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When doing a distinguishing test between aldehydes and ketones using Tollen's reagent, is it necessary to convert $\ce{Ag+}$ ions into $\ce{Ag(NH3)2+}$ in order to oxidize the aldehyde?

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Yes, it is important to convert your silver ion to diamminesilver(I) complex. If you do not add ammonia to silver nitrate, the silver ion is reduced so quickly that colloidal silver metal would appear and the solution would become a black, cloudy liquid and you won't get the silver mirror when it reacts with aldehyde.

It is also important that you maintain the pH of the reaction. During the preparation of Tollen reagent, you will see the use of sodium hydroxide. This is very important for maintaining the pH. During the whole reaction, the pH is between 8-10 and it goes even faster if the pH is beyond 10. The reaction also becomes more sensitive. This has been explained in details in this journal by going through the exact mechanism of the reaction.

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    $\begingroup$ Since a black cloudy liquid is produced, would it mean that Ag+ can still be used to distinguish between aldehydes and ketones? The only difference is that now the observation is no longer a silver mirror $\endgroup$
    – john
    May 24 at 5:36
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    $\begingroup$ @john No, it will create a false indication. The clear indication for the presence of aldehyde is the formation of mirror. If you are using only silver ions, then it will give the cloudy solution irrespective of the organic compound you are using. So, you can't distinguish aldehyde from the other organic compounds. So, presence of ammonia and basic pH is important for the formation of silver mirror. $\endgroup$ May 24 at 6:00

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