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Glossary A Aprotic Solvent: Polar aprotic solvents are solvents that will dissolve many salts, but lack an acidic hydrogen. These solvents generally have intermediate dielectric constants and polarity. Although discouraging use of the term "polar aprotic", IUPAC describes such solvents as having both high dielectric constants and high dipole moments, an example being acetonitrile. Other solvents meeting IUPAC's criteria include DMF, HMPA, and DMSO D DMSO: Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula (CH3)2SO. This colorless liquid is an important polar aprotic solvent that dissolves both polar and nonpolar compounds and is miscible in a wide range of organic solvents as well as water. E

 

 

Elimination Reaction: An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism. In most organic elimination reactions, at least one hydrogen is lost to form the double bond: the unsaturation of the molecule increases. L Leaving Group: A leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. Leaving groups can be anions or neutral molecules, but in either case it is crucial that the leaving group be able to stabilize the additional electron density that results from bond heterolysis. Common anionic leaving groups are halides such as Cl−, Br−, and I−, and sulfonate esters, such as tosylate (TsO−). Common neutral molecule leaving groups are water and ammonia.

N

Nucleophile: A nucleophile is a chemical species that donates an electron pair to an electrophile to form a chemical bond in relation to a reaction. All molecules or ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.

P

Protic Solvent: A protic solvent is a solvent that has a hydrogen atom bound to an oxygen (as in a hydroxyl group) or a nitrogen (as in an amine group). In general terms, any solvent that contains labile H+ is called a protic solvent. The molecules of such solvents readily donate protons (H+) to reagents.

S

Substitution Reaction:Substitution reaction (also known as single displacement reaction or single replacement reaction) is a chemical reaction during which one functional group in a chemical compound is replaced by another functional group. Substitution reactions are of prime importance in organic chemistry.

 

 

Substitution reactions in organic chemistry are classified either as electrophilic or nucleophilic depending upon the reagent involved. W Williamson ether synthesis: The Williamson ether synthesis is an organic reaction, forming an ether from an organohalide and an alcohol. This reaction was developed by Alexander Williamson in 1850.Typically it involves the reaction of an alkoxide ion with a primary alkyl halide via an SN2 reaction. This reaction is important in the history of organic chemistry because it helped prove the structure of ethers.

The general reaction mechanism is as follows.

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Non-nucleophilic base: As the name suggests, a non-nucleophilic base is an organic base that is a poor nucleophile. Normal bases are also nucleophiles, but often chemists seek the proton-removing ability of a base without any other functions. Characteristic of non-nucleophilic bases is their steric bulk, which allows protons to attach to the basic atom but prevents alkyl groups from doing so.

A variety of amines and nitrogen heterocycles are useful bases of moderate strength (pKa of conjugate acid around 10-13)

• N,N-Diisopropylethylamine, or DIPEA (also called Hünig's Base) • 1,8-Diazabicycloundec-7-ene, or DBU—a favorite for the E2 eliminatio

reaction • 2,6-Di-tert-butylpyridine, a weak non-nucleophilic base • Phosphazene bases, such as t-Bu-P4

Non-nucleophilic bases of high strength are usually anions. For these species the pKa's of the conjugate acid is around 35-40.

• Lithium diisopropylamide, or LDA

 

 

• Silicon-based amides, such as sodium and potassium bis(trimethylsilyl)amide (NaHMDS and KHMDS, respectively)

• Lithium tetramethylpiperidide, or LiTMP (harpoon base)

Other strong non-nucleophilic bases are sodium hydride and potassium hydride. These compounds are dense, salt-like materials that are insoluble and operate by surface reactions.

Some reagents are of high basicity (pKa of conjugate acid around 17) but of modest but not negligible nucleophilicity. Examples include sodium tert-butoxide and potassium tert-butoxide.

Vicarious nucleophilic substitution:

In organic chemistry, the Vicarious nucleophilic substitution is a special type of nucleophilic aromatic substitution in which a nucleophile replaces a hydrogen atom on the aromatic ring and not leaving groups such as halogen substituents which are ordinarily encountered in SNAr. This reaction type was reviewed in 1987 by Polish chemists Mieczysław Mąkosza and Jerzy Winiarski.

It is typically encountered with nitroarenes and especially with charlotte nucleophiles, resulting in alkylated arenes: the new substituent can take the ortho or pera positions, reversing the selectivity for the meta position that is usually observed with such compounds under electrophilic substitution. Carbon nucleophiles carry an electron-withdrawing group and a leaving group: the nucleophile attacks the aromatic ring, and excess base can eliminate to form an exocyclic double bond which is successively protonated under acidic condition, restoring aromaticity.

 

 

 

 

Time line

Year Description Image

1929 The terms nucleophile and electrophile were introduced by Christopher Kelk Ingold in 1929, replacing the terms anionoid and cationoid proposed earlier by A. J. Lapworth in 1925. The word nucleophile is derived from nucleus and the Greek word φιλος, philos for love.

1953 TheMany schemes attempting to quantify relative nucleophilic strength have been devised. first such attempt is found in the Swain–Scott equation derived in 1953.

This free-energy relationship relates the pseudo first order reaction rate constant(in water at 25 °C), k, of a reaction, normalized to the reaction rate, k0, of a standard reaction with water as the nucleophile, to a nucleophilic constant n for a given nucleophile and a substrate constant s that depends on the sensitivity of a substrate to nucleophilic attack (defined as 1 for methyl bromide).

 

 

1972

The Ritchie equation, derived in 1972, is another free-energy relationship. Where N+ is the nucleophile dependent parameter and k0 the reaction rate constant for water. In this equation, a substrate-dependent parameter like s in the Swain–Scott equation is absent. The equation states that two nucleophiles react with the same relative reactivity regardless of the nature of the electrophile, which is in violation of the Reactivity–selectivity principle. For this reason this equation is also called the constant selectivity relationship.

1994 In the Mayr-Patz equation (1994). The second order reaction rate constant k at 20°C for a reaction is related to a nucleophilicity parameter N, an electrophilicity parameter E, and a nucleophile-dependent slope parameter s. The constant s is defined as 1 with 2-methyl-1-pentene as the nucleophile.

 

 

Weblinks : 1.www.chem.memphis.edu/parrill/chem3311/2003/NucElec. 2. www.sparknotes.com › ... › Organic Chemistry: Intro to Organic

3.www.mhhe.com/physsci/chemistry/carey/.../ch08leavinggroups.html4.

4.www.masterorganicchemistry.com/.../what-makes-a-good-nucleophile/

5. chemwiki.ucdavis.edu › ... › SN2 › Leaving Groups

Suggestive Reading

William Brown, Christopher Foote, Brent Iverson, Eric Anslyn, Organic Chemistry, Enhanced Edition

 

 

Named Organic Reactions By Thomas Laue, Andreas Plagens

Smith, March. Advanced Organic Chemistry 6th ed. (501-502)

P. S. Kalsi, Organic Reactions And Their Mechanisms