= Deoxyribose
{title2=$\mathrm C_5\mathrm H_{10}\mathrm O_4$}
{wiki}
In the usual <DNA> context, <deoxyribose> means 2-deoxy-D-<ribose>, a five-<carbon> <monosaccharide> with <molecular formula> $\mathrm C_5\mathrm H_{10}\mathrm O_4$. Relative to D-<ribose>, the <hydroxy group> at C2 is replaced by <hydrogen>. The locant 2 matters: it identifies which position loses its <oxygen>. See https://iupac.qmul.ac.uk/BlueBook/P10.html#P-102.5.3[Favre and Powell, IUPAC Blue Book, P-102.5.3: deoxy sugars] and https://iupac.qmul.ac.uk/class/nucle.html[IUPAC: nucleic acids and their component sugars].
The paired <Fischer projections> in <ribose> make the change concrete. C2 has <hydrogen> on both sides, while C3 and C4 retain their <hydroxy groups> on the right. C2 is therefore not a <stereocenter>: exchanging its two identical <hydrogen> substituents cannot produce a distinct configuration. The unchanged C3 and C4 still carry <stereochemical> information.
Replacing $\mathrm{OH}$ by $\mathrm H$ removes one <oxygen> but leaves the total <hydrogen> count unchanged: the <hydrogen> belonging to the removed <hydroxy group> is replaced by a <hydrogen> bonded directly to <carbon>. Thus $\mathrm C_5\mathrm H_{10}\mathrm O_5$ becomes $\mathrm C_5\mathrm H_{10}\mathrm O_4$. This comparison describes structures, rather than a complete <chemical reaction> or its mechanism.
In <DNA>, <deoxyribose> is incorporated as a ring in each <nucleotide>, with <phosphate> connections building the backbone. Its C2′ bears <hydrogen> where <RNA>'s <ribose> bears a <hydroxy group>. This small local difference lets the <sugar> structure identify the two kinds of <nucleic acid>; the <nucleobases> supply their sequence information. See https://www.ncbi.nlm.nih.gov/books/NBK26883/[Alberts et al.: nucleotides as the subunits of DNA and RNA].
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