American chemist whose studies demonstrated that carbon can have a valence (i.e., affinity for electrons) of two as well as a valence of four, thus greatly advancing the understanding of theoretical organic chemistry.

Brought to the United States by his father, Nef studied at Harvard University (A.B., 1884) and at the University of Munich (Ph.D., 1886), where he was a student of Alfred von Baeyer. After working a year in Baeyer's laboratory, Nef taught at Purdue University from 1887 to 1889 and at Clark University from 1889 to 1892, when he resigned to become professor of chemistry at the University of Chicago. Nef was a leader in establishing graduate study in the United States, bringing with him standards and techniques of the university organic-chemistry laboratories of Europe.

Nef's major research was in the chemistry of isocyanides, nitroparaffins, and fulminates, from the last of which came his work on carbon valence. His research resolved a disagreement between the German chemist Friedrich A. Kekule von Stradonitz (Kekule von Stradonitz, August), who had proposed the single valence of carbon as four, and Scottish chemist Archibald S. Couper (Couper, Archibald Scott), who proposed the variable valences of carbon as four and two. Nef's findings also enhanced the value of Couper's system of writing the structural formulas of organic compounds.