Organic reactions are often classified using two words: one identifying the type of REAGENT and the other the type of REACTION.
Nucleophiles are electron pair donors, which use that pair to form a new bond to the substrate. If you are into A2 chemistry and interested in transition metal complex ions [and let's face it they ARE pretty amazing!] then nucleophiles could be called LIGANDS for these function in the same way!
Electophiles are electron pair acceptors; they accept a pair of electrons FROM the substrate to form a new bond.
Free radicals are very reactive intermediates which carry a single unpaired electron.
Alkanes are boring. Their olde name of "paraffins" was much more apt since it is derived from some Latin: "parum affinis" meaning 'little affinity'. See the benefits of a classical education? Since they contain only C-C and C-H bonds that are scarcely polar, electrophiles and nucleophiles are denied access, so we are left with free radicals. Since the substrate is Saturated, only Substitution can take place. Remember that free radicals [as their name implies] have no manners, and will give mixtures of products and multiple substitution with limited control. Their reactions with haloalkanes also give HX (g).
Alkenes contain a weak pi bond and, being UNsaturated, will suffer ADDITION reactions. The two pi type electrons are readily accessible to electrophiles, hence electrophilic addition reactions are the norm for alkenes. Whatever else happens the double bond does NOT break, for if it did the molecule would fall into two pieces!
Having put a halogen atom into the molecule we have now activated the substrate in a different way. The saturated haloalkanes now contain a polar C-Hal bond that will be subject to nucleophilic attack on the positively charged Carbon atom. Their reactions will therefore typically be nucleophilic substitution. Remember that the length and therefore strength of the C-Hal bond dictates reactivity NOT polarity, so the most reactive is C-I although the most polar is C-Cl.
Posted by dcox at September 25, 2009 09:30 PM