Question

Rationalize the following statements/observations (a) Alkanes are unreactive when compared with alkenes. (b) In the case of isomeric alkanes, the boiling points decrease with increase in branching of the chain. (c) Normal alkanes with even number of carbon atoms melt at a higher temperature than the adjacent alkane having odd number of carbon atoms.

Short answer

Answer: Alkanes with an even number of carbon atoms generally have a higher melting point than their adjacent odd-numbered counterparts because they can be packed more closely and efficiently in the crystal lattice due to their symmetrical shape. This results in stronger intermolecular forces (van der Waals forces) between the molecules in the solid phase, requiring more energy to overcome these forces and thus leading to higher melting points for even-numbered alkanes.

Step-by-step solution

Statement (a): Alkanes are unreactive when compared with alkenes.

Alkanes are hydrocarbons that consist of only single bonds between carbon atoms. They are also referred to as saturated hydrocarbons. Due to the presence of only single bonds, alkanes are relatively stable and have lower reactivity than alkenes. Alkenes, on the other hand, are unsaturated hydrocarbons containing carbon-carbon double bonds. The presence of the double bond imparts higher reactivity in alkenes compared to alkanes, as the double bond can be easily broken to form new compounds. Thus, alkanes are considered unreactive when compared to alkenes.

Statement (b): In the case of isomeric alkanes, the boiling points decrease with increase in branching of the chain.

Boiling point is influenced by various factors such as molecular size, molecular weight, and intermolecular forces. In the case of isomeric alkanes, the molecular weight is the same, so the main factor affecting the boiling point is the shape of the molecule and the resulting intermolecular forces. For isomeric alkanes with greater branching, the surface area of the molecule decreases. This reduces the van der Waals forces (London dispersion forces) between the molecules, which are dominant intermolecular forces in alkanes. As a result, less energy is required to overcome the intermolecular forces in more branched isomeric alkanes, and their boiling points decrease with increasing branching of the chain.

Statement (c): Normal alkanes with even number of carbon atoms melt at a higher temperature than the adjacent alkane having odd number of carbon atoms.

The difference in melting points between normal alkanes with odd and even numbers of carbon atoms is influenced by the packing efficiency of the molecules in the solid phase. In normal alkanes with even numbers of carbon atoms, the molecules can be packed more closely and efficiently in the crystal lattice due to their symmetrical shape. This results in stronger intermolecular forces (van der Waals forces) between the molecules in the solid phase. In contrast, normal alkanes with odd numbers of carbon atoms have a less symmetrical shape and cannot be packed as closely or efficiently in the crystal lattice. This leads to weaker intermolecular forces in the solid phase. As a result, normal alkanes with even numbers of carbon atoms have higher melting points than adjacent alkanes with odd numbers of carbon atoms, as more energy is required to overcome the stronger intermolecular forces in the more efficiently packed even-numbered alkanes.

Key concepts

Saturated Hydrocarbons

Understanding the nature of alkanes begins with recognising them as saturated hydrocarbons. What does this mean? It implies that alkanes have single carbon-carbon (C-C) bonds and the maximum possible number of hydrogen atoms bonded to the carbon skeleton. This saturation of hydrogen atoms leads to a stable molecular structure.

With no double or triple bonds, alkanes exhibit less reactivity compared to their unsaturated counterparts—alkenes and alkynes—which contain double and triple bonds, respectively. This stability is due to the strength of the single bonds and the lack of a readily accessible site for reaction without breaking these strong bonds.

For students, why does this matter? When predicting the behaviour of alkanes in chemical reactions—like combustion or halogenation—it's crucial to remember that they are less likely to undergo reactions that involve breaking their stable C-C single bonds. This fairly inert nature is a defining characteristic of saturated hydrocarbons such as methane (CH₄), ethane (C₂H₆), and other higher members of the alkane series.

Boiling Point of Isomeric Alkanes

Isomers are compounds with the same molecular formula but different structural arrangements. When analyzing isomeric alkanes, the boiling point is a critical property to understand. The key factor here is the influence of molecular shape on intermolecular forces.

Greater branching in an alkane reduces its surface area, diminishing van der Waals forces (a type of intermolecular attraction). This means that branched alkanes need less energy to transition into a gaseous state—hence, they have lower boiling points. Students should note that despite having the same molecular weight, isomeric alkanes will not boil at the same temperature.

For example, compare n-butane and iso-butane. Iso-butane, with its branched structure, has a lower boiling point than n-butane due to its decreased surface area and, consequently, weaker intermolecular forces. This concept is essential when predicting the physical properties of isomeric alkanes and their behaviour during distillation or under changing temperature conditions.

Melting Point Trends in Alkanes

When considering the melting points of alkanes, the trend among the 'normal' or unbranched alkanes stands out. Specifically, normal alkanes with an even number of carbon atoms tend to have higher melting points compared to adjacent alkanes with an odd number of carbon atoms.

This phenomenon is because alkanes with even numbers of carbons pack more neatly and efficiently in a solid crystal lattice due to their symmetrical structure. This tight packing enhances the van der Waals forces between alkane molecules, meaning more heat is needed to melt the crystal—hence, a higher melting point.

In contrast, odd-numbered alkanes have a slightly staggered or asymmetrical structure. This prevents tight packing in the solid state, resulting in a lower melting point. Understanding this could be pivotal for students when considering the physical state of an alkane at a given temperature, and it highlights the importance of molecular symmetry on physical properties.