2-Bromopropane is treated separately with (i) sodium ethanethiolate in ethanol and (ii) potassium tert-butoxide in tert-butanol, each at $50^{\circ }\mathrm{C}$. The major products are, respectively,

Thiolates are excellent nucleophiles but weak bases (thiols are far more acidic than alcohols), so with a secondary halide they give the SN2 product, the sulfide. tert-Butoxide is a strong, bulky base and a poor nucleophile, so it removes a beta proton in E2 and gives propene. The bulky ether would require SN2 by a hindered nucleophile at a secondary carbon.