Studies of the electrochemical reduction of 1,5-dibromo-, 1,5-diiodo-, 1-bromo-5-chloro-, and 1-ch]oro-5-iodopentane at carbon electrodes in dimethylformamide containing tetramethylammonium perchlorate have been carried out with cyclic voltammetry and controlled-potential electrolysis. Cyclopentane, arising from the intramolecular cyclization of an electrogenerated 5-halopent-l-yl carbanion, is a significant electrolysis product. Moreover, depending on the identity of the 1,5-dihalopentane and the presence or absence of a proton source, one can obtain substantial yields of n-pentane, 1-pentene, 1-chloropentane, and 1,10-dichlorodecane as well as smaller amounts of cis-and trans-2-pentene, 5-chloro-1-pentene, 1,4-pentadiene, n-decane, 1-decene, n-pentadecane, 1-pentadecene, and n-eicosane. Experiments involving the use of several different deuterium ion or atom donors have been employed to determine the pathways by which these products are formed. In addition, we offer some comparisons concerning the electrochemical behavior of 1,3-dihalopropanes, 1,4-dihalobutanes, and 1,5-diha]opentanes at carbon cathodes.Only a limited amount of research concerning the electrochemistry of 1,5-dihalopentanes has been published, all of which pertains to the electrochemical reduction of 1,5-dibromopentane at mercury cathodes. 1-~ In a polarographic study of the reduction of 1,5-dibromopentane in dimethylformamide containing tetramethylammonium bromide and 5% water, Z~vada et al. 1 concluded that the starting material is reduced in a pair of two-electron steps which cannot be resolved into discrete waves. Rift 2'3 investigated the electrolytic reduction of 1,5-dibromopentane at a mercury pool in dimethylformamide containing tetra-n-butylammonium perehlorate and postulated that n-pentane is produced through a carbanion mechanism. However, different results were reported by Casanova and Rogers, ~ who found that controlled-potential electrolysis of 1,5-dibromopentane at mercury in dimethylformamide containing various tetraalkylammonium salts afforded a dialkylmercury species (CsHIoHgC~HI0) in high yield, whereas constant-current electrolysis of the same starting material gave only n-pentane. Avaca et al. ~ examined the electrochemical behavior of 1,5-dibromopentane in dimethyliormamide or acetonitrile containing various tetraalkylammonium salts at a mercury cathode and observed that controlled-potential eleetrolyses yielded mercury-containing solids with the general formula (C~HIoHg)~.In this paper we discuss the electrochemical reductions of 1,5-dibromo-, 1,5-diiodo-, l-bromo-5-ehloro-, and lchloro-5-iodopentane at carbon cathodes in dimethylformamide containing tetramethylammonium perehlorate; the electrochemistry of the last three 1,5-dihalopentanes has not been described previously. In performing this work, we have avoided the possible formation of organomercury compounds by not using mercury cathodes. We report for the first time the occurrence of electroreduetive intramolecular eyclization of each of the four 1,5-dihalopenfanes ...