Rank the following dienes in order of increasing stability:

trans-1,3-pentadiene, cis-1,3-pentadiene, 1,4-pentadiene, and 1,2-pentadiene.
rank the dienes from least stable to most stable. to rank items as equivalent, overlap them.

1,2-pentadiene < 1,4-pentadiene < cis-1,3-pentadiene < trans-1,3-pentadiene

Explanation:

1,3-pentadiene have two double bonds which are in conjugation, due to which delocalization occur. Delocalization of electrons more provide stability to 1,3-pentadiene as compared to 1,4-pentadiene and 1,2-pentadiene.

Among cis form and trans form of 1,3-pentadiene, trans form is more stable beacuse of less steric hindrance.

1,2-pentadiene is an example of cumulated diene. 1,4-pentadiene is an isolated diene. Cumulated dienes are less stable as two double bonds are present on the same carbon which leads to high electron density.

So, 1,2-pentadiene will be the least stable.

Order of dienes from least stable to most stable is as follows:

1,2-pentadiene < 1,4-pentadiene < cis-1,3-pentadiene < trans-1,3-pentadiene