Re: Why is benzene carcinogenic and toluene not?

Wow, what a great question!
You are absolutely correct: while benzene is carcinogenic, most of its simple derivatives are not. I looked at the Material Safety Data Sheets (MSDS) through Vermont Safety Information Resources, Inc. (SIRI, go to http://www.siri.org/msds/index.html for more) for aniline, benzoic acid, phenol, and toluene, and found none of them to be carcinogenic. For that matter, bromobenzene and chlorobenzene weren't listed as carcinogens; however, both benzene and nitrobenzene were. I noticed that benzene was linked specifically to leukemia, so I went to MEDLINE (via PubMed: http://www.ncbi.nlm.nih.gov/PubMed/) and searched for "benzene" and "leukemia". I found several articles describing the cellular and biochemical connections between benzene and leukemia including one very recent paper by Martyn T. Smith, et al. from Nina Holland's lab at Berkeley goes into some detail on this. They found that it is not benzene but one of its metabolic products, hydroquinone (also used for photographic developers), that may be responsible for causing leukemia by interfering with chromosomal migration during mitosis. This would explain why the substituted forms of benzene are not carcinogens: the substituents prevent the compound from being metabolized to hydroquinone. This may also explain why nitrobenzene is carcinogenic, since the nitrate group is readily released to yield unsubstituted benzene. I'm not sure why phenol wouldn't be metabolized directly to hydroquinone to have the same effect, but it may be that the high toxicity of phenol prevents high enough exposure to become a cancer risk. For more on the chemistry and substitution of aromatic rings, you should consult an organic chemistry textbook: benzene substitution is a BIG topic, and a great deal is known about its chemistry.