In an organic reaction, the organic compound called the substrate reacts with a suitable attacking species called the reagent to form products.
The formation of products may occur either directly from the reactants through a transition state or through the formation of one or more intermediates.
Sometimes by products are also formed from intermediates.
Organic molecules ————–> [Intermediates]——–> Products + By products
Substrate is that reactant which supplies carbon for the formation of a new bond while the other reactant is called the reagent.
A Sequential account of each step, describing details of electron movement, energetics during bond cleavage and bond formation and the rates of transformation of reactants into products is called the reaction mechanism.
Electrophiles are electron loving chemical species. Their attraction for electrons is due to the presence of an electron deficient atom in them. Electrophiles may be either positively charged or electrically neutral chemical species i.e.
Positive electrophiles: H+ , Cl+, Br+, I+, NO2+, NO+ , R+
Neutral electrophiles: R· , CR2, SO3, BF3, AlCl3 , FeCl3 , SnCl4
Since both positively charged and neutral electrophiles are short by a pair of electrons they have a strong tendency to attract electrons from other sources and hence behave as Lewis acids.
Electrophiles always attack the substrate molecule at the site of highest electron density.
All the positively charged species do not always act as electrophiles. Only those positively charged species which can accept a pair of electron can act as electrophiles. Thus NH4+ , Na+, Ca2+ cannot act as electrophiles since all of these already have an octet of electron in their respective valence shells.
Nucleophiles are nucleus loving chemical species.
Since the nucleus of any atom is positively charged, therefore, nucleophiles must be electron rich chemical species containing at least one lone pair of electrons.
They may be either negatively charged or neutral chemical species:
Negative nucleophiles: – H‾, BH4‾ , AlH4‾ , Cl‾, Br‾, I‾, R‾, OH‾, CN‾, C6H5O‾
Neutral nucleophiles: H2O , NH3, RNH2, R2NH, ROH, RSH, ROR
Both negatively charged and neutral nucleophiles contain at least one unshared pair of electrons, they have a strong tendency to donate this pair of electrons to electron deficient species and hence behave as Lewis bases.
Nucleophiles always attack the substrate molecule at the site of lowest electron density.