Chapter 15

Prep of alcohol by reduction of Aldehyde and Ketones Via Hydrogenation exothermic but slow w/o a catalyst potential catalysts: Pd Pt Ni, Ru Via sodium borohydride Aldehyde?NaBH4, polar protic solvent ?primary alcohol Can be ketone instead of aldehyde 1NaBH reacts with 4 aldehydes or ketones NaBH their mechanisms are similar to Grinard reactions with aldehydes and ketones except they act as hydride donors instead of carbanion sources Hydrogen in product bound to carbon comes from NaBH4 Hydrogen on O comes from solvent for NaBH4 Via Lithium aluminum hydride Aldehyde ? LiAlH4, Diethyl ether, H2O ? primary alcohol Diethyl either can be used as a solvent or THF 1 LiAlH4 reacts with 4 aldehydes or ketones Can be ketone instead of aldehyde mechanism same as for NaBH except the reduction and hydrolysis need to be done in separate steps Does not reduce isolated Double bonds neither dose NABH4 Esters And Carboxilic acides to alcohols LiALH is only thing that reduces a carboxilic acid to a primary alcohol does it the same way it does an aldehyde or ketone esters are reduced by either LiAlH or NaBH in the same way as described above but produce two alcohols consisting of the atoms on either side of the origional C-O single bond Alcohols from Epoxides Grignard reagent + Epoxide ?Diethly either, H3O ?primary alcohol the resulting alcohol has the R group from the grinard reagent on the opposite side as the carbon connected to the alcohol group organolithium also react in the same way the epoxide ring is cleaved through the nuc from the gringard Diol Prep same ways as discussed before but with twice as much reagent Nomenclature is the same as alcohol but there are two locants needed and -diol replaces -ol Vicinal diols have their hydroxyl groups on adjacent carbons 1,2-ethanediol and 1,2-propanediol are frequently encountered and go by the acceptable common nomenclature of ethylene glycol and propylene glycol respectively Vicinal diol prep called dihydroxylation Step 1: Alkene + Osmium tetraoxide ? cyclic osmate ester two Os bonded to the carbons that were double bonded Step 2: add oxidizing such as Tert butyl hydroperoxide and get vicinal diol both oxogens come from the psmiumtetraoxide the Tert butyl hydroperoxide can be regenerated Happens via syn addition Can prepare a enantiomerically pure vicinial diol Alkene ? K2OsO2(OH)4, K3Fe(CN)6 ? R or S- alkyldiol Alcohols to ethers Heat 2 moles of primary alcohol in the presence of and acid catalyst a condensation rxn because two molecules combine to form larger one while liberating a smaller molecule elimination would happen if you used secondary or tertiar alcohols Mechanism O on alcohol accepts proton rom acid oxomonium ion is attacked by O on anohter alcohol and H2O breaks free O on third alcohol or conjugate base takes proton from dialkyloxonium ion Diols can react intramolecularly to form cyclic ethers when a 5 or 6 member ring results for this starting alcohol can have any substitution Fischer Esterification ROH + R?C=O -OH ? R?C=O -OR +H2O equilibrium lies slightly to products side if you add benzene it will distil with the water creating a more favored product side same reaction possible when you replace the carboxilic acid with a acyl chloride you also get HCL instead of H2O use weak base catalist e.g. pyridine You could also use acid anhydrides and it woudl work the same R?C=O -OC=O -R? produces a carboxilic acid instead of HCL Important point is that the C-O bond of the alcohol remains intact in all these rxns configuration fo the acyl groups is conserved in all these reactions Oxidation of Alcohols primary alcohols are oxidized to a aldehyde and will continue to carboxilic acids unless oxidation is stopped frequently with the use of chroium (VI) Oxidization via Chromic acid (H2CrO4) is common To stop at an aldehyde usually use PCC (C5H5NH+ ClCrO3-) or PDC (C5H5NH)22+ Cr2O72- in an anhydrous media such as CH2Cl2 Tertiary alcohols can not undergo oxidation because thy have no hydroxyl bearing carbon Mechanism O in alcohol attacks the Cr in condensation to release an H2O Water then attacks the h on the Hydrogen agencent to the oxogen on the original alcohol allowing a Db to form between the c and O and the bond between the O and the chromite to break Oxidative cleavage of Vicinial Diols When treated with HIO4 vicineal diols C-C bond cleaves forming 2 aldehydes or ketones and HIO3 Same reaction can happen on cyclic diols and give dicarbonyl compounds happens faster when substitutes are cis Thiols Nomenclature ame nomenclature and structural rules as alcohols but replace the O with S and -ol with -thiol Named as a substitute it is a mercapto or sulfanyl group The S-H bond is less polar than the o-h means no hydrogen bonding but S-H are stronger acids than alcohols becuase of weaker S-H bond conjugate bases are called alkanethiolate These weak bases undergo SN2 reactions even wiht secondary alkylhalides Prepared via SN rxn with H2S conjugate base from alkyl halide Oxidation of C=S compounds does not occur only sulfer can be oxidized sulfer can be called sulfenic acid (with one O) sulfinic acid (with 2 Os) or sulfonic acid (with 3 Os) 2 Thiols oxidize to form disulfides with just the O2 in the air the S-S bond is significantly stronger than the H-S bonds because of its covalency