2.5 Physical Properties, Functional Groups, and Intermolecular Interactions

Now that we have reviewed some of the basic concepts of molecular geometry and polarity, it’s time to explore how these factors affect the physical properties of compounds. We can begin to understand these influences by examining the boiling points, melting points, and water solubilities of some representative compounds, as shown in Table 2-4.

Table 2-4 is titled, physical properties of representative compounds. The table has seven columns and nine rows. The rows represent different compounds. The columns represent the molar mass, boiling point, melting point, water solubility, dipole moment, and dominant intermolecular interaction for the compounds. Data are included in the accompanying table. | Compound Molar mass (grams per mol) Boiling point (degree Celsius) Melting point (degree Celsius) Solubility in water (grams per 100g of water) Dipole moment (D) Dominant intermolecular interaction | Sodium methanoate also knows as sodium formate, where a sodium cation is bonded to a carboxylate ion. Not applicable Greater than 253 253 77 Not applicable Ion-ion | Methanoic acid, also known as formic acid, where a hydrogen atom is bonded to a carboxyl group. 46 101 8 Infinite 1.4 Hydrogen bonding | Ethanol, which has a two-carbon chain where carbon 1 is bonded to a hydroxyl group. 46 78 Negative 114 Infinite 1.7 Hydrogen bonding | Ethanal, also known as acetaldehyde, where a carbon atom is bonded to three hydrogen atoms and an aldehyde group. 44 20 Negative 117 Greater than 100 3.0 Dipole-dipole | Dimethyl ether, where an oxygen atom is bonded to two methyl groups. 46 Negative 25 Negative 139 6.9 1.3 Dipole-dipole | Propene with a three-carbon chain, where a double bond exists between carbon 1 and 2. 42 Negative 48 Negative 185 0.00061 0.3 Induced dipole-induced dipole | Propane with a three-carbon chain, connected by single bonds. 44 Negative 45 Negative 188 0.00039 0 Induced dipole-induced dipole | Ethane, with a two- carbon chain, connected by single bonds. 30 Negative 89 Negative 183 0.006 0 Induced dipole-induced dipole

All of the compounds in Table 2-4 are covalent except sodium methanoate (sodium formate, Na+ –OCH═O). The covalent compounds are all similar in size, shape, and molar mass, too, with the exception of ethane, which is roughly 30% lighter. Therefore, their different physical properties are due mainly to differences in the functional groups present. Thus:

Different functional groups in a compound can lead to significantly different physical properties.

Connections Sodium methanoate is used as a dye activator in fabric dyeing processes because it helps promote the fixation of a dye to the fabric. It has also been used in the food industry as a preservative and flavor enhancer.

Connections Formic acid is found in the venom of ants, and has several uses in industry and agriculture. It is used in the production of leather and in dyeing textiles, and is also used to treat animal feed because of its properties as a preservative and an antibacterial agent.

Magnified picture of a red ant.

YOUR TURN 2.9
SHOW ANSWERS

Circle the functional group that is present in each covalent compound in Table 2-4 and identify the compound class to which each molecule belongs.

Methanoic acid has a CO2H group and is a carboxylic acid; ethanol has an OH group and is an alcohol; ethanal has a C═O group and is an aldehyde; dimethyl ether has a C—O—C group and is an ether; propene has a C═C group and is an alkene; propane and ethane have no functional groups.

Why do functional groups have such a profound effect on the physical properties of organic compounds? Functional groups can differ in the atoms they possess or in the arrangement of those atoms in space, both of which will impact how charge is distributed within a molecule. This will affect the ways in which various species attract (or repel) each other, so-called intermolecular interactions (also called intermolecular forces). We will examine the following types of intermolecular interactions:

●  Ion–ion interactions,

●  Dipole–dipole interactions,

●  Hydrogen bonding,

●  Induced dipole–induced dipole interactions (or London dispersion forces), and

●  Ion–dipole interactions.

The first four of these intermolecular interactions are discussed in Section 2.6 in the context of boiling points and melting points. We examine the fifth and final intermolecular interaction in Section 2.7 in the context of a compound’s solubility in a given solvent.

Even though these intermolecular interactions are given different names, they all originate from the same fundamental law: opposite charges attract. As a result, the strength of each intermolecular interaction depends on the concentrations of charge involved.

All else being equal, the greater the concentrations of charge that are involved in an intermolecular interaction, the stronger is the resulting attraction.