Boiling Point Of Alkanes

Alkanes are a kind of hydrocarbon, which are organic molecules made up exclusively of carbon and hydrogen atoms. They’re also known as saturated hydrocarbons since they exclusively have C-C single bonds. Alkenes, alkynes, and aromatic chemicals are examples of additional hydrocarbon sub-categories. Because they contain C-C double and C-C triple bonds, this category is referred to as unsaturated hydrocarbons.

Crude oil and natural gas contain alkanes, which are crucial components. As a result, they are not only the primary source of energy but also the most valuable resource for the chemical industry. Alkanes were once known as paraffins (from the Latin parum affinis, which means “little related”) because of their poor reactivity and inability to combine with water.

Linear n-alkanes, branched alkanes, and cycloalkanes are the three types of alkanes. CnH2n+2 is the empirical formula for linear and branching alkanes, whereas CnH2n is the formula for cycloalkanes.

Properties of alkanes

Alkanes are colourless and odourless hydrocarbons. They have weak Van Der Waals attraction forces. At 298K, alkanes with 1-4 carbon atoms are gases, those with 5-17 carbon atoms are liquids, and those with 18 or more carbon atoms are solids. Alkanes’ structure: All carbon atoms in alkanes are sp3 hybridised, which means they create four sigma bonds with either carbon or hydrogen atoms.

The odour of several higher alkanes is distinct. Scents can be bound by proteins contained in the membranes of olfactory neurons. A signal carrying information about the fragrance is transmitted to the brain, which is triggered by a conformational change in these proteins. As a result, alkanes are part of a class of substances known as pheromones, which are signalling molecules utilised in nature to facilitate communication between members of the same species.

Boiling points

The boiling points displayed are for “straight-chain” isomers, of which there are several. At room temperature, the first four alkanes are gases, and solids do not exist until around C17H36, however, this is approximate since various isomers have different melting and boiling temperatures. There are an incredible amount of isomers by the time an alkane has 17 carbons.

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 a molecule like methane, these forces will be quite tiny, but they will grow as the molecules become larger. As a result, the boiling temperatures of alkanes increase in proportion as the size of alkanes increases .

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 extremely short distances between molecules.

Boiling point of hydrocarbons

Organic compounds’ boiling points can reveal a lot about their physical qualities and structural traits. When the vapour pressure of a liquid equals the air pressure, it boils. The kinetic energy of molecules determines vapour pressure. Temperature, as well as the mass and velocity of molecules, affect kinetic energy. When the temperature approaches the boiling point, the average kinetic energy of the liquid particles is sufficient to overcome the attraction forces that keep molecules in the liquid state.

The balance between molecules in the gaseous and liquid states causes vapour pressure. When liquid molecules have enough kinetic energy, they can escape off the surface and transform into a gas.

The table on the left lists a number of alkanes having the general formula CnH2n+2, along with their names, formulae, and physical characteristics. What does the melting and boiling data show as a whole? The melting and boiling temperatures of alkanes increase as the chain length (number of carbons) increases.

Longer chain molecules have greater boiling temperatures because they become twisted around and entangled in one other, similar to spaghetti strands. Short chain molecules, which have only weak forces of attraction for one other, require more energy to separate them.

Conclusion

CnH2n+2 is their general formula. They have a bond angle of 109.5 degrees and tetrahedral shape. Alkanes float because their density is lower than that of water. With increasing molecular mass,  density rises. Apart from weak Van der Waals forces, London forces, and dispersion forces, alkane molecules are held together by weak intermolecular interactions. AS the Van Der Waals force increases with increasing molecular weight, the boiling point increases. The boiling point of straight chain alkanes is greater than that of their structural isomers. Melting Point: It rises with increasing molecular weight because larger alkanes are often solids, making it difficult to break the intermolecular forces of attraction between them. Even-numbered alkanes pack better in the solid phase than odd-numbered alkanes, forming a well-organized structure that is difficult to break, resulting in a higher melting point for even-numbered alkanes than odd-numbered alkanes.