For isomers, the more branched the chain, the lower the boiling point tends to be. It is evident that there is an increase of 20-30 degrees for each carbon atom added to the chain. For the same number of carbon atoms, increased branching leads to more compact molecule that can pack more closely in a solid lattice. As a rule of thumb, as packing efficiency increases, melting point increases. Alkanes with stronger intermolecular van der waals forces have higher boiling point. Why? Linear or less branched isomeric alkanes have higher boiling points as compared to branched alkanes assuming that both have comparable molecular masses. However, the first few members are exceptions where the increase in boiling point is relatively more. For instance, let's say you're planning to go to a nice long vacation (ask me, I'd say to Hawaii). Hence, the boiling point of an alkane chain decreased with an increase in branching. The intermolecular forces (often called van der Waal's forces) depend on attractions caused by quantum fluctuations in the surface electrons of the molecule. Going from “branched” to “highly branched” makes a molecule more compact and sphere-like. For example: propane will have a lower boiling point than butane. Starting with the simplest branched compound, as you increase branching, you will increase the melting point, but decrease the boiling point. But you've decided to carry just one suitcase for the trip. -boiling points decrease as branching increases,-branched alkanes have less surface area in contact so the intermolecular forces are fewer and weaker,-therefore less energy is required to overcome the intermolecular forces, Van der Waals dispersion forces are smaller for shorter molecules and only operate over very short distances between one molecule and its neighbors. So there are weaker dipole to dipole forces and less energy is required to overcome these forces hence the boiling point decreases. Among isomeric alkanes, branching decreases the boiling points. A similar reasoning is given whenever we're to compare the melting points of trans and cis-2-butene. Treat the n-hydrocarbon as a special case, and ignore it for the time being. The intermolecular attractive forces are stronger and melting point higher. A variety of alkanes with the generic formula C n H 2n+2 are given in the table at the left with names, formulas, and physical properties. Alkenes and alkynes: The melting points and boiling points of alkenes and alkynes increase with an increase in molecular weight. As a result, the Van der Waals force also decreases which can be overcome at a relatively lower temperature. As the branching of an alkane increases, the boiling point decreases. As branching increases, the surface area of the molecule decreases which results in a small area of contact. The basic assumption that this happens is flawed. Hence, the boiling point of an alkane chain decreases with an increase in branching. Hence the melting point increases, as branching increases. With increase in the branching, the surface area of the molecule decreases and vander waals forces of attraction decreases which can be overcome at a relatively lower temperature. As the chain length (numbers of carbons) increases the melting and boiling points of the alkanes gradually increase for these compounds. Similarly, the boiling point of alkanes increases as the length of the carbon chain increases. What is the general trend in the melting and boiling data? This is due to the fact that branching of the chain makes the molecule more compact and thereby decreases the surface area. Therefore, the boiling points of the alkanes increase with molecular size. 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