Information AboutElectrophile |
| CATEGORIES ABOUT ELECTROPHILE | |
| physical organic chemistry | |
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The electrophiles attack the most electron-populated part of a Nucleophile . The electrophiles frequently seen in the Organic Syntheses are cations such as H+ and NO+, polarlized neutral molecules such as HCl , Alkyl Halide s, Acyl Halide s, and Carbonyl Compound s, polarlizable neutral molecules such as Cl2 and Br2, Oxidizing Agent s such as organic Peracid s, chemical species that do not satisfy the Octet Rule such as Carbene s and Radicals , and some of lewis acids such as BH3 and DIBAL . ELECTROPHILES IN ORGANIC CHEMISTRY Alkenes Electrophilic Addition is one of the three main forms of reaction concerning Alkenes . They consist of:
Addition of halogens These occur between alkenes and electrophiles, often halogens as in Halogen Addition Reaction s. Common reactions include use of bromine water to Titrate against a sample to deduce the number of double bonds present. For example, Ethylene + Bromine → 1,2-dibromoethane : :C2H4 + Br2 → BrCH2CH2Br This takes the form of 3 main steps shown belowLenoir, D.; Chiappe, C. ''Chem. Eur. J.'' 2003, ''9'', 1036.; : #Forming of a π-complex #:The electrophilic Br-Br molecule interacts with electron-rich alkene molecure to form a π-complex 1. #Forming of a three-membered bromonium ion #:The alkene is working as an electron donor and bromine as an electrophile. The three-membered Bromonium Ion 2 consisted with two carbon atoms and a bromine atom forms with a release of Br−. #Attacking of bromide ion #:The bromonium ion is opened by the attack of Br− from the back side. This yields the Vicinal dibromide with an Antiperiplanar configuration. When other nucleophiles such as water or alcohol are existing, these may attack 2 to give an alcohol or an ether. This process is called AdE2 mechanism. intermediate may be predominant instead of '''3''' if the alkene has a cation-stabilizing substituent like phenyl group. There is an example of the isolation of the bromonium ion '''2'''.Brown, R. S. Acc. Chem. Res. 1997, 30, 131. Addition of hydrogen halides Hydrogen halides such as hydrogen chloride (HCl) adds to alkenes to give alkyl halide in Hydrohalogenation . For example, the reaction of HCl with ethylene furnishes chloroethane. The reaction proceeds with a cation intermediate, being different from the above halogen addition. An example shown below: : #Proton (H+) adds (by working as an electrophile) to one of the carbon atoms on the alkene to form cation 1. #Chloride ion (Cl−) combines with the cation 1 to form the adducts '''2''' and '''3'''. In this manner, the Stereoselectivity of the product, that is, from which side Cl− will attack relies on the types of alkenes applied and conditions of the reaction. At least, which of the two carbon atoms will be attacked by H+ is usually decided by Markovnikov's Rule . Thus, H+ attacks the carbon atom which carries the less number of substituents so as to the more stabilized carbocation (with the more stabilizing substituents) will form. This process is called A-SE2 mechanism. Hydrogen Fluoride (HF) and hydrogen iodide (HI) react with alkenes similarly and Markovnikov-type products will be given. Hydrogen bromide (HBr) also takes this pathway, but sometimes a radical process competes and a mixture of isomers may form. Hydration One of the more complex Hydration Reaction s utilises Sulfuric Acid as a Catalyst . This reaction occurs in a similar way to the addition reaction but has an extra step in which the OSO3H group is replaced by an OH group, forming an alcohol: :C2H4 + H2O → C2H5OH As you can see the H2SO4 does not take part in the overall reaction, however it does take part but remains unchanged so is classified as a catalyst. This is the reaction in more detail: : #The H-OSO3H molecule has a δ+ charge on the initial H atom, this is attracted to and reacts with the double bond in the same way as before. #The remaining (negatively charged) −OSO3H ion then attaches to the Carbocation . Forming ethyl hydrogensulphate (upper way on the above scheme). #When water (H2O) is added and the mixture headed ethanol (C2H5OH) is produced, the "spare" hydrogen atom from the water goes into "replacing" the "lost" hydrogen and thus reproduces sulfuric acid. Another pathway in which water molecule combines directly to the intermediate carbocation (lower way) is also possible. This pathway become predominant when aqueous sulfuric acid is used. Overall this process adds a molecule of water to a molecule of ethene. This is an important reaction in industry as it produces Ethanol , which is the alcohol having various purposes including fuels and starting material for other chemicals. ELECTROPHILICITY SCALE Several methods exist to rank electrophiles in order of reactivity Electrophilicity Index Chattaraj, P. K.; Sarkar, U.; Roy, D. R. Chem. Rev. ; (Review); 2006; 106(6); 2065-2091. and one of them is devised by Robert Parr ''Electrophilicity Index'' Parr, R. G.; Szentpaly, L. v.; Liu, S. J. Am. Chem. Soc. ; (Article); 1999; 121(9); 1922-1924. with the '''electrophilicity index''' ω given as: : with the Electronegativity and Chemical Hardness . This equation is related to classical equation for Electrical Power : : where is the Resistance ( Ohm or Ω) and is Voltage . In this sense the electrophilicity index is a kind of electrophilic power. Correlations have been found between electrophilicity of various chemical compounds and reaction rates in biochemical systems and such phenomena as allergic contact dermititis. A electrophilicity index also exists for radical react with a preference for electron-poor reaction sites. SEE ALSO REFERENCES |
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