9.2.3. 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. Due to the weak forces, 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. They are colourless and odourless.

Alkanes, which are hydrocarbons consisting only of carbon and hydrogen, exhibit various physical properties that stem from their molecular structure. They are considered almost non-polar because of the similarity in the electronegativity between carbon and hydrogen, which means they lack a significant charge difference across the molecule. As a result, they do not dissolve well in water (a polar solvent), which is a principle often summed up by the phrase 'like dissolves like'. For instance, smaller alkanes (with fewer carbon atoms, 1-4) exist in gaseous forms due to their low molecular weights. As the number of carbon atoms increases (5-17), they transition to liquids due to increased van der Waals forces, and they become solids when they contain 18 or more carbon atoms, reflecting even stronger intermolecular attractions.

Think of alkanes like a group of friends at a party. The lighter alkanes (like methane and ethane) just float around and socialize lightly, representing gases. As more friends (carbon atoms) join, they start to crowd around (liquid state) and eventually become a solid group when they all squeeze together tightly (like paraffin wax).

What do you think about solubility of alkanes in water based upon non-polar nature of alkanes? Petrol is a mixture of hydrocarbons and is used as a fuel for automobiles. Petrol and lower fractions of petroleum are also used for dry cleaning of clothes to remove grease stains. On the basis of this observation, what do you think about the nature of the greasy substance?

Due to their non-polar characteristics, alkanes do not mix well with polar solvents like water. This principle is significant in everyday applications, such as in cleaning. Since grease is made up of higher alkanes, which are also non-polar, it can be effectively dissolved by other non-polar solvents in petrol rather than by water. This property explains why petrol is often used in dry cleaning\u2014it's effective at breaking down oily stains because the components of grease and oil are chemically compatible with the hydrocarbons in petrol.

Consider trying to mix oil and water in a salad dressing. No matter how hard you shake them, the oil (non-polar) and water (polar) will separate because they don\u2019t want to mix. This is akin to how alkanes behave with water. However, if you spill cooking oil on a cotton cloth, you could easily clean it with a non-polar solvent like petrol, just as grease stains are removed during dry cleaning.

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. This is due to the fact that the intermolecular van der Waals forces increase with increase of the molecular size or the surface area of the molecule.

The boiling point of alkanes increases with the number of carbon atoms primarily because larger alkanes have greater surface areas, leading to stronger van der Waals forces between their molecules. As you might expect, more energy (in the form of heat) is needed to break these interactions within the liquid phase to transition it to a gas phase. The boiling points of alkanes reflect their increasing size and molecular complexity as we move down the group in the periodic table.

Imagine stacking different sizes of pillows\u2014if you have a small pillow, it takes little effort to lift it; however, as you stack on larger pillows, it becomes much harder to lift them all at once. Similarly, small alkanes boil off easily as gases, while larger alkanes require more energy to boil due to their stronger intermolecular forces.

Chemical properties of alkanes are quite inert towards acids, bases, oxidising and reducing agents. However, they undergo substitution reactions under certain conditions. One or more hydrogen atoms of alkanes can be replaced by halogens, nitro group and sulphonic acid group.

Alkanes are generally unreactive due to their saturated nature, which means they are stable and do not readily react with most reagents. However, they can undergo substitution reactions, particularly halogenation, where hydrogen atoms are replaced by halogen atoms when exposed to heat or ultraviolet light. This involves a series of steps\u2014initiation, propagation, and termination\u2014 to ultimately produce halogenated hydrocarbons, which are of industrial significance.

Think of halogenation like a game of musical chairs. The alkanes (the players) are stable when the music (heat or UV light) is off. However, when the music starts, some players (hydrogens) need to leave their seats (be replaced by halogens). The last players remaining might shift their positions based on who remains (the final products formed).

Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat. The general combustion equation for any alkane is: CnH2n+2 + (3n+1)O2 \u2192 nCO2 + (n+1)H2O.

The combustion of alkanes is an important reaction that yields carbon dioxide and water by oxidizing the hydrocarbons. This reaction releases a significant amount of heat energy, making alkanes desirable as fuels. In practical terms, when alkanes like methane combust, they are used in stoves, heaters, and engines, providing energy for numerous applications. This reaction is carefully controlled in engines to maximize efficiency and reduce pollutants.

Think of combustion like lighting a campfire. When you ignite a log (alkane) with enough oxygen around, it burns brightly, giving off warmth (energy) and turning to ash (carbon dioxide and water droplets). Similarly, in internal combustion engines, alkanes are like fuel logs that provide the fire needed to run the vehicle.

Alkanes on heating with a regulated supply of dioxygen or air at high pressure and in the presence of suitable catalysts give a variety of oxidation products.

Controlled oxidation of alkanes offers a method to selectively convert them into useful products, often at high pressures and in the presence of catalysts. This process allows for the formation of alcohols, aldehydes, or even carboxylic acids, depending on the conditions used. Through careful control of temperature and reagents, chemists can direct the reaction to produce a desired outcome, which is crucial in the synthesis of various organic chemicals.

Picture a chef cooking in a kitchen. If they turn the heat too high, they may burn the food, but with the right temperature and seasoning (catalysts), they can create a delicious meal (desired chemical products). This selective cooking mirrors how chemists control oxidation reactions to obtain specific organic compounds.

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