CHAPTER 14: Unit 4. Physical Properties of carboxylic Acids

Physical Properties of carboxylic Acids

Learning Objectives

1. Compare the boiling points of carboxylic acids with alcohols of similar molar mass.
2. Compare the solubilities of carboxylic acids in water with the solubilities of comparable alkanes and alcohols in water.

1. Low-molar mass carboxylic acids are liquids at room temperature and have sharp and unpleasant odors. Long-chain acids are wax-like solids. The carboxyl group is polar and forms hydrogen bonds to other carboxyl groups or other molecules. Thus carboxylic acids have relatively high boiling points and those with lower molar masses are soluble in water. Due to H-bonding interaction, they form a dimer.
   Carboxylic acids have stronger intermolecular forces than an ester, giving them higher boiling points and melting points when comparing compounds of similar molecular weight.How does soap clean away dirt?Soaps are salts of carboxylic acids. They have many carbon atoms in a long chain of hydrocarbon. The ionic end is called polar end and the carbon chain of C-C and C-H bonds are called the nonpolar ends. They are called hydrophilic (water seeking) and hydrophobic ( water repelling) end respectively.Dissolving soap in water forms spherical droplets having the ionic bonds on the surface and the nonpolar tails pack together in the interior. The spherical droplets are called a micelle.  In this arrangement, the ionic bonds can interact with polar solvent water and bring the nonpolar greasy hydrocarbon portion of the soap into solution. Nonpolar tails of the soap molecules are so well sealed off from the water so they can interact with the nonpolar grease molecules and be washed down the drain with water since micelles are soluble in water.Two common pain relievers that are also anti-inflammatory agents contain a carboxyl group-aspirin & ibuprofen. Another example is Naproxen. 
2. Acidity: Soluble carboxylic acids behave as weak acids, donating protons to water molecules. The portion of the acid molecule left after proton loss is called a carboxylate ion which has resonating structures.
   The equilibrium constant of this reaction is small, usually it is expressed in terms of pKa values. Higher the pKa values, weaker the acid.Below is the list of pKa values of common carboxylic acids.Below is the table to show relative acidity of

IN CLASS PRACTICE PROBLEMS

Rank the above molecules in order of increasing boiling points.

Homework Exercises

1. Which compound has the higher boiling point—CH₃CH₂CH₂OCH₂CH₃ or CH₃CH₂CH₂COOH? Explain.

2. Which compound has the higher boiling point—CH₃CH₂CH₂CH₂CH₂OH or CH₃CH₂CH₂COOH? Explain.

3. Which compound is more soluble in water—CH₃COOH or CH₃CH₂CH₂CH₃? Explain.

4. Which compound is more soluble in water—CH₃CH₂COOH or CH₃CH₂CH₂CH₂CH₂COOH? Explain.

Answers

1. CH₃CH₂CH₂COOH because of hydrogen bonding (There is no intermolecular hydrogen bonding with CH₃CH₂CH₂OCH₂CH₃.)

3. CH₃COOH because it engages in hydrogen bonding with water (There is no intermolecular hydrogen bonding with CH₃CH₂CH₂CH₃.)