Organic Chemistry

Physical Properties of Alkyl Halides

Physical Properties of Alkyl Halides will be discussed such as dipole moment, London force, Dipole–dipole attractions, densities of common alkyl halides.

Structure of Alkyl Halides

– In an alkyl halide, the halogen atom is bonded to an carbon atom.

– The halogen is more electronegative than carbon, and the bond is polarized with a partial positive charge on carbon and a partial negative charge on the halogen.

– The dipole moment (μ) is given in debyes (D):

μ = 4.8 × δ × d

where δ is the amount of charge separation, and (d) is the bond length

– The electronegativities of the halogens increase in the order

– The carbon–halogen bond lengths increase as the halogen atoms become bigger (larger atomic radii) in the order

– These two effects oppose each other, with the larger halogens having longer bonds but weaker electronegativities.

– The overall result is that the bond dipole moments increase in the order

– A molecular dipole moment is the vector sum of the individual bond dipole moments.

– Molecular dipole moments are not easy to predict because they depend on the bond angles and other factors that vary with the specific molecule.

– The following table lists the experimentally measured dipole moments of the halogenated methanes.

– Notice how the four symmetrically oriented polar bonds of the carbon tetrahalides cancel to give a molecular dipole moment of zero.

Physical Properties of Alkyl Halides

– Now we will discuss Physical Properties of Alkyl Halides

– Two types of intermolecular forces influence the boiling points of alkyl halides.

– The London force is the strongest intermolecular attraction in alkyl halides.

– London forces are surface attractions, resulting from coordinated temporary dipoles.

– Molecules with larger surface areas have larger London attractions, resulting in higher boiling points.

– Dipole–dipole attractions (arising from the polar bond) also affect the boiling points, but to a smaller extent.

– Molecules with higher molecular weights generally have higher boiling points because they are heavier (and therefore slower moving), and they have greater surface area.

– The surface areas of the alkyl halides vary with the surface areas of halogens.

– We can get an idea of the relative surface areas of halogen atoms by considering their van der Waals radii.

– The following Figure shows that an alkyl fluoride has nearly the same surface area as the corresponding alkane; thus its London attractive forces are similar.

Physical Properties of Alkyl Halides — Space-filling drawings of the ethyl halides. The heavier halogens are larger, with much greater surface areas. As a result, the boiling points of the ethyl halides increase in the order F < Cl < Br < I.

– The alkyl fluoride has a larger dipole moment, however, so the total attractive forces are slightly greater in the alkyl fluoride, giving it a higher boiling point.

– For example, the boiling point of n-butane is 0 °C, while that of n-butyl fluoride is 33 °C.

– The other halogens are considerably larger than fluorine, giving them more surface area and raising the boiling points of their alkyl halides.

– With a boiling point of 78 °C, n-butyl chloride shows the influence of chlorine’s much larger surface area.

– This trend continues with n-butyl bromide (bp 102 °C) and n-butyl iodide (bp 131 °C).

– The following Table lists the boiling points and densities of some simple alkyl halides.

– Notice that compounds with branched, more spherical shapes have lower boiling points as a result of their smaller surface areas.

– For example, n-butyl bromide has a boiling point of 102 °C, while the more spherical tert-butyl bromide has a boiling point of only 73 °C. This effect is similar to the one we saw with alkanes.