2 2市 Structures Reading:Gray:(2-14) OGC:(3.8) VI-1
Resonance Resonance Structures Reading: Gray: (2-14) OGC: (3.8) VI-1
Resonance Structures two structural possibilities for [C3H]: H H H H H H C一C= C= C: H H H H however,the actual molecule is neither of the two structures: H H H H H V-2
C C H H H H H C C C H H H H H C C C H H H H H C Resonance Structures [C3H5] : – two structural possibilities for - - however, the actual molecule is neither of the two structures: -δ -δ VI-2
Resonance Structures:Another Example H Two structural possibilities for benzene: H H H Each C-C bond can be thought of as a H H 1.5-electron bond in the actual structure V-3
H C C H H H H C H Resonance Structures: Another Example Two structural possibilities for benzene: C C C H C C H H H H C H C C C H C C H H H H C H C C C Each C - C bond can be thought of as a 1.5-electron bond in the actual structure VI-3
Formal Charge Formal Charge:the difference between the number of valence shell electrons for an element and the number of electrons surrounding it in a molecule (including half of the bond electrons and all the free electrons) Example: The leftmost carbon has 3 bonds H with 2 electrons each,plus 2 free electrons;so it has 5 electrons surrounding it.The formal charge H equals the valence electrons minus surrounding electrons: 4-5=-1 Formal Charge V-4
C C H H H H H C - Formal Charge Formal Charge: the difference between the number of valence shell electrons for an element and the number of electrons surrounding it in a molecule (including half of the bond electrons and all the free electrons) Example: The leftmost carbon has 3 bonds with 2 electrons each, plus 2 free electrons; so it has 5 electrons surrounding it. The formal charge equals the valence electrons minus surrounding electrons: 4 - 5 = –1 = Formal Charge VI-4
“Reasonable”Resonance Structures 1.Form octets if possible 2.Maximize bonding 3.Distribute formal charges in a reasonable way -minimize if possible -more negative on more electronegative atoms -avoid same sign on neighboring atoms V-5
“Reasonable” Resonance Structures 1. Form octets if possible 2. Maximize bonding 3. Distribute formal charges in a reasonable way -minimize if possible -more negative on more electronegative atoms -avoid same sign on neighboring atoms VI-5