# Why are phosphate-phosphate bonds weaker than phosphate-water bonds?

According to the Wikipedia entry on ATP,

ATP is an unstable molecule in unbuffered water, in which it hydrolyses to ADP and phosphate. This is because the strength of the bonds between the phosphate groups in ATP is less than the strength of the hydrogen bonds (hydration bonds), between its products (ADP + phosphate), and water

Why are the bonds between a water and a phosphate stronger than the bonds between two phosphates?

I'm studying first year chemistry, so an explanation that appealed to basic principles would be best.

(I've thought about this for a bit, and looked at the molecular diagrams in hope of seeing the answer. I'd include my thoughts on the matter in this question, however those thoughts amount to a half dozen inconclusive lines of speculation, so I opted to leave them out. Saying that to say, I have put effort into trying to figure out the answer, but I haven't gotten anywhere.)

• Ouch it hurts how wrongly Wikipedia is describing this >__<*
– Jan
Oct 27 '15 at 19:46

ATP is an unstable molecule in unbuffered water, in which it hydrolyses to ADP and phosphate. This is because the strength of the bonds between the phosphate groups in ATP is less than the strength of the hydrogen bonds (hydration bonds), between its products (ADP + phosphate), and water acid anhydrides are generally prone to hydrolysis into two acid molecules.

Ignore that description Wikipedia just gave you, it is just blatantly bad from every perspective.

Phosphorous(V) is generally a strong electrophile and will be readily attacked nucleophilicly by water molecules — think of an $\mathrm{S_N2}$ mechanism. On the other side, $\ce{ROPO3^2-}$ is a good leaving group, too.

While the exact details may or may not be like I presented here, all acid anhydrides can easily hydrolysed similarly, be it disulphonic acid ($\ce{H2S2O7}$), organic acid anhydrides ($\ce{R-CO-O-CO-R}$) etc. Technically, the $\ce{P-O-P}$ bond in ATP/ADP is no different from the bonds in $\ce{P4O10}$ which we all know to be very readily hydrolysed.

• My class has gotten to organic chemistry yet, so $S_{N}2$ bonds aren't comfortable terrain for me. However, I watched the relevant Khan Academy videos after reading your answer. It seems to me that in the phosphate groups in ATP, the phosphorous atom is overloaded with electrons. Consequently, it's nucleophilic. The hydrogens in water are partially positive, so they're electrophilic. As such, bonding with the hydrogen atoms is a more appealing option (so to speak) for the phosphorous atom. Is that, very roughly, the idea?
– Hal
Oct 27 '15 at 22:55
• @Hal No. The phosphorous is extremely deprieved of electrons because it has four very electronegative oxygens clinging onto it, making it very positive. On the other hand, the oxygen in water is, of course, rather negative. So the oxygen's lone pair interacts with phosphorous.
– Jan
Oct 28 '15 at 8:04
• Many thanks. I really wanted to understand this.
– Hal
Oct 28 '15 at 13:04