20.S: Carboxylic Acids and Nitriles (Summary)

Concepts & Vocabulary

20.0 Chapter Objectives and Introduction to Carboxylic Acids

• Carboxylic acids are defined as molecules that have a hydroxyl group bonded to a carbonyl group (RCOOH).
• Fatty acids are naturally occurring carboxylic acids that are important components of lipids.
• Fatty acids can either be saturated (all carbon-carbon bonds are single bonds) and unsaturated (one or more carbon-carbon double bonds).
• Several other functional groups can be prepared from carboxylic acids and are considered derivatives (acyl halides, anhydrides, esters, amides and nitriles).

20.1 Naming Carboxylic Acids and Nitriles

• Carboxylic acids are named following IUPAC rules with ending -oic acid.
• When carboxylic acids are named as an attachment to a ring, add -carboxylic acid to the name of the cyclic compound.
• Nitriles are named following IUPAC rules with the ending -nitrile or -carbonitrile.

20.2 Structure and Properties of Carboxylic Acids

• Carboxylic acids have trigonal planar shape around the carbonyl carbon and due to resonance structures, all the atoms of the functional group are co-planar.
• Conjugation in carboxylic acids increases acidity of the hydroxyl hydrogen compared to alcohols. The conjugate base (carboxylate) is stabilized by resonance.
• Carboxylic acids form hydrogen bonded dimers yielding higher boiling points than for other molecules of similar size.
• Small carboxylic acids are often soluble in water due to hydrogen bonding. Larger carboxylic acids are often slightly soluble in water, though when converted to a carboxylate salt by treating with base, become water soluble.

20.3 Biological Acids and the Henderson-Hasselbalch Equation

• Carboxylic acids are fully protonated in acidic solutions.
• Carboxylic acids are fully dissociated in basic solutions.
• The Henderson-Hasselbalch equation can be used to relate pH, pKa and ratio of concentrations of acid and conjugate base.

20.4 Substituent Effects on Acidity

• Withdrawing groups (through inductive or resonance effects) can stabilize carboxylates, increasing acidity of carboxylic acids.
• Donating groups (through inductive or resonance effects) destabilize carboxylates, decreasing acidity of carboxylic acids.
• Nearly all substituents in ortho positions increase acid strength whether donating or withdrawing, this is called the ortho-effect.

20.5 Preparing Carboxylic Acids

• Carboxylic acids can be prepared through oxidation of several functional groups including: 1^o alcohols, aldehydes, alkyl arene side chains, and alkenes.
• Carboxylic acids can also be prepared by carboxylation of Grignard reagents and hydrolysis of nitriles.

20.6 Reactions of Carboxylic Acids: An Overview

• There are four main categories of carboxylic acid reactions:
  • Acid/base reaction (salt formation)
  • Nucleophilic acyl substitution
  • Reduction
  • Substitution alpha to the carbonyl

20.7 Chemistry of Nitriles

• Nitriles have linear geometry due to the sp hybrid orbitals on carbon and nitrogen.
• Nitriles are less basic than amines.
• Nitriles can be prepared from aldehydes, ketones, or amides.
• Nitriles react similarly to carbonyl compounds where nucleophiles will attack the carbon end of the nitrile.
• Nitriles can be hydrolyzed to carboxylic acids under basic or acidic conditions.
• Nitriles can be reduced to amines by hydride reduction.
• Nitriles can be converted to aldehyde by reaction with DIBALH.
• Nitriles can be converted to ketones by reaction with organometallic compounds.

20.8 Spectroscopy of Carboxylic Acids and Nitriles

• IR of carboxylic acids typically show a very strong and broad OH stretch from about 2500 to 3300 cm^-1 as well as a strong carbonyl stretch around 1710 cm^-1.
• ¹H NMR of carboxylic acids show the OH proton between 10-12 ppm as well as hydrogens on carbon adjacent to the carbonyl around 2-3 ppm.
• ¹³C NMR of carboxylic acids show the carbonyl carbon at 160-180 ppm.
• Mass spectra of carboxylic acids often have fragments for OH (loss of 17) and COOH (loss of 45).
• The most identifying characteristic of IR of nitriles is the CN triple bond stretch which appears near 2250 cm^-1.
• ¹H NMR of nitriles show protons on the carbon next to the nitrile at around 2-3 ppm.
• ¹³C NMR of nitriles show the nitrile carbon in the 115-120 ppm range.
• Mass spectra of nitriles will often not have a visible molecular ion, but instead exhibit a strong M-1 peak.

Skills to Master

• Skill 20.1 Name carboxylic acids and nitriles using IUPAC rules.
• Skill 20.2 Draw the structure of carboxylic acids and nitriles from the IUPAC name.
• Skill 20.3 Describe the geometries and approximate bond angles of carboxylic acids and nitriles.
• Skill 20.4 Explain the acidity of carboxylic acids based on conjugate base stabilization.
• Skill 20.5 Explain physical properties of carboxylic acids.
• Skill 20.6 Describe the structure of carboxylic acids in buffered solutions.
• Skill 20.7 Use the Henderson-Hasselbach equation to relate pH, pK_a, and acid/conjugate base concentrations.
• Skill 20.8 Explain inductive effects on acidity.
• Skill 20.9 Explain relative acidity of aromatic acids.
• Skill 20.10 Draw mechanisms for preparing carboxylic acids including:
  • Oxidations
  • Carboxylation of Grignard reagents
  • Hydrolysis of Nitriles
• Skill 20.11 List the four categories of carboxylic acid reactions.
• Skill 20.12 Draw reactions for preparing nitriles including formation from:
  • aldehyde or ketone
  • 1^o amide
• Skill 20.13 List general reactions of nitriles.
• Skill 20.14 Draw mechanisms for reactions of nitriles including:
  • Hydrolysis to acids (acidic and basic mechanisms)
  • Reduction to 1^o amines
  • Conversion to aldehyde by reaction with DIBALH.
  • Conversion to ketones by reaction with organometallic compounds.
• Skill 20.15 Plan a synthesis of a ketone from a nitrile.
• Skill 20.16 Use IR, NMR and MS to identify carboxylic acids and nitriles.

Summary of Reactions

Carboxylic Acid Preparation

Nitrile Preparation

Reaction of Nitriles