What is the mechanism of an aldehyde reacting with fehlings solution and tollens reagent?
I know it is oxidised to a carboxylic acid, but i want to know the mechanism.
Thank you.
yes
We can't draw mechanisms/diagrams/pictures on this forum. I suggest you look at google. I'm sure there is on on the web. Try this site. I see some electrons being pushed. If this does'nt help use google.
https://www.google.com/search?q=mechanism+aldehyde+fehling%27s+solution&client=firefox-a&hs=oiD&rls=org.mozilla:en-US:official&channel=sb&tbm=isch&tbo=u&source=univ&sa=X&ei=LxQ6VLeQMsPIggTQ2ILYAw&ved=0CCIQsAQ&biw=1024&bih=609
When an aldehyde reacts with Fehling's solution or Tollens reagent, it undergoes oxidation to form a carboxylic acid. Let's look at the mechanisms for both reactions:
1. Fehling's solution:
Fehling's solution is a mixture of copper(II) sulfate (CuSO4) and sodium potassium tartrate (NaKC4H4O6). The reaction with an aldehyde occurs in two successive steps:
Step 1: Formation of Cu(I) intermediate
- The aldehyde (RCHO) acts as a reducing agent, and it is oxidized to a carboxylate anion (RCOO-) while reducing the Cu(II) in CuSO4 to Cu(I).
- This step involves the transfer of two electrons from the aldehyde to the Cu(II) ion, resulting in the formation of an unstable Cu(I) intermediate.
Step 2: Formation of carboxylic acid
- The Cu(I) intermediate reacts with hydroxide ions (OH-) present in the Fehling's solution, forming a copper(I) hydroxide precipitate (Cu2O) and regenerating the Cu(II) ion.
- Simultaneously, the carboxylate anion (RCOO-) reacts with Cu(II), forming a water-soluble copper(II) carboxylate.
- Overall, the aldehyde is oxidized to a carboxylic acid, while the Cu(II) in Fehling's solution is reduced to Cu2O.
2. Tollens reagent:
Tollens reagent consists of silver nitrate (AgNO3) and ammonia (NH3) in aqueous solution. The reaction with an aldehyde proceeds through the following steps:
Step 1: Formation of diammine silver(I) complex
- The aldehyde is oxidized by the silver ion (Ag+) present in Tollens reagent to form a carboxylate anion (RCOO-) and reduce Ag+ to Ag.
- This step involves the transfer of two electrons from the aldehyde to Ag+ ions, resulting in the formation of an unstable Ag(I) intermediate.
Step 2: Auto-reduction of Ag(I) complex and formation of silver mirror
- The Ag(I) intermediate, also known as Tollens' silver complex, undergoes auto-reduction in the presence of excess ammonia (NH3).
- This self-reduction process generates silver metal (Ag), which deposits as a shiny silver mirror on the inside of the reaction vessel.
- Simultaneously, the hydroxide ions (OH-) from the excess ammonia react with the carboxylate anion (RCOO-), producing a water-soluble salt of the carboxylic acid.
- Overall, the aldehyde is oxidized to a carboxylic acid, while Ag+ in Tollens reagent is reduced to metallic silver.
It is important to note that both Fehling's solution and Tollens reagent are commonly used to distinguish aldehydes from ketones. Aldehydes are reactive and easily oxidized, while ketones do not undergo these reactions.