Nucleophilic addition

In organic chemistry, a nucleophilic addition reaction is an addition reaction where a chemical compound with an electron-deficient or electrophilic double or triple bond, a π bond, reacts with electron-rich reactant, termed a nucleophile, with disappearance of the double bond and creation of two new single, or σ, bonds. The reactions are involved in the biological synthesis of compounds in the metabolism of every living organism, and are used by chemists in academia and industries such as pharmaceuticals to prepare most new complex organic chemicals, and so are central to organic chemistry. Addition reactions require the presence of groups with multiple bonds in the electrophile (due to the fact that double bonds and even triple bonds can both lack electron rich sources): carbon–heteroatom multiple bonds as in carbonyls, imines, and nitriles, or carbon–carbon double or triple bonds. The lack of electron rich sources is due to the fact that these bonds are partially empty, even though they remain connected, since the region occupying the orbital is essentially dead. This electrophilic behavior is defined as empty space since everything inside is basically without any source of electricity except from outside the bond, since bonds tend to want to attract more to themselves(whether this be electric or non-electric can differ in most situations). The addition of the nucleophile means the continuous addition of a negative charge throughout the reaction, unless an electrophile also makes itself present to form a complete structure with no charge at all. The negative charge being continuous throughout the reaction until the formation of an intermediate, bearing the charge, thus is the addition reaction we have before us. Once this meets an electrophile, then the intermediate formed with the negative charge can thus be neutralized to form a complete structure via a type of bond.

Addition to carbon–heteroatom double bonds

Nucleophilic addition reactions of nucleophiles with electrophilic double or triple bond (π bonds) create a new carbon center with two additional single, or σ, bonds. Addition of a nucleophile to carbon–heteroatom double or triple bonds such as >C=O or -C≡N show great variety. These types of bonds are polar (have a large difference in electronegativity between the two atoms); consequently, their carbon atoms carries a partial positive charge. This makes the molecule an electrophile, and the carbon atom the electrophilic center; this atom is the primary target for the nucleophile. Chemists have developed a geometric system to describe the approach of the nucleophile to the electrophilic center, using two angles, the Bürgi–Dunitz and the Flippin–Lodge angles after scientists that first studied and described them.

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 and there are not any other controlling issues such as chelation with a Lewis acid, the reaction product is a racemate. Addition reactions of this type are numerous. When the addition reaction is accompanied by an elimination the reaction type is nucleophilic acyl substitution or an addition-elimination reaction.

Addition to Carbonyl groups

With a carbonyl compound as an electrophile, the nucleophile can be:

• water in hydration to a geminal diol (hydrate)
• an alcohol in acetalisation to an acetal
• a hydride in reduction to an alcohol
• an amine with formaldehyde and a carbonyl compound in the Mannich reaction
• an enolate ion in an aldol reaction or Baylis–Hillman reaction
• an organometallic nucleophile in the Grignard reaction or the related Barbier reaction or a Reformatskii reaction
• ylides such as a Wittig reagent or the Corey–Chaykovsky reagent or α-silyl carbanions in the Peterson olefination
• a phosphonate carbanion in the Horner–Wadsworth–Emmons reaction
• a pyridine zwitterion in the Hammick reaction
• an acetylide in alkynylation reactions.
• a cyanide ion in cyanohydrin reactions

In many nucleophilic reactions, addition to the carbonyl group is very important. In some cases, the C=O double bond is reduced to a C-O single bond when the nucleophile bonds with carbon. For example, in the cyanohydrin reaction a cyanide ion forms a C-C bond by breaking the carbonyl's double bond to form a cyanohydrin.

Addition to Nitriles

With nitrile electrophiles, nucleophilic addition take place by:

• hydrolysis of a nitrile to form an amide or a carboxylic acid
• organozinc nucleophiles in the Blaise reaction
• alcohols in the Pinner reaction.
• the (same) nitrile α-carbon in the Thorpe reaction. The intramolecular version is called the Thorpe–Ziegler reaction.
• Grignard reagents to form imines. The route affords ketones following hydrolysis or primary amines following imine reduction.

Addition to carbon–carbon double bonds

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

In step 2 the negatively charged carbanion combines with (Y) that is electron-poor to form the second covalent bond. Ordinary alkenes are not susceptible to a nucleophilic attack (apolar bond). Styrene reacts in toluene with sodium to 1,3-diphenylpropane through the intermediate carbanion:

Another exception to the rule is found in the Varrentrapp reaction. Fullerenes have unusual double bond reactivity and additions such has the Bingel reaction are more frequent. When X is a carbonyl group like C=O 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 an active hydrogen compound the reaction is known as 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.