due to the polarity of the C-X bond a partial positive charge is generated on the carbon atom and hence the carbon atom becomes an electrophilic centre at which a nucleophile can attack. Problem . Using curved-arrow notation, show the formation of reactive intermediates when the following covalent bonds undergo heterolytic cleavage. (a) CH –SCH , (b) CH –CN, (c) CH –Cu Problem .
Giving justification, categorise the following molecules/ions as nucleophile or electrophile: Nucleophiles: HS – ,C H O – ,(CH ) N:H N: – These species have unshared pair of electrons, which can be donated and shared with an electrophile. Electrophiles : BF ,C1 + H –C + =O,N + O . Reactive sites have only six valence electrons; can accept electron pair from a nucleophile. Problem .
Identify electrophilic centre in the following: CH CH=O, CH CN, CH I. Among CH HC * =O, H CC * ≡ N, and H C * –I, the starred carbon atoms are electrophilic centers as they will have partial positive charge due to polarity of the bond. . .
Electron Movement in Organic Reactions The movement of electrons in organic reactions can be shown by curved-arrow notation. It shows how changes in bonding occur due to electronic redistribution during the reaction. To show the change in position of a pair of electrons, curved arrow starts from the point from where an electron pair is shifted and it ends at a location to which the pair of electron may move. Presentation of shifting of electron pair is given below : (i) from π bond to adjacent bond position (ii) from π bond to adjacent atom (iii) from atom to adjacent bond position Movement of single electron is indicated by a single barbed ‘fish hooks’ (i.e.
half headed curved arrow). For example, in transfer of hydroxide ion giving ethanol and in the dissociation of chloromethane, the movement of electron using curved arrows can be depicted as follows: . . Electron Displacement Effects in Covalent Bonds