# Ambigious nature of aldehydic group in glucose

In aqueous solution, α-anomer and β-anomer of glucose remain in equilibrium with each other along with a small amount of glucose in open chain form ($0.02~\%$). The open chain form in the solution contains a free aldehydic group and hence gives several reactions involving free aldehyde.

Though little, the open chain form of glucose reacts with many reagents which are used to treat free aldehydic groups. Therefore, glucose in aqueous solution also reacts with those reagents.

$$\ce{glucose_{(aq)} + HCN -> gluco cyanohydrin}$$

$$\ce{glucose_{(aq)} + H2NOH -> gluco oxime}$$

$$\ce{glucose_{(aq)} + H2NNHPh -> gluco phenylhydrozone}$$

$$\ce{glucose_{(aq)} + 3H2NNHPh-> gluco osazone}$$

$$\ce{glucose_{(aq)} + Br2_{(aq)}-> gluconic acid}$$

Glucose also answers Tollen's reagent test, Fehling's solution test and Benedict's solution test.

However, as interesting it may seem, glucose is a two-face molecule. Here are some reactions which glucose does not like:

$$\ce{glucose_{(aq)} + NaHSO3 -> no reaction}$$

$$\ce{glucose_{(aq)} + {2,4-DNP} -> no reaction}$$

$$\ce{glucose_{(aq)} + {Schiff's reagent} -> no reaction}$$

Why is that glucose does not react with $\ce{NaHSO3}$ or $\ce{{2,4-DNP}}$ or Schiff's reagent while it answers many other reactions where the free aldehydic group is involved?

I am looking for a convincing answer. Most answers in web tell that some are strong and some aren't strong enough. If you give enough time, all the carbonyl reactions should work with glucose because there will always be $0.02~\%$ in open chain form. If some of it gets used up, then the equilibrium will shift and hence more open chain form of glucose will be formed.

• glucose does react with brady's reagent to give an osazone,do make the correction – Supernova Mar 20 '17 at 10:54
• My textbook says it doesn't. Can you cite some reference? I would expect it to react with DNP. Isn't the mechanism for reaction with phenylhydrazine with glucose same the reaction of DNP with glucose? So, if glucose can form hydrazones and osazones then it should react with Schiff's reagent, DNP, etc. too. – Yashas Mar 20 '17 at 10:56
• So you say that it will work. Agreed, but given enough "time" is the crunch factor here. A bit of math using the Arrhenius formulae, considering the extremely low collision frequency, would lead that time factor to be around $\ce{4.6 * 10^10}$ seconds – Supernova Mar 20 '17 at 10:57
• chemistry.stackexchange.com/questions/22269/does-glucose-react-with-bradys-reagent – Supernova Mar 20 '17 at 10:59
• I do agree that DNP reacts with carbonyl compounds but glucose seems to show a weird behavior. The answerer of that question quoted text from the DNP wiki page (which says that DNP reacts with carbonyl compounds) and then wrote that glucose should, therefore, react with it. He/She could be someone just like me who expects glucose to react in such a manner. But as you see, glucose behaves differently, we should be careful before drawing such conclusions. – Yashas Mar 20 '17 at 11:06