[uzi4u2]

1st )    which statements are true for the chlorination of benzene ?

       1)the reaction is a nucleophilic substitution
       2)the reaction is an electrophilic addition
       3)the reaction is an electrophilic substitution
       4)the catalyst is FeCl3
       5)in the transiton state one of the ring c atoms is in sp3 hybrid state
 
 optional answers :     a) 1,4      b)1,4,5      c)3,4,5     d)1,2,3      e)2,5

2nd)      in which of the following reactions does heterolysis occur ?

         1)halogenation of alkanes
         2)hydrogenation  of ethene
         3)addition of HCL into alkenes
         4)halogen substitution in alkylhalogenides

 optional answers :     a)3,4      b)1,2,3,4     c)only 4    d)1,2,3      e)1,2


3rd)      which property of buffers is dependent on the actual concentration of their components

          1)buffer capacity
          2)ph
          3)ph range bufferd
          4)poh
         
                 now on this question answer 1 and 3 are basicly the same idea but i think its 3 which is
                 better .


thank you for your help 


3rd)       

[xiankai]

1st) note that statements 1,2 and 3 are mutually exclusive, since halogenation of benzene undergoes one main mechanism. with hindsight (d) is out for sure. now, if u can recall the mechanism for benzene reactions, it is electrophilic substitution, because the delocalised pi electrons makes it to a certain extent, unreactive. FeCl₃ is a known catalyst for halogenation of benzene. lastly, when the arenium ion has been formed after substituting a delocalised electron with H, a C atom contains 4 bonds. therefore it is (c)

2nd) in reaction 1 homolysis occurs; the free radical Cl is formed to substitute the H.

in reaction 2, homolysis occurs again as the free radical H is formed. when the double bond is cleaved, the electrons are not split among the C atoms, but donated to the H radicals.

in reaction 3, there is heterolysis as the pi electrons polarise the H-Cl bond, which eventually leads to Cl getting all the electrons as partially positive H is attracted to the pi electrons.

in reaction 4, i am not so sure as i have only been taught about substitution along the alkyl chain, but it may be heterolytic, because the partial polarity of C-X where X is a halogen may be sufficient to stimulate something like reaction 3.

therefore my best bet is (a), but don't forget i dont know reaction 4

3rd) i have not studied buffers, alas.

[uzi4u2]

thanks for your help !   
anybody knows the last question by anychance ?

[Mitch]

What's the definition of buffer capacity?

[uzi4u2]

umm... i dunno 
I guess its the certin lvl which a buffer can stand at which when it reaches that point it can no longer keep the same p.h

[Borek]

Buffer capacity