Chem 334 - Summer 1998 - Organic Chemistry I
Dr. Carl C. Wamser Dr. Carl C. Wamsernomenclature
classification ( 1° , 2° , 3° - based on the C with the OH group)
common - alkyl alcohol
Greek letters denote positions away from functional group
IUPAC - -ol suffix with number prefix (priority functional group)
chain must include the C with the alcohol
number chain so alcohol gets lower number
a generic IUPAC name:
(stereochem)-{n-substituent}{cyclo}alkan{e}
prefixes (alphabetical) suffix (priority functional group)
structure and properties
tetrahedral O, similar to water
polar C-O and O-H bonds, nonpolar C-C and C-H bonds
(amphoteric - both hydrophilic and hydrophobic parts)
H-bonding (~ 5 kcal/mol) holds molecules together
leads to relatively high boiling points and water solubility
compare data from Table 8-1: CH4 CH3Cl CH3OH
compound type: nonpolar polar polar, H-bonding
boiling point: -162° -24° +65°
water solubility: very low low miscible
reactions as an acid
R-O-H <=====> R-O- + H+ (very weak acid)
alcohols are generally weaker acids than water
acidity order (of alcohols): CH3OH > H2O> 1° > 2° > 3°
pKa 15.5 15.7 15.9 17 18
basicity order (of alkoxide anions): 3° > 2° > 1° > OH- > CH3O-
caused by solvation effects:
larger anions are not well-stabilized by solvation
but in the gas phase, the order is reversed (polarizability effect)
substituent effects: electron-withdrawal stabilizes anions (inductive effect)
EtOH (15.9) Cl(CH2)2OH (14.3)
CF3CH2OH (12.3) CF3(CH2)2OH (14.6) CF3(CH2)3OH (15.4)
reactions as a base
R-O-H + H+ <=====> R-OH2+ (weak base)
alcohols are generally weaker bases than water
acidity order (of oxonium ions): 3° > 2° > 1° > CH3OH2+ > H3O+
pKa -3.8 -3.2 -2.4 -2.2 -1.7
consequences of alcohol acid-base reactions
in aqueous solution, very little RO- or ROH2+ is ever present
(illustrate KOH in EtOH, where K = 0.6 )
strong bases or strong acids are needed to make RO- or ROH2+
(e.g., NaNH2 + ROH or HCl + ROH )
synthetic methods for alcohols
1) industrial syntheses (large scale, economical, use of catalysts)
water gas: C + H2O -----> CO + H2
Fischer-Tropsch synthesis: CO + 2 H2 ----> CH3OH
hydration of alkenes: C=C + H2O ----> H-C-C-OH
oxo process: C=C + CO + 2 H2 ----> H-C-C-CH2-OH
2) biosyntheses (the way Nature does it)
fermentation of sugars (ethanol - grain alcohol)
destructive distillation (methanol - wood alcohol)
3) laboratory syntheses (convenient, safe, general)
a) nucleophilic substitutions:
R-X + OH- -----> R-OH + X-
for 1°, direct SN2 is OK
for 3°, need SN1 conditions:
R-X + H2O -----> R-OH + HX
for 2°, need an alternate method to avoid elimination as the major product
(acetate method)
2° R-X + NaOOCCH3 ----> R-OOCCH3 ----> R-OH
b) reductions of carbonyl (C=O) containing compounds:
ketones ----> 2° alcohols
aldehydes ----> 1° alcohols
heterogeneous catalysis (H2 on Pt, Pd, or Ni surfaces)
R2C=O + H2 -----(cat)---> R2CH-OH
hydride reductions (addition of H- to carbonyl group)
R2C=O + NaBH4 (or LiAlH4) ---------> R2CH-OH
organic redox reactions
review balancing of redox reactions
1) separate into oxidation and reduction half-reactions
2) balance each half-reaction
balance atoms other than O and H by inspection
balance charges by adding H+ or OH- (depending on conditions)
balance O atoms by adding H2O as needed
balance H atoms by adding ( H atoms )
3) combine the two half-reactions so that the ( H atoms ) cancel out
the oxygen functional groups (including multiple bonds)
alkanes alcohols, ethers carbonyls (C=O) carboxyls (O-C=O) CO2
ox. state 0 +1 +2 +3 +4
oxidations of alcohols
1° alcohol ---> aldehyde or carboxylic acid
2° alcohol ---> ketone
3° alcohol - no reaction
Cr(VI) as an oxidant: Na2Cr2O7 / H2SO4 / H2O (very strong oxidant)
CrO3 / pyridine / HCl (PCC) - converts 1° alcohols to aldehydes only
mechanism involves intermediate chromic esters
biological redox reactions with NADH and NAD+
(stereospecific with alcohol dehydrogenase)
organometallic compounds
R-X + 2 Li ----> LiX + R-Li (alkyllithium)
R-X + Mg ----> R-Mg-X (Grignard reagent)
the R-M bonds are polarized
R is a strong base and strong nucleophile
organometallic reactions
as a base - R-M + H2O ----> R-H + MOH
transmetallation - R-M + M'X ----> R-M' + MX
as a nucleophile - R-M + C=O ----> R-C-O-M
organometallics in synthesis
Grignard reagents: R-X + Mg --(ether)--> R-Mg-X
R-Mg-X + C=O ----> R-C-O-Mg --(H+, H2O)--> R-C-O-H
(alcohol synthesis)
syntheses of 1° alcohols (from CH2O)
syntheses of 2° alcohols (from aldehydes - RCHO)
syntheses of 3° alcohols (from ketones - R2CO)
strategy of synthetic reaction sequences
1) work backwards from the target molecule (retrosynthetic analysis)
2) visualize the construction of the carbon framework
(reactions that make new C-C bonds are especially valuable)
3) make sure the functional groups and stereochemistry come out right
possible pitfalls to avoid:
1) minimize the number of steps (to optimize yields)
2) avoid multifunctional compounds (possible interferences)
3) avoid structural or other constraints (know the mechanism)
the simple organometallic approach to building new bonds from alcohols:
convert alcohol to an organometallic reagent
convert alcohol to a carbonyl compound
add organometallic to carbonyl compound to make a new, larger alcohol
repeat as necessary to create the new bonds needed