[lutesium]

Dear Sir/Ma'am

I used to synthesise these useful reagents in my lab but I preferred to synthesise Bromoacetic Acid rather than Chloro- because of easier handling of Bromine... But I cannot understand something:

According to the Chemistry rules Cl is more reactive than Br but according to my sight Br2 reacted much more easier than Cl2! (sometimes -in the absence of direct sunlight- the chlorination may take up to one day) -and there was no need to use Red Phosphorus or Sulphur as halogen carriers!-

Can you explain me why this contradiction occurs???

[lutesium]

Noone knows the answer???

[agrobert]

Are you using a solvent?

[lutesium]

No! In the synthesis of Chloro/Bromo Acetic Acids no solvent is used... Actually the solvent is acetic acid itself after you distill off the acetic acid a molten mass should form which upon cooling should crystallyze to a mass! I think the question lies in here: Why is Br2 a better nucleophile than Cl2 while Br2 is a better leaving group than Cl2???


Lutesium...

[RBF]

I'm not sure of the answer to the original question, but this appears to be the Hell-Volhard-Zelinksii (sp?) reaction.  However, P or PBr₃ normally is used for both chlorination and bromination reactions.  In any case, neither chlorine or bromine act as nucleophiles here-they are electrophiles, with the enol form of the acyl halide behaving as the nucleophile.
The Br-Br bond is weaker than the Cl-Cl bond, so that may be the reason bromine reacts faster. 

[lutesium]

Thank you very much RBF I do alo think so! But the electronegativity of Cl is higher than Br this means that it must react at a faster rate than Br... For example F2 reacts explosively with water which has caused scientists to get blind!!! (I believe that it has reacted with the water on their eyes to produce HF which is a really corrosive reagent!) I really cannot understand this electronegativity issue Yes RBF you're right Br-Br bond is weaker than Cl-Cl bond but Chemistry rules say that Cl-Cl bond (which should be much stronger than Br-Br bond) must react much more easier than Br2... Observations also tell the same as we do (Br2 reacts much more easier than Cl2) but I'm really puzzled! Please help me!!!


Lutesium...