Note # 1:  I just received word that we will have working hoods by next Tuesday!  There is no need to prepare for the Eucalyptus lab!!

Note # 2:  A copy of the Molecular Modeling Workbook with CD is on two-hour reserve in the library.

Note # 3:  The next homework assignment is posted on the web.  DO NOT GET BEHIND!

Note # 4:  I will have several tickets to the Science Lecture for Jan. 17 - Olive Sacks.  If you are interested in attending, please let me know.  First come, first served!

Chapter 8 Reactions of Alkalis:  Radicals

A.  Radical Substitution Reactions
    I.  Chlorination and Bromonation of Alkalis
        a.  Initiation:  homolytic cleavage of chlorine and bromine.
            1.  indicate with one barb arrow, signifying the movement of one electron.
            2.  homolytic bond dissociation energies on p. 130
            3.  2 free radicals are formed (unpaired electron)
        b.  Propagation:  a radical reacts and forms a new radical
            1.  the halogen radical abstracts a hydrogen from the alkane
                a.  a hydrogen halide and an alkyl radical is formed
            2.  the alkyl radical reacts with the halogen
                a.  an alkyl halide and a radical halogen is formed
        c.  Termination:  two radicals combine to form a molecule
                a.  reaction comes to an end by decreasing the number of radicals present.
    II.  Monohalogenation
        A.  One hydrogen is replaced by a halogen.
            1.  maximized by having an excess alkane reagent present
    III.  Di or multiple halogenation
        A.  More hydrogens are replaced by halogens

B.  Product Distribution
    I.  Statistically:  depends on the number of different types of hydrogens available.
    II.  Ease of hydrogen abstraction:  depends on the stability of the radical
        a.  radical stability:  tertiary >  secondary  >  primary  >  methyl radical
        b.  experimental evidence concludes that at room temperature, it is 3.8 times easier for a chlorine radical to abstract a hydrogen from a secondary carbon and 5 times easier to extract a hydrogen from a tertiary carbon.
        c.  experimental evidence concludes that at 125 oC, it is 82 times easier for a bromine radical to abstract a hydrogen from a secondary carbon and 1600 times easier to extract a hydrogen from a tertiary carbon.
    III.  Predict the product distribution by multiplying the number of kinds of hydrogen by the reactivity.

C.  Calculations of change in enthalpy.  p. 130
    I.  Change in enthalpy = the amount of needed to break a bond minus the amount of energy released when a bond forms.
    II..  Reactivity-selectivity principle:  the greater the reactivity of a species, the less selective it will be.
        a.  Reaction of a chlorine radical with an alkane is exothermic
            1.  the transition state resembles the reagents
            2.  less than 1 kcal difference in activation energy
        b.  Reaction of a bromine radical with an alkane is endothermic
            1.  the transition state resembles the products
            2.  about 1.5 kcal difference in activation energy.
    III.  Florine too reactive, iodine not reactive enough.