Chemistry · Haloalkanes And Haloarenes · NEET
The C-X bond is polar, so haloalkane molecules have dipole-dipole attraction on top of van der Waals forces. They are also much heavier than the parent alkane. Both stronger intermolecular forces and higher molecular mass mean more energy is needed to separate the molecules, so the boiling point is higher.
Branching makes the molecule more round and compact. A compact shape has a smaller surface area, so neighbouring molecules touch each other over a smaller area. This weakens the van der Waals forces, so less energy is needed to boil it. That is why 2-bromo-2-methylpropane (most branched) has the lowest boiling point of its isomers.
R-I > R-Br > R-Cl > R-F. As you go down the halogen group the atom gets bigger and has more electrons, so the van der Waals (dispersion) forces get stronger and the mass rises. Iodides boil highest, fluorides lowest. NEET often hides this in a mixed list, so line up mass first.
Bromo, iodo and polychloro derivatives are HEAVIER than water (density greater than 1 g/mL), so they sink. Density increases with more carbon atoms, more halogen atoms, and heavier halogen. For example n-C3H7Cl is 0.89, n-C3H7Br is 1.335, and n-C3H7I is 1.747 g/mL.
To dissolve, water must break its own hydrogen bonds and the haloalkane must break its own attractions. Haloalkanes cannot form strong hydrogen bonds with water, so the new attractions released are weaker than the hydrogen bonds broken. Not enough energy is released, so they stay only very slightly soluble. They dissolve well in organic solvents instead.
The para isomer is more symmetrical, so its molecules pack neatly into the crystal lattice. Better packing means stronger lattice forces, so more energy is needed to melt it. Boiling points of the isomers are nearly the same, but the para isomer melts highest. This symmetry point is a common NEET one-mark idea.
The hydrolysis reaction (with aq. NaOH) that takes place at the slowest rate, among the following, is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Pure alkyl halides are colourless. Bromides and iodides slowly develop colour when left in light. Many volatile ones have a sweet smell.
Methyl chloride, methyl bromide, ethyl chloride and some chlorofluoromethanes are gases at room temperature. Higher members are liquids or solids.
Density increases from chloride to bromide to iodide for the same carbon skeleton, because the halogen atom gets heavier. More carbon atoms and more halogen atoms also raise the density.
They dissolve well in organic solvents such as other haloalkanes, benzene and ether, where the new attractions formed are similar in strength to the ones broken. They stay only very slightly soluble in water.
NEET regularly asks you to arrange isomers or a mixed set in order of boiling point, or to spot the densest or least soluble compound. Knowing the three simple rules (mass up, chain up, branching down) lets you answer these one-mark questions fast without any calculation.
