The alkyl halide required to give butyl sec-butyl ether would be sec-butyl bromide (CH3CHBrCH2CH3). The nucleophile in its conjugate base form that would react with the alkyl halide would be butoxide ion (C4H9O-). The product formed upon the reaction would be butyl sec-butyl ether (CH3CH2CH2OCH(CH3)CH2CH3).