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Download Alcohols General formula R-OH hydroxyl group Nomenclature
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Alcohols General formula R-OH hydroxyl group Nomenclature IUPAC: name as for alkane dropping the -ane and adding -anol. Indicate the position of the hydroxyl group with a number. Number the longest alkane chain beginning at the end nearer the hydroxyl group. e.g. Trivial name IUPAC name CH 3 OH methanol methyl alcohol CH 3CH 2 OH H ethanol ethyl alcohol CH 3 C CH3 2-propanol isopropanol C OH triphenylmethanol trityl alcohol OH CH 3CH 2 3-methyl-3-hexanol H 3C C OH CH 2CH 2CH 3 1 2 3 HOCH2CH=CH 2 CH 3CHOHCH=CHCH 3 OH 2-propen-1-ol allyl alcohol 3-butene-2-ol cyclopentanol CH 2CH 2OH 2-phenylethanol H 3C OH HOCH2CH 2OH HOCH2CHCH 2OH OH cis-3-methylcyclopentanol 1,2-ethanediol ethylene glycol 1,2,3-propantriol glycerol Properties of Alcohols Boiling Points B.Pt.oC -161.5 -33 100 19 M.W. 16 17 18 20 Compound CH 4 NH3 H 2O HF B.Pt.oC -161.5 -24 64.5 -88.6 -42 -24 M.W. 16 50.5 32 30 44 46 Compound CH 4 CH 3Cl CH 3OH CH 3CH 3 CH 3CH 2CH 3 CH 3OCH3 The low molecular weight alcohols, with 4 carbons or less, are water soluble. The hydrocarbons and aryl halides are not. There are several types of intermolecular bonding: London or dispersion forces, dipole-dipole interactions and hydrogen bonding. Hydrogen bonding is the strongest type of intermolecular bonding. Hydrogen bonds exist between molecules having an H attached to a strongly electronegative atom such as O, N, or F. These forces are present in water and can build up a 3-D network. δ + δH O δ+H δ+ δδ+ δH O H O + Hδ δ+H + + δ δ δ- O H δ- O H δ- δ+ + Hδ δ+H O H Hδ+ Alcohols can also hydrogen bond in this way. etc. δO δ+ δH O δ+H δ+ δH O R R R - δO R δ δ+H O R δ + O Hδ δ+H R These hydrogen bonds have to be broken before the molecules break apart to form vapour and so they account for the relatively high boiling points. Synthesis of Alcohols Grignard reactions Generalized reaction O C + carbonyl group RMgX ether Grignard reagent OMgX C R addition complex H3O+ Preparation of Grignard reagent ether R-X + Mg RMgX X = Br or I Mechanism of the addition of Grignard reagents to the carbonyl group δO OMgX δ+ + δδ MgX + C C C C OH C R + Mg+X- Formation of primary, secondary and tertiary alcohols by the Grignard Reactions Primary alcohols O ether C + RMgX H H formaldehyde e.g. CH 3 CH CH 3 + Mg Br O C H OMgX H C OMgBr CH 3 C CH H H CH 3 ether H H C H R primary alcohol H R OH H 3O+ OH H3O+ CH3 C CH H H CH3 2-methyl-1-propanol Secondary alcohols O ether C + R'MgX R H OMgX C R R' aldehyde CH 3CH 2MgBr + CH3 ether CH 3 H R C H R' secondary alcohol H e.g. O C OH H3O+ OMgBr H 3O + C H CH2CH 3 CH 3 OH C H CH 2CH3 2-butanol Tertiary alcohols R e.g. O C R' + R"MgX R C R" H3O+ OH R C R' R" tertiary alcohol R' OH OMgBr MgBr O + ether CH3 CCH3 O MgBr and OMgX ether C H3C C CH3 H 3O+ H 3C C CH3 OH C + benzophenone triphenylmethanol Triphenylmethanol is an interesting compound. When treated with acid, it forms a very stable carbocation which has 10 resonance structures. Hydration of Alkenes Acid-catalyzed addition of water to an alkene General reaction C C H 2SO4 + H2 O C OH e.g. CH 3 CH 3 C=CH2 CH 3-CH-CH3 75% CH 3 H 2SO4 + H2 O follows Markovnikov rule get both syn and anti addition OH H 2SO4 CH3CH=CH2 + H2 O H C CH3-C-CH 3 OH 85% Oxymercuration-demercuration reaction General reaction C C O + H2 O + Hg(OCCH 3 )2 mercuric acetate C + NaBH4 OH HgOCOCH 3 O e.g. CH 3 1) Hg(OCCH 3)2 C O THF + HOCCH 3 C acetic acid OH HgOCOCH 3 C + OH H 3C C OH H C follows Markovnikov rule get both syn and anti addition OH 2) OH-, NaBH 4 90% O 1) Hg(OCCH 3)2 CH3CH 2CH=CH2 2) OH-, NaBH 4 OH CH 3CH 2CHCH3 95% In oxymercuration-demercuration reactions, acid is not required so that carbocations are not formed and rearrangement does not occur CH 3 CH 3-C-CH=CH 2 CH 3 H 3O + O CH 3 + CH 3C CHCH3 CH 3 1) Hg(OCCH 3)2 2) OH-, NaBH 4 CH 3 CH3-C-CH-CH3 + CH 3 CH 3 OH CH3-C CH-CH3 CH 3 94% CH 3 CH3-C CH-CH3 OH CH 3 70% rearranged product Hydroboration - oxidation This is a very important reaction since it gives Anti-Markovnikov addition and both the H and the OH add syn C C + BH3 δ+ C C H B H δ- Because there is a cyclic compled formed, both B and H add to the same side of the double bond. H H2O2, OH- C OH H C e.g. + BO3 - H2O2, OH-, 0o C 3 CH3CH=CH2 + BH3 B-(CH2CH2CH3)3 3 CH3CH2CH2OH tri-(1-propyl) borane 1-Propanol Notice the different products from hydration and hydroboration of an alkene OH H2O, H2 SO4 H3C C CH3 Markownikov addition H3C C CH2 CH 3 CH3 CH3 1) BH3 2) H2O 2, OH-, 0o C H C CH CH OH 3 2 CH3 OH H 3C anti-Markovnikov addition H2O, H2 SO4 1) BH3 2) H2O2, OH-, 0o C H3C H OH syn addition, stereospecific reaction very useful Hydrolysis of alkyl halides This reaction was studied in the context of SN 1 and SN2 reactions. Reactions of Alcohols Reactions with active metals i.e., the hydrogen of the OH is slightly acidic 2RO-M+ + H2 2ROH + 2M M = Li, Na, K fast reaction Mg, Ca, Al very slow reaction 2CH 3CH2-Na+ + e.g. 2CH3CH2OH + 2Na 2 H 3C CH3 C OH CH3 CH3 C O-K+ CH3 2 H3C + 2K H2 + H2 potassium t-butoxide CH3 6 H3C C H OH 2 Al + 2Al O CH3 CH CH3 3 + 3H2 Dehydration to Alkenes This reaction has been studied in the synthesis of alkenes H3O+ H2O + C C C heat H OH Mechanism: OH C C + H3 O+ H C H + OH2 C slow fast fast C fast + OH2 C C + H2 O H C H + C fast slow C C + H3 O+ Note that all of the reactions are reversible. Therefore the alkene has to be removed from the solution as rapidly as it is formed. The slow rate-determining step is the formation of the carbocation. Tertiary alcohols react more rapidly than secondary which react more rapidly than primary. e.g. CH3 CH2 20% H2SO4 + H2 O H3C C CH3 H3C C OH 90oC CH3 CH3 CH2 Dehydration follows the H3C OH Saytzeff rule - the most H 3PO4 , 120oC + H2 O highly substituted alkene + is formed as the major product. This is also the OH more stable alkene. 83% very small amount o H3PO4 , 200 C + H2 O Conversion to Alkyl Halides HX RX R-OH or PX3 + H2 O or P(OH)3 This is one of the most important methods of synthesis of alkyl halides OH Br e.g. + 100oC HBr + H2 O cyclohexanol CH3 H3C + C OH CH3 Mechanism H3O+ CH3 H3C C CH3 H3C and CH2Cl + HCl benzyl chloride benzyl alcohol but CH3CH2OH + C Cl + H2 O CH3 i.e. SN 1 reaction Clnotice that another reaction of carbocations is loss of H+ to yield an alkene. In this example, the nucleophile Clis in large excess and so the formation of the alkyl halide is favored. H3C CH3 + OH2 CH2OH CH3 25oC HCl (conc) C+ CH3 The benzylic cation is more stable than a tertiary carbocation. no reaction since the chloride ion is not a strong enough nucleophile HCl HCl - ZnCl2 + CH3CH2 OH HOZnCl + CH3 CH2Cl ZnCl ClPrimary alcohols and methyl alcohols react with phosphorus trihalides by an SN2 reaction CCl4 CH3CH2CHCH3 + PBr3 CH3CH2CHCH3 80oC OH Br CCl4 CH3CH2CH2CH2Cl 80oC Using PBr3 there is no carbocation formed and so we do not get rearrangements and this is usually the preferred reagent for converting alcohols to alkyl bromides. CH3CH2CH2CH2OH + Mechanism R-CH2-O-H + Br P Br Br This is an SN 2 type of reaction PCl3 Br+ R-CH2-O-P-Br2 H R-CH2Br + HOPBr2 Oxidation of Alcohols Primary alcohols RCH2OH + CrO3 chromic oxide O R C H aldehyde pyridine RCH2OH + K2 Cr2O 7 H 2SO4 Secondary alcohols H R C R' O R C R' ketone H2SO4 + K2 Cr2O7 OH Tertiary alcohols R" R C R' + K2 Cr2O 7 N pyridine O R C OH carboxylic acid H 2SO4 no reaction black tar if the temperature is high enough! oxidation would require breaking of C-C bond OH Examples CH 2OH Primary CH3CH2CH2OH 1-propanol CrO3/pyridine O CH3CH2C H CrO3/pyridine 0oC benzyl allcohol H 2SO4 Secondary H 3C CH CH 3 + K2 Cr2O7 2-propanol OH + K2 Cr2O7 H 2SO4 Cr3+ green O H3C C CH 3 2-propanone (acetone) O H 2SO4 cyclohexanone Unsaturated H 2C CH CH 2 OH CrO 3/pyridine 0oC O benzaldehyde CH3CH2CH2COOH butanoic acid This reaction was the basis of the breathalyzer test: Cr2 O7 = orange OH C 0oC propanal CH3CH2CH2CH2OH + K2 Cr2O 7 H O H2C CH C H propenal Using a stronger oxidising agent would oxidise the double bond by ethanol oxidation