Aliphatic and aromatic compounds exhibit different reactivity patterns when subjected to nucleophilic substitution reactions due to their distinct structural and electronic properties.Aliphatic compounds are open-chain hydrocarbons that can be saturated alkanes or unsaturated alkenes and alkynes . They generally undergo nucleophilic substitution reactions more readily than aromatic compounds. The reactivity of aliphatic compounds in nucleophilic substitution reactions depends on the type of carbon atom being attacked primary, secondary, or tertiary and the nature of the leaving group.For example, in a primary alkyl halide like ethyl bromide CH3CH2Br , the nucleophilic substitution reaction can occur via two mechanisms: SN1 or SN2. In the SN2 mechanism, a nucleophile e.g., hydroxide ion, OH- attacks the carbon atom bonded to the bromine atom, leading to the formation of ethanol CH3CH2OH and the release of a bromide ion Br- . The reaction occurs in a single concerted step with a transition state where the nucleophile and the leaving group are both partially bonded to the carbon atom.Aromatic compounds, on the other hand, are characterized by a ring of atoms with delocalized -electrons, which results in a stable, resonance-stabilized structure. This stability makes aromatic compounds less reactive towards nucleophilic substitution reactions compared to aliphatic compounds. The aromatic ring tends to undergo electrophilic substitution reactions rather than nucleophilic substitution reactions.However, nucleophilic substitution can occur in aromatic compounds if the ring has an electron-withdrawing group e.g., nitro group that activates the ring for nucleophilic attack. For example, in 2,4-dinitrochlorobenzene, the nitro groups -NO2 withdraw electron density from the benzene ring, making the carbon atom bonded to the chlorine atom more susceptible to nucleophilic attack. In this case, a nucleophile e.g., hydroxide ion, OH- can attack the carbon atom, leading to the formation of 2,4-dinitrophenol and the release of a chloride ion Cl- . This reaction is called nucleophilic aromatic substitution SAr .In summary, aliphatic compounds are generally more reactive towards nucleophilic substitution reactions than aromatic compounds due to their lack of resonance stabilization. Aromatic compounds can undergo nucleophilic substitution if they have electron-withdrawing groups that activate the ring for nucleophilic attack.