7/23/2019 elocalization.docx http://slidepdf.com/reader/full/elocalizationdocx 1/18 elocalization and Resonance Structures Rules In resonance structures, the electrons are able to move to help stabilize the molecule. This movement of the electrons is calleddelocalization . 1. Resonance structures should have the same number of electrons, do not add or subtract any electrons. (You can check the number of electrons by counting them !. "ll resonance structures must follo# the rules of #riting $e#is %tructures. &. The hybridization of the structure must stay the same. '. The skeleton of the structure can not be changed (only the electrons move. . Resonance structures must also have the same amount of lone pairs. Formal Charge )ven though the structures look the same, the formal charge (*+ may not be. *ormal charges are charges that are assigned to a specific atom in a molecule. If computed correctly, the overall formal charge of the molecule should be the same as the oidation charge of the molecule (the charge #hen you #rite out the empirical and molecular formula -e #ant to choose the resonance structure #ith the least formal charges that add up to zero or the charge of the overall molecule. The euation for finding *ormal +harge is/ Formal Charge = (number of valence electrons in free orbital) - (number of lone-pair electrons) - ( \( \frac{!{"! \) number bond pair electrons) The formal charge has to eual the molecule0s overall charge. ). +% 2 has an overall charge of 21, so the $e#is structure0s formal charge has to eual 21. %ee $e#is %tructure for more information. Example 2: Thiocyanate Ion Consider the thiocyanate (CNS - ) ion. SOLUTION 1. Find the Lewis Structure of the molecule. (Remember the Lewis Structure rules.)
elocalization and Resonance StructuresRulesIn resonance structures, the electrons are able to move to help stabilize the molecule. This movement of the
electrons is calleddelocalization.
1. Resonance structures should have the same number of electrons, do not add or subtract any electrons.
(You can check the number of electrons by counting them
!. "ll resonance structures must follo# the rules of #riting $e#is %tructures.
&. The hybridization of the structure must stay the same.
'. The skeleton of the structure can not be changed (only the electrons move.
. Resonance structures must also have the same amount of lone pairs.
Formal Charge
)ven though the structures look the same, the formal charge (*+ may not be. *ormal charges are charges that areassigned to a specific atom in a molecule. If computed correctly, the overall formal charge of the molecule should be
the same as the oidation charge of the molecule (the charge #hen you #rite out the empirical and molecular
formula -e #ant to choose the resonance structure #ith the least formal charges that add up to zero or the charge
of the overall molecule.
The euation for finding *ormal +harge is/
Formal Charge = (number of valence electrons in free orbital) - (number of lone-pair electrons) - ( \( \frac{!{"!
\) number bond pair electrons)
The formal charge has to eual the molecule0s overall charge.
). +%2 has an overall charge of 21, so the $e#is structure0s formal charge has to eual 21.
%ee $e#is %tructure for more information.
Example 2: Thiocyanate Ion
Consider the thiocyanate (CNS-) ion.
SOLUTION
1. Find the Lewis Structure of the molecule. (Remember the Lewis Structure rules.)
Resonance #$bridsResonance %tructures are a representation of a Resonance Hybrid , #hich is the combination of all resonance
structures. Though the *ormal +harge closest to zero is the most accepted structure, in reality the correct $e#is
structure is actually a combination of all the resonance structures (and hence is not solely describe as one.
1. 3ra# the $e#is %tructure 4 Resonance for the molecule (using solid lines for bonds.
!. -here there can be a double or triple bond, dra# a dotted line (22222 for a bond.&. 3ra# only the lone pairs found in all resonance structures, do not include the lone pairs that are not on all of
0he abo$e resonance structures show that the electrons are delocalied within the molecule and throu%h this #rocess the molecule %ains e
5one with both of its o##osite char%es creates a neutral molecule and throu%h resonance it is a stable molecule. 0he e/tra electron tha
ne%ati$e char%e on either terminal o/y%en can be delocalied by resonance throu%h the terminal o/y%ens.
8enene is an e/tremely stable molecule and it is accounted for its %eometry and molecular orbital interaction" but most im#ortantly
resonance structures. 0he delocalied electrons in the benene rin% ma3e the molecule $ery stable and with its characteristics of a nucleo
react with a stron% electro#hile only and after the first reacti$ity" the substituted benene will de#end on its resonance to direct the ne/t #oreaction to add a second substituent.
0he ne/t molecule" the !mide" is a $ery stable molecule that is #resent in most biolo%ical systems" mainly in #roteins. 8y stud
s#ectrosco#y and :-Ray crystallo%ra#hy it is confirmed that the stability of the amide is due to resonance which throu%h molecular orbitcreates almost a double bond between the Nitro%en and the carbon.
Some structural resonance conformations are the ma;or contributor or the dominant forms that the molecule e/ists. For e/am#le" if wabo$e rules for estimatin% the stability of a molecule" we see that for the third molecule the first and second forms are the ma;or contrib
o$erall stability of the molecule. 0he nitro%en is more electrone%ati$e than carbon so" it can handle the ne%ati$e char%e more than carbowith a ne%ati$e char%e is the least fa$orable conformation for the molecule to e/ist" so the last resonance form contributes $ery little for ththe on.
0he <ybrid Resonance forms show the different Lewis structures with the electron been delocalied. 0his is $ery im#ortant for the reacti$
benene because in the #resence of an electro#hile it will react and the formation of another bond will be directed and determine by relon% #air of electrons delocalied in the aromatic substituted rin% is where it can #otentially form a new bond with an electro#hile" as it i
are three #ossible #laces that reacti$ity can ta3e #lace" the first to react will ta3e #lace at the para #osition with res#ect to the chloro suthen to either ortho #osition.
References1. 6etrucci, Ralph 7., et al. General Chemistry: Principles and Modern Applications. e# 8ersey/ 6earson
6rentice 7all, !99:. 6rint.
!. "hmad, -an2Yaacob and ;akaria, <at =. >3ra#ing $e#is %tructures from $e#is %ymbols/ " 3irect )lectron
6airing "pproach.> 8ournal of +hemical )ducation/ 8ournal ::.&/ n. pag. -eb. <arch !999. $ink to this
