When cis-4-chlorocyclohexanol is treated with sodium hydroxide in ethanol, it gives mainly the substitution product trans-1,4-cyclohexanediol (1). Under the same reaction conditions, trans-4-chlorocyclohexanol gives 3-cyclohexenol (2) and the bicyclic ether (3).

Sodium hydroxide (NaOH) is a strong base, and ethanol is used as a solvent in this reaction. It's important to note that under these conditions, the nucleophilic substitution reaction will dominate over elimination reactions.

In both cis-4-chlorocyclohexanol and trans-4-chlorocyclohexanol, the chlorine atom is the leaving group because it is bonded to a carbon and can depart as a chloride ion (Cl-).

Sodium hydroxide (NaOH) is the source of the nucleophile in this reaction: hydroxide ion (OH-). The hydroxide ion will replace the leaving group (chlorine) in the starting compounds.

When cis-4-chlorocyclohexanol reacts with sodium hydroxide, the hydroxide ion replaces the chlorine atom via an SN2 mechanism to form trans-1,4-cyclohexanediol (1). It is essential to remember that the SN2 mechanism leads to inversion of the stereochemistry at the substitution site.

The reaction of trans-4-chlorocyclohexanol with sodium hydroxide yields two products: 3-cyclohexenol (2) and the bicyclic ether (3). These products result from an elimination reaction instead of a substitution reaction (which occurred in the case of cis-4-chlorocyclohexanol). The hydroxide ion acts as a base, removing a neighboring hydrogen atom, leading to the elimination of chloride and formation of a double bond, yielding 3-cyclohexenol (2). A secondary reaction occurs to form the bicyclic ether (3) through intramolecular nucleophilic attack on the neighboring hydroxyl group. In conclusion, treating cis-4-chlorocyclohexanol with sodium hydroxide in ethanol results mainly in the substitution product trans-1,4-cyclohexanediol (1). However, under the same conditions, trans-4-chlorocyclohexanol produces 3-cyclohexenol (2) and the bicyclic ether (3). This indicates that the stereochemistry of the starting compounds plays a significant role in the reaction outcome.