= Monosaccharide ring-chain interconversion
In <water>, D-<glucose> mainly occupies cyclic forms. The C5 <hydroxy group> adds to the C1 <aldehyde>, forming a cyclic <hemiacetal>; reversible opening restores the <aldehyde>. Reclosure can give either <anomer>. Their changing proportions cause <mutarotation>, the approach of <optical rotation> to an equilibrium value. This pathway involves changing <chemical bonds>, rather than simply rotating a <hydroxy group> about a single <chemical bond>.<Organic Chemistry by John McMurry/25.5 Cyclic Structures of Monosaccharides: Anomers>[McMurry, Organic Chemistry, §25.5: intramolecular hemiacetal formation and final mutarotation discussion]{ref}
\Image[/glucose-anomers-ring-chain-connection.png]
{title=Two D-<glucose> <anomers> connect through the same open-chain <aldose>}
{description=Original <Haworth projections> share an open-chain <Fischer projection>. Both rings have the C5 $\mathrm{CH_2OH}$ group up and C2/C3/C4 <hydroxy groups> down/up/down. The C1 <hydroxy group> is down in alpha-D-glucopyranose and up in beta-D-glucopyranose. Ring <carbon>-<hydrogen> <chemical bonds> are omitted. Arrows indicate reversible connectivity, not measured <reaction rates> or relative populations. The <Haworth projection> is symbolic; actual rings are puckered.}
In this conventional <Haworth projection>, C5's $\mathrm{CH_2OH}$ group points up. The alpha <anomer>'s C1 <hydroxy group> points down, on the opposite face; the beta <anomer>'s points up, on the same face. C2/C3/C4 retain down/up/down <hydroxy groups>. These are above/below relationships to a reference ring plane, not a claim that the ring is physically flat.<Nomenclature of Carbohydrates (Recommendations 1996) by Alan D. McNaught/2-Carb-5. Cyclic forms and their representation>[Nomenclature of Carbohydrates (Recommendations 1996) by Alan D. McNaught; IUPAC–IUBMB, Nomenclature of Carbohydrates (1996), 2-Carb-5.4 and 5.7: Haworth representation and nonplanar conformations]{ref}
The <anomeric carbon> is C1, the existing <carbonyl group> <carbon> that becomes a new <stereocenter>; no extra <carbon> <chemical atom> appears. The two <anomers> differ at that centre while retaining the other configurations.<Nomenclature of Carbohydrates (Recommendations 1996) by Alan D. McNaught/2-Carb-6. Anomeric forms; use of α and β>[Nomenclature of Carbohydrates (Recommendations 1996) by Alan D. McNaught; IUPAC–IUBMB, Nomenclature of Carbohydrates (1996), 2-Carb-6.1–6.2: anomeric centre and configurational relationship]{ref}
Trace the ring as C1–C2–C3–C4–C5–O5–C1, where O5 denotes the <oxygen> originally in the C5 <hydroxy group>. It contains five <carbon> <chemical atoms> and one <oxygen>; C6 remains attached outside the ring. The C1 <carbonyl group> <oxygen> becomes the C1 <hydroxy group> <oxygen>. This accounts for every <oxygen>: O5 in the ring and five <hydroxy groups> at C1, C2, C3, C4 and C6. Ring closure preserves $\mathrm C_6\mathrm H_{12}\mathrm O_6$. A six-membered ring and a six-<carbon> <sugar> are thus different counts.
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