nucleophile

Examples of nucleophile in the following topics:

• Nucleophilicity

  • Electrophile: An electron deficient atom, ion or molecule that has an affinity for an electron pair, and will bond to a base or nucleophile.
  • The most reactive nucleophiles are said to be more nucleophilic than less reactive members of the group.
  • The nucleophilicities of some common Nu:(–) reactants vary as shown in the following
  • The cumulative results of studies of this kind has led to useful empirical rules pertaining to nucleophilicity:
  • (ii) For a given period of the periodic table, nucleophilicity (and basicity) decreases on moving from left to right.

• Nucleophilic Substitution

  • Nucleophilic substitution is a reaction that can occur uni- or bimolecularly in which a nucleophile replaces a leaving group at a carbon.
  • Nucleophilic substitution is a superset of reactions that involve an electron-rich nucleophile bonding to an electrophilic carbon atom and displacing a stable leaving group.
  • Generally, the form of a nucleophilic substitution reaction can be expressed as:
  • In unimolecular nucleophilic substitution (SN1), a leaving group is replaced by a nucleophile in a two-step process.
  • In SN2, the nucleophile "pushes" the leaving group off the carbon in the R group.

• Nucleophilicity & Basicity

  • then reactions with much weaker nucleophiles or bases may take place.
  • Some confusion in distinguishing basicity (base strength) and nucleophilicity (nucleophile strength) is inevitable.
  • Nucleophilicity is a more complex property.
  • For two or more molecules incorporating nucleophilic atoms of the same kind and charge, the stronger base is usually the stronger nucleophile.
  • In each of these pairs the weaker base is the stronger nucleophile.

• The Alkyl Moiety

  • We can now piece together a plausible picture of how nucleophilic substitution reactions of 1º and 2º-alkyl halides take place.
  • The nucleophile must approach the electrophilic alpha-carbon atom from the side opposite the halogen.
  • The diagram below shows this process for an anionic nucleophile.
  • The consequence of rear-side bonding by the nucleophile is an inversion of configuration about the alpha-carbon.
  • Neutral nucleophiles react by a similar mechanism, but the charge distribution in the transition state is very different.

• Nucleophilicity of Phosphorus Compounds

  • The chemistry of phosphines and the related phosphite esters is dominated by their strong nucleophilicity and reducing character.
  • The nucleophilicity of trivalent phosphorus results in rapid formation of phosphonium salts when such compounds are treated with reactive alkyl halides.

• Acid-Base Catalysis

  • As we have noted, many common organic reactions proceed by bonding between nucleophilic and electrophilic sites in the reactant molecules.
  • Three examples are shown in equations 1 through 3; electrophiles are colored red, and nucleophiles are colored blue.
  • In the former addition reaction, bromine (an electrophile) attacks the nucleophilic double bond of 1-butene to give an electrophilic cyclic-bromonium intermediate (enclosed in square brackets) accompanied by a nucleophilic bromide ion.
  • Thus, acid chlorides are very reactive with a wide range of nucleophiles, including water and alcohols (eq. 3).
  • We can use a stronger nucleophile than water, such as hydroxide anion.

• Substitution

  • This apparent nucleophilic substitution reaction is surprising, since aryl halides are generally incapable of reacting by either an SN1 or SN2 pathway.
  • To explain this, a third mechanism for nucleophilic substitution has been proposed.
  • Three additional examples of aryl halide nucleophilic substitution are presented on the right.
  • Nitrogen nucleophiles will also react, as evidenced by the use of Sanger's reagent for the derivatization of amino acids.
  • Some distinguishing features of the three common nucleophilic substitution mechanisms are summarized in the following table.

• Reactive Intermediates

  • Electrophile: An electron deficient atom, ion or molecule that has an affinity for an electron pair, and will bond to a base or nucleophile.
  • Nucleophile: An atom, ion or molecule that has an electron pair that may be donated in bonding to an electrophile (or Lewis acid).
  • Using these definitions, it is clear that carbocations ( called carbonium ions in the older literature ) are electrophiles and carbanions are nucleophiles.
  • In this sense they are electrophiles, but the non-bonding electron pair also gives carbenes nucleophilic character.
  • The importance of electrophile / nucleophile terminology comes from the fact that many organic reactions involve at some stage the bonding of a nucleophile to an electrophile, a process that generally leads to a stable intermediate or product.

• Nucleophilicity of Sulfur Compounds

  • Thiolate conjugate bases are easily formed, and have proven to be excellent nucleophiles in SN2 reactions of alkyl halides and tosylates.
  • Although the basicity of ethers is roughly a hundred times greater than that of equivalent sulfides, the nucleophilicity of sulfur is much greater than that of oxygen, leading to a number of interesting and useful electrophilic substitutions of sulfur that are not normally observed for oxygen.

• Addition Reactions

  • There are two main types of polar addition reactions: electrophilic addition and nucleophilic addition.
  • In the related addition-elimination reaction, an addition reaction is followed by an elimination reaction; in most reactions, this involves addition to carbonyl compounds in nucleophilic acyl substitution.
  • In nucleophilic addition reactions, the nucleophile donates an electron pair to the electrophile (one of the atoms in the double bond).
  • In hydrohalogenation, the nucleophile is the halogen.
  • Top to bottom: electrophilic addition to alkene, nucleophilic addition of nucleophile to carbonyl, and free radical addition of halide to alkene.