11. FORMAL CHARGES 4 Ch 1.6
where v= # of valence
e (also Per. Table #), Lp= total # of lone pair e and b= total # of bonding
Try using this equation with the Ns and Os in CH3NO2, hydronium ion, hydroxide ion, water, ammonia, ammonium ion, etc. The general rule is ;
A four-bonded N is +, a three-bonded O is + a single-bonded O is -ve, a two-bonded N are -ve and, a triply bonded C can be either + or - dependent whether it has 6 or 8 electrons (see below). [ C: means 8 electrons and thus a carbanionC- ]
||+ (cation)||neutral||- (anion)|
||three bonds only
CH3+ (and CH2=CH+)
CH4 ( & CH2=CH2 & HCtripleCH)
|three bond WITH 1 lone pair
||four bonds NH4+
|three bonds + 1 lone pair NH3
|two bond + 2 lone pairs NH2-|
||three bonds H3O+ [one lone pair] (& >C=OH+)||two bonds H2O+ 2 lone pairs||one bond + 3 lone pairs HO-
(and CH3CO2- :i.e. acetate)
VERY REACTIVE CARBON i.e.REACTION INTERMEDIATES
CARBOCATIONS, CARBON RADICALS & CARBANIONS are known to exist for a short time (called "intermediates") during the course of many OC rxns. They are usually too unstable to be isolated [though some especially stable ones have been.
Carbocations are 6 electron species ("electrophiles") hybridised sp2 and are thus trigonal planar in shape with an empty 2p orbital; they react very rapidly with electron-rich ("nucleophilic") sites in other molecules in order to achieve a filled shell (or "octet"), i.e. CH3+ (Note: in some older organic chemistry books (Sykes!) , these species were called "carbonium" ions.)
Carbon radicals are 7 electron species hybridised sp2 (also "electrophiles" cf. carbocations), 1 electron in the 2p orbital: they react with electron rich species or other radicals --> 8 shared electrons, i.e. CH3.
8 electron species ("nucleophiles") again usually hybridised sp2
and are trigonal planar with the 2p orbital containing a lone pair of electrons;
these anions react very rapidly with electron-poor ("electrophilic")
species i.e. CH3-.
N.B. Some textbooks may claim that the methyl carbanion is sp3 hybridised. For the purposes of this course, it is best to consider this unlikely and that ALL CARBANIONS SHOULD BE CONSIDERED hybridised sp2.