Alcohols Lecture-3
Reactions of alcohols
Compare acidity constants of alcohols
Relative acidity
$H_2O>ROH>RC \equiv CH>H_2>NH_3>RH$
all alcohols except methanol are weaker acids than water
Basicity
Basicity order
$CH_3O^-<OH^-<CH_3CH_2O^-<(CH_3)_3CO^-<CH \equiv C^-H<H^-<NH_2^-<CH_3CH_2^-$
Electronic effect R
Extent of solvation of conjugate base
Reactions replacing
(A) Reactions replacing H atom $1^o>2^o>3^o$
(1) With active metals $(Na, K, Al, Mg)$
$2ROH+2Na \rightarrow 2RONa+H_2 \uparrow$
$ROH+M \rightarrow RO^-M^+ \frac{1}{2}H_2 \uparrow$
(2) With grignard’s reagents react with active hydrogen
$ROH (stronger \hspace{1mm} acids) +R’MgX \rightarrow R’H+ROMgX$
Organic acids
(3) with organic acids in presence of conc $H_2SO_4$- esters
Reaction
(4) With acid chloride or anhydride $(RCOCl or (RCO)_2O))$ $\rightarrow $ ester
Phosphoric acid
(5) With phosphoric acid
Good leaving group
(6) OH $\rightarrow$ good leaving group
With alkane/arene sulfonyl chloride
$RSO_2Cl$
$ArSO_2Cl$
$RSO_2Cl$ methane sulfonyl chloride $\rightarrow$ mesylates methane sulfonate ester
$CF_3SO_2Cl \rightarrow $ triflates
$ArSO_2Cl PTSCl \rightarrow$ tosylates
$R-\ddot O_{..}H+ClSO_2R’ \xrightarrow [{-psyr HCl}]{Pry} ROSO_2R’+HCl$
Halogen acids
Reactions replacing OH group
With halogen acids HI-(temperature below HI)HBr-(conc HBr or KBr/NaBr + $H_2SO_4 \rightarrow$ nylux) HCl-(conc HCl+anhydrous$ZnCl_2$ lewis acids $\rightarrow$ heat)
Substitution reaction $\rightarrow$ acid catalyzed
$R-OH + HX \rightarrow RX + H_2O$
prommates OH, makes it a good leaving group
Lucas reagent
$3^o>2^o>1^o$
$1^o$, $CH_3CH_2CH_2CH_2CH_2OH(n-pentyl \hspace{1mm} alcohol) \xrightarrow [{anhydrous ZnCl_2, \Delta}]{HCl} CH_3CH_2CH_2CH_2CH_2Cl$(n-pentyl chlorids)
$2^o$, $>-OH( isopropylalcohol) \xrightarrow [{NaBr,H_2SO_4,reflux}]{conc HBr or} >-Br$
$3^o$, $t-butylalcohol \xrightarrow [{RT}]{conc HCl}$ t-butyl chloride
Phosphorus halides
(2) With phosphorus halides $(PCl_5,PBr_3, P/I_3, P/Br) \rightarrow $ allyl halides
$1^o$ or $2^o$ alcohals
no carbocation formation
no rearrangement especially at/ under $0^oC$
$PBr_3$ preferred reagent food alkyl bromide fomation
Thionyl chloride
(3) With thionyl chloride $\rightarrow$ allyl chloride
Oxidation
(C) Reactions involving both R and OH groups
Oxidation, dehydro generation, dehydration
OXIDATION of alcohols
a number of oxidizing agents
($1^o \rightarrow$ $2^o \rightarrow$ $3^o) \rightarrow$
Mn (VII) and Ge(VI)
Mn(VII) $KMnO_4 \rightarrow$ acidified $KMnO_4$, alkaline $KMnO_4$
$1^o \rightarrow$ acid
$RCH_2OH \xrightarrow {KMnO_4} R-COOM$
No loss of C atoms
$R-CH_2OH \xrightarrow [{KMnO_4,OH^-,H_2O, \Delta}]{basic aq.} R-COO^-K^+ \xrightarrow {H^+} RCOOH+MnO_2\downarrow$brown ppt
Cr(VI) $\rightarrow CrO_3/dil H_2SO_4$ in aq acetone chromic acid $H_2CrO_4$
Jones reagent
$H_2CrO_4$ or $Na_2Cr_2O_7$ in $H_2SO_4$ water
$RCH_2OH(1^o), Cr_2O_7^{2-}$(orange red)
$R-CH=O+Cr^{2+} \xrightarrow {Cr_2O_7^{2-}} RCOOH$
Mechanism of oxidation
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