The Deoxy-hexose sugars have four potentially acidic sites (i.e., C1-HO1, C2-HO2, C3-HO3 and C4-HO4, see Figure 1c). Based on the predicted acidity values, the OH groups in equatorial position in CH3-pentose sugars have the weakest acidity site. For instance, C4-HO4 site of L-fucose and C2-HO2 site of L-rhamnose have the acidity values of 355 and 369 kcal.mol-1, respectively. The strongest acidic site in L-rhamnose is C3-HO3 with ∆Hacid = 349 kcal.mol-1 and in L-fucose it is C1-HO1 with∆Hacid = 346 kcal.mol-1 (Figure 1a). While, the calculated acidity values (∆Hacid ) of C1-HO1, C2-HO2, C3-HO3 and C4-HO4 sites in aldo-pentose sugars are in the range 343- 355 kcal.mol-1 (Table 2). As seen in Table 2, the strongest acidic sites are C2-HO2 in L-lyxose, C3-HO3 in D-ribose, and C4-HO4 in L-arabinose with the∆Hacid values of 343, 345, 348 kcal.mol-1, respectively (Figure 1b). It is concluded that OH group in the axial position in aldo-pentose is the strongest acidic site and thus the most favored deprotonation site. Moreover, the acidity of ethanol and 2-propanol are 378.3 and 375.1 (kcal.mol-1) in the gas phase, respectively [42]. In comparison with these alcohols, the acidity values of methyl-pentose and aldo-pentose sugars are 346-369 and 343-355 kcal.mol-1 (Table 2); these results suggest the role of H-bonds in increasing the acidity of Deoxy-hexose sugars. As shown in Scheme 1 (as a sample) and Figure 2, the conjugate bases of these sugars are stabilized via intramolecular H-bonds, which are studied in details using AIM and NBO analyses in this study. The geometries of the conjugate bases of these sugars are discussed in the following section.