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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Gius on April 07, 2007, 11:37:40 PM

Title: Fehling's Solution- Strange results
Post by: Gius on April 07, 2007, 11:37:40 PM

I was working with Fehling's solution in the lab today with several unknown compounds. 

One of them, which was aqueous, was clear and colourless....its structure is either that of formaldehyde or a 4 carbon ketone/aldehyde

The results that baffled me was the fact that when the unknown was mixed with fehling's at room temperature, no reaction occurred.   As soon as it was heated (after a minute), the cupric precipitate formed.  I just don't understand why heat was needed for the reaction to occur.  To my knowledge, heat would be needed to dissolve the unknown...but the unknown was already aqueous!!

I was debating whether it had to do anything with the compound (specifically the 4 C compound) converting between a ring and a hemiacetal...with the hemiactal doing the reducing (I believe)...would this reaction be driven by heat?

help me out guys.  Thanks

Title: Re: Fehling's Solution- Strange results
Post by: Gius on April 07, 2007, 11:39:58 PM

Normally I would take an IR...but the solution was aqueous and would ruin the salt disks for the IR

Title: Re: Fehling's Solution- Strange results
Post by: Yggdrasil on April 08, 2007, 02:33:15 AM

Many reactions won't occur at room temperature because the activation energy of the reaction is too high for the reaction to proceed at a significant rate.  By increasing the temperature of the reaction mixture, you increase the number of molecule in the reaction with sufficient kinetic energy to overcome the activation energy barrier and react.  This increases the rate of reaction and allows for the formation of an observable amount of product.

Title: Re: Fehling's Solution- Strange results
Post by: Gius on April 08, 2007, 01:23:20 PM

I'm not sure if it is as simple as that....but that is the best guess I've heard thus far.  I thought Fehling's reactions had low activation energy...I could be wrong

Title: Re: Fehling's Solution- Strange results
Post by: Yggdrasil on April 08, 2007, 02:22:50 PM

At least according to Wikipedia (http://en.wikipedia.org/wiki/Fehling_solution) Fehling's test requires heating.

Title: Re: Fehling's Solution- Strange results
Post by: Gius on April 08, 2007, 10:35:03 PM

Quote from: Yggdrasil on April 08, 2007, 02:22:50 PM

At least according to Wikipedia (http://en.wikipedia.org/wiki/Fehling_solution) Fehling's test requires heating.

Yeah, it usually does if the compound of interest is insoluable...especially aldehydes

Title: Re: Fehling's Solution- Strange results
Post by: Gius on April 09, 2007, 02:37:38 AM

Despite the fact that I said it is either formaldehyde or a 4 Carbon compound, I'm starting to think its actually glucose

does it make sense that heat is needed to drive the equilibrium of glucose from cyclic form to straight chain...the straight chain is the reducing form...which is what's needed

make sense?