[Altered State]

In this exercise I need to obtain para-cyanophenol from benzene.
I've drawn the mechanism I just worked out in ChemDraw. Can you confirm if it is OK or not?



I'm not sure of the last steps. Make all sense to me, but since I'm a student and this is my first year learning Organic Chemistry, maybe I'm missing something elemental (or not that elemental)

[orgopete]

Can one not make a cyano compound by diazotization and cyanide substitution?

[Altered State]

Can one not make a cyano compound by diazotization and cyanide substitution?

I suppose so...
Does that mean this is right? Any clarification?

[orgopete]

It means p-bromobenzonitrile will be the product and not the cyano-diazonium xsalt.

[Altered State]

It means p-bromobenzonitrile will be the product and not the cyano-diazonium xsalt.

So the 5th step is wrong... What would you suggest? I'm out of ideas right now... 

[willug]

p-bromobenzonitrile would be ok. You could use some Cu catalysed chemistry to get you to the phenol in one step...

[Altered State]

p-bromobenzonitrile would be ok. You could use some Cu catalysed chemistry to get you to the phenol in one step...

Can you be more specific? I do not know that reaction.

And I'm also thing that I might be confused about what are you refering to...
You mean that, when I have Br-Ph-N≡N and I add NaCN, Br won't get out, N2 will do, and I'm getting Br-Ph-CN?

In that case, I can get rid of Br and obtain phenol using that Cu chemistry I do not know, but, is there any simplier way? I don't think I really need that Br if I'm not using it as leaving group to allow CN to enter...

[orgopete]

I wasn't in class, so I can only guess what reactions may have been covered. If benzene were chlorinated and then nitrated, could you make p-hydroxybenzonitrile from it?

[Altered State]

I can obtain hydroxi from chlorine using NaOH, but I'm almost surr that reaction wasn't covered in class.

[orgopete]

If nucleophilic aromatic substitution has not been covered, then it should not be used.

If the synthesis were carried out stepwise, first A and then B, you should choose a group for A that is an ortho/para director.

[opsomath]

One minor issue, you don't need FeBr3 to brominate aniline; aniline will react with all the halogens including iodine on its own. (Source: I used to make this compound in large batches)

As orgopete correctly indicates, NaCN would substitute the diazo group before it would react with Br. (N2+ is the best leaving group in the world)

Here are two options for you to install a CN group:

1) You can convert Ar-N2+ to Ar-CN using CuCN, a variant of the Sandmeyer reaction.

2) You can covert amide, Ar-CONH2, to Ar-CN by dehydration. P2O5 or SOCl2 are often used for this.

Of course, you will need to figure out for yourself at what step you should use one of these

[Altered State]

Thank you both.

This is the final mechanism I worked out:


Benzene  ---(HNO3/H2SO4)--> Nitro-benzene ---(Reduction H2 Pd/C)---> Aniline ---(Cl2)---> para-chloroaniline ---(NaNO2, HCl, 0ºC)--> Cl-Ar-N2+  ---(CuCN)---> Cl-Ar-CN ---(NaOH)---> CN-Ar-Cl

But since you have gave me few ways to do some of the steps (and I love it), I'm not sure this is the best one.
Any clarification would be apreciated.

[opsomath]

I would suggest making the OH group by hydrolysis of a diazonium (Ar-N2+, heat with water) -> Ar-OH instead of Ar-SN2

[Altered State]

I would suggest making the OH group by hydrolysis of a diazonium (Ar-N2+, heat with water) -> Ar-OH instead of Ar-SN2

So by adding 2 aniline group and converting in diazonium, 1 heated with water and another one to get CN group using CuCN? (obviously in different steps, first I would obtain phenol and then add another NH2 to get it transformed into CN).

May you write the whole mechanism if this is not what you ment, please?

[opsomath]

That could work. Basically put in a nitro group instead of chlorine in your last synthesis, then reduce it and convert it into an OH later. That will get complicated though.

Honestly, have you thought about just putting on the chlorine first? See where that takes you.