When 2-bromopentane is heated with an alcoholic solution of potassium hydroxide (ethanolic KOH), it undergoes dehydrohalogenation ($\beta$-elimination). According to Zaitsev's (Saytzeff's) rule, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms. Thus, pent-2-ene (81%) is formed as the major product over pent-1-ene (19%) . Furthermore, pent-2-ene can exist as cis and trans isomers. The trans-isomer is more stable than the cis-isomer due to lesser steric hindrance between the alkyl groups. Hence, trans-2-pentene is the main product formed.