Introduction to Alkyl Halides

Introduction to Alkyl Halides

An alkyl halide is another name for a halogen-substituted alkane. The carbon atom, which is bonded to the halogen atom, has sp3 hybridized bonding orbitals and exhibits a

tetrahedral shape. Due to electronegativity differences between the carbon and halogen atoms, the s covalent bond between these atoms is polarized, with the carbon atom becoming slightly positive and the halogen atom partially negative. Halogen atoms increase in size and decrease in electronegativity going down the family in the periodic table. Therefore, the bond length between carbon and halogen becomes longer and less polar as the halogen atom changes from fluorine to iodine.

Physical properties

Alkyl halides have little solubility in water but good solubility with nonpolar solvents, such as hexane. Many of the low molecular weight alkyl halides are used as solvents in reactions that involve nonpolar reactants, such as bromine. The boiling points of different alkyl halides containing the same halogen increase with increasing chain length. For a given chain length, the boiling point increases as the halogen is changed from fluorine to iodine. For isomers of the same compound, the compound with the more highly-branched alkyl group normally has the lowest boiling point. Table summarizes data for some

representative alkyl halides.

TABLE 1 Boiling Points (°C) of Alkyl Halides

Flouride Chloride Bromide Iodine

Group bp bp bp bp

Methyl -78.4 -28.8 -3.6 42.5

Ethyl -37.7 13.1 38.4 72

Propyl -2.5 46.6 70.8 102

Isopropyl -9.4 34 59.4 89.4

Butyl 32 78.4 101 130

Sec-butyl 68 91.2 120

Tert-butyl 51 73.3 100

Nomenclature

Alkyl halides are named using the IUPAC rules for alkanes. Naming the alkyl group attached to the halogen and adding the inorganic halide name for the halogen atom creates common names.

Nucleus and Nucleophiles

A nucleus is any atom that has a partial or fully positive charge associated with it. A nucleophile is an atom or group that is attracted to a source of partial or full positive charge. Alkyl halides act as a nucleus because of the great electronegativity differences between the carbon atom and the halogen atom directly bonded to it. This great electronegativity difference causes the electron density in the overlap region between the carbon and halogen atoms to be pulled toward the halogen atom. This shifting of electron density in the molecule makes the carbon atom partially positive (the nucleus) and the halide ion partially negative (the incipient leaving group).

Figure illustrates the effect of electronegativity differences on bond polarity.

Electrons in the overlap region between the carbon and the halogen atoms are attracted to the more electronegative halogen atom. The carbon atom, which now has less of a share of the bonding electrons, becomes partially positive, and the halogen atom, which has a greater share of these electrons, becomes partially negative.

Remember that a nucleophile is a substance that has a pair of electrons that it can donate to another atom. The weaker the forces of attraction holding the electron pair to the original molecule, the more readily this molecule will share the electrons and the stronger the resulting nucleophile will be. The weakest held electrons on an atom are the nonbonding electron pairs. Electrons in p bonds, although held more strongly than nonbonding electrons, are also loosely held and easily shared, making unsaturated compounds relatively good Nucleophiles.

Because they possess a negative charge, anions are always better Nucleophiles than their conjugate acids.