Melting Points of the Alkanes

Alkanes are a class of chemicals that possess single covalent connections between carbon and hydrogen atoms. Saturated hydrocarbons are what they’re called. This group of chemicals are made up of solitary covalent connections between the carbon and hydrogen atoms in the molecules . A homologous series with the chemical formula C_nH_2n+2 is also included .

The alkane family of hydrocarbons is the most basic. Carbon and hydrogen are the only elements in them. Each carbon atom makes four bonds, while each hydrogen atom makes one. Chemists prefer line-angle formulas to condensed structural formulas because they are simpler and faster to draft. Another shortened form of structural formulas for alkanes exists.

Properties of alkanes

At room temperature, alkanes can exist as gases, liquids, or solids. Gases include methane, ethane, propane, and butane; liquids include pentane through hexadecane; and solids include homologues greater than hexadecane.

Branched alkanes with the same carbon content usually have lower boiling points than unbranched alkanes. Because the van der Waals forces between molecules of unbranched alkanes are stronger, this happens. In the natural world, these forces can be dipole dipole, dipole induced dipole, or induced dipole induced dipole. Because of their larger surface areas, unbranched alkanes have higher van der Waals forces of attraction.

Alkanes that are solid are often soft and have low melting temperatures. Strong repulsive forces between electrons on nearby atoms, which are in close contact in crystalline materials, cause these properties. The strong repulsive forces balance out the weak van der Waals attraction forces.

Alkanes, on the other hand, are virtually totally insoluble in water. The van der Waals forces of attraction between alkane molecules and water molecules would have to be stronger than the dipole dipole forces between water molecules for alkanes to dissolve in water. This isn’t the case at all.

Melting and boiling points of alkanes

Alkanes are nonpolar, colourless, and odourless chemicals that are not very reactive and have limited biological action. Alkanes are gaseous for short carbon chains, volatile liquids with densities around 0.7 g/mL for moderate carbon lengths, and solids for long carbon chains due to their relatively weak London dispersion forces. There is a clear relationship between the size and shape of molecules and the strength of the intermolecular forces (IMFs) that causes changes in physical states for molecules with the same functional groups.

There is no substantial bond polarity because there is no major difference in electronegativity between carbon and hydrogen. The molecules themselves are polarised to a low degree. The only attraction between one molecule and its neighbours will be Van der Waals dispersion forces in the case of a fully symmetrical molecule like methane. For the molecule like methane, these attractive forces will be quite tiny, but they will grow as the molecules become larger. As a result, the free rising temperatures of alkanes rise in proportion to their molecular size.

The boiling point of isomers will be lower as the more branched the chain is. Shorter molecules have less Van der Waals dispersion forces, which only function at very short distances between molecules. Short, fat molecules (those with a lot of branching) have a tougher time fitting together as tightly as long, thin molecules.

Boiling and melting points of some alkanes

Boiling points of cycloalkanes

The boiling point of a cycloalkane is roughly 20 K higher than that of a straight chain alkane.

There is no substantial bond polarity between the carbon and the hydrogen since the electronegativity of the two elements is very similar. The polarity of the molecules is low. The only attraction between one molecule and its neighbours will be Van der Waals dispersion forces in the case of a fully symmetrical molecule like methane. These forces will be quite a little for the molecule like methane, but as the molecule size becomes larger, they start to grow. Alkanes’ boiling temperatures rise in direct proportion as their molecular size grows. 

The more branched the chain is when you have isomers, the lower the boiling point tends to be. Shorter molecules have less Van der Waals dispersion forces, which only function at very short distances between molecules. Short, fat molecules (with a lot of branching) have a harder time lying as close together as long, thin molecules.

Conclusion 

Because of the weak intermolecular interactions between alkane molecules, they have low melting and boiling points. methane, ethane, propane, and butane are all gases at the room temperature,

There are more electrons in a molecule as the carbon chain lengthens. This indicates that the molecules have more (relatively) stronger intermolecular forces. As a result, breaking these forces requires more energy, resulting in higher melting or boiling temperatures.