3-Chloro-1-pentene reacts with sodium ethoxide in ethanol to produce 3-ethoxy-1-pentene. The reaction is second order, first order in 3-chloro-1-pentene and first order in sodium ethoxide. In the absence of sodium ethoxide, 3-chloro-1-pentene reacts with ethanol to produce both 3-ethoxy-1-pentene and 1-ethoxy-2-pentene. The first reaction proceeds via an mechanism. The stereochemistry of the product is . The second reaction proceeds via an mechanism. The stereochemistry of 3-ethoxy-1-pentene is . The stereochemistry of 1-ethoxy-2-pentene is . For the second reaction, draw the structure of the intermediate's resonance contributor that leads to the formation of 3-ethoxy-1-pentene.

In the first case, what we have is an SN2 bimolecular reaction. It should be recalled that sodium ethoxide is a strong nucleophile. Strong nucleophiles lead to SN2 substitution in secondary alkyl halides where SN1 or SN2 mechanisms are probable. The observed mechanism now depends on the actual nucleophile used in the reaction. Since a strong nucleophile is used, 3-ethoxy-1-pentene is obtained as shown in the image attached to this answer.

In the second case, a weaker nucleophile, ethanol is used. The use of a weak nucleophile leads to an SN1 mechanism. The 3-Chloro-1-pentene forms a secondary carbocation as shown in the image attached. This secondary carbocation can re-arrange itself to give the two products; 3-ethoxy-1-pentene and 1-ethoxy-2-pentene as shown in the reaction mechanism in the image attached. The structure of the contributor leading to the formation of 3-ethoxy-1-pentene is clearly shown in the image attached.