Organic Chemistry

Properties

Figure 118

Equatorial (Dibro-

momethyl)cyclohexane

Figure 119 1,6-Dichloro-2,5-

dimethylhexane

1,1-Dichloro-3-

methylcyclobutane

44.2 Physical properties.

R-X bond polarity: C—F > C—Cl > C—Br > C—I

atom| electronegativity| difference from C(= 2.5) |

F 4.0 1.5

Cl 3.0 0.5

Br 2.8 0.3

I 2.5 0.0

The difference in electronegativity of the carbon-halogen bonds range from 1.5 in C-F to
almost 0 in C-I. This means that the C-F bond is extremely polar, though not ionic, and
the C-I bond is almost nonpolar.

Physical appearance: Haloalkanes are colourless when pure. However bromo and iodo
alkanes develop colour when exposed to light. Many volatile halogen compounds have sweet
smell.

Boiling point: Haloalkanes are generally liquids at room temperature. Haloalkanes gen-
erally have a boiling point that is higher than the alkane they are derived from. This is
due to the increased molecular weight due to the large halogen atoms and the increased
intermolecular forces due to the polar bonds, and the increasing polarizabilty of the halogen.

For the same alkyl group, the boiling point of haloalkanes decreases in the order RI > RBr

RCl > RF.This is due to the increase in van der Waals forces^1 when the size and mass
of the halogen atom increases.

For isomeric haloalkanes, the boiling point decrease with increase in branching. But boiling
points of dihalobenzenes are nearly same; however the para-isomers have higher melting
points as it fits into the crystal lattice better when compared to ortho- and meta-isomers.

1 https://en.wikipedia.org/wiki/van%20der%20Waals%20forces