CHEM Lecture 5

Transcription

1 CEM 494 Special Topics in Chemistry Illinois at Chicago CEM Lecture 5 Prof. Duncan Wardrop ctober 8, 2012

2 CEM 494 Special Topics in Chemistry Illinois at Chicago Chapter 15 Alcohols and Alkyl alides

3 CEM 494 Special Topics in Chemistry Illinois at Chicago Functional Groups Alcohols and Alkyl alides

4 Functional Groups functional group: a defined connectivity for a specific group of atoms ( 2) within a molecule since alkanes are chemically inert, functional groups are responsible for chemical reactivity under specific conditions and also the physical, chemical and biological properties of organic molecules b.p C Inert to acids, bases, oxidizing & reducing agents C C vs. C C Ethane Ethanol b.p. = 78.4 C Reacts with acids, bases, oxidizing & reducing agents Biologically Active! Slide

5 Examples of Functional Groups alkene alkyne alcohol alkyl halide C C C C C C X Cl Slide 5

6 Examples of Functional Groups ether C sulfide C C S C thiol amine C N C S Me S S 2N N2 Slide 6

7 Carbonyl Functional Groups: Carboxylic Acid Derivative carbonyl group C pervasive common name C C 3 acetyl acet- carboxylic acid ester acid halide amide C C C C X C N 3 C C 3 C C C 2 C 3 3 C C Cl 3 C C N Ph acetic acid ethyl acetate acetyl chloride acetanilide Slide 7

8 Carbonyl Functional Groups aldehyde ketone C C C C 3 C C 3 C C C3 acetaldehyde acetone Slide 8

9 Time to Memorize Functional Groups! Functional Group (F.G.) C C C C F. G. Class Example IUPAC Name Trivial Name Alkene cyclohexene cyclohexene Alkyne 3 methylbut 1 yne isopropyl acetylene C C C 3 1,3 Diene 2 methylbuta 1,3 diene isoprene C C C C C C X C C C C C C S X Allene 1 chloropropa 1,2 diene chloroallene Cl Arene 1,4 dimethylbenzene p xylene Phenol 4 methylphenol p cresol Aryl halide Br bromobenzene bromobenzene Cl Alkyl halide 2 chloropropane isopropyl chloride Alcohol butan 1 ol n butanol Ether methoxybenzene anisole Epoxide Thiol 7 oxabicyclo[4.1.0]heptane (R) 2 (4 methylcyclohex 3 enyl)propane 2 thiol cyclohexene oxide grapefruit thiol many already encountered - alkenes, alkynes, arenes not alkanes study functional group handout from website - learn to draw & name F.G.s make flash cards you will be asked to identify and name functional groups on quiz 3 & first exam S Slide 9

10 IUPAC (Substitutive): Alkyl alides Br C 3 Br C C 3 C 2 CC 2 CC 2 C 3 3-bromo-5-methylheptane Steps: 1. Identify and number the longest continuous chain of carbons. 2. Follow all previous rules and conventions for naming/numbering alkane chains. 3. Name the compound according to the figure below. Conventions: Previous conventions apply (e.g., first point of difference rule). alogens and alkyl groups are considered to have equal rank when deciding numbering. If two numbering schemes give same locant, choose numbering that lists substituents alphabetically. Subsituent names for halogens are fluoro, chloro, bromo, & iodo. parent locant halo substituent parent chain name Slide 10

11 Cl IUPAC (Substitutive): Alcohols 4 5-chloro-2,3-dimethyl-pentan-1-ol 3 5 C 2 C 2 CC C CC 2 1 or 5-chloro-2,3-dimethyl-1-pentanol Steps: 1. Identify and number the longest continuous chain of carbons to give the - group the lowest locant. 2. Name the parent by replacing -e with -ol (e.g. pentane becomes pentanol). 3. Name the compound according to the figure below. Conventions: Previous conventions apply (e.g., first point of difference rule). Alcohols outrank (have priority over) halogens and alkyl groups when considering numbering scheme. Alcohol locant may be placed before the parent name (e.g. 1-pentanol) or after ( e.g. pent-1-ol). Cl locant parent (drop last e ) ol parent (drop last e ) locant ol Slide 11

12 Two Substitutive Nomenclatures vs. Functional Group Class Nomenclature substitutive and 2004 name are preferred old habits are hard to break Slide 12

13 Classification of Substituted Carbons count the number of carbons bonded to the carbon atom you wish to classify: one = primary (1º) two = secondary (2º) three = tertiary (3º) four = quaternary (4º) Slide 13

14 Self Test Question What is the IUPAC (substitutive) name for the following molecule? 8 7 Br A. 3-hydroxy-4,5,6-triethyl-7-bromooctane B. 2-bromo-3,4,5-triethyloctan-3-ol C. 7-bromo-4,5,6-triethyloctan-3-ol D. 4-(3-bromo-1,2-diethylbutyl)-3-hexanol E. 6-(1-bromoethyl)-4,5-diethyl-3-octanol Slide 14

15 CEM 494 Special Topics in Chemistry Illinois at Chicago Properties of Alcohols and Alkyl alides

16 Alcohols and Alkyl alides Are Polar red = higher electron density (partial negative charge) blue = lower/ deficient electron density (partial positive charge) Slide 16

17 Effect of Structure on Boiling Points only London dispersion forces aka: induced-dipole/ induced-dipole induced/induced dipole-dipole attractive force dipole/induced-dipole induced/induced dipole-dipole attractive force dipole/induced-dipole hydrogen bonding C 3 C 2 C 3 (propane) C 3 C 2 F (fluoroethane) C 3 C 2 (ethanol) MW boiling point (ºC) Dipole moment (µ) Slide 17

18 Review: induced-dipole//induced-dipole (London Dispersion Forces) C C C C C more atoms = more electrons = more induced dipoles = more attractive forces = C C C C C higher boiling point Slide 18

19 Dipole/Induced-Dipole C C F permanent dipole in one molecule induces a temporary dipole in a non-polar region of another molecule C C F more C-X bonds = more dipole/dipole-induced attractive forces Slide 19

20 Dipole/Dipole C C C F F C attractive force between two permanent dipoles not necessarily only at the polar covalent bond: region of partial positive charge in one molecule attracted to region of negative charge in another molecule. Slide 20

21 ydrogen Bonding is a Strong Dipole/Dipole Attractive Force Slide 21

22 Boiling Point Trends in same class (i.e. X = F) boiling point increases as MW increases; more atoms = more attractive van der Waals forces = higher boiling point alcohols have significantly higher boiling points than similar halides; strong hydrogen bonding attractive forces boiling point increases from fluorine to iodine for same series; polarizability of halogen increases down periodic table Slide 22

23 Boiling Point Trends (Cl, Br, I nly) boiling point increases with increasing number of halogens despite CCl 4 having no molecular dipole, it has the highest boiling point induced-dipole/induced-dipole forces are the greatest because it has the greatest number of chlorine atoms Slide 23

24 Polarizability and Teflon Teflon = polytetrafluoroethylene (PTFE) F F F F F F F F F F n F F more bonds to F (low polarizability) = less/weaker induceddipole/induced-dipole attractive forces Slide 24

25 Polarizability polarizability: the ease of distortion of the electron cloud of a molecular entity by an electric field; flexibility, squishiness of the electron cloud bromomethane (b.p. = 3 ºC) fluoromethane (b.p. = -78 ºC) polarizability increases down the periodic table; larger orbitals; more polarizable = better able to momentarily generate induced-dipole = stronger induced-dipole/ induced-dipole forces = more attractive forces = higher boiling point Slide 25

26 Self Test Question Rank the following in order of increasing boiling point. a. b. c. d. F F Cl F F highest lowest A. a, b, c, d B. b, c, d, a C. b, d, c, a D. a, c, b, d E. d, b, c, a Slide 26

27 CEM 494 Special Topics in Chemistry Illinois at Chicago rganic Reactions Preparation of Alkyl alides Chapter: 15

28 Preparation of Alkyl alides from Alcohols C X Substituition + + C X alcohol hydrogen halide alkyl halide water 1, 2 & 3 alcohols react irreversible reaction; no equilibrium here more reactive reactants & substrates = faster reaction Slide 28

29 Reactivity of ydrogen alides Increasing Reactivity of ydrogen alides Toward Alcohols F < Cl < Br < I least reactive (slowest) most reactive (fastest) stronger acid (lower pka) = more reactive increased reactivity = faster reaction remember: irreversible reaction; no equilibrium here Slide 29

30 Reactivity of Alcohols more substituted alcohol = more reactive increased reactivity = faster reaction remember: irreversible reaction; no equilibrium here Slide 30

31 igher Energy of Reactants = Increased Reactivity C 3 + Br C 3 + Cl = transition state E a E a C 3 X + 2 higher energy reactants = if transition states are the same, then lower activation energy (Ea) = faster reaction = more reactive Slide 31

32 Lower Energy Transition States = Increased Reactivity higher energy transition states = + Cl E a E a if reactant energies are close, then lower activation energy (Ea) = = transition state RCl + 2 faster reaction = more reactive We will explore why 3º alcohols provide lower energy transition states on Thursday. Slide 32

33 More Examples more reactive hydrogen halide (Br) is needed for less reactive secondary alcohols think about the pattern of the reaction ignore parts of the molecule that don t react reaction conditions: generally, above/below rxn arrow Slide 33

34 Alternative Conditions A mixture of sodium bromide and sulfuric acid may be used in place of Br. reagents generally placed above/below reaction arrow inorganic products usually omitted (assumed) Slide 34

35 1º & 2º Alcohols Unreactive Toward Cl no rxn + Cl X Cl + Alternative Reagent for Preparation of Alkyl Chlorides thionyl chloride S Cl Cl SCl2 SCl 2 Cl + S 2 + Cl K 2 C 3 thionyl chloride reacts rapidly with 1º and 2º alcohols byproducts of the reaction are S 2 (g) and Cl(g) base is needed to neutralize Cl: e.g. K 2 C 3, pyridine Slide 35

36 Alkyl alides & Purple Pills Cl S Cl Me N N S N (SCl 2 ) N Cl S-Alkylation S-xidation N N S N S N N N Esomeprazole Nexium Slide 36

37 Self Test Question Predict the organic product of the following reaction... Cl a. b. Cl c. 3 C C 3 3 C C 3 d. Br Cl? 25 ºC 3 C C 3 Br Cl 3 C C 3 Br e. Cl Cl Cl 3 C CCl 3 Cl Cl 3 C C 3 A. a B. b C. c D. d E. e Slide 37

38 Self Test Question Which of the following transformations is unlikely to generate the product indicated? primary alcohols and Cl are insufficiently reactive x a. b. Cl Cl 25 ºC Cl 25 ºC Cl A. a. B. b. c. d. SCl 2 K 2 C 3 Cl 2 hν Cl Cl C. c. D. d. Slide 38

39 Substitution Reaction hydroxyl group halide R + X R X + alcohol hydrogen halide alkyl halide water ydroxyl group is being substituted (replaced with) a halide Slide 39

40 CEM 494 Special Topics in Chemistry Illinois at Chicago Mechanisms of Substitution Reactions Chapter 15

41 Substitution: ow Does it appen? break bond break bond make bond make bond R + X R X + alcohol hydrogen halide alkyl halide water mechanism: a generally accepted series of elementary steps that show the order of bond breaking and bond making elementary step: a bond making and/or bond breaking step that only involves one transition state Slide 41

42 Ingold-ughes Mechanistic Designators Designates type of process Letter # N or E Designates molecularity Nucleophilic or Electrophilic Example Me Me Me -Br Me Me Me Br S N 1 Substitution, nucleophilic, 1st order Rate = k x [t-bu] Slide 42

43 Nucleophilic Substitution (S N 1) Step ne Proton Transfer (Protonation) Cl pka = -3.9 fast & reversible alkyloxonium ion Cl Slide 43

44 Step ne Potential Energy Diagram Step ne Proton Transfer (Protonation) transition state: energy maximum along reaction coordinate for one elementary step; usually involves partial bond making and partial bond breaking intermediate: energy minimum along the reaction coordinate; species with a finite lifetime; neither reactant, nor product ammond Postulate: structure of the transition state looks most like its closest energy reactant or intermediate Slide 44

45 Mechanism: Nucleophilic Substitution (S N 1) Step Two Dissociation (Ionization) slow Me Me Me 2 carbocation (t-butyl cation) Slide 45

46 Step Two Potential Energy Diagram Step Two Dissociation (Ionization) largest activation energy (Ea) endothermic, slowest carbocation intermediate is much higher in energy than an oxonium ion carbocations do not have a full octet, whereas oxonium ions do structure of transition state most resembles the closest energy neighbor, the carbocation (ammond Post.) Slide 46

47 Mechanism: Nucleophilic Substitution (S N 1) Step Three Carbocation Capture Me Me Me Cl fast Cl carbocation (t-butyl cation) t-butyl chloride Cation = Electrophile Anion = Nucleophile Slide 47

48 Step Three Potential Energy Diagram Step Three Carbocation Capture fast step because small activation energy; positive and negative atoms bond fast products are much lower in energy since they are neutral; exothermic reaction transition state looks most like its closest neighbor, the carbocation intermediate (very little C-Cl bond formation at transition state) (ammond Postulate) Slide 48

49 Nucleophiles Add to Electrophiles nucleophile: nucleus loving; Lewis base; electron pair donor; forms bonds with a nucleus that can accept electrons; does not necessarily have to be negatively charged; has available, filled orbitals! electrophile: electron loving; Lewis acid; electron pair acceptor; forms bonds by accepting electrons from other atoms; does not necessarily have to be positively charged; has available, empty orbitals! Me Me Me Cl Cation is Electrophile empty 2pz orbital Chloride is Nucleophile filled n orbital (: = lone pair) Slide 49

50 Complete Mechanism Cl fast & reversible alkyloxonium ion Cl slow Me Me Cl fast Cl Me carbocation (t-butyl cation) 2 t-butyl chloride Slide 50

51 Complete Potential Energy Diagram mechanism only valid for 3º & 2º alcohols reaction is only as fast as its slowest step rate determining step (RDS) slowest step (largest Ea) = rate determining step (RDS) here, slowest step is carbocation formation Protonation carbocation formation carbocation capture here, RDS is unimolecular Slide 51

52 Naming the Mechanism: Ingold Notation R + X R X + alcohol hydrogen halide alkyl halide water S N 1 S: Substitution N: Nucleophilic the alcohol functional groups is being substituted with a halide the halide doing the substitution is a nucleophile 1: 1st order (unimolecular) the RDS is carbocation formation; this step is unimolecular (1st order) Slide 52

53 Self Test Question Consider the S N 1 mechanism for the formation of 2- bromobutane. Which structure best represents the highest energy transition state in this mechanism? a. δ + Br δ c. + Br A. a. B. b. b. δ + δ + + Br d. + Br C. c. D. d. Slide 53