Hydrocarbons are compounds composed of carbon and hydrogen atoms. Alkanes are hydrocarbons. As the number of carbons increases in straight-chain alkanes, there is a steady gradation of properties which can be easily compared and predicted. The properties of branched alkanes vary considerably and are hard to predict because of other intervening forces that come into play. The boiling points of a few alkanes are given below:
What is the most likely boiling point of 2,3-dimethylbutane?
|A|| 58o C|
|B|| 63.3o C|
|C|| 68.7o C|
|D|| 75.8o C|
Ans. A This question tests your knowledge of intermolecular forces and other interactions that give rise to some of the chemical and physical properties of compounds. Let's take a look at the compound 2,3-dimethylbutane. We do not have a clue about the precise properties of the compound asked in this question. But what we do know is the boiling point of one of its relatives. In the table given in the passage, we are told that the boiling point of 3-methylpentane is 63.3o C. If we compare these two compounds, we see that they have the same number of carbons and hydrogens – both are six-carbon alkanes. If the boiling point of 3-methylpentane is 63.3o C, then 2,3-dimethylbutane will have a boiling point lower than that. The only choice with a lower value is Choice A.