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Showing posts with label Basic Strength Order:. Show all posts
Showing posts with label Basic Strength Order:. Show all posts

Thursday, January 23, 2020

Wednesday, January 22, 2020

What is correct basicity order of pyridine, pyridazine, pyrimidine and pyrazine ?


Pyridine is more basic as compared to pyridazine, pyrimidine and pyrazine because pyridazine, pyrimidine and pyrazine containing two nitrogen atoms which are exerted inductive effect to each other so availability of lone pair for donation is less.

But pyridazineis  more basic as expected, it may be due to electrostatic repulsion of the lone pairs of the vicinal nitrogens. This repulsion destabilizes the non-protonated structure and therefore makes protonation easier.

Why pyridine is more basic than Pyrrole?

In case of Pyrrole  the lone pair electrons of the nitrogen atom is involved in conjugated system of pi electrons of five membered ring leading to greater stability of the molecule due to acquiring aromatic character. Hence less available for donation.
While in case pyridine already has a stable conjugated system of three double bonds in an aromatic ring, like benzene. Hence the lone pair electrons on the N atom in pyridine is localized and  more available for donation and  easily donated to a H+ ions. Therefore, pyridine is a stronger base than Pyrrole.

Why pyrimidine is less basic than pyridine?



Pyrimidine is less basic than pyridine because (–I effect) negative inductive effect of nitrogen atom to another nitrogen atom that cause decreases electron density of each other. This makes it less willing to donate its lone pair. This is not count that number of lone pairs to explain basicity.


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Tuesday, December 31, 2019

Imidazole is more basic than pyridine? Why?


Imidazole and its derivatives form an interesting and important class of heterocyclic aromatic amines.
The basic strength of imidazole is approximately 100 times more basic than pyridine. Because protonation of imidazole yields an ion that is stabilized by the electron delocalization represented in the resonance structures given as below:
As seen in above figure the electrostatic potential map of the conjugate acid of imidazole (imidazolium ion) is consistent with the resonance description that shows both nitrogens as equivalent.
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Biological Important of Imidazole and structure:


Imidazole and its derivatives form an interesting and important class of heterocyclic aromatic amines.
An imidazole ring is a structural unit in two biologically important compounds, histidine and histamine. 
Histidine is one of the amino acid building blocks of proteins and is directly involved in key proton-transfer processes. The drop in blood pressure associated with shock is a result of the formation of histamine, which stimulates the dilation of blood vessels.

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Pyridine is almost 1 million times less basic than piperidine? Why?


We know that if nitrogen is part of an aromatic ring, however, its basicity decreases markedly.  So in pyridine nitrogen is part of an aromatic ring hence its basicity is much more less than piperidine. The difference between the two lies in the fact that the nitrogen lone pair occupies an sp3- hybridized orbital in piperidine versus an sp2-hybridized one in pyridine. As we have noted on several occasions, electrons in orbitals with more s character are more strongly held than those with less s character. For this reason, nitrogen holds on to its unshared pair more strongly in pyridine than in piperidine and is less basic.

Cyclohexylamine amine is the stronger base than Aniline? Why?


We can say that aniline is a weaker base than cyclohexylamine because the electron pair on nitrogen of aniline is strongly held by virtue of being delocalized into the π system of the aromatic ring. The unshared pair in cyclohexylamine is localized on nitrogen, less strongly held, and therefore “more available” in an acid–base reaction.

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Tetrahydroquinoline amine is the stronger base than Tetrahydroisoquinoline? Why?



We can say that Tetrahydroisoquinoline  is a weaker base than Tetrahydroquinoline because the electron pair on nitrogen of Tetrahydroisoquinoline is strongly held by virtue of being delocalized into the π system of the aromatic ring. The unshared pair in Tetrahydroquinoline is localized on nitrogen, less strongly held, and therefore “more available” in an acid–base reaction.

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