Compounds of Halogens: Oxides and Oxoacids

Chlorine, bromine and iodine form a range of oxides, most of which are unstable, powerfully

oxidising, and (in several cases) explosively decompose. Dichlorine oxide ($\text{Cl}_2\text{O}$,

Cl at $+1$) is a yellowish-brown gas, obtained by passing chlorine over dry mercury(II) oxide,

with a bent structure at oxygen. Chlorine dioxide ($\text{ClO}_2$, Cl at $+4$) is an

odd-electron, paramagnetic, bent molecule, used industrially as a bleaching agent for wood pulp

and, in water treatment, as a disinfectant. Dichlorine heptoxide ($\text{Cl}_2\text{O}_7$, Cl

at $+7$), the anhydride of perchloric acid, is the most stable of chlorine's oxides, formed from

two $\text{ClO}_3$ tetrahedral-type units joined through a single bridging oxygen.

More systematically important is the family of oxoacids, in which chlorine (taken as the

representative halogen) is found in the oxidation states $+1$, $+3$, $+5$ and $+7$:

hypochlorous acid $\text{HOCl}$ ($+1$), chlorous acid $\text{HOClO}$ ($+3$), chloric acid $\text{HOClO}_2$ ($+5$) and perchloric acid $\text{HOClO}_3$ ($+7$). In every member of

this series, the acidic hydrogen is always attached through oxygen (as $\text{Cl-O-H}$, never

directly to chlorine), and chlorine itself is $sp^3$ hybridised throughout, coordinated to one

$-\text{OH}$ group plus a number of additional (non-hydroxyl, typically double-bonded or

resonance-delocalised) oxygens that increases from zero in $\text{HOCl}$ to three in

$\text{HOClO}_3$.

Two clear, opposite trends run across this series as the number of oxygen atoms (and hence

chlorine's formal oxidation state) increases. Acid strength increases,

$\text{HOCl} < \text{HOClO} < \text{HOClO}_2 < \text{HOClO}_3$: each additional, strongly

electronegative oxygen atom withdraws electron density from the O-H bond (making it easier to

lose the proton) and, once the proton has been lost, stabilises the resulting negative charge on

the conjugate-base anion by spreading it over an increasing number of resonance-equivalent

oxygen atoms -- more oxygens to delocalise the negative charge over means a more stable,

lower-energy conjugate base, and hence a stronger acid. Oxidising power falls in exactly the

reverse order, $\text{HOCl} > \text{HOClO} > \text{HOClO}_2 > \text{HOClO}_3$: hypochlorous acid,

with chlorine in its lowest positive oxidation state here, is reduced (gains electrons) most …