A @T06446@ by which one @M03986@ (the @S06082@ ) is converted into another by the formation of one (and only one) two-centre @B00697@ between the substrate and another molecular entity and which involves no other changes in @C01274@ in the substrate. For example, the formation of an acyl @C00907@ by attachment of carbon monoxide to a @C00812@ (R⁺): The product of an attachment may also be the @A00138@ of the two reactants, but not all adducts can be represented as the products of an attachment. (For example, the Diels–Alder @C01496@: results in an @A00138@ of buta-1,3-diene and ethene, but the reaction cannot be described as an attachment since bonds are formed between more than two centres.)