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Physical Chemistry
27 Feb 2008 13:52:03 IST
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iberis
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1 Mar 2008 18:50:37 IST
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Conjugation occurs when a p orbital is present adjacent to a double bond.
1.This p orbital may contain a single electron as in case of radicals. Eg. CH2=CH-CH2.
2. It may be an empty p orbital as in case of cations. Eg. CH2=CH-CH2+
3. Or it may be the p orbital of another double bond. Eg. CH2=CH-CH=CH2
Presence of a p orbital adjacent to a double bond leads to delocalization of the 
bond. This delocalization is called conjugation.
bond. This delocalization is called conjugation.Conjugated radicals, cations are more stable than non-conjugated ones.
Resonance is mere existence of two or more structures of a compound that differ only in positions of electrons.
We can draw various resonating structures of the above-described conjugated systems.
CH2=CH-CH=CH2 <--->+CH2-CH=CH-CH2- <--->-CH2-CH=CH-CH2+
Difference btw resonance n conjugation is that resonance doesn?t necessarily require a p orbital on the atom adjacent to the double bond.
Eg. +CH2-O-CH3 <---> CH2= O+ -CH3
Here although a double bond is present, the delocalization is only btw 2 atoms... the atoms adjacent to the double bond are not involved.
I hope it is clear...
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1 Mar 2008 19:07:16 IST
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CONJUGATION IS THE ALTERNATE SEQUENCE OF DOUBLE AND SINGLE BONDS IN A MOLECULE .
CONJUGATION IS ATTAINED FOR EXTRA STABILITY OF THE MOLECULE AND TO SATISFY THE VALENCY DUE TO HIGH SYMMETRY
RESONANCE IS ENTIRELY DIFFERENT FROM CONJUGATION RESONANCE IS A EFFECT WHICH OCCURS IN CONJUGATED MOLECULES.
THUS DIENE CONJUGATED MOLECULES SHOW MORE ACIDITY AND STABILITY.
CH2=CH2-CH=CH2
CONJUGATION IS ATTAINED FOR EXTRA STABILITY OF THE MOLECULE AND TO SATISFY THE VALENCY DUE TO HIGH SYMMETRY
RESONANCE IS ENTIRELY DIFFERENT FROM CONJUGATION RESONANCE IS A EFFECT WHICH OCCURS IN CONJUGATED MOLECULES.
THUS DIENE CONJUGATED MOLECULES SHOW MORE ACIDITY AND STABILITY.
CH2=CH2-CH=CH2
1 Mar 2008 21:25:01 IST
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@SAVALKAR
Firstly, conjugation is not restricted to compounds containing alternate single and double bonds. Like i said, it is also observed in radicals n cations provided they are present on atoms adjacent to a double bond.
Secondly, resonance is not entirely different from conjugation. Its just that conjugated systems ALSO show resonance but resonance is not only restricted to conjugated systems, it is also observed in other cases...
1 Mar 2008 22:05:21 IST
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Re:What exactly is conjugation and why does it take place?
A simple pi bond where the two p orbitals overlap sideways is shown by
Conjugated double bond system
A conjugated double bond system looks like this
The diagram shows those molecular orbitals.
To stress again - the diagram shows only one of the delocalised molecular orbitals. Remember that both of the red bits in the diagram are part of the same orbital.
The interaction of the two double bonds with each other to produce a delocalised system of pi electrons over all four atoms is known as conjugation. Conjugation in this context literally means "joining together".
In reality, if you start by overlapping four atomic orbitals, you will end up with four molecular orbitals.
The four electrons will go into the two lowest energy of these - two in each. That means that you get two pi bonding orbitals. We just draw one of these for simplicity - the other one has a different shape.
The four electrons will go into the two lowest energy of these - two in each. That means that you get two pi bonding orbitals. We just draw one of these for simplicity - the other one has a different shape.
There are also two pi anti-bonding orbitals, but these are normally empty.
For most purposes, we ignore these entirely - although not for this topic because energy from light can promote electrons from a pi bonding orbital into one of the anti-bonding orbitals .
For most purposes, we ignore these entirely - although not for this topic because energy from light can promote electrons from a pi bonding orbital into one of the anti-bonding orbitals .
Recognising conjugated double bonds in a molecule
You can recognise the presence of conjugated double bonds in a molecule containing more than one double bond because of the presence of alternating double and single bonds.
The double bonds don't have to be always between carbon atoms. All of the following molecules contain conjugated double bonds, although in the last case, the conjugation doesn't extend over the whole molecule:
However, although the next molecule contains two double bonds, they aren't conjugated. They are separated by two single bonds.
The reason why it is important to have the double and single bonds alternating is that this is the only way you can get all the p-orbitals overlapping sideways.
In the last case, you will get sideways overlap at each end of the molecule to get two individual pi bonds. But the extra single bond in the middle stops them from interacting with each other.
In the last case, you will get sideways overlap at each end of the molecule to get two individual pi bonds. But the extra single bond in the middle stops them from interacting with each other.
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