3.4
Physical Properties of Alkanes
Alkanes are nonpolar molecules due to the presence of only carbon and hydrogen atoms. The electronegativity difference between carbon and hydrogen is…
A series of unbranched alkanes in which successive members differ by a −CH2− group is called a homologous series, and each alkane in this series is a homologue.
Alkanes are nonpolar due to the small electronegativity difference between carbon and hydrogen, implying that only weak dispersion forces exist between their molecules.
The strength of these forces increases proportionally with carbon chain length. The first four alkanes are gases at room temperature and atmospheric pressure; moderate carbon chains, from C5 to C17, are liquids. Homologues beyond C17 are solids.
Dispersion forces also influence the different physical properties of alkanes.
Within a homologous series, as surface area increases, the dispersion forces between molecules also increase. Because more energy is required to separate the molecules, the boiling points of unbranched alkanes rise with each additional carbon.
Unlike straight-chain alkanes, branched alkanes are compact and more spherical, which reduces the area of interaction and the strength of the intermolecular forces. Therefore, compared to the unbranched form, branched isomers boil at lower temperatures.
While the boiling points of straight-chain alkanes rise gradually with the number of carbons, their melting points do not increase steadily. Instead, the trend alternates between the even and odd members of the homologous series.
In the crystalline state, even alkanes pack closely in a zig-zag arrangement. Consequently, the molecules experience stronger attractions, leading to higher melting points.
In comparison, odd alkanes pack less tightly due to their parallel arrangement, resulting in weaker interactions and lower melting points.
The melting points of branched alkanes are dictated by their molecular symmetries.
Branched alkanes with substantial symmetry melt at temperatures higher than the unbranched hydrocarbon. In comparison, asymmetrical branching leads to a lower melting point.
Alkanes are insoluble in water due to their inability to form hydrogen bonds. Additionally, as their densities are lower than 1 g/cm3, they float on water.
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Q1: Why are alkanes nonpolar molecules?
Alkanes are nonpolar because they contain only carbon and hydrogen atoms with minimal electronegativity difference, resulting in a zero dipole moment. This nonpolar nature means only weak dispersion forces exist between alkane molecules, which significantly influence their physical properties and behavior.
Q2: How does carbon chain length affect the boiling point of straight-chain alkanes?
Boiling points of straight-chain alkanes increase proportionally with carbon chain length because longer chains have greater surface area. Increased surface area strengthens dispersion forces between molecules, requiring more energy to separate them. The first four alkanes are gases, C5 to C17 are liquids, and alkanes beyond C17 are solids at room temperature.
Q3: Why do branched alkanes have lower boiling points than their straight-chain isomers?
Branched alkanes are compact and spherical in shape, which reduces their surface area compared to unbranched forms. With less surface area available for intermolecular contact, the dispersion forces between branched molecules are weaker. Consequently, branched isomers require less energy to boil and have lower boiling points than straight-chain alkanes.
Q4: What causes the odd-even pattern in alkane melting points?
Straight-chain alkanes show alternating melting points due to differences in crystal packing. Even-numbered alkanes pack tightly in a zig-zag arrangement, experiencing stronger intermolecular attractions and higher melting points. Odd-numbered alkanes pack less efficiently in a parallel arrangement, resulting in weaker interactions and lower melting points.
Q5: How does molecular symmetry affect the melting points of branched alkanes?
Branched alkanes with substantial molecular symmetry melt at higher temperatures than their unbranched counterparts. For example, neopentane, a symmetrically branched isomer, melts at −16.5°C, much higher than n-pentane at −129.8°C. Asymmetrical branching, conversely, leads to lower melting points due to less efficient crystal packing.
Q6: Why are alkanes insoluble in water?
Alkanes are insoluble in water because they cannot form hydrogen bonds with water molecules. Additionally, alkanes have densities lower than 1 g/cm³, causing them to float on water. Their nonpolar nature and inability to interact with polar water molecules prevent dissolution.
Q7: What is a homologous series of alkanes?
A homologous series is a sequence of unbranched alkanes where successive members differ by exactly one −CH₂− group. Each member is called a homologue. Members within a homologous series show gradual changes in physical properties due to increasing carbon chain length and surface area, making them useful for studying trends in alkane behavior.