniform At any given instant, however, because electrons move, the electrons and therefore the charge may not be uniformly distributed Electrons may, in one instant, be slightly accumulated on one part of the molecule, and, as a consequence, a small temporary dipole will occur Ch. 2-49

50 Dispersion forces (London forces) This temporary dipole in one molecule can induce opposite (attractive) dipoles in surrounding molecules These temporary dipoles change constantly, but the net result of their existence is to produce attractive forces between nonpolar molecules Ch. 2-50

51 Two important factors determine the magnitude of dispersion forces The relative polarizability of electrons of the atoms involved The electrons of large atoms such as iodine are loosely held and are easily polarized, while the electrons of small atoms such as fluorine are more tightly held and are much less polarizable Ch. 2-51

52 δ δ δ δ δ δ δ δ I I C I I stronger dispersion forces δ δ δ δ δ δ δ δ I I C I I δ δ δ F F C F F weaker dispersion forces δ δ F C F F F δ Ch. 2-52

53 The relative surface area of the molecules involved The larger the surface area, the larger is the overall attraction between molecules caused by dispersion forces Ch. 2-53

54 e.g. Pentane vs. Neopentane (both C 5 12 ) δ δ δ δ δ δ δ δ δ δ+ larger surface area stronger dispersion forces δ δ δ δ + δ δ C C C C C δ δ δ δ δ δ δ δ δ δ+ Pentane (bp 36 o C) smaller surface area weaker dispersion forces δ Neopentane (bp 9.5 o C) δ δ δ δ C C C C C Ch. 2-54

55 13C. Boiling Points The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the pressure of the atmosphere above it the boiling points of liquids are pressure dependent, and boiling points are always reported as occurring at a particular pressure Ch. 2-55

56 Examples C 3 t Bu C3 NO 2 bp: 245 o C / 760 mmg (74 o C / 1 mmg) O bp: 260 o C / 760 mmg (140 o C / 20 mmg) 1 atm = 760 torr = 760 mmg Ch. 2-56

57 13D. Solubilities A general rule for solubility is that like dissolves like in terms of comparable polarities Polar and ionic solids are usually soluble in polar solvents Polar liquids are usually miscible Nonpolar solids are usually soluble in nonpolar solvents Nonpolar liquids are usually miscible Polar and nonpolar liquids, like oil and water, are usually not soluble to large extents Ch. 2-57

58 e.g. MeO and 2 O are miscible in all proportions hydrogen bond 3 C δ O O δ Ch. 2-58

59 ydrophobic means incompatible with water ydrophilic means compatible with water ydrophobic portion ydrophilic group O Decyl alcohol A typical detergent molecule O O S O O Na Ch. 2-59

60 13E. Guidelines for Water Solubility Organic chemists usually define a compound as water soluble if at least 3 g of the organic compound dissolves in 100 ml of water Usually compounds with one to three carbon atoms are water soluble, compounds with four or five carbon atoms are borderline, and compounds with six carbon atoms or more are insoluble Ch. 2-60

61 14. Summary of Attractive Electric Forces Ch. 2-61

62 15. Infrared Spectroscopy Ch. 2-62

63 The position of an absorption band (peak) in an IR spectrum is specified in units of wavenumbers ( ) ν ν = 1 λ E = hν E ν (λ = wavelength in cm) (E = energy) (ν = frequency of radiation) ν = E = c λ hc λ Ch. 2-63

64 Ch. 2-64

65 Characteristic IR absorptions Intensity: s = strong, m = medium, w = weak, v = variable Group Freq. Range (cm -1 ) Intensity Alkyl C (stretching) (m s) Alkenyl C (stretching) C= (stretching) cis-rc=cr trans-rc=cr Alkynyl C (stretching) C C (stretching) ~ (m) (v) (s) (s) (s) (v) Ch. 2-65

66 Characteristic IR absorptions Intensity: s = strong, m = medium, w = weak, v = variable Group Freq. Range (cm -1 ) Intensity Aromatic Ar (stretching) - monosubstituted - o-disubstituted - m-disubstituted - p-disubstituted ~ (v) (very s) (very s) (s) (s) (very s) (very s) Ch. 2-66

67 Characteristic IR absorptions Intensity: s = strong, m = medium, w = weak, v = variable Group Freq. Range (cm -1 ) Intensity Alcohols, Phenols & Carboxylic Acids O (stretching) - alcohols & phenols (sharp, v) (dilute solutions) - alcohols & phenols (broad, s) (hydrogen bonded) - carboxylic acids (broad, v) (hydrogen bonded) Ch. 2-67

68 Characteristic IR absorptions Intensity: s = strong, m = medium, w = weak, v = variable Group Freq. Range (cm -1 ) Intensity Aldehydes, Ketones, Esters, Carboxylic Acids, Amides C=O (stretching) Aldehydes (s) (s) Ketones Esters Carboxylic Acids Amides (s) (s) (s) (s) Amines N (m) Nitriles C N (m) Ch. 2-68

69 16. Interpreting IR Spectra IR spectrum of octane Ch. 2-69

70 IR spectrum of toluene Ch. 2-70

71 16B. IR Spectra of ydrocarbons IR spectrum of 1-heptyne Ch. 2-71

72 IR spectrum of 1-octene Ch. 2-72

73 16B. IR Spectra of Some Functional Groups Containing Carbonyl Functional Groups O O O R R R R OR (aldehyde) (ketone) (ester) cm cm cm -1 O O R O (carboxylic acid) cm -1 R NR 2 (amid) cm -1 Ch. 2-73

74 Alcohols and phenols The IR absorption of an alcohol or phenol O group is in the cm -1 range, and most often it is broad Ch. 2-74

75 Carboxylic Acids IR spectrum of propanoic acid Ch. 2-75

76 Amines 1 o and 2 o amines give absorptions of moderate strength in the cm -1 region 1 o amines exhibit two peaks in this region due to symmetric & asymmetric stretching of the two N bonds 2 o amines exhibit a single peak 3 o amines show no N absorption because they have no such bond A basic p is evidence for any class of amines Ch. 2-76

77 Amines IR spectrum of 4-methylaniline Ch. 2-77

78 RN 2 (1 Amine) Two peaks in cm -1 region R 2 N (2 Amine) One peak in cm -1 region symmetric stretching asymmetric stretching Ch. 2-78

79 END OF CAPTER 2 Ch. 2-79