The chemistry of alkyl dihalides in zeolite NaX at room temperature

Alkyl dihalides adsorbed in NaX zeolite undergo room temperature, substitutional dehalogenation, single dehydrohalogenation, combined dehalogenation-dehydrohalogenation, or two consecutive dehydrohalogenations. The preference for a particular reaction depends on the nature and relative positions of the halogens, and on alkyl chain length. Nuclear magnetic resonance chemical shifts and mass spectrometric assignments to the reagents and products indicate the particular reactions taking place. All of the alkyl dihalides containing the iodo group (diiodoalkanes) studied (alpha alpha, alpha beta, alpha gamma) undergo only substitutional dehalogenation to form zeolite bound iodoalkoxy. The same chemistry was observed with both short and long chain diiodoalkanes. The dichloroalkane and dibromoalkane chemistry is dependent on the relative halogen positions and the alkyl chain length. The alpha, alpha dichloroalkanes and dibromoalkanes examined by us undergo single dehydrohalogenation, with the exception of 2.2-dichloropropane, which forms a mixture of dehydrohalogenation and substitutional dehalogenation products. The chemistry of alpha, beta alkyl dihalides (Cl, Br) in NaX is dependent on the carbon chain length. The dihaloethanes (Cl, Br) undergo both substitutional dehalogenation and dehydrohalogenation. As the chain length increases to three carbon atoms or more, only dehydrohalogenation reactions take place. No alkyne products were observed with a, p dichloroalkanes and dibromoalkanes, an indication that NaX is too weak a base to initiate sequential dehydrohalogenations. The chemistry of alpha, gamma dichloroalkanes and dibromoalkanes were also dependent on the chain length. The alpha, gamma dihalopropanes (Cl, Br) in NaX form a propene-framework bound product, associated with dehydrohalogenation and subsequent substitutional dehalogenation. The alpha, gamma dihalobutanes (Cl, Br) undergo two consecutive dehydrohalogenations to form dienes. (C) 2008 Elsevier Inc. All rights reserved.