In the account in Gen 2-3 both Adam and Eve sin by disobeying God and eating of the tree of the knowledge of good and evil. However, Eve is not mentioned in Romans 5 (though Paul does mention her elsewhere, e.g. 1 Tim 2:14). This is presumably because Paul is drawing the contrast between Adam and Jesus, disobedience vs. obedience (see Rom 5:15-19). It serves his purpose better to focus on Adam.
This contrast between Adam and Jesus is one uses repeatedly in his letters. He calls Jesus “the last Adam” (1 Cor 15:45). This contrast between the two Adams is beautifully expressed in Philippians 2:5-11. Both Jesus and Adam were made in the “image of God” (v6) but whereas Adam strove to be like God, Jesus did not seek equality with God (v6). Instead he humbled himself and was obedient even to the point of death (v8). So unlike Adam, who was cast out of Paradise, Jesus was exalted and given the name above every name (v9).