The mechanism for the nucleophilic substitution reaction between 1-bromobutane and potassium cyanide in an aprotic solvent is an SN2 Substitution Nucleophilic Bimolecular reaction. Here's a step-by-step description of the mechanism:1. Potassium cyanide KCN dissociates into potassium K+ and cyanide CN- ions in the aprotic solvent. The cyanide ion acts as a nucleophile due to its negative charge and the presence of a lone pair of electrons on the carbon atom.2. The cyanide ion attacks the electrophilic carbon atom the one bonded to the bromine in 1-bromobutane from the backside, opposite to the leaving group Br- . This backside attack leads to the inversion of the stereochemistry at the electrophilic carbon.3. As the cyanide ion forms a bond with the electrophilic carbon, the carbon-bromine bond starts to break, and the bromide ion Br- is released as a leaving group.4. The final product is butanenitrile also known as butyronitrile , with the bromide ion and potassium ion as side products.Now, if the solvent is changed to a protic solvent, the reaction rate of the SN2 reaction will decrease. This is because protic solvents can form hydrogen bonds with the nucleophile CN- , which increases the solvation shell around the nucleophile and hinders its ability to attack the electrophilic carbon. This stabilization of the nucleophile by the protic solvent reduces its reactivity, leading to a slower reaction rate for the SN2 reaction.