Chapter 17: Alcohols and Phenols O C O H H sp3 phenol alcohol (aromatic alcohol) pKa~ 16-18 pKa~ 10 Alcohols contain an OH group connected to a saturated carbon (sp3) Phenols contain an OH group connected to a carbon of a benzene ring O H H O C C C C chemistry dominated by the keto form enol keto 76 O O O O R' H H R H R R R O water alcohol ether peroxide S S S R' R H R R R S thiols thioether disulfides Alcohols are classified as primary (1°), secondary (2°) or tertiary (3°), which refers to the carbon bearing the hydroxy group methanol primary secondary tertiary R R H R C O C O C O C O R R H H H H H R H H H H 1° carbon 2° carbon 3° carbon 77 39 17.1: Nomenclature: 1. In general, alcohols are named in the same manner as alkanes; replace the -ane suffix for alkanes with an -ol for alcohols CH 3CH 2CH 2CH 3 CH 3CH 2CH 2CH 2OH OH butane 1-butanol 2-butanol 2. Number the carbon chain so that the hydroxyl group gets the lowest number 3. Number the substituents and write the name listing the substituents in alphabetical order. 4. For phenols, follow benzene nomenclature and use phenol as the parent name. The carbon bearing the -OH group gets number 1. 78 OH OH NO OH 2 NO2 2-methyl-2-pentanol 3-phenyl-2-butanol 3,4-dinitrophenol Many alcohols are named using non-systematic nomenclature H C OH 3 OH OH C OH OH HO OH H3C HO H3C benzyl alcohol allyl alcohol tert-butyl alcohol ethylene glycol glycerol (phenylmethanol) (2-propen-1-ol) (2-methyl-2-propanol) (1,2-ethanediol) (1,2,3-propanetriol) 79 40 17.2: Properties of alcohols and phenols: Hydrogen bonding: The structure around the oxygen atom of an alcohol or phenol is similar to that in water and is sp3 hybridized Alcohols and phenols have much higher boiling points than similar alkanes and alkyl halides H2O CH3CH2CH2CH3 CH3CH2CH2CH2Cl CH3CH2CH2OH MW=18 MW=58 MW=92.5 MW=74 bp= 100° C bp= -0° C bp= 77° C bp= 116° C C6H6 C6H6OH C6H6CH3 C6H6CH2OH MW=78 MW=94 MW=92 MW=108 bp= 80° C bp= 182° C bp= 110° C bp= 203° C 80 Alcohols and phenols, like water, can form hydrogen bonds: non covalent interaction between a hydrogen atom (δ+) involved in a polar covalent bond, with the lone pair of a heteroatom (usually O or N), which is also involved in a polar covalent bond (δ-) O H N H C O C O !+ !- !- !+ !- !+ O H N H C O H H O O H H R H R R O ! ! O! O H O ! H ! H ! H H H H H H ! O ! O O O O ! O ! ! H H H H ! H H H R R O O O R H H H H O O H H Hydrogen-bonds are broken H H O when the alcohol reaches its bp, H which requires additional energy81 41 17.3: Properties of alcohols and phenols: acidity and basicity: Like water, alcohols are weak Brønsted bases and weak Brønsted acids. The nature of the R group can significantly influence the basicity or acidity H O O + - R H + H X R H :X oxonium ion H H O + O R O + O R H H H H alkoxide ion CH3OH CH3CH2CH2CH2OH CH3CH2CH(OH)CH3 (CH3)C-OH MW = 32 MW = 74 MW = 74 MW = 74 bp= 65° C bp = 116° C bp = 99° C bp = 82° C pKa ~ 15.5 pKa ~ 16 pKa ~ 17 pKa ~ 18 The steric environment around the oxygen atom can influence the physical properties of an alcohol 82 Solvation: upon acid dissociation the alkoxide ion is stabilized by solvation through hydrogen bonding between water and the negatively charge oxygen. The steric environment around the negatively charge oxygen influences the solvation effect H H O + O R O + O R H H H H H O O O H H H H H H H H H H C H C O H O H H C O H O H H C H C H H H O H H H O H H O H Acidity: methanol > 1° alcohol > 2° alcohol > 3° alcohol Reflects the ability water to stabilized the resulting alkoxide though solvation 83 42 Electronic factors that influence acidity: inductive and resonance effect CH3CH2OH FCH2CH2OH F2CHCH2OH F3CCH2OH (F3C)3CCH2OH pKa ~ 16.0 14.4 13.3 12.4 5.4 F3C !+ Electron-withdrawing groups make an C O F C alcohol a stronger acid by stabilizing 3 the conjugate base (alkoxide) F3C X OH X= -H -Cl -Br -NO2 -CH3 -OCH3 -NH2 pKa ~ 9.9 9.38 9.35 7.15 10.16 10.21 10.46 X OH OH X ~ pKa X= -Cl 9.38 8.85 -NO2 7.15 8.28 -OCH3 10.21 9.65 84 -CH3 10.17 10.16 Phenols are much more acidic than aliphatic alcohols: a benzene ring is generally considered electron withdrawing (inductive effect) the benzene ring stabilizes the negative charge of the phenoxide ion through resonance (Fig. 17.3, p. 595) 85 43 Electron-withdrawing substituents make a phenol more acidic by stabilizing the phenoxide ion through delocalization of the negative charge and through inductive effects Electron-donating substituents make a phenol less acidic by destabilizing the phenoxide ion (resonance effect) The location of the substituent relative to the phenol is important 86 17.4: Preparation of alcohols: OH H O+ 3 Markovnikov addition H H a) B2H6, THF Anti-Markovnikov b) NaOH, H2O2 Overall syn addition of BR2 OH H–OH across the π-bond OH OH a) Hg(OAc)2, H2O b) NaBH 4 Markovnikov HgOAc a) OsO4 b) NaHSO3 OH O O Syn addition of -OH Os groups O O OH Hydration of alkenes (Ch. 7.4) Oxymercuration of alkenes (Ch. 7.4) Hydroboration of alkenes (Ch. 7.5) Di-hydroxylation of alkenes (Ch. 7.8) 87 44 17.5: Alcohols from reduction of carbonyl compounds Figure 10.5 (Chapter 10.10) Increasing oxidation state C C C C C C O C OH C O C CO2 OR Cl Cl Cl C Cl C Cl C Cl C Cl Cl Cl Cl C NH2 C NH C N 88 17.5: Alcohols from reduction of carbonyl compounds add the equivalent of H2 across the π-bond of the carbonyl to give an alcohol H O O [H] aldehyde (R or R´= H) → 1° alcohol C C R H R R' R' ketone (R and R´≠ H) → 2° alcohol [H]: sodium borohydride: NaBH4, ethanol reduces aldehydes to 1° alcohols and ketones to 2° alcohols lithium aluminum hydride (LAH): LiAlH4, ether reduces aldehydes, carboxylic acids, and esters to 1° alcohols and ketones to 2° alcohols In general, NaBH4 and LiAlH4 will not reduce C=C. M M O EtOH or H O+ OH ether O 3 H: C C C H H 89 45 17.6: Alcohols from reaction of carbonyl compounds with Grignard reagents Alkyl halides will react with some metals (M0) in ether or THF to form organometallic reagents Grignard reagent- organomagnesium ether R-X + Mg(0) o r T H F R-Mg(II)-X X= Cl, Br, or I R can be a variety of groups: 1°-, 2°-, 3°-alkyl, aryl or vinyl - + ! ! _ Carbanions: nucleophile C MgX C reacts with electrophile – R-MgX ≡ R: 90 ! - Grignard reagents react with aldehydes or ketones O ! + to give alcohols C MgX MgX + O ether O H3O OH R: C C C R R If, carbonyl = H2C=O → 1° alcohol = aldehyde → 2° alcohol = ketone → 3° alcohol Br 0 1) H2C=O Mg , ether MgBr + 2) H3O OH O OH Br Mg0, ether MgBr 1) H + 2) H3O OH 0 O Mg , ether 1) CH CH Br CH3CH2MgBr 3 2 + 2) H3O 91 46 Grignard reagents react with esters to give 3° alcohols O 1) 2 H3CMgBr 2) H O+ HO 3 CH3 CH2CH3 O CH3 _ H3C H3CMgBr + the H3O MgX _ O O CH3 CH OCH2CH3 3 Some functional groups are incompatible with Grignard reagents Grignard reagents are very strong bases as well as highly reactive nucleophiles O _ O H O+ O H C MgBr 3 H 3 _ + CH4 R O R O MgX R OH Carboxylic acids are simply deprotonated by Grignard reagents and do not give addition products 92 Grignard reagents will deprotonate alcohols 0 _ Mg + H3O HO Br HO MgBr O H HO H _ BrMg Other incompatible groups: -CO2H, -OH, -SH, NH2, CONHR (amides) Reactive functional groups: aldehydes, ketones, esters, amides, halides, -NO2, -SO2R, nitriles 93 47 17.7 Some reactions of alcohols A. Reactions involving the C-O bond Dehydration to alkenes: E1 mechanism (reactivity: 3° > 2° >> 1°) requires strong acid catalyst (H2SO4) water is a much better leaving group than HO- usually follows Zaitzev’s rule H2SO4 OH H3C CH3 H2C H2O, THF C C C CH2CH3 H3C C CH2CH3 - H2O H3C H H3C CH3 94 Dehydration to alkenes with POCl3 E2 mechanism- requires an anti-periplanar conformation between leaving group and the hydrogen that is being lost mild conditions, requires a base (pyridine) H OH POCl3, pyridine H H CH3 CH3 H POCl3, OH pyridine CH CH3 3 H 95 48 Conversion of an alcohol to an alkyl halide (R-OH → R-X) (Chapter 10.7) 1. Substitution reaction of alcohols with HX R-OH + HX R-X + H2O Works better with more substituted alcohols H H H R H C OH << R C OH < R C OH < R C OH H H R R Methyl Primary (1°) Secondary (2°) Tertiary (3°) increasing reactivity SN1 mechanism involving a carbocation intermediate: rearrangements, stereochemistry (racemization) 96 2.