9.2.3.1. Physical properties

Alkanes are almost non-polar molecules because of the covalent nature of C-C and C-H bonds and due to very little difference of electronegativity between carbon and hydrogen atoms. They possess weak van der Waals forces.

Alkanes, which are composed solely of carbon and hydrogen, do not have a significant charge separation in their bonds. This is due to the similar electronegativity values of carbon and hydrogen, which means that the electrons are shared relatively equally between these atoms. As a result, alkanes do not have a positive or negative end, making them non-polar. Non-polar molecules have weak attractions to each other, known as van der Waals forces, which are much weaker than ionic or hydrogen bonds.

Think of alkanes as being similar to oil. Oil floats on water and doesn't mix with it because it is non-polar (like alkanes) and water is polar. Just like oil, alkanes do not interact significantly with water or other polar substances.

The first four members, C1 to C4 are gases, C5 to C17 are liquids and those containing 18 carbon atoms or more are solids at 298 K.

At room temperature (approximately 25°C or 298 K), the physical state of alkanes changes with the number of carbon atoms they contain. For the smallest alkanes (like methane, ethane, propane, and butane), they are gaseous because their molecular weight is low and they do not have enough van der Waals forces to hold them in a liquid or solid form. As we progress to alkanes with more carbon atoms, such as pentane to heptadecane (C5 to C17), they become liquids, as the increased molecular weight allows for greater van der Waals forces. Finally, alkanes with 18 or more carbon atoms are solids, as the long molecular chains allow for solid structures to form through these intermolecular attractions.

Picture how different oils behave. Light oils like gasoline (which is mostly made of alkanes C5-C10) are liquids, while heavier oils for industrial use can become greases or solids at lower temperatures. Just as certain oils change state based on temperature and carbon chain length, so do alkanes.

They are colourless and odourless.

Alkanes do not have a distinct color or smell, which is one of their key characteristics. This is because their molecular structure does not absorb light in the visible spectrum, making them appear colorless. Furthermore, the absence of functional groups that typically give off odors (like alcohols or amines) contributes to their odorless nature. This property makes them safe to handle in small quantities and easy to use as fuels.

Think about natural gas, which is primarily methane. It is colorless and odorless, making it difficult to detect leaks. To make it safer, companies add a special odor (like rotten eggs) to warn users of gas leaks, highlighting how we usually expect compounds to have recognizable colors or smells.

What do you think about solubility of alkanes in water based upon non-polar nature of alkanes?

Given that alkanes are non-polar substances, they do not dissolve in polar solvents like water. This follows the principle of 'like dissolves like,' meaning that polar solvents tend to dissolve polar solutes, and non-polar solvents dissolve non-polar solutes. Therefore, when alkanes are introduced to water, they tend to separate rather than mix.

This is much like mixing oil and water. If you've ever tried to mix them, you may have noticed that they separate, with oil floating above the water. This visual separation helps illustrate the principle that non-polar substances like alkanes do not mix with polar substances like water.

Boiling point (b.p.) of different alkanes are given in Table 9.2 from which it is clear that there is a steady increase in boiling point with increase in molecular mass.

As the molecular size of alkanes increases, their boiling points tend to increase as well. This rise in boiling points is primarily due to the increase in van der Waals forces, which are stronger in larger molecules because of higher molecular mass and greater surface area for interaction. Thus, larger alkanes require more energy (in the form of heat) to break these intermolecular forces compared to smaller alkanes.

Imagine trying to boil a pot of water with a few ice cubes versus a large block of ice. The larger block takes much more energy (heat) to melt and boil than just a few cubes, similar to how larger alkanes need more heat to reach their boiling point.

An interesting observation is made by lower boiling points for branched chain alkanes compared to their straight-chain isomers.

Branched-chain alkanes tend to have lower boiling points than their straight-chain counterparts because branching reduces the surface area available for intermolecular contact. This decrease in surface area leads to weaker van der Waals forces in branched alkanes, meaning that they require less energy to reach boiling point and can transition to gas at lower temperatures compared to straight-chain isomers.

Think of it like a group of people trying to fit through a door. If the group is all lined up in a straight line (like straight-chain alkanes), they take up more space and are slower to get through. But if some of the people are standing off to the side (like branched alkanes), they can move through the door more quickly and easily because they are taking up less space.

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