Question

Which of the following series arranges alkanes into increasing boiling point?.

Answer 1

Answer: arrange a number of given straight-chain alkanes in order of increasing or decreasing boiling point or melting point.

arrange a series of isomeric alkanes in order of increasing or decreasing boiling point.

explain the difference in boiling points between a given number of alkanes.

Explanation:Table 3.5.1 describes some of the properties of some straight-chain alkanes.

There is not a significant electronegativity difference between carbon and hydrogen, thus, there is not any significant bond polarity. The molecules themselves also have very little polarity. A totally symmetrical molecule like methane is completely non-polar, meaning that the only attractions between one molecule and its neighbors will be Van der Waals dispersion forces. These forces will be very small for a molecule like methane but will increase as the molecules get bigger. Therefore, the boiling points of the alkanes increase with molecular size.

For isomers, the more branched the chain, the lower the boiling point tends to be. Van der Waals dispersion forces are smaller for shorter molecules and only operate over very short distances between one molecule and its neighbors. It is more difficult for short, fat molecules (with lots of branching) to lie as close together as long, thin molecules.

The boiling points shown are for the "straight chain" isomers of which there is more than one. The first four alkanes are gases at room temperature, and solids do not begin to appear until about  C17H36 , but this is imprecise because different isomers typically have different melting and boiling points.