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Why is pyrrole a weaker base than dimethylamine (using the pKb as reference)?
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Adi Pantilimon
Why is pyrrole a weaker base than dimethylamine (using the pKb as reference)?
The lone pair of electrons of N in pyrrole is delocalized .It is part of the pi system, thus involved in aromatization. This lone pair is no longer available for a proton hence it is a weak base. On the other hand, aniline is already aromatic.Though the lone pair of electrons of -NH2 group of aniline is delocalized over the benzene ring, it is not involved in aromatization. This lone pair is still available for a proton, hence aniline is stronger base than pyrrole.
The lone pair of electrons of N in pyrrole is delocalized .It is part of the pi system, thus involved in aromatization. This lone pair is no longer available for a proton hence it is a weak base. On the other hand, aniline is already aromatic.Though the lone pair of electrons of -NH2 group of aniline is delocalized over the benzene ring, it is not involved in aromatization. This lone pair is still available for a proton, hence aniline is stronger base than pyrrole.
The lone pair in pyrrole participates in the aromatic pi-electron system. Protonation of pyrrole causes that lone pair to be donated to a proton, destroying the aromaticity. This is why pyrrole is a fairly weak base. Imidazole has two nitrogen atoms, and one of them has a lone pair that does not participate in the aromatic ring, and is therefore much more available for protonation.
The lone pair in pyrrole participates in the aromatic pi-electron system. Protonation of pyrrole causes that lone pair to be donated to a proton, destroying the aromaticity. This is why pyrrole is a fairly weak base. Imidazole has two nitrogen atoms, and one of them has a lone pair that does not participate in the aromatic ring, and is therefore much more available for protonation.
First, a reality check. What are the respective pKbs (remember, the smaller the pKb the stronger the base). The pKb values for pyrrole and dimethylamine are 13.6 and 3.7 so, indeed, pyrrole is a weaker base than dimethylamine…by nearly 10 orders of magnitude.
Now, as to why. Below is shown the structure of pyrrole and dimethylamine. In the case of pyrrole, there are 4 electrons from the double-bond in the 5-membered ring. This is an anti-aromatic configurations (anti-aromatic is if the number of electrons = 4n, n=1,2,etc). If the lone-pair on the ring nitrogen is donated, it will bring the number of π-electrons to 6, and aromatic number of electrons (aromatic - 4n+2 electrons, for pyrrole, n=1). So, as noted, in pyrrole the electrons on nitrogen are tied up in the aromatic system. Now, if the ring nitrogen gets protonated, the system will revert to being anti-aromatic. This is not the case for dimethyl amine (no aromatic/anti-aromatic considerations). The 10 orders of magnitude difference reflects the energy required to go from aromatic to anti-aromatic.
First, a reality check. What are the respective pKbs (remember, the smaller the pKb the stronger the base). The pKb values for pyrrole and dimethylamine are 13.6 and 3.7 so, indeed, pyrrole is a weaker base than dimethylamine…by nearly 10 orders of magnitude.
Now, as to why. Below is shown the structure of pyrrole and dimethylamine. In the case of pyrrole, there are 4 electrons from the double-bond in the 5-membered ring. This is an anti-aromatic configurations (anti-aromatic is if the number of electrons = 4n, n=1,2,etc). If the lone-pair on the ring nitrogen is donated, it will bring the number of π-electrons to 6, and aromatic number of electrons (aromatic - 4n+2 electrons, for pyrrole, n=1). So, as noted, in pyrrole the electrons on nitrogen are tied up in the aromatic system. Now, if the ring nitrogen gets protonated, the system will revert to being anti-aromatic. This is not the case for dimethyl amine (no aromatic/anti-aromatic considerations). The 10 orders of magnitude difference reflects the energy required to go from aromatic to anti-aromatic.
The lone pair of electrons of N in pyrrole is delocalized .It is part of the pi system, thus involved in aromatization. This lone pair is no longer available for a proton hence it is a weak base.
On the other hand, aniline is already aromatic.Though the lone pair of electrons of -NH2 group of aniline is delocalized over the benzene ring, it is not involved in aromatization. This lone pair is still available for a proton, hence aniline is stronger base than pyrrole.
The lone pair of electrons of N in pyrrole is delocalized .It is part of the pi system, thus involved in aromatization. This lone pair is no longer available for a proton hence it is a weak base.
On the other hand, aniline is already aromatic.Though the lone pair of electrons of -NH2 group of aniline is delocalized over the benzene ring, it is not involved in aromatization. This lone pair is still available for a proton, hence aniline is stronger base than pyrrole.
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The lone pair in pyrrole participates in the aromatic pi-electron system. Protonation of pyrrole causes that lone pair to be donated to a proton, destroying the aromaticity. This is why pyrrole is a fairly weak base. Imidazole has two nitrogen atoms, and one of them has a lone pair that does not participate in the aromatic ring, and is therefore much more available for protonation.
The lone pair in pyrrole participates in the aromatic pi-electron system. Protonation of pyrrole causes that lone pair to be donated to a proton, destroying the aromaticity. This is why pyrrole is a fairly weak base. Imidazole has two nitrogen atoms, and one of them has a lone pair that does not participate in the aromatic ring, and is therefore much more available for protonation.
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First, a reality check. What are the respective pKbs (remember, the smaller the pKb the stronger the base). The pKb values for pyrrole and dimethylamine are 13.6 and 3.7 so, indeed, pyrrole is a weaker base than dimethylamine…by nearly 10 orders of magnitude.
Now, as to why. Below is shown the structure of pyrrole and dimethylamine. In the case of pyrrole, there are 4 electrons from the double-bond in the 5-membered ring. This is an anti-aromatic configurations (anti-aromatic is if the number of electrons = 4n, n=1,2,etc). If the lone-pair on the ring nitrogen is donated, it will bring the number of π-electrons to 6, and aromatic number of electrons (aromatic - 4n+2 electrons, for pyrrole, n=1). So, as noted, in pyrrole the electrons on nitrogen are tied up in the aromatic system. Now, if the ring nitrogen gets protonated, the system will revert to being anti-aromatic. This is not the case for dimethyl amine (no aromatic/anti-aromatic considerations). The 10 orders of magnitude difference reflects the energy required to go from aromatic to anti-aromatic.
First, a reality check. What are the respective pKbs (remember, the smaller the pKb the stronger the base). The pKb values for pyrrole and dimethylamine are 13.6 and 3.7 so, indeed, pyrrole is a weaker base than dimethylamine…by nearly 10 orders of magnitude.
Now, as to why. Below is shown the structure of pyrrole and dimethylamine. In the case of pyrrole, there are 4 electrons from the double-bond in the 5-membered ring. This is an anti-aromatic configurations (anti-aromatic is if the number of electrons = 4n, n=1,2,etc). If the lone-pair on the ring nitrogen is donated, it will bring the number of π-electrons to 6, and aromatic number of electrons (aromatic - 4n+2 electrons, for pyrrole, n=1). So, as noted, in pyrrole the electrons on nitrogen are tied up in the aromatic system. Now, if the ring nitrogen gets protonated, the system will revert to being anti-aromatic. This is not the case for dimethyl amine (no aromatic/anti-aromatic considerations). The 10 orders of magnitude difference reflects the energy required to go from aromatic to anti-aromatic.
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