Tollens' Fehling's and Benedict's Test

 Three Visual “Tests” For The Presence of Aldehydes: Benedict’s, Fehlings, and Tollens’ Tests

We’ve seen previously that aldehydes are a functional group that can be oxidized relatively easily to carboxylic acids. For example, oxidation of alcohols with a “strong” oxidant like chromic acid (H2CrO4) results in an aldehyde that is quickly oxidized further to a carboxylic acid.

During this process, the aldehyde is oxidized and the oxidizing agent is reduced. Another way of framing this is to say that the aldehyde is the reducing agent in this process.

 The list of reagents that can be used to oxidize aldehydes to carboxylic acids is loooong. Of these, a few methods stand out in providing a particularly clear visual indication that the reaction has proceeded to completion. 

Three “visual” tests for aldehydes that you might encounter in an introductory organic chemistry lab are the following:

• Fehling's Solution:- where an aldehyde changes the color of a blue Cu(II) solution to red Cu(I)  [as Cu2O]. 
• Benedict's Solution:- a slightly modified version of Fehling’s solution
• Tollens' Test :- where aldehyde oxidation results in a beautiful “mirror” of silver metal to precipitate on the reaction vessel.

Importantly, ketones don’t react under any of these conditions.  The above tests were also a useful way of distinguishing aldehydes from ketones in the dark days before IR and NMR spectroscopy made this routine.

Carbohydrates: Reducing Sugar

What’s a reducing sugar, and why is it important? Here’s a quick summary. Full details in the post below.

Mutarotation

1-Mutarotation Is The Change In Optical Rotation Observed When Pure α- or β- Anomers Are Dissolved In Water (or other solvents)

In our recent post on ring-chain tautomerism, we said that there are two isomers of D-glucose in its 6-membered ring (“pyranose”) form.

These two diastereomers – which, to make matters more confusing, are called “anomers” in the context of sugar chemistry –  differ in the orientation of the hydroxyl group on C-1. (Note that C-1 is a hemiacetal. )

• ➡️In the “alpha” (α) anomer, the OH group on C-1 is on the opposite side of the ring as the chain on C-5.
• ➡️In the “beta” (β) anomer, the OH group on C-1 is on the same side of the ring as the C-5 substituent.

• Each of these two forms can be synthesized and isolated as pure compounds.

  
  • ●The alpha (α)  anomer of D-glucose has a specific rotation of +112 degrees in water.
  • ●The beta (β)  anomer of D-glucose has a specific rotation of +19 degrees. (18.7 actually, but rounding up to 19).

  Here’s the interesting thing. When either anomer is dissolved in water, the value of the specific rotation changes over time, eventually reaching the same value of +52.5°. 

• • ●The specific rotation of α-D-glucopyranose decreases from +112° to +52.5°.
  • ●The specific rotation of β-D-glucopyranose increases from +19° to +52.5°.

  This behaviour is called mutarotation (literally, “change in rotation”).

• Hold on.  Isn’t specific rotation of a molecule supposed to remain the same?

  Yes – if it is indeed the same molecule! 

  And therein lies the answer to the puzzle. For when the solutions whose specific rotations have changed to +52.5° are analyzed, they are found to no longer consist of 100% alpha (α) or 100% beta (β) anomers, but instead a ratio of alpha (α) (36%) and beta (β) (64% ) isomers.

Calculation of pKa

Calculation of pKa

SN1 and SN2 Reaction

SN1 and SN2 Reaction By Nind Kishor Gupta

Oxide, Superoxide, Peroxide

Oxide, Superoxide, Peroxide