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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.
Related Questions:

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.

Related Questions:

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.

Related Questions:

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.

Related Questions:

Phenyl group is known to extract negative inductive effect, but each phenyl ring in biphenyl is more reactive than benzene towards Electrophilic substation. Why?



It is because, in biphenyl, one of the phenyl groups acts as donor and the other as electron acceptor. This increases electron density on benzene ring and facilitates Electrophilic attack at ortho and para position.

Are all the five bonds of PCl5 equivalent? Justify your answer.


PCl5 has Sp3 hybridisation and trigonal bipiramidal geometry. PCl5 has three equivalent equatorial and two equivalent axial P – Cl bonds. However, due to greater bond pair – bond pair repulsions, the axial P – Cl bonds are longer and hence different from the three equatorial bonds

Halides of Nitrogen Family:

Monday, December 30, 2019

Silianol (SiH3OH) is more acidic than methanol (CH3OH) why?


We know that acidic strength of an acid also depend upon stability of its conjugate base. So silianol is more acidic than methanol (H3C-OH) because conjugate base silianol (H3Si-OH) stablised by dispersal of negative charge in H3Si-O- ion by 2pπ—3dπ back bonding

Hence methanol is less acidic than silianol.

For more detail on Back bonding click here

Related Questions: 

(1) Although anhydrous aluminium chloride is covalent but its aqueous solution is ionic in nature. Why?

(2) Why Ga has small size than Al exceptionally

(3) Why aqueous solution of borax reacts with two moles of acids ?

(4) What is structure of solid Ortho Boric acid ?

(5) What is the structure of trimetaboric acid and trimetaborate ion?  

(6) Why Borazine is more reactive than benzene towards Electrophic Aromatic substitution reactions ?

(7) Why Borazine (B3N3H6) is also known as inorganic benzene ?.

(8) Why B-F bond length in BF3 is shorter (130 pm) than B-F bond Iength in BF4- (143 pm)?. Explain.

(9) Why B-F do not exist as dimer?. Explain.

(10) Although anhydrous aluminium chloride is covalent but its aqueous solution is ionic in nature. Why?

(11) Why Boric acid become strong acid in the presence of cis 1,2-diol or 1,3-diol ?

Trisilyl amine, N(SiH3)3 is planar whereas trimethyl amines N(CH3)3 is pyramidal. Explain why?.


In N(SiH3)3, N attains sp2 hydridisation and the lone pair of N is involved in dpi-ppi  back bonding by getting itself delocalized on to empty 3d – orbitals of silicon.

But in N(CH3)3, N is sp3 hybridised in which three of the hybrid orbitals are used in forming s  - bonds with NH3 groups, while the lone pair is present in the fourth hybrid orbital. Thus the molecule is pyramidal.
For More deatils click here Back Bond theory:

What is the molecular formula of Borax ?

(A) Na2[B4O3(OH)4].6H2O
(B) Na2[B4O5(OH)4].6H2O
(C) Na2[B4O5(OH)4].8H2O
(D )Na2[B4O6(OH)2].8H2O

The structure of Borax is Na2[B4O5(OH)4].8H2O.


Hence option (C) is the correct answer for details click on link given below

Three-center four-electron Bridge bond (3C-4e Bond): Structure of AlCl3:

(1) Al2Cl6 is neither hypovalent nor hypovalent rather its octet is complete. We will used  MOT here  it cannot act as Lewis acid  due to crowding in spite having vacant d orbital’s   however AlCl3  act as Lewis acid .

(2) Al2Cl6 contains six bonds having two bridge bonds (3c-4e) and four bonds is (3C-4e) bridge bonds are lies perpendicular to plane.

(3) The bond lengths of terminal Al−Cl bonds are shorter (206 pm) while bond length of bridged Al−Cl bonds are longer (221 pm) and also outer bond angle (Cl−Al−Cl) are greater (118) than inner bond angle (Al−Cl−Al).

(4) Maximum six atoms are lies in same plane which four terminal Cl and two aluminium atoms.

(5) AlCl3 in vapor phase or in non-polar solvent is dimeric Al2Cl6 hence sp3 hybridised having 3c-4e bonds.

Related Questions: 

(1) Why aqueous solution of AlCl3 is acidic in nature ?

(2) What happen when aq AlCl3 react with Acid or Base?

(3) Although anhydrous aluminium chloride is covalent but its aqueous solution is ionic in nature. Why?

(4) Why BF3 do not exist as dimer?. Explain.

(5) Why B-F bond length in BF3 is shorter (130 pm) than B-F bond Iength in BF4- (143 pm)?. Explain.

(6) B-F bond length in BF3 is shorter than B-F bond length in (BF4)- why?

(7) When B2H6 is allowed to react with following Lewis bases, then how may given Lewis base form adduct through symmetrical Cleavage of B2H6.

(8) What is product of reaction between diborane (B2H6) and ammmonia (NH3)?

(9) Why methylation of Diborane (B2H6) replace four hydrogen only ?

(10) What is Use of Boric Acid?

(11) What is use of Orthoboric acids?

(12) What is basicity of "Boric acid" ?

(13) Why Boric acid exist in solid state ?

(14) What is structure of solid Ortho Boric acid ?

(15) What is effect of heat on Borax?

(16) What is the structure of trimetaboric acid and trimetaborate ion?

(17) What is the Sodium per borate ,give the structure and its uses?

(18) Why aqueous solution of borax reacts with two moles of acids ?

(19) What is the molecular formula of Borax ?

(20) Why Boric acid become strong acid in the presence of cis 1,2-diol or 1,3-diol ?

(21) Why Borazine is more reactive than benzene towards Electrophic Aromatic substitution reactions ?

(22) Why Borazine (B3N3H6) is also known as inorganic benzene ?.

(23) Four-center two-electron bond (4C-2e Bond): Structure of AlCl3:

(24) What is the difference between the structure of AlCl3 and diborane?


What is the difference between the structure of AlCl3 and diborane?

The hydrogen bridged dimer B2H6 contains two three centre (3C-2e bond), two electron bonds, A three-centre bond uses two electron to link three atoms, and  four two centre, two electron bond (2C-2e). 3C-2e bridge bond is perpendicular to plane in which 2C-2e bond present.
In contrast the bridge bonding in Al2Cl6 contains two three centre (3C-4e bond), four electron bonds  which can be described in terms of electron pair bond in which a chlorine atom bonded to one aluminium atom act as a Lewis base by donating a lone pair of electrons to the aluminium atom which acts as a Lewis acid.

Related Questions: 

(1) Why aqueous solution of AlCl3 is acidic in nature ?

(2) What happen when aq AlCl3 react with Acid or Base?

(3) Although anhydrous aluminium chloride is covalent but its aqueous solution is ionic in nature. Why?

(4) Why BF3 do not exist as dimer?. Explain.

(5) Why B-F bond length in BF3 is shorter (130 pm) than B-F bond Iength in BF4- (143 pm)?. Explain.

(6) B-F bond length in BF3 is shorter than B-F bond length in (BF4)- why?

(7) When B2H6 is allowed to react with following Lewis bases, then how may given Lewis base form adduct through symmetrical Cleavage of B2H6.

(8) What is product of reaction between diborane (B2H6) and ammmonia (NH3)?

(9) Why methylation of Diborane (B2H6) replace four hydrogen only ?

(10) What is Use of Boric Acid?

(11) What is use of Orthoboric acids?

(12) What is basicity of "Boric acid" ?

(13) Why Boric acid exist in solid state ?

(14) What is structure of solid Ortho Boric acid ?

(15) What is effect of heat on Borax?

(16) What is the structure of trimetaboric acid and trimetaborate ion?

(17) What is the Sodium per borate ,give the structure and its uses?

(18) Why aqueous solution of borax reacts with two moles of acids ?

(19) What is the molecular formula of Borax ?

(20) Why Boric acid become strong acid in the presence of cis 1,2-diol or 1,3-diol ?

(21) Why Borazine is more reactive than benzene towards Electrophic Aromatic substitution reactions ?

(22) Why Borazine (B3N3H6) is also known as inorganic benzene ?.

(23) Four-center two-electron bond (4C-2e Bond): Structure of AlCl3:

(24) What is the difference between the structure of AlCl3 and diborane?

Chloroform is more acidic than fluoroform why?


We know that acidic strength of the acid also depends upon stability of conjugate bases, so for relative strength of acid, we need to check the relative stabilities of their conjugate bases.

                                                     CF3-, CCl3-, CBr3- CI3-

We are expecting the acidic strength haloform acids as CHF3, CHCl3, CHBr3, CHI3 in decreasing order. Because Fluorine is most electronegative atom so it would be stabilize CF3- more, as electronegativity decreases from F to I the stability of conjugate -ve ion would be but that is not correct the actual order is CHCl3 > CHF3 > CHBr3 > CHI3.

This is because there is effective back bonding in CCl3- and hence the negative charge partially gets stabilised by back donation to the vacant 3d orbitals of Cl. Thus, CHCl3 is a stronger acid than CHF3 and also among them due to 2pπ-3dπ back bonding.



The acidic strengths of the other three haloforms can be compared the inductive effects of their anions. F is very electronegative and hence stabilises the negative charge on the C atom. So, CHF3 is a better acid than CHBr3, and the least acidic is CHI3.

The overall acidic strength order is, CHCl3 > CHF3 > CHBr3 > CHI3.

Related Questions:

  1. How is base strength related to the availabihty of the electron-pair?
  2. Amines are more basic than ammonia why?
  3. What is relative basic strength order 1° amines , 2°amines and 3° amines ? Explain:
  4. Give an explanation for the fact that Guanidine NH=C(CH3)2 is a stronger base than most of amines?
  5. Arrange in correct order of basic Character of aniline, pyrrol, pyridine and piperidine?
  6. What is correct basicity order of pyridine, pyridazine, pyrimidine and pyrazine ?
  7. Give an explanation for the fact that Guanidine NH=C(CH3)2 is a stronger base than most of amines?
  8. Arrange in correct order of basic Character of aniline, pyrrol, pyridine and piperidine?
  9. What is correct basicity order of pyridine, pyridazine, pyrimidine and pyrazine ?
  10. Why pyridine is more basic than Pyrrole?
  11. Why pyrimidine is less basic than pyridine?
  12. Imidazole is more basic than pyridine? Why?
  13. Biological Important of Imidazole and structure:
  14. Pyridine is almost 1 million times less basic than piperidine? Why?
  15. Cyclohexylamine amine is the stronger base than Aniline? Why?
  16. Tetrahydroquinoline amine is the stronger base than Tetrahydroisoquinoline? Why?
  17. Arrange the following in the order of increasing basicity : p-Toluidine, N, N-Dimethyl-p-toluidine, p-Nitroaniline, Aniline. (I.I.T.1986)
  18. Arrange the following in the increasing order of their acid strength : Methyl amine, Dimethyl amine, Aniline, N-methyl aniline (I.I.T, 1988).