Alcohols R-O-H Classification CH3, 1o, 2o, 3o Nomenclature: Common names: “alkyl alcohol” IUPAC: parent = longest continuous carbon chain containing the –OH group. alkane drop -e, add –ol prefix locant for –OH (lower number for OH)
CH3 CH3 CH3CHCH2CHCH3 CH3CCH3 OH OH 4-methyl-2-pentanol tert-butyl alcohol 2-methyl-2-propanol 2o 3o CH3 HO-CHCH2CH3 CH3CH2CH2-OH sec-butyl alcohol n-propyl alcohol 2-butanol 1-propanol 2o 1o
Physical properties of alcohols: polar + hydrogen bonding relatively higher mp/bp water insoluble! (except for alcohols of three carbons or less) CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2-OH hydrophobic hydrophilic
Oldest known organic synthesis: “fermentation” Sugar + yeast ethyl alcohol + CO2 Grape juice => “wine” Barley => “beer” Honey => “mead” Rice => “sake” ~5-11% ethanol
Alcohols, synthesis: 1. 2. 3. 4. Hydrolysis of alkyl halides (CH3 or 1o) 5. 6. 7. 8.
NR some 1o/2o R-H R-X R-OH Acids Bases Active metals Oxidation Reduction Halogens
Alcohols, reactions: R-|-OH With HX With PX3 (later) RO-|-H As acids Ester formation Oxidation
1. Reaction of alcohols with HX: (#1 synthesis of RX) R-OH + HX R-X + H2O a) HX: HI > HBr > HCl b) ROH: 3o > 2o > CH3 > 1o c) May be acid catalyzed d) Rearrangements are possible except with most 1o alcohols.
CH3CH2CH2CH2-OH + NaBr, H2SO4, heat CH3CH2CH2CH2-Br n-butyl alcohol n-butyl bromide 1-butanol 1-bromobutane CH3 CH3 CH3C-OH + HCl CH3C-Cl (room temperature) tert-butyl alcohol tert-butyl chloride 2-methyl-2-propanol 2-chloro-2-methylpropane CH3CH2-OH + HI, H+, heat CH3CH2-I ethyl alcohol ethyl iodide ethanol iodoethane
Mechanism? CH3-OH and most 1o alcohols react with HX via SN2 mechanism 3o and 2o react with HX via SN1 mechanism Both mechanisms include an additional, first step, protonation of the alcohol oxygen: R-OH + H+ R-OH2+ “oxonium ion”
Whenever an oxygen containing compound is placed into an acidic solution, the oxygen will be protonated, forming an oxonium ion.
Mechanism for reaction of an alcohol with HX: CH3OH or 1o alcohols:
Mechanism for reaction of an alcohol with HX: 2o or 3o alcohols:
May be catalyzed by acid. SN2 rate = k [ ROH2+ ] [ X- ] SN1 rate = k [ ROH2+ ] Acid protonates the -OH, converting it into a better leaving group (H2O), increasing the concentration of the oxonium ion, and increasing the rate of the reaction.
Rearrangements are possible (except with most 1o alcohols): CH3 CH3 CH3CHCHCH3 + HBr CH3CCH2CH3 OH Br Br- CH3 CH3 [1,2-H] CH3 CH3CHCHCH3 CH3CHCHCH3 CH3CCH2CH3 OH2+ + + 2o carbocation 3o carbocation
Most 1o? If large steric requirement… CH3 CH3 CH3CCH2-OH + HBr CH3CCH2CH3 CH3 Br neopentyl alcohol 2-bromo-2-methylbutane CH3 CH3 CH3 CH3CCH2-OH2+ CH3CCH2+ CH3CCH2CH3 CH3 CH3 + 1o carbocation 3o carbocation [1,2-CH3]
With PX3 ROH + PX3 RX PX3 = PCl3, PBr3, P + I2 No rearrangements ROH: CH3 > 1o > 2o CH3 CH3 CH3CCH2-OH + PBr3 CH3CCH2-Br neopentyl alcohol 2,2-dimethyl-1-bromopropane
Dehydration (later)
As acids. With active metals: ROH + Na RONa + ½ H2 With bases: ROH + NaOH NR! CH4 < NH3 < ROH < H2O < HF
CH3CH2OH + NaOH H2O + CH3CH2ONa WA WB SA SB CH3CH2OH + CH3MgBr CH4 + MgBr(OCH2CH3) SA SB WA WB CH3OH + NaNH2 NH3 + CH3ONa SA SB WA WB
Ester formation. CH3CH2-OH + CH3CO2H, H+ CH3CO2CH2CH3 + H2O CH3CH2-OH + CH3COCl CH3CO2CH2CH3 + HCl CH3-OH + CH3SO2Cl CH3SO3CH3 + HCl Esters are alkyl “salts” of acids.
oxidation states of carbon CH4 CH3OH CH2O HCO2H CO2 -4 -2 0 +2 +4 reduction -
Oxidation Oxidizing agents: KMnO4, K2Cr2O7, CrO3, NaOCl, etc. Primary alcohols: CH3CH2CH2-OH + KMnO4, etc. CH3CH2CO2H carboxylic acid Secondary alcohols: OH O CH3CH2CHCH3 + K2Cr2O7, etc. CH3CH2CCH3 ketone Teriary alcohols: no reaction.
Primary alcohols ONLY can be oxidized to aldehydes: CH3CH2CH2-OH + C5H5NHCrO3Cl CH3CH2CHO pyridinium chlorochromate (PCC) aldehyde or CH3CH2CH2-OH + K2Cr2O7, special conditions
Alcohols, synthesis: 1. 2. 3. 4. Hydrolysis of alkyl halides (CH3 or 1o) 5. 6. 7. 8.
Alcohols, reactions: R-|-OH With HX With PX3 (later) RO-|-H As acids Ester formation Oxidation