There has been some discussion lately about alternative developers. The following rule first formulated by Kendall and later extended by Pelz enables the determination as to whether a particular organic chemical is a developer or not. In order for a chemical to be a developer it must fit the rule A - (C=Z)n - A' A and A' must be a chemical group - OH, -NH2, - NHR, -NRR', where R and R' are various substitution groups C is carbon Z is either carbon C or nitrogen N n is 0 or any integer and represents the number of carbon-carbon or carbon-nitrogen double bonds between the groups A and A' The rule is not perfect and there are some exceptions but works remarkably well. Examples are, group (a) HO - (C=C)n - OH when n is 1 you have catechol. when n = 2 you have hydroquinone. group (b) HO - (C=C)n - NH2 when n = 0 you have hydrazine when n = 2 you have paraminophenol when n = 2 and A' is the substituted amino group - NHCH3 you have Metol. group (c) H2N - (C=C)n - NH2 includes the phenylenediamines group (d) HO - (C=N)n - NH2 includes the phenidones The usual keto structural fragment of the heterocyclic ring O=C - N H must be rewritten as the enolic form HO - C=N - for the phenidones. This form shows the - OH group and the C=N bond necessary to fit the rule. group (e) H2N - (C=N)n - NH2 Superadditivity The combined activity of two developing agents is greater than the sum of each of them used individually. While not part of the KP rule, for any two developing agents to exhibit superadditivity one must be of group (a) and the other from group (b) or group (d). Examples of superadditive mixtures are hydroquinone and Metol and hydroquinone and phenidone. Taken from Mason "Photographic Processing Chemistry."