Physical Properties of Organic Compounds: Boiling and Melting Points

Physical Properties of Organic Compounds

Hydrocarbons

Hydrocarbons are nonpolar, with melting and boiling points that depend on the length of the carbon chain. If the chain is branched, the melting and boiling points decrease. Due to their structure, alkenes and alkynes have slightly higher melting and boiling points than alkanes. Aromatic hydrocarbons are typically liquid or solid at room temperature.

Halogenated derivatives have higher melting and boiling temperatures than hydrocarbons due to the C-X bond.

Alcohols

Alcohols contain at least one -OH group. They have higher melting and boiling points than hydrocarbons because they can form hydrogen bonds; consequently, there are no gaseous alcohols at room temperature. If the alcohol is the main functional group, the suffix -ol is used; otherwise, the prefix hydroxy- is applied.

Esters

Esters have low melting and boiling points that increase with molar mass. The physical and chemical properties of low molecular weight esters generally correspond to a combination of the properties of their constituent groups. As the molecular weight of the hydrocarbon portion increases, it exerts a predominant effect over the oxygen-containing functional groups, which is why wax esters are solid, soft, and insoluble in water.

Aldehydes and Ketones

Carbonyl compounds have lower boiling points than alcohols of similar molecular weight. There are no significant differences between the boiling points of aldehydes and ketones of the same molecular weight. Short-chain carbonyl compounds are soluble in water, but solubility decreases as the chain length increases.

Carboxylic Acids

The presence of double bonds leads to a lower melting point. The melting points of dicarboxylic acids are very high, and they are highly soluble in alcohols.

Ethers

Ethers are named by placing the particle -oxy- between the two radicals, or more commonly, by naming the two radicals in alphabetical order followed by the word ether. The boiling points of ethers are similar to those of alkanes, and their water solubility is comparable to that of alcohols.

Amines

Primary and secondary amines have higher boiling and melting points than hydrocarbons but lower than alcohols because the N-H bond is less polarized than the O-H bond. Tertiary amines lack N-H bonds, making them nonpolar with lower boiling and melting points than primary and secondary amines.

Amides

Amides are highly polar and possess higher boiling and melting points than carboxylic acids. They typically exist as crystalline solids.