Nucleophile: A chemical species seeking a nucleus with which to form a bond. To do this the species must have a pair of non- bonding electrons available to form a bond. A nucleophile is also a Lewis base. Lewis base: An electron pair donor in a chemical reaction (section 14). Electrophile: A chemical species seeking a pair of electrons on a second species with which to form a bond. An electrophile is also a Lewis acid. Lewis acid: An electron pair acceptor in a chemical reaction (section 14). Free-radical: A chemical species with an unpaired valence electron seeking an electron with which to form a bond. (see section 1) Adduct: The product of an addition reaction. Carbocation: A trivalent species with a positive charge, an electrophile and a Lewis acid Example: (CH3)3C+, the positive carbon having only six valence electrons. The mechanistic classification is combined with the structural classification and may include the reaction order (1st or 2nd) of the rate-determining elementary step and further information on the substrate. Example: CH3Cl + I¯ CH3I + Cl¯ nucleophilic substitution second order, S N2, (nucleophilic attack of iodide ion on carbon replacing chlorine in one step, synchronous making and breaking of bonds). In representing details of bond breaking and making electron movement is shown using a "curly arrow", for electrons moving as pairs, and a "fish hook", for electrons moving singly. The tail of the arrow is close to the electron(s) moving and the head shows where they are moving to. Bond breaking and making is described as heterolytic or homolytic. Heterolytic bond breaking or making : The pair of electrons of a bond move together Example: breaking (CH3)3C–Br (CH3)3C+ + :Br¯ making and breaking I:¯ + CH 3–Cl I–CH3 + :Cl¯ Homolytic bond breaking: Electrons of a bond divide evenly to give two free-radicals Example: CH 3O-OC2H5 CH3O• + •OC2H5 . Homolytic bond making: Pairing of an unpaired electron with a second electron from a bond of the substrate to form a new bond and a new radical; or dimerisation of two radicals to give a molecule Examples: Cl• + CH2=CH2 Cl-CH2-CH2• • + • Br Br2 Full reaction schemes showing electron movement are given in the examples of mechanisms below. Note how it is not necessary to show non-bonding electron pairs. The tail of the curly arrow implies the presence of an electron pair. Examples of the major mechanistic classes Note: (1) that electron pairs and H atoms are not necessarily shown. They can be implied from the symbolism, or from the number of valence electrons in the structure shown. (2) how important it is to do "electron book-keeping", and show formal charge.
Posted on: Sun, 11 Aug 2013 14:34:21 +0000
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