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 multiple bonds like carbonyls, imines or nitriles
- 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 hydration to a gemdiol (hydrate)
- an alcohol in acetalisation to an acetal
- an hydride in reduction to an alcohol
- an aliphatic amine to an imine
- an aromatic amine to a Schiff base
- a enolate ion in an Aldol condensation
- a Grignard reagent
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
The nitrile group is an electrophile in the Blaise reaction with organozinc nucleophiles and with alcohols in the Pinner reaction.
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.
Perfluorinated alkenes (alkenes that have all hydrogens replaced by fluorine) are highly prone to nucleophilic addition, for example by fluoride ion from caesium fluoride or silver(I) fluoride to give a perfluoroalkyl anion.
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