Physical Properties of Alkyl Halides-Organic Halides

Organic Halides

The halogen atom of an alkyl halide is attached to an sp3-hybridized carbon. The arrangement of groups around the carbon atom, therefore, is generally tetrahedral. Because halogen atoms are more electronegative than carbon, the carbon–halogen bond of alkyl halides is polarized; the carbon atom bears a partial positive charge, the halogen atom a partial negative charge:

Physical Properties of Alkyl Halides-Organic Halides
Physical Properties of Alkyl Halides-Organic Halides

Halogen atom size increases as we go down the periodic table: fluorine atoms are the smallest and iodine atoms the largest. Consequently, the carbon–halogen bond length increases and carbon–halogen bond strength decreases as we go down the periodic table (Table 6.1). Maps of electrostatic potential (see Table 6.1) 

Physical Properties of Alkyl Halides-Organic Halides
Physical Properties of Alkyl Halides-Organic Halides

at the van der Waals surface for the four methyl halides, with ball-and-stick models inside, illustrate the trend in polarity, C-X bond length, and halogen atom size as one progresses from fluorine to iodine substitution. Fluor methane is highly polar and has the shortest C-X bond length and the strongest C-X bond. Iodomethane is much less polar and has the longest C-X bond length and the weakest C-X bond. In the laboratory and in industry, alkyl halides are used as solvents for relatively nonpolar compounds, and they are used as the starting materials for the synthesis of many compounds.

 As we shall learn in this chapter, the halogen atom of an alkyl halide can be easily replaced by other groups, and the presence of a halogen atom on a carbon chain also affords us the possibility of introducing a multiple bond. Alkyl halides are classified as primary (1°), secondary (2°), or tertiary (3°) according to the number of carbon atoms directly bonded to the carbon bearing the halogen (Section 2.5). Compounds in which a halogen atom is bonded to a sp2-hybridized carbon are called vinylic halides or phenyl halides. 

The compound CH2-CH-Cl has the common name vinyl chloride, and the group CH2 CH is commonly called the vinyl group. Vinylic halide, therefore, is a general term that refers to a compound in which a halogen is attached to a carbon atom that is also forming a double bond to another carbon atom. Phenyl halides are compounds in which a halogen is attached to a benzene ring (Section 2.4B). Phenyl halides belong to a larger group of compounds that we shall study later, called aryl halides.

Physical Properties of Alkyl Halides-Organic Halides
Physical Properties of Alkyl Halides-Organic Halides

Together with alkyl halides, these compounds comprise a larger group of compounds known simply as organic halides or organohalogen compounds.

Physical Properties of Organic Halides

Most alkyl and aryl halides have very low solubilities in water, but as we might expect, they are miscible with each other and with other relatively nonpolar solvents. Dichloromethane (CH2Cl2, also called methylene chloride), trichloromethane (CHCl3, also called chloroform), and tetrachloromethane (CCl4, also called carbon tetrachloride) are sometimes used as solvents for nonpolar and moderately polar compounds. Many chloroalkanes, including CH2Cl2, CHCl3, and CCl4, have a cumulative toxicity and are carcinogenic, however, and should therefore be used only in fume hoods and with great care. Table 6.2 lists the physical properties of some common organic halides.

 

Physical Properties of Alkyl Halides-Organic Halides
Physical Properties of Alkyl Halides-Organic Halides

 Exercise:

Physical Properties of Alkyl Halides-Organic Halides
Physical Properties of Alkyl Halides-Organic Halides

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