 Such cross-cutting classifications are frequently found in organic chemistry, where compounds can be classified according to their so-called functional groups. Functional groups are specific combinations of atoms within a molecule that will cause the molecule to engage in certain reactions and to have other physical and chemical properties that are characteristic of that group. For example, alcohols are organic compounds containing a hydroxyl group −OH bound to a carbon atom of an alkyl group or derivative of an alkyl group. Alcohols undergo characteristic reactions such as esterification. Since other hydrogen atoms in an alcohol molecule may be substituted by another functional group, the resulting molecule will have properties characteristic of both functional groups and may be classified accordingly. Benzyl alcohol, C6H5CH2OH (or BnOH), is obtained from methane, CH4, by replacing one hydrogen atom by an alcohol-forming hydroxyl group, −OH, and another by the phenyl group −C6H5 (Ph) (the phenyl group plus the CH2 from the methane is the benzyl group, i.e. Bn is PhCH2). Thus benzyl alcohol may be classified either as an alcohol, or as an aromatic benzene derivative, since it participates in the characteristic reactions of the latter, such as electrophilic aromatic substitution or hydrogenation of the benzene ring. If the hierarchy requirement on a system of natural kinds is correct, then not all these cross-cutting classifications pick out natural kinds. The claim that the hierarchy requirement is too stringent for scientific kinds has been defended by Khalidi (1998) and Tobin (2010b).