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_ chemistry + Some Basic Principles and Techniq, ors? hilic reagents possess ia, plecttOP eo an electron defici, nO) centre. This is why they have a lust for coe, a arcaok a electron rich centres, Electrophilic peeps, and following two types : eagents, are (i positively charged electrophiles : Positive}, and carbocations possess a deficiency of ele y charged, it fore act as electrophiles. Some ere and, on below ones are, , St, + WYO, NO, NO2, Cl’, Br*, 1, CaN, HHS 2 rT, CoHsNo (diazonium, Hi (1° carbocations) C, e RCH} ), RoCH (2° carbocations),, , 63° carbocations), etc., , ji) Neutral electrophiles : Some neutral ‘, , ; species which, essa deficiency of electrons act as ‘, e ae and neutral electrophiles., (a) Species like BFg, AICI, etc., in whi, rom has only six Gees which the central, , FE, | ca, =D: a— Al, (Boron has 6 electrons in idence, the valence shell) the valence shell), , (b) Free radicals, carbenes and nitrenes:, H, , |, , , , H—C °c Hi, aici eat be, (carbon has 7 electrons) oan, alia N —COO0C;Hs, Slocum eee, , It is to be noted that each free radical is short of one, electron while carbenes and nitrenes are short of a pair of, electrons. ;, , Thus, both positively charged and neutral electrophiles, are short of a pair of electrons (execpt free radicals which, are short of 1 electron only). This is why they have a strong, tendency to attack at electron rich centres and behave as, , , , ORGANIC REACTIONS AND THEIR MECHANISM, , , , 677, , Lewis aci, ‘Gewik an They always attack the substrate molecule at, 'e of highest electron density., , 12.10.2 Nucleophilic Reagents or Nucleophiles, , x ae nucleophilic (nucleo : nucleus, philic : loving;, ae ae loving) reagents or nucleophiles are those, ae hich possess an affinity for the positive charge. The, ucleophiles are electron rich species and possess at least, , one lone pair of electrons. Therefore, in chemical reactions,, they attack at those centres which are deficient in electrons., Nucleophilic reagents are of following two types :, , la Negatively charged nucleophiles : The species having, an available electron pair and a negative charge act as, negatively charged nucleophiles. For example,, , OH’, OR, CN”, H, Cl, Br, 1, RCH3, RoCH , RoC,, RCOO , etc., , These speices possess an available electron pair along, , with a negative charge, ¢.g.,, , , , =0-—H, no oa, , (hydroxyl ion), , , , (chloride ion), i (carbanion), , 2. Neutral nucleophiles : In some neutral molecules, such as H,0, R—OH (alcohlos), R—O—R (ethers), NH3,, R—NH, (amines), etc., the hetero atom possesses one or, more lone pair of electrons., , , , H—O—H R—O—H R—, (water) (alcohol) (ether), H, es |, R—NH) H—N;, (amine) |, H, (ammonia), , Such molecules are able to donate a lone pair of, electrons to an electron deficient centre and therefore act, as nucleophiles., , Thus, both negatively charged and neutral, nucleophiles possess lone pair of electrons and can donate, it to an electron deficient species. Therefore, in chemical, reactions, they behave as Lewis bases and attack the, substrate molecule at the site of lowest electron density.