[Hijaz Aslam]

Two different mechanisms of Gattermann-Koch reaction is given below.





The following points puzzles me:
1.  Both the mechanisms are different. (In the former case the Chloride(which has a negative charge after cleaving from Hydrogen atom) ion gets added to the carbon atom which has a partial negative charge too (or am I wrong?). In the latter mechanism the reaction takes a completely different path. - Which one is correct?

2. In the above reaction, I guess that all the reactants are added into a test tube simultaneously (or is it not?). So, how do we know which reactant attacks which other reactant first? This is a general doubt regardless of the above reaction. How do we know the attack that takes place first?

3.  In the mechanism it's seen that Anhydrous Aluminium Chloride partially involves in the reaction with a weak bond (I guess). - Is that how Anhy.AlCl₃ works (I heard that they suppress the involvement of water in the reaction. Does the above mechanism explain how it suppress the action of water)?

[OrgXemProf]

The top reaction sequence doesn't qualify as a "mechanism' - - it's more like a summary of what goes on during aluminum trichloride catalyzed reaction of carbon monoxide with benzene. The bottom sequence of steps is much more revealing and explains the role of the catalysts.

Some points to consider:
 
1. It's unlikely that CO would react with benzene in the absence of a catalyst. So, how do HCl and AlCl3 promote reaction of CO with benzene in the bottom reaction sequence?
2. Does benzene suffer electrophilic or nucleophilic attack? Does benzene undergo substitution or addition as a result of reaction with CO + AlCl3 + HCl?
3. Does AlCl3 function as a Lewis acid or a Lewis base in this reaction?
4. Which reaction would be likely to proceed more rapidly: reaction of CO + HCl + AlCl3 with benzene or with xylene (i. e., 1,4-dimethylbenzene), each of which affords a single monoaldehyde product?
5. Aluminum chloride undergoes hydrolysis in water; what are the likely hydrolysis products?

[Hijaz Aslam]

Some points to consider:
 
1. It's unlikely that CO would react with benzene in the absence of a catalyst. So, how do HCl and AlCl3 promote reaction of CO with benzene in the bottom reaction sequence?
2. Does benzene suffer electrophilic or nucleophilic attack? Does benzene undergo substitution or addition as a result of reaction with CO + AlCl3 + HCl?
3. Does AlCl3 function as a Lewis acid or a Lewis base in this reaction?
4. Which reaction would be likely to proceed more rapidly: reaction of CO + HCl + AlCl3 with benzene or with xylene (i. e., 1,4-dimethylbenzene), each of which affords a single monoaldehyde product?
5. Aluminum chloride undergoes hydrolysis in water; what are the likely hydrolysis products?

1. HCl and AlCl₃ attacks CO which consequently gives the Carbon atom a positive charge, which then reacts with benzene undergoing electrophilic reaction.

2. From the above point possibly Electrophilic Reaction, (I've read in a post that, Aryl groups mostly undergo Electrophilic reactions, but it can also undergo Nucleophilic Reaction, when the Nucleophile attacks a Hydrocarbon and the Carbon atom of this Hydrocarbon undergoes Electrophilic attack upon the Aryl group, (which is a manipulation of Electrophilic Reaction). But does that count here? )
In the mechanism, the hydrogen atom attached to the sp₂ hybridized Carbon of the Benzene gets detached while the CO,HCl,AlCl₃ derivative combines with that Carbon. So can we call it Substitution?

3. As 1,4-dimethylbenzene contains Electron Donating groups (methyl-) it increases the nucleophilicity of the Carbon atom (which undergoes the attachment process) and hence xylene proceeds more rapidly. (Or am I wrong?)

4. Al³⁺(aq) + 6H2O(l)  [Al(H2O)6]³⁺(aq) . Is that right?

Thanks for your support OrgXemProf 

[OrgXemProf]

Your Point #2: Nucleophilic attack upon a benzene ring vs. electrophilic attack by the benzene ring upon the nucleophilie might be considered the equivalent of someone's fist hitting you in the nose, or your nose hitting someone's fist. We normally classify reactions from the standpoint of fist hitting nose rather than the reverse, so I'd stick with the reactive agent (electrophile or nucleophile) attacking the benzene ring. Capisce?

Your Point #3: Correct. Electron donating groups in the benzene ring facilitate the rate-determining step (attack by the electrophile upon the benzene ring).

Your Point #4: As an aside, I would call your attention to the amphoteric nature of Al(OH)3: See, e. g.,  http://depts.washington.edu/chem/facilserv/lecturedemo/AmphotericNatureAlOH3-UWDept.ofChemistry.html; also, http://en.wikipedia.org/wiki/Amphoterism