Nucleophilic addition

In organic chemistry, a nucleophilic addition reaction is an addition reaction where in a chemical compound a pi bond is removed by the creation of two new covalent bonds by the addition of a nucleophile.

Addition reactions are limited to chemical compounds that have multiply-bonded atoms:

• molecules with carbon – hetero double bonds like C=0 or C=N
• molecules with carbon – carbon double bonds or triple bonds

Addition to carbon – hetero double bonds

Addition reactions of a nucleophile to carbon – hetero double bonds such as C=0 or C=N show a wide variety. These bonds are very polar (large difference in electronegativity) and carbon carries a partial positive charge. This makes this atom the primary target for the nucleophile.

YH + R₁R₂C=0 → YR₁R₂C-O^- + H⁺ → YR₁R₂C-OH

This type of reaction is also called a 1,2 nucleophilic addition. The stereochemistry of this type of nucleophilic attack is not an issue, when both alkyl substituents are dissimilar, the reaction product is a racemate. Addition reactions of this type are numerous. With a carbonyl compound as an electrophile, the nucleophile can be:

• water in hydrolysis to a gemdiol
• an alcohol in acetalisation to an acetal
• an hydride in reduction to an alcohol
• an aliphatic amine to a imine
• an aromatic amine to a Schiff base
• a enolate ion in a Aldol condensation

With miscelleneous electrophiles:

• hydrolysis of a nitrile to a amide or a carboxylic acid
• addition of an alcohol to a isocyanate to form a urethane

Addition to carbon – carbon double bonds

Y-Z + -C=C- → Y-C-C-Z

The driving force for this reaction is the formation of an nucleophile Y^- that forms a covalent bond with a electron-poor unsaturated system -C=C- (step 1). The negative charge on Y is transferred to the carbon – carbon bond.

step (1) Y^- + -C=C(X)-Z → Y-C-C(X)^--

In step 2 the negatively charged carbanion combines with (Z) that is electron-poor to form the second covalent bond.

step (2) Y-C-C(X)^-- + Z → Y-C-C(X)-Z

Ordinary alkenes are not susceptible to a nucleophilic attack (apolar bond). Styrene reacts in toluene with sodium to 1,3-diphenylpropane ^[1] through the intermediate carbanion:

Ph-CH₃ + Na → Ph-CH₂^- + NaH

Ph-C=CH₂ + Ph-CH₂^- → Ph-CH₂CH₂CH₂-Ph

When X is a carbonyl group like C=0 or COOR or a cyanide group (CN), the reaction type is a conjugate addition reaction. The substituent X helps to stabilize the negative charge on the carbon atom by its inductive effect.

In addition when Y-Z is a active hydrogen compound the reaction type a Michael Reaction.

References

• ^[1] Sodium-catalyzed Side Chain Aralkylation of Alkylbenzenes with Styrene Herman Pines, Dieter Wunderlich J. Am. Chem. Soc.; 1958; 80(22)6001–6004.
• March jerry; (1885). Advanced Organic Chemistry reactions, mechanisms and structure (3rd ed.). New York: John Wiley & Sons, inc. ISBN 0–471–85472–7