Types of Hydrogen in Isopropylbenzene

Isopropylbenzene's molecular formula, C9H12, suggests that only two different components make up this compound: carbon and hydrogen. This is only partially correct, however. As a result of their orientations with respect to different carbons, individual hydrogens within isopropylbenzene are considered to be "of a different order." Practically speaking, this means certain hydrogens are more susceptible to a common reaction known as "brominization" than others and, as such, are considered to be different entities.

Aromatic Hydrogens

• The most reactive variety of hydrogen found in isopropylbenzene is "aromatic" hydrogen, of which there are five, each bonded to one of the carbons in benzene's hexagonal "ring." When isopropylbenzene is placed in the presence of a compound that contains bromine, these five aromatic hydrogens are the bromine atoms' easiest target. Spatially, benzene's rigid ring shape derives from the double bonding of all its carbons to one another; the hydrogens are left protruding from the main structure.

Aliphatic Hydrogens

• Next along the chain are the "aliphatic" hydrogens, which bond to the two carbon atoms of isopropylbenzene that are neither part of the benzene ring nor directly bonded to it. As these carbons come off the so-called "benzylic" carbon that connects the isopropyl radical (C3H7) to the benzene ring (C6H5), their respective hydrogens, like the aromatic ones, are left vulnerable to any bromine atoms floating in solution nearby. Aliphatic hydrogens occur in three varieties -- primary, secondary and tertiary -- of which those found in isopropylbenzene qualify as the first.

Benzylic Hydrogens

• The least reactive of all hydrogens in isopropylbenzene is the lone "benzylic" hydrogen, which is bonded to the carbon connecting the isopropyl radical to the benzene ring. As is the case with aliphatic hydrogens, benzylic hydrogens occur in primary, secondary and tertiary orders -- and since isopropylbenzene's benzylic carbon has but one hydrogen coming off it, this hydrogen is considered tertiary. It is the most stable hydrogen in the entire compound, shielded by a benzene ring on one side and a T-shaped isopropyl radical on the other.