= N-acetylgalactosamine
{c}
<N-acetylgalactosamine> is an <amino sugar> derivative whose carbon2 nitrogen carries an <acetyl group>. In biological contexts the D sugar is usual; the systematic framework is 2-acetamido-2-deoxygalactose. The neutral free sugar has <molecular formula> $\mathrm{C_8H_{15}NO_6}$. A broad name and a specific beta-D anomer are different levels of chemical specification.<N-acetylgalactosamine in ChEBI>[CHEBI:28800 and narrower CHEBI:28037; beta-specific CHEBI:28497]{ref}
An original atom-counting exercise starts with <galactose>, $\mathrm{C_6H_{12}O_6}$. Replacing its carbon2 $\mathrm{OH}$ by $\mathrm{NH_2}$ gives <galactosamine>, $\mathrm{C_6H_{13}NO_5}$. Replacing one nitrogen-bound hydrogen with an <acetyl group> adds $\mathrm{C_2H_2O}$ overall, producing $\mathrm{C_8H_{15}NO_6}$. This is structural bookkeeping, not a proposed one-step laboratory synthesis.<Nomenclature of Carbohydrates (Recommendations 1996) by Alan D. McNaught>[Sections2-Carb-14 and2-Carb-25: amino-deoxy sugars and nitrogen substitution]{ref}
<N-acetylgalactosamine> and <N-acetylglucosamine> differ in carbon4 configuration. That <epimerization> is separate from changing anomeric configuration or adding/removing the <acetyl group>. A common formula alone cannot identify which arrangement is present.
In <dermatan sulfate>, residues of <N-acetylgalactosamine> form part of the linked sugar chain. A linked residue, the free monomer and an activated nucleotide-sugar donor should not be assigned the same complete molecular formula merely because they share this sugar name.<Dermatan sulfate in ChEBI>[CHEBI:18376: uronic-acid/amino-sugar repeat description]{ref}
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