* Polarity * Flourine Chlorine and Bromine are all more electronegative than carbon, as a result the C-X bond is polarized, slightly positive charge on carbon and negative on the halogen. * Electronegativity of C is less than 2.5 which is why it is stil polar with I (2.5 also). * Boiling Point * Constitutional isomers with branched chains have lower boiling points than unbranched chains. * Branched chains have a more spherical arrangement which means a smaller surgace area and smaller van der Waals forces. * Haloalkanes have higher boiling point than alkanes. (same size) * This is due to the dipole moment in the haloalkane. The larger the halogen the greater its polarizability. The higher the polarizabelity the higher the boiling point. I >Br>Cl>F>H * Fluoroalkanes are comparable to hydrocarbons of the same weight. It has a low boiling point compared to the hydrocarbon. * Density * Densities of liquids haloalkanes are greater than those of comparable hydrocabons. (large mass to volume ratio of halogen. * Bond Length * C-X bonds are weaker than C-H except for (C-F). * A radical is formed when a molecule breaks into 2 and each atom retains one electrion (Homolytic bond cleavage. When one atom keeps both electrons its heterolytic cleavage.
* HX or X2 is added to alkenes, or they can replace an –OH. * Regioselectivity * Treating propane with bromine gives mixture consisting of 2-bromopropane (92%)and 1-bromopentane(8%). * Bromine is substituted in for a secondary hydrogen, as opposed to one of the six primary hydrogens. * Chlorination is much less regioselective. 57 to 43 percent. * 1800:80:1 as compared to 5:4:1 * Energetics * Delta H = summation of BDE of bonds broken – bonds formed.
8.5 Mechanism of Halogenation of Alkanes * Formation of Radicals *