arboxylic Acids arboxylic acids have one property that distinguishes them from most other organic compounds they re acidic. Now not as acidic as fuming sulfuric acid, but still pretty darned acidic. The acidity of these compounds arises from the resonance stabilization of the conjugate base (bond lengths equalize): arboxylic acids undergo hydrogen bonding much like alcohols. owever, in a carboxylic acid you have both a hydrogen bond donor and acceptor in the same molecule. These acids thus form highly stable dimers (as shown below). For small acids (e.g. acetic acid) this dimer cannot even be broken down by distillation which explains why the boiling point for these acids is so high (the molecular weight is essentially doubled. The acidity of these compounds can be modified by changing the substituents on the alkyl portion of the acid. Electron withdrawing groups make the molecule more acidic (by stabilizing the negative charge on the conjugate base), while electron-donating groups make it less acidic, by destabilizing that same charge. You should know this relationship already! It s the same one that determines good leaving groups and electrophiles, the stability of carbocations and carbanions, it is a major force in organic chemistry. Again: Electron withdrawing groups stabilize negative charges Electron donating groups destabilize negative charges. Protonation 3 + favored or - protonation at carbonyl is favored by resonance stabilization while protonation at oxygen is disfavored by partial positive charge on carbon. 1
Preparation of carboxylic acids: You ve already seen a veritable plethora of methods for the preparation of carboxylic acids. Let me run a few by you just to make sure everyone is up-tospeed: 1) xidation of a PIMAY alcohol with Na 2 r 2 ; e.g. Na 2 r 2 7 3 2 3 2 2-4, 2 2) xidation of aromatic side chains containing benzylic hydrogens with KMn 4 KMn 4 KMn 4 oxidation of alkenes or alkynes is also possible although not common; e.g. Ph KMn 4 Ph 18-crown-6, 25 18-crown-6 is used to make KMn 4 soluble in benzene 3) xidative cleavage of internal and terminal alkynes with 3. 3 2 4) xidation of aldehydes with silver. Ag 2, N 4 (Tollens reagent) only the aldehyde is oxidized, and not the 2 alcohol + Ag (mirror) There are a few other ways to get carboxylic acids out of various compounds. For example, nitriles (formed by the reaction of an alkyl halide with KN) can be hydrolyzed under acidic (milder) or basic (very harsh) conditions to give the corresponding acid. If the nitrile is hydrolyzed under basic conditions, the amide is the compound more commonly isolated (rather high temperatures are required to get the acid under basic conditions): 2
A somewhat more common method for the preparation of carboxylic acids is the carboxylation of Grignard reagents. This is really a straightforward reaction a Grignard reagent is simply quenched with dry ice (solid 2 ). The Grignard attacks the carbonyl group to form the acid, which is relatively inert to further attack. In fact, any of the -M reagents we ve talked about in the past will do this carboxylation reaction: General features of carboxylic acids - strong acids protonate the carbonyl group of carboxylic acids see above discussion - carboxylic acids react as Bronsted-Lowry acids, i.e. proton donors. They are strong organic acids; even a weak base such as Na 3 is strong enough to deprotonate carboxylic acids see Table 19.3 Again, acidity of these compounds arises from the resonance stabilization of the conjugate base (bond lengths equalize): see Figure 19.7 the more stable the conjugate base, the more acidic the compound. - Electron withdrawing groups stabilize a conjugate base, making a carboxylic acid more acidic 3
- Electron donating groups destabilize a conjugate base, making a carboxylic acid less acidic page 703-704 ubstituted Benzoic acids - electron donating groups activate benzene to electrophilic attack and hence make the benzoic acid derivative less acidic - electron withdrawing groups deactivate benzene to electrophilic attack and hence make the benzoic acid derivative more acidic see Figure 19.8 ulfonic Acids - very strong acids (pka values ~ -7) because their conjugate bases are resonance stabilized and all resonance structures delocalize a negative charge on oxygen; sulfonate ions are weak bases hence make good leaving groups in nucleophilic substitution :B - B e.g. (Ts) Amino Acids - most natural occurring amino acids have the amino group bonded to the α carbon and hence are called α-amino acids N 2!-amino acid Glycine, the simplest amino acid has =. When, the α-carbon is a stereogenic center and two enantiomers are possible: N 2 2 N L amino acid D amino acid 4
Except for when = 2, the α carbon has the configuration. The natural occurring enantiomer of an amino acid is assigned as the L isomer and its unnatural enantiomer is the D isomer. Amino acids are never uncharged neutral compounds; they exist as zwitterionic salts hence they have high melting points and are soluble in water N 2 N 3!-amino acid zwitterion An amino acid can exist in three different forms depending on p of the solution: - at low p, the amino acid has a net positive charge due to protonation of the carboxylate anion - at p = ~7, the amino acid exists as the zwitterion (no net charge) - at high p, the ammonium cation is deprotonated and the amino acid has a net negative charge The isoelectric point (pi) is the p at which the amino acid exists primarily in its neutral form (as a zwitterion): pi = [pka () + pka (N 3+ )]/2 5