Chapter 17: Alcohols and Phenols. Based on McMurry s Organic Chemistry, 7 th edition

Transcription

1 Chapter 17: Alcohols and Phenols Based on McMurry s Organic Chemistry, 7 th edition

2 Alcohols and Phenols Alcohols contain an OH group connected to a a saturated C (sp 3 ) They are important solvents and synthesis intermediates Phenols contain an OH group connected to a carbon in a benzene ring Methanol, CH 3 OH, called methyl alcohol, is a common solvent, a fuel additive, produced in large quantities Ethanol, CH 3 CH 2 OH, called ethyl alcohol, is a solvent, fuel, beverage Phenol, C 6 H 5 OH ( phenyl alcohol ) has diverse uses - it gives its name to the general class of compounds OH groups bonded to vinylic, sp 2 -hybridized carbons are called enols 2

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4 Why this Chapter? To begin to study oxygen-containing functional groups These groups lie at the heart of biological chemistry 4

5 17.1 Naming Alcohols and Phenols General classifications of alcohols based on substitution on C to which OH is attached Methyl (C has 3 H s), Primary (1 ) (C has two H s, one R), secondary (2 ) (C has one H, two R s), tertiary (3 ) (C has no H, 3 R s), 5

6 IUPAC Rules for Naming Alcohols Select the longest carbon chain containing the hydroxyl group, and derive the parent name by replacing the -e ending of the corresponding alkane with -ol Number the chain from the end nearer the hydroxyl group Number substituents according to position on chain, listing the substituents in alphabetical order 6

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8 Naming Phenols Use phenol (the French name for benzene) as the parent hydrocarbon name, not benzene Name substituents on aromatic ring by their position from OH 8

9 17.2 Properties of Alcohols and Phenols The structure around O of the alcohol or phenol is similar to that in water, sp 3 hybridized Alcohols and phenols have much higher boiling points than similar alkanes and alkyl halides A positively polarized OH hydrogen atom from one molecule is attracted to a lone pair of electrons on a negatively polarized oxygen atom of another molecule This produces a force that holds the two molecules together These intermolecular attractions are present in solution but not in the gas phase, thus elevating the boiling point of the solution 9

10 Properties of Alcohols and Phenols: Acidity and Basicity Weakly basic and weakly acidic Alcohols are weak Brønsted bases Protonated by strong acids to yield oxonium ions, ROH

11 Alcohols and Phenols are Weak Brønsted Acids Can transfer a proton to water to a very small extent Produces H 3 O + and an alkoxide ion, RO, or a phenoxide ion, ArO 11

12 Acidity Measurements The acidity constant, K a, measures the extent to which a Brønsted acid transfers a proton to water [A ] [H 3 O + ] K a = and pk a = log K a [HA] Relative acidities are more conveniently presented on a logarithmic scale, pk a, which is directly proportional to the free energy of the equilibrium Differences in pk a correspond to differences in free energy Table 17.1 presents a range of acids and their pk a values 12

13 pka Values for Typical OH Compounds 13

14 Relative Acidities of Alcohols Simple alcohols are about as acidic as water Alkyl groups make an alcohol a weaker acid The more easily the alkoxide ion is solvated by water the more its formation is energetically favored Steric effects are important 14

15 Inductive Effects Electron-withdrawing groups make an alcohol a stronger acid by stabilizing the conjugate base (alkoxide) (alcohol) (alcohol) 15

16 Generating Alkoxides from Alcohols Alcohols are weak acids requires a strong base to form an alkoxide such as NaH, sodium amide NaNH 2, and Grignard reagents (RMgX) Alkoxides are bases used as reagents in organic chemistry 16

17 Phenol Acidity Phenols (pk a ~10) are much more acidic than alcohols (pk a ~ 16) due to resonance stabilization of the phenoxide ion Phenols react with NaOH solutions (but alcohols do not), forming salts that are soluble in dilute aqueous solution A phenolic component can be separated from an organic solution by extraction into basic aqueous solution and is isolated after acid is added to the solution 17

18 Nitro-Phenols Phenols with nitro groups at the ortho and para positions are much stronger acids 18

19 17.3 Preparation of Alcohols: A Review Alcohols are derived from many types of compounds The alcohol hydroxyl can be converted to many other functional groups This makes alcohols useful in synthesis 19

20 Review: Preparation of Alcohols by Regiospecific Hydration of Alkenes Hydroboration/oxidation: syn, non-markovnikov hydration Oxymercuration/reduction: Markovnikov hydration 20

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22 MARKOVNIKOV S RULE Named for Vladimir W. Markovnikov ( ), this rule originally stated that the addition of HX (Xhalogen) to an unsymmetrically substituted alkene occurs with attachment of the hydrogen to the carbon atom of the double bond having more hydrogens. Markovnikov addition: (a) Markovnikov product via a tertiary carbocation and (b) anti-markovnikov product (minor product) via a much less stable primary carbocation. 22

23 Addition of non-symmetrical reagent to a non-symmetrical alkene, then two isomeric products that are constitutional isomers can be obtained. For example, the reaction of HCl with propene gives 1-chloropropane and 2-chloropropane. Normally, 2-chloropropane is the major product. Since one product is favoured over the other, the reaction is said to be regioselective. If 2-chloropropane were the only product then the reaction is said to be regiospecific 23

24 Preparation of Alcohols by Regiospecific Hydration of Alkenes 24

25 1,2-Diols Cis-1,2-diols from hydroxylation of an alkene with OsO 4 followed by reduction with NaHSO 3 Trans-1,2-diols from acid-catalyzed hydrolysis of epoxides 25

26 17.4 Alcohols from Reduction of Carbonyl Compounds Reduction of a carbonyl compound in general gives an alcohol Note that organic reduction reactions add the equivalent of H 2 to a molecule 26

27 Reduction of Aldehydes and Ketones Aldehydes gives primary alcohols Ketones gives secondary alcohols 27

28 Reduction Reagent: Sodium Borohydride NaBH 4 is not sensitive to moisture and it does not reduce other common functional groups Lithium aluminum hydride (LiAlH 4 ) is more powerful, less specific, and very reactive with water Both add the equivalent of H - (+1H) 28

29 Mechanism of Reduction The reagent adds the equivalent of hydride to the carbon of C=O and polarizes the group as well 29

30 Reduction of Carboxylic Acids and Esters Carboxylic acids and esters are reduced to give primary alcohols (+2H) LiAlH 4 is used because NaBH 4 is not effective 30

31 17.5 Alcohols from Reaction of Carbonyl Compounds with Grignard Reagents Alkyl, aryl, and vinylic halides react with magnesium in ether or tetrahydrofuran to generate Grignard reagents, RMgX Grignard reagents react with carbonyl compounds to yield alcohols 31

32 Reactions of Grignard Reagents with Carbonyl Compounds 32

33 Reactions of Esters and Grignard Reagents Yields tertiary alcohols in which two of the substituents carbon come from the Grignard reagent Grignard reagents do not add to carboxylic acids they undergo an acid-base reaction, generating the hydrocarbon of the Grignard reagent 33

34 Grignard Reagents and Other Functional Groups in the Same Molecule Can't be prepared if there are reactive functional groups in the same molecule, including proton donors 34

35 Mechanism of the Addition of a Grignard Reagent Grignard reagents act as nucleophilic carbon anions (carbanions, : R ) in adding to a carbonyl group The intermediate alkoxide is then protonated to produce the alcohol 35

36 17.7 Some Reactions of Alcohols Two general classes of reaction

37 Conversion of alcohols into alkyl halides: - 3 alcohols react with HCl or HBr by S N 1 through carbocation intermediate - 1 and 2 alcohols converted into halides by treatment with SOCl 2 or PBr 3 via S N 2 mechanism 37

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39 Conversion of Alcohols into Tosylates Reaction with p-toluenesulfonyl chloride (tosyl chloride, p-toscl) in pyridine yields alkyl tosylates, ROTos Formation of the tosylate does not involve the C O bond so configuration at a chirality center is maintained Alkyl tosylates react like alkyl halides 39

40 Stereochemical Uses of Tosylates The S N 2 reaction of an alcohol via a tosylate, produces inversion at the chirality center The S N 2 reaction of an alcohol via an alkyl halide proceeds with two inversions, giving product with same arrangement as starting alcohol 40

41 Dehydration of Alcohols to Yield Alkenes The general reaction: forming an alkene from an alcohol through loss of O-H and H (hence dehydration) of the neighboring C H to give bond Specific reagents are needed 41

42 Acid- Catalyzed Dehydration Tertiary alcohols are readily dehydrated with acid Secondary alcohols require severe conditions (75% H 2 SO 4, 100 C) - sensitive molecules don't survive Primary alcohols require very harsh conditions impractical Reactivity is the result of the nature of the carbocation intermediate 42

43 Figure 17.6 Mechanism of the acidcatalyzed dehydration of an alcohol to yield an alkene. The process is an E1 reaction and involves a carbocation intermediate. 43

44 Dehydration with POCl 3 Phosphorus oxychloride in the amine solvent pyridine can lead to dehydration of secondary and tertiary alcohols at low temperatures An E2 via an intermediate ester of POCl 2 (see Figure 17.7) 44

45 Fjgure 17.7 Mechanism of the dehydration of secondary and tertiary alcohols by reaction with POCI 3 in pyridine. The reaction is an E2 process. 45

46 Conversion of Alcohols into Esters 46

47 17.7 Oxidation of Alcohols Can be accomplished by inorganic reagents, such as KMnO 4, CrO 3, and Na 2 Cr 2 O 7 or by more selective, expensive reagents Pyridinium ChloroChromate 47

48 Oxidation of Primary Alcohols To aldehyde: pyridinium chlorochromate (PCC, C 5 H 6 NCrO 3 Cl) in dichloromethane Other reagents produce carboxylic acids 48

49 Oxidation of Secondary Alcohols Effective with inexpensive reagents such as Na 2 Cr 2 O 7 in acetic acid PCC is used for sensitive alcohols at lower temperatures 49

50 Mechanism of Chromic Acid Oxidation Alcohol forms a chromate ester followed by elimination with electron transfer to give ketone The mechanism was determined by observing the effects of isotopes on rates 50

51 How Alcohol Is Metabolized in the Human Body Alcohol dehydrogenase converts methanol into formaldehyde (H 2 CO) and aldehyde dehydrogenase turns this formaldehyde into a formic acid radical. Both formaldehyde and formic acid are highly poisonous and quickly dead to blindness and death. 51

52 17.8 Protection of Alcohols Hydroxyl groups can easily transfer their proton to a basic reagent This can prevent desired reactions Converting the hydroxyl to a (removable) functional group without an acidic proton protects the alcohol 52

53 Methods to Protect Alcohols Reaction with chlorotrimethylsilane in the presence of base yields an unreactive trimethylsilyl (TMS) ether The ether can be cleaved with acid or with fluoride ion to regenerate the alcohol 53

54 Protection-Deprotection An example of TMS-alcohol protection in a synthesis 54

55 17.9 Phenols and Their Uses Industrial process from readily available cumene Forms cumene hydroperoxide with oxygen at high temperature Converted into phenol and acetone by acid 55

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57 17.10 Reactions of Phenols The hydroxyl group is a strongly activating, making phenols substrates for electrophilic halogenation, nitration, sulfonation, and Friedel Crafts reactions Reaction of a phenol with strong oxidizing agents yields a quinone Fremy's salt [(KSO 3 ) 2 NO] works under mild conditions through a radical mechanism 57

58 Quinones in Nature Ubiquinones mediate electron-transfer processes involved in energy production through their redox reactions 58

59 17.11 Spectroscopy of Alcohols and Phenols Characteristic O H stretching absorption at 3300 to 3600 cm 1 in the infrared Sharp absorption near 3600 cm -1 except if H-bonded: then broad absorption 3300 to 3400 cm 1 range Strong C O stretching absorption near 1050 cm 1 (See Figure 17.11) Phenol OH absorbs near 3500 cm -1 59

60 Nuclear Magnetic Resonance Spectroscopy 13 C NMR: C bonded to OH absorbs at a lower field, 50 to 80 1 H NMR: electron-withdrawing effect of the nearby oxygen, absorbs at 3.5 to 4 (See Figure 17-13) Usually no spin-spin coupling between O H proton and neighboring protons on C due to exchange reactions with moisture or acids Spin spin splitting is observed between protons on the oxygen-bearing carbon and other neighbors Phenol O H protons absorb at 3 to 8 60

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62 Mass Spectrometry Alcohols undergo alpha cleavage, a C C bond nearest the hydroxyl group is broken, yielding a neutral radical plus a charged oxygen-containing fragment Alcohols undergo dehydration to yield an alkene radical anion 62