4 alkyl and wherein the carbon based moiety can itself be further substituted when a substituent is monovalent such as for example f or cl it is bonded to the atom it is substituting by a single bond when a substituent is more than monovalent such as o which is divalent it can be bonded to the atom it is substituting by more than one bond i e a divalent substituent is bonded by a double bond for example a c substituted with o forms a carbonyl group c u2550 o which can also be written as u201c co u201d u201c c o u201d or u201c c u2550 o u201d wherein the c and the o are double bonded when a carbon atom is substituted with a double bonded oxygen u2550 o group the oxygen substituent is termed an u201c oxo u201d group when a divalent substituent such as nr is double bonded to a carbon atom the resulting c u2550 nr group is termed an u201c imino u201d group when a divalent substituent such as s is double bonded to a carbon atom the results c u2550 s group is termed a u201c thiocarbonyl u201d group alternatively a divalent.