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The doformation of a bond is to a rst a approximation
proportional to the square
of the extension of compression _
_
Where K = 1470 for CC bond! and K = "#40 for a double bond
C==C$
an extension of 0$00% nm in a CC bond requires an ener&' of
the order of_ ()ariation in its len&th of *+,! -here as the
same amount of ener&' is capableof e.ectin& an an&ular
deformation of more than 10+ of the inter)alenc' an&le
of a tetrahedral carbon atom$a useful concept has been put
for-ard! based upon comparati)e
examination of )arious bond len&ths$ /n a bond one
assi&ns to each atom an
atomic co)alenc' radius$ This ctitious radius r is such that the
sum! _! shall beequal to the len&th d of the bond $ the )alue
of the concept of the co)alenc'
radius of an atom lies in the fact that this quantit' is
independent of the natureof the atom to -hich it is bonded$
____
2or example! CC = 0$1%4 nm! from -hich one calculates rc = 0$077
nm$C3 = 0$10# nm! from -hich r3 = 0$10#0$77 = 0$0*" nm$ This )alue
is fouond in
)irtuall' all 3 bonds$ Table 1$" sho-s the principal mean
co)alenc' radii$ neestimate the precision of calculations made -ith
the aid of co)alenc' radii! usin&
the follo-in& molecule! of -hich the &eometr' has been
experimentall'
determined5
_____
*0 or&anic stereochemistr'
______
/n6uence of conu&ation on molecular &eometr'
8'stematic de)iations from theor' are e)ident in measurements of
interatomicdistances in s'stem in shich double and sin&le bonds
alternate! or in -hich a
double bond an atom carr'in& a lone pair are lin9ed$
2or example! in amines _________ the mean )alue of the C: bond
distance is
about 0$147 nm! -here as in amides _________ the distance C: is
about 0$1*;nm$ /n butadiene______ the central CC bond is shorter
than a CC bond in a
saturated molecule! and the double bonds are sli&htl'
len&thened b' comparison-ith the double bond in a
monoolifen5
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______
The t-o double bonds of butadiene cannot considered
independentl' of each
other$ Conu&ation bet-een the t-o double bonds is
responsible for the peculiar
beha)iour of butadiene (and is e)ident also in is chemical
reacti)it',$ /t arisesfrom the o)erlap of the t-o _ electron
clouds$ /f one considers the atomic p
orbitals of four carbon atoms! quantum mechanics sho-s that it
is possible toconstruct four molecular orbitals$ /n the &round
state the four _ electrons occup'
in pairs the orbitals _ dan _$ :ote that onl' the orbital _
recalls the classical
formula of butadiene! -ith stron& electron densit' bet-eem _
and _!_ and _$ Themolecular orbital _ is sich that t-o _ electron
are )er' delocalsi bet-een the formulae$ /n this )er'
empiricaltreatment one tries in a -a' to construct a photot picture
of the bonds in a
molecule b' superimposin& a number of incomplete
dia&rams$ /n eachcontributin& structure the atoms ha)e ha)e
the same positions! onl' the
disposition of the )alence electrons chan&e$
___________________
___
___
2i&ure 1$;
/n &ure 1$; se)eral contributin& structure of mesomers
of butadiene are sho-n!
formulae " and * &i)in& some double bond character of
the central bond ofbutadiene$
/f one limits oneself to loo9in& at the resonance 1?@* one
notices an analo&'
-ith the description of the _ s'stem b' the molecular orbitals
_!_$ 3o-e)er! *corresponds to a delocali -hereas t-o electrons onl'
are distributedbet-een the three CC bonds in the _ orbital$
The )ariation in the len&ths of bonds is not the onl'
consequence of conu&ation$2or conu&ation to be e.ecti)e a
lateral o)erlap of the p atomic orbitals is also
required! necessitatin& the coplanarit' of the t-o double
bonds$ utadiene and
the amides are discussed on pa&e 11#$ /t is possible for
planar clinic s'stem -ithalternatin& sin&le and double
bonds to sho- an enhanced conu&ation -hich
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manifests itself b' a set of properties (stabilit'!
susceptibilit' to electrophilic
substitution! etc, characteristic of aromaticit'$ en
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_____________
2i&ure 1$#
f the atoms to a smaller distanc than _ is ener&eticall'
unfa)ourable due to
existence of repulsi)e forces! called )an der -aals forces$ /f!
on the other hand!one considers t-o atoms at a &reat distance
from each other (_,! a potential
cur)e sho- that an approach of the atom reduces the ener&'
of the s'stem$ The
forces -hich exist bet-een the atoms are thus attracti)eD the'
are sometimescalled Eondon forces$ The' are proportional to _!
-hereas the )an de -aals forces
are proportional to _$ t the distances _ the attracti)e and
repulsi)e interactionterms of the )an der -aals forces are equal$
/t is useful to assume that _ is equal
to the sum of t-o distances characteri
