Melting and Boiling Points of Organic Compounds

Organic compounds have a significant role in our life because they are widely used in everywhere.

Thousands of organic compounds are discovered so far by scientists in the world. With discovering lot of compounds, organic chemistry was born. In this chapter we are going to discuss how melting and boiling points of organic compounds vary.

Boiling points of ethane (an alkane compound) and ethanoic acid (a carboxylic acid compound) is listed below and they are two different types of organic compounds.

Boiling point of methane and ethanoic acid

  • Methane (CH4): -161.50C - You see, methane (simplest alkane) has a very low boiling point.
  • Ethanoic acid (CH3COOH): 118.10C . Ethanoic acid has a higher boiling point than water.
  • Methane and ethanoic acid are belongs to two types of organic compounds and according to the type of type, there is a considerable boiling point difference. These thing will be explained in detail later in this tutorial.

Boiling points of same compound type

Ethanol and n-propanol belongs to alcohol group. Boiling points of ethanol and n-propanol are 78.240C and 970C respectively. Boiling point of ethanol is smaller than n-propanol. You, will see more examples of these type of compounds later in this tutorial.

Written by: Heshan Nipuna, updated: 2020/02/25

Following facts are important when we compare melting and boiling point values of organic compounds. Later, in this tutorial, we are going to look, how these factors are affected for boiling points of organic compounds.

  • Relative Molecular mass
  • Dipole dipole interactions
  • Ability of making hydrogen bonds - very important. When there are hydrogen bonds between molecules and they are strong melting and boiling points are increased.
  • Structure of carbon chain

Melting and boiling points of alkanes, alkenes, alkynes

Alkanes, alkenes and alkynes are hydrocarbons and we are going to see the trend of melting and boiling points of them.

Boiling points of alkanes

Alkanes are non polar molecules. Therefore, there are only Van der Waals forces between alkane molecules. Van der Waals forces are the weakest intermolecular forces type. When relative molecular mass increases in alkane compounds, melting and boiling points values are also increased.

melting and boiling points of alkanes

Melting and boiling points increases alkane, alkene, alkyne respectively.

Consider ethane, ethene, ethyne

Ethane is an alkane which is a non-polar compound. Ethyne (alkyne compound) has the highest melting and boiling point.

melting and boiling points of alkanes, alkenes, alkynes

When number of hydrogen bonds and strength of hydrogen bonds increases, melting and boiling points increases.

See this example. Compare boiling points of ethanol and ethyl chloride compounds. Both organic compounds have two carbon atoms. Ethanol's boiling point is 78.370C. Ethyl chloride's boiling point is 12.30C. We know ethanol can make strong hydrogen bonds. But, there are only di-pole di-pole attractions between ethyl chloride molecules. Di-pole di-pole attractions are weaker than hydrogen bonds. Due to ability of making hydrogen bonds, boiling point of ethanol is high.

Boiling point of benzene

Benzene, an aromatic organic compound which has the boiling point of 80.10C. Benzene forms weak Van der Waals forces. But benzene has a higher molecular mass compared with simple alkanes such as methane, ethane..

Due to higher molecular mass, boiling points of benzene is higher than simple alkanes.

Alcohols, aldehyde, ketone and carboxylic acids

  • All of alcohols and carboxylic acids can form hydrogen bonds.
  • Carboxylic acids forms most strong and highest number of hydrogen bonds among them.
  • So, carboxylic acids has highest melting and boiling points.
  • Dipole dipole interactions between aldehydes and ketones molecules are less strong than hydrogen bonds in alcohols.
  • Therefore melting and boiling points of alcohols are higher than aldehydes and ketones.
melting and boiling points of alkanes aldehydes ketones alcohols carboxylic acids

Melting and boiling points of alkyl halide compounds

Halogen atom is more electro negative than carbon atom. So C-X bond is polarized. There are dipole dipole interaction between alkyl halide compounds. These interactions are much stronger than intermolecular forces between alkanes,

Melting and boiling points of alkyl halide compounds are much higher than alkanes.

melting and boiling points of alkanes alkyl halides aldehydes ketones alcohols carboxylic acids.jpg

When relative molecular mass of organic compound increases, melting and boiling points also increase.

Now we discuss some problems by comparing different elements and compounds which have different melting and boiling points. These problems very important in examinations. Study them carefully.

Related articles

Oxidation Numbers of Elements in Periodic Table Metal characteristics in the periodic table Why compounds have different boiling points? - Reasons for boiling point variations of O2, HBr, ethanol Organic Chemistry and compounds Alkali metals reactions and occurrence Alkaline earth metals reactions and occurrence Inorganic chemistry reactions and occurrence

Related lewis structures

P2O5 lewis structure OH- lewis structure Ammonium ion (NH4+) lewis structure H2CO3 lewis structure