What is hyper-conjugation or Baker-Nathan effect ?
There is evidence that a C-H
bond which is adjacent to a pi bond
can take part in a delocalized system . The simplest example is
propylene a C- H bonding orbital in the
methyl group which is orthogonal to the plane containing the carbon atom
interacts with the adjacent p-orbital to form delocalized molecular orbital .
This is termed hyper-conjugation or Baker-Nathan effect.
For example , hyper-conjugation of toluene and propylene are as follows ,
What is no-bond resonance ?
The delocalization of
sigma-electrons or lone pair of electrons
into adjacent pi-orbitals or p-orbitals is called hyper-conjugation.
In hyper-conjugation , each alpha hydrogen in each resonating structure remain separate that is each C – H remain separate .
For this reason, hyper-conjugation is also called‘no-bond resonance’.
In hyper-conjugation , each alpha hydrogen in each resonating structure remain separate that is each C – H remain separate .
For this reason, hyper-conjugation is also called‘no-bond resonance’.
Baker Nathan effect:
A completely different approach to this problem of 1,2 -addition is the use of the Baker-Nathan effect. Generally, the I effect of alkyl group is t-butyl> isopropyl > ethyl > methyl .
This order of I effect has been used satisfactorily explain various physical data. In some reactions this order is reversed .
For example, Baker and Nathan examined kinetically the reaction between p-substituted benzyl bromides and pyridine.
The reaction was carried in acetone,and was shown to be entirely SN2 . Thus reaction may be formulated.
Application of hyper-conjugation:
Why propene is more stable than ethene ?
Propene is more stabler than ethene due to greater hyper-conjugation effect for
propene .But this effect is absent in ethene.
For this, the heat of hydrogenation value of ethene is greater than propene.
For this, the heat of hydrogenation value of ethene is greater than propene.
The hyper-conjugation of ethene and propene are shown below.
Wh2-methyl-but-2-ene is more stable than 2-methyl-but-1-ene?
Similarly , 2-metyhyl –but-2-ene is more stable than
2-methyl –but-1-ene . Because , 2-metyhyl
–but-2-ene contains 9 alpha hydrogen whereas
2-methyl –but-1-ene have five alpha-hydrogen.
So, 2-metyhyl –but-2-ene exhibit nine resonating structure which is more than 2-methyl –but-1-ene. Consequently, 2-metyhyl –but-2-ene is more stable than 2-methyl –but-1-ene.
So, 2-metyhyl –but-2-ene exhibit nine resonating structure which is more than 2-methyl –but-1-ene. Consequently, 2-metyhyl –but-2-ene is more stable than 2-methyl –but-1-ene.
Stability of
carbo cations or free radicals :
Stability of carbo cations or free radicals depends on
hyper-conjugation effect. Primary, secondary and tertiary carbocation contains
3, 6 and 9 alpha-hydrogen atom respectively.
So, tertiary carbo cation or free radical have nine resonating structure . whereas secondary carbocation or free radical have six and primary carbocation have have three resonating structure.
So, tertiary carbo cation or free radical have nine resonating structure . whereas secondary carbocation or free radical have six and primary carbocation have have three resonating structure.
Consequently. Tertiary butyl radical have highest and primary carbo-cation have lowest stability.
Summary:
What is Baker Nathan effect ?
Why hyperconjugation is also called no-bond resonance?
Why propene is more stable than ethene ?
Why2-methyl-but-2-ene is more stable than2-methyl-but-1-ene?
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