(a) Electrophiles : The word electrophile is derived from electron + philia. ‘Philia’ refers to love/affinity/attraction. Thus, any species which has an attraction or affinity for electrons is termed as an electrophile. Species that qualify as electrophiles may either be positively charged or without any formal charge, i.e., they are electron deficient species which do not have a complete octet around them. Examples include group 13 halides like BCl3,BF3,AlCl3, etc., or N+O2,H+,CH3C+O, etc.
(b) Nucleophiles : ‘Nucleo’ refers to any positively charged centre. Thus, species that are attached to or have an affinity for a positively charged centre are termed as nucleophiles. (Nucleo + phile = Nucleus loving). Nucleophiles may either be negatively charged such as Cl–,Br–,I–,F– , CH3COO–,OH–,CN–, etc. or be electron rich species without any negative charge. The best example of such a nucleophile is H2O molecule. Here, the oxygen atom has two lone pairs of electrons and therefore acts as a nucleophile.