Carboxylic Acids and Reduction/Oxidation
• Addition of H (or “H-”) Reduction 2 • Loss of O2 or O
carboxylic aldehyde alcohol alkane acid (or ketone)
• Loss of H 2 Oxidation • Addition of O2 or O
+ Neither oxidation nor reduction: Addition or loss of H , H2O, or HX
Carboxylic acids are the most oxidized functional group of carbon.
Carboxylic Acids via Oxidation
Chromate oxidation of primary alcohols:
H2SO4 aldehyde carboxylic alcohol acid
Oxidation of aldehydes with Tollens’ reagent:
1. Ag2O, NH3 2. H O+ 3 Selective for aldehydes; will not oxidize alcohols.
aldehyde carboxylic acid Carboxylic Acids via Oxidation
Oxidative cleavage of alkynes:
Oxidation of benzylic carbons:
Purifying Carboxylic Acids by Acid-Base Extraction
Acidic and basic organic molecules can be separated from other substances by manipulating their protonation state and solubility.
Let’s say we run a reaction.
Na2Cr2O7 + unreacted
H2SO4 + Na+ 3+ We want this… + Cr(H2O)6 + organic + HSO - …but not these. side products 4
How do we isolate our desired product from the other materials, without using distillation or chromatography? Acid-Base Extraction
Step 1: Remove inorganics from organics via differential solubility.
(sept) funnel - Na+ HSO4 Cr(H O) 3+ H2O 2 6
CHCl3 + organic side densities: products (H2O) = 1 g/mL (CHCl3) = 1.48 g/mL
A large density difference ensures the two liquids separate completely and quickly. (Much faster than, say, oil and water.)
Step 2: Add basic water to deprotonate carboxylic acid, and transfer it to the aqueous phase.
- Na+ HSO4 3+ discard Cr(H2O)6 H2O layer
+ organic + organic side side products products
add discard NaOH/ H2O CHCl3 layer
+ organic side products Step 3: Re-acidify carboxylate to return it to organic solution. Remove solvent in vacuo to yield pure acid.
HCl/ H2O, fresh CHCl3 discard
remove solvent in rotary evaporator
Chemistry student, removing solvent in a “rotovap”
Nucleophilic Addition to Carbonyls
Heteroatom electron donors add reversibly to C=O groups.
Equilibrium driven + to the left by entropy.
Also true for Cl , HS , NH3 nucleophiles.
Hydrogen anions (“hydride”) and carbon anions add irreversibly to C=O groups.
+ Reaction driven to the right by instability of - - charge on H , RCH2 . +