Chemistry · Ch 12 — Basic Concepts of Organic Reactions
Electron Movement in Organic Reactions
Electron Movement in Organic Reactions
Beyond simply classifying a species as a nucleophile or electrophile, every organic reaction can be understood in a deeper, unified way by following the movement of electrons -- that is, tracking how electron density redistributes itself as the reaction proceeds. Exactly how the electrons move depends jointly on the nature of the substrate, the nature of the reagent, and the prevailing reaction conditions.
As already introduced, this electron flow is represented on paper with curved arrows: a full curved arrow shows a pair of electrons moving, while a half-headed curved arrow shows the movement of a single electron (used for homolytic/radical steps). Every one of these electron movements results in either the breaking or the forming of a bond -- whether a sigma bond or a pi bond.
The book groups all such electron movement into exactly three types:
Type 1 -- A lone pair becomes a bonding pair. An existing lone pair on some atom moves in to form a brand-new bond. Two examples: (i) hydroxide ion's lone pair attacks an incoming proton , forming a new O-H bond and giving neutral water, --; (ii) an alkoxide-type oxygen's lone pair moves into the adjacent C-O region to form a new bond, converting a singly-bonded, negatively-charged oxygen into a neutral carbonyl oxygen, .
Type 2 -- A bonding pair becomes a lone pair. This is the mirror image of Type 1: an existing bond breaks, and its electron pair is retained as a lone pair on one of the two atoms, while the other atom departs (often as a cation). Two examples: (i) neutral water loses a proton -- an O-H bonding pair becomes a fresh lone pair on oxygen, generating hydroxide ion and releasing ; (ii) a neutral carbonyl compound's bond breaks -- the C=O bonding pair becomes a lone pair on oxygen, generating a negatively-charged alkoxide oxygen and leaving the carbon positively charged. …
Worked out. Two worked examples. First: hydroxide ion, H-O: with a lone pair and a negative charge, is attacked by a proton H+; a curved arrow runs from one of oxygen's lone pairs to the incoming H+, forming a new O-H bond and neutral water, H-O-H. Second: an alkoxide-type oxygen carrying a lone pair and negative charge, singly bonded to a CH2 group, has a curved arrow running from that lone pair into the C-O bond region, forming a new C=O pi bond as the oxygen's lone pair becomes a bonding pair -- giving the neutral carbonyl compound O=CH2 (both hydrogen …
Worked out. The reverse of Type 1, again with two worked examples. First: neutral water, H-O-H, loses a proton -- a curved arrow runs from one O-H bonding pair up onto oxygen, leaving behind hydroxide ion, H-O: with a fresh lone pair and a negative charge (with the departing H+ shown separately). Second: a neutral carbonyl compound, :O=CH2, has a curved arrow running from the C=O pi bonding pair up onto oxygen, breaking the pi bond and leaving an alkoxide-type oxygen with an extra lone pair and negative charge, singly bonded to a now electron-deficient, posit …
Worked out. Four worked examples, each a bond breaking and its electron pair immediately forming a different bond in the same step. (i) A B-H bonding pair (drawn as a hydridic H3B-H fragment) is attacked by an external proton H+; the curved arrow runs from the B-H bond to the incoming H+, forming a new H-H bond and leaving neutral BH3. (ii) An alkene pi bond adjacent to a carbocation centre (CH2+) curves around to form a new pi bond further along the chain, shifting the double bond and the positive charge in an allylic-type rearrangement. (iii) A C-H sigma bond next to a carbocation (H2C-CH2 with a positive charge on the adjacent carbon) migrates: the curved arrow runs from the C-H bond onto the cationic carbon, forming a new C=C pi bond (giving ethylene, H2C=CH2) while releasing the migrating hydrogen as H+. (iv) The reverse: an alkene, H2C=CH2, is attacked by an external H+; the curved arrow runs from the C=C pi bond to the incoming …