Properties and Trends of Organic Compounds
Boiling point and melting point
Boiling point is the temperature at which a liquid's vapour pressure equals the external pressure, causing it to change into a gas.
Melting point is the temperature at which a solid becomes a liquid.
Increasing chain length
As the carbon chain length increases in a homologous series , boiling and melting points tend to increase.
Longer carbon chains have stronger dispersion forces between molecules which results in higher energy requirements to break these intermolecular interactions during boiling or melting.
Effect of branching
Branched isomers typically have lower boiling and melting points compared to their straight-chain counterparts, as can be seen in the graph above. The reduced surface area of branched molecules leads to less efficient packing which weakens the dispersion forces and thus requires less energy to boil or melt.
Influence of functional groups
Compounds with hydroxyl or carboxyl functional groups , such as carboxylic acids and alcohols, exhibit higher boiling or melting points than equivalent alkanes with a similar chain length.
The presence of -OH (hydroxyl) or -COOH (carboxyl) groups allows for hydrogen bonding between molecules. This significantly increases boiling or melting points because hydrogen bonds are strong and therefore take more energy to break.
Compounds such as esters and haloalkanes, with functional groups -COO (ester) and -Cl (chloro) respectively, also exhibit higher boiling or melting points than alkanes of similar molecular weight. Dipole-dipole interactions between molecules are not as strong as hydrogen bonds but still can significantly influence the physical properties and contribute to the increase in boiling or melting point of polar compounds.
The graph below illustrates how molecular size, intermolecular forces, and functional groups influence the boiling point trends of various compounds.
Title: Boiling Point Trends of Organic Compounds : The Influence of Molecular Size, Intermolecular Forces, and Functional Groups
Viscosity
Viscosity is a measure of a fluid's resistance to flow.
Increasing chain length
As the carbon chain length increases in a homologous series, viscosity typically increases.
Larger molecules have a larger molecular weight and tend to have greater dispersion forces between molecules leading to increased resistance to flow.
Effect of branching
Increased branching in molecules reduces viscosity.
Branched isomers typically have lower viscosity compared to their straight-chain counterparts. The reduced surface area of branched molecules leads to less efficient packing leading to fewer intermolecular interactions and lower resistance to flow.
Influence of functional groups
Alcohols and carboxylic acids generally exhibit higher viscosity compared to alkanes of similar molecular weight because of hydrogen bonding due to the -OH (hydroxyl) and COOH (carboxyl) groups respectively. Dipole-dipole interactions are not as strong as hydrogen bonds but still can significantly influence the resistance to flow of polar compounds. Stronger intermolecular forces lead to greater resistance to flow.
Example
Ethanoic acid contains a carboxyl group (-COOH), which allows for strong hydrogen bonding between molecules. Chloroethane contains a halogen (-Cl) that can participate in dipole-dipole interactions. The boiling point of ethanoic acid is much higher due to the presence of hydrogen bonding, which is a stronger intermolecular force than dipole-dipole interactions.
Check your understanding of the physical properties of organic compounds by answering the following questions.
