Aliphatic amines contain only H and alkyl substituents. Alkyl and aryl nitro compounds behave differently. Organic polysulfides generally have the formulae RSR, where R = alkyl or aryl. Unlike other alkyl ethers, dimethyl ether resists autoxidation. Alkyl amines characteristically feature tetrahedral nitrogen centers. Typical Lewis bases are conventional amines such as ammonia and alkyl amines. They react with alkyl halides to form ester. Branched alkanes are named as a straight-chain alkane with attached alkyl groups. It also converts alcohols to alkyl chlorides. The reaction with alkyl halides leads to alkylation of the nitrogen atom. Ethylamine undergoes the reactions anticipated for a primary alkyl amine, such as acylation and protonation. Alkenes can be prepared indirectly from alkyl amines. Urea is byproduct of converting alkyl halides to thiols via a S-alkylation of thiourea. Polymerization is initiated by alkyl lithium compounds. Sodium iodide is used for conversion of alkyl chlorides into alkyl iodides.