Monosaccharides: The Anomeric Centre

These flashcards cover the key vocabulary and concepts regarding monosaccharides, the anomeric centre, mutarotation, and the anomeric effect as presented in the lecture.

Ring-chain tautomerism

The equilibrium between the open-chain and closed-chain (ring) forms of a sugar, also known as tautomerisation.

Anomeric centre

The hemiacetal carbon in a cyclic sugar (such as $C1$ for aldoses or $C2$ for ketoses) that becomes a new stereocentre during the ring-closing reaction.

Anomers

A specific type of epimer and diastereomer where two sugars differ in stereochemical configuration only at the anomeric carbon.

$\alpha$ anomer

The configuration where the hydroxyl ($OH$) group at the anomeric centre is on the opposite face to the group on the other side of the ring oxygen ($O$) that determines $D$ or $L$ notation.

$\beta$ anomer

The configuration where the hydroxyl ($OH$) group at the anomeric centre is on the same face as the group on the other side of the ring oxygen ($O$).

Mutarotation

The change in optical rotation of cyclic sugars that occurs when a pure anomer equilibrates in solution through ring-chain tautomerisation, following an addition-elimination $SN1$-type mechanism.

Anomeric effect

An electronic effect where electronegative substituents ($OH$, $OR$, $F$, $Cl$, $Br$) on a pyranose ring prefer to occupy an axial position rather than an equatorial position.

Hyperconjugation (n ightarrow ext{\sigma}^* donation)

A stabilising interaction in the axial position where a lone pair ($n$) on the ring oxygen is anti-periplanar to the $C-X$ bond and donates into its antibonding orbital ($\sigma^*$).

1,3-diaxial interactions

Steric repulsions that occur between axial substituents and other groups located above or below the plane of the ring, which typically favour the equatorial ($\beta$) anomer.

Reducing sugar

Any sugar (usually a mono- or di-saccharide) containing a hemiacetal moiety that is in equilibrium with an aldehyde form, allowing it to act as a reducing agent towards metal salts.

Benedict's test

A rapid visual test for reducing sugars or aldehydes; a modified version was historically used as a diagnostic test for diabetes to quantify glucose in urine.

Hemiacetal

A functional group formed by the reaction of a carbonyl (aldehyde or ketone) and an alcohol; in sugars, this represents the closed-chain ring form.