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Electromeric though a bit outdated is
important for understanding changes in electronic density in a molecule in the
presence for other species. This also involves movement of electrons but in
this case due to some external agent. For example if a positive charge like H+
is brought near a double bond (say CH2=CH2), the double
bond which is electron rich (a double bond has pi electrons, remember?), the
bond is polarized towards the proton. Inductive effect:
Due to polarisation of molecules in sigma bond an electronic
effect is produced which is called inductive effect. This is only because of
the electronegativity difference between the atoms at either ends of the bond.
So bond polarity is created because the more electronegative atom pulls more
electrons toward itself. In this the molecule with more electronegativity has
slightly negative charge whereas the other molecule has positive charge. When
the electronegative atom is connected to the chain of carbon atoms then the
positive charge is transferred to other atom and this is electron withdrawing
effect and is also called –I effect. On the other hand some groups like
alkyl groups are electron releasing in nature, this electron releasing nature
is +I effect.
The strength of this effect is dependent on the
distance between the main group and the substituent group. This effect gets
weaker as the distance between these two groups increases
Resonance is a phenomenon where a molecule is represented in
more than one form when a single Lewis structure cannot represent all of its
properties. An example is 1CH2=2CH-3CH=4CH2,
which from the Lewis structure shown consists of two double bonds and a single
bond. So C1-C2and C3-C4 bond lengths should be substantially shorter than that
between C2 and C3, but all bonds are found to be of the same length in reality.
So the above representation of bonds and electrons is not entirely accurate.
Hyperconjugation is very similar to resonance,
sometimes referred to a No-bond resonance or Baker-Nathan effect. In case of
classical resonance we had seen the involvement of lone pair of electrons and
pi bonds (double/triple bonds). In hyperconjugation single bonds are involved
in the electron delocalization circuitry. This effect is still not fully
understood in detail but would serve the purpose of basic organic chemistry. This positive charge is stabilized by
hyperconjugation as follows:
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