Statistical Distribution
One cheminformatics study identified 849,574 unique substituents up to 12 non-hydrogen atoms large and containing only C,H,N,O,S,P,Se and the halogens in a set of 3,043,941 molecules. Fifty common substituents are found in only 1% of this set, and 438 in 0.1%. 64% of the substituents are unique to just one molecule. The top 5 consists of the phenyl, chlorine, methoxy, hydroxyl, and ethyl substituent. The total number of organic substituents in organic chemistry is estimated at 3.1 million, creating a total of 6.7×1023 molecules. An infinite number of substituents can be obtained simply by increasing carbon chain length. For instance, the substituents methyl (-CH3) and pentyl (C5H11).
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