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

UP PGT CHEMISTRY 2019 RESULT

UPPGT 2016 RESULT

         Acidity of Aldehydes and Ketones

           For alkylation reactions of enolate anions to be useful, these intermediates must be generated in high concentration in the absence of other strong nucleophiles and bases. The aqueous base conditions used for the aldol condensation are not suitable because the enolate anions of simple carbonyl compounds are formed in very low concentration, and hydroxide or alkoxide bases induce competing SN2 and E2 reactions of alkyl halides. It is necessary, therefore, to achieve complete conversion of aldehyde or ketone reactants to their enolate conjugate bases by treatment with a very strong base (pKa > 25) in a non-hydroxylic solvent before any alkyl halides are added to the reaction system. Some bases that have been used for enolate anion formation are: NaH (sodium hydride, pKa > 45), NaNH2 (sodium amide, pKa = 34), and LiN[CH(CH3)2]2 (lithium diisopropylamide, LDA, pKa 36). Ether solvents like tetrahydrofuran (THF) are commonly used for enolate anion formation. With the exception of sodium hydride and sodium amide, most of these bases are soluble in THF. Certain other strong bases, such as alkyl lithium and Grignard reagents, cannot be used to make enolate anions because they rapidly and irreversibly add to carbonyl groups. Nevertheless, these very strong bases are useful in making soluble amide bases. In the preparation of lithium diisopropylamide (LDA), for example, the only other product is the gaseous alkane butane.