Cl2O7 is an endergonic molecule, meaning it is intrinsically unstable, decomposing to its constituent elements with release of energy:
2 Cl2O7 → 2 Cl2 + 7 O2 (ΔH° = –132 kcal/mol)
Cl2O7 is bent with Cl−O−Cl angle of 118.6° giving the molecule C2symmetry. The terminal Cl−O distances are 1.709 Å and the Cl=O distances are 1.405 Å. In this compound, chlorine exists in its highest formal oxidation state of +7, although the bonding in this molecule is significantly covalent.
It also reacts with alkenes to give alkyl perchlorates. For example, it reacts with propene in carbon tetrachloride solution to yield isopropyl perchlorate and 1-chloro-2-propyl perchlorate.
Dichlorine heptoxide is a strongly acidic oxide, and in solution it forms an equilibrium with perchloric acid.
Although it is the most stable chlorine oxide, Cl2O7 is a strong oxidizer as well as an explosive that can be set off with flame or mechanical shock, or by contact with iodine. Nevertheless, it is less strongly oxidising than the other chlorine oxides, and does not attack sulfur, phosphorus, or paper when cold. It has the same effects on the human body as elemental chlorine, and requires the same precautions.
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