Petroleum ether followed by dichloromethane is used to separate fluorene and 9-fluorenone. Why would reversing the order in which these solvents are used be unwise

9-fluorenone binds more strongly to the stationary phase.

Explanation:

9-fluorenone is more polar than fluorene  because it possesses a carbonyl group. Recall that the stationary phase is alumina and alumina is a polar adsorbent. Alumina is a top choice for the separation of components that contain a weakly or moderately polar component.  These polar components  of the mixture are retained for a longer time and more selectively by the adsorbent.

The fluorene is eluted first with the Petroleum ether, this makes it easier to elute the 9-fluorenone with the dichloromethane. The 9-fluorenone will be slower because it is more strongly attracted to the alumina owing to its polar carbonyl group.

Fluorene moves faster because its' polarity is the same as Petroleum ether and also opposite of the alumina. If the order of solvents is reversed and dichloromethane is first used, the 9-fluorenone binds more strongly to the alumnina making it difficult to elute fluorene using Petroleum ether.

away from the curb

explanation:

the wheels would then hit the curb if it started to rolling, decreasing the chance of an accident.