Pharmacist Blog about what I encounter and find useful/interesting

I currently have examinations so I'll have a break from blogging.

Alkanes and radicals Addition of radicals on an alkene Look at the two reactions below, the first reaction is in normal circumstances, which will result in a normal Markovnikov reaction  ( with the addition of a   protic acid  HX to an  alkene , the acid hydrogen (H) becomes attached to the carbon with fewer   alkyl   substituents , and the  halide  (X) group becomes attached to the carbon with more alkyl substituents). In the second reaction, peroxide is used as condition, this will result in an anti-Markovnikov  reaction. The peroxide will generate Br radicals in small amounts (this is the initiation for this anti-reaction).  Let's see the peroxide mechanism in detail. As stated above, an alkyl peroxide is a radical initiator. The electrophile will add on the sp2 carbon which carries the most hydrogens (where also the radical with most alkyl groups will be formed). The termination reactions will have several other outcomes....

Alkanes and radicals Selectivity & reactivity principle Radical bromation is more selective than radical chlorination. Have a look on following illustrations to get more insight in this process: Why is it that the Bromation radical is more selective? This can be explained using the Hammand postulate. Bromation is an endothermic process, while chlorination is exothermic . The postulate states the following: Endothermic process: a rather productish (radical) transition state Exothermic: a rather reagentic (alkane) transition state Thus the Bromine atom can see the difference between the 1°, 2° and 3° hydrogens more clearly.  The more reactive a particle is, the less likely it will react selective. ( More reactivity = less selectivity, vice versa ). Alkanes undergo Bromation and Chlorination, but no Fluoration or Iodisation. Iodisation simply does not occur, and Fluoration is a way to heavy reaction to be useful.

Alkanes & radicals Relative stabilities of alkyl radicals Alkyl  groups stabilize carbocations about 5 to 10 times better than when alkyl groups have to stabilize radicals:  Radicals: Resonance >> Hyperconjugation Carbocations: Hyperconjugation >> Resonance Hyperconjugation makes the carbocations more stabilized than when hyperconjugation occurs with radicals (not so stable). This is explained due to the fact that in carbocations, both electrons sit in the same binding orbital, with radicals however, one of the electrons is sited in the anti-binding orbital.  Product spread The product spread (product outcome) is determined by CHANCE and REACTIVITY. The chance (or probability) is based on the relative amount of primary and secundary protons (in the example below 6:4), but secundary hydrogens are more reactive than primary hydrogens, this means both chance and reactivity determine the outcome of the reaction.  Thus to det...