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).

Fluorine is more electronegative than chlorine even then, p-flurobenzoic acid is weaker acid than p-chlorobenzoic acid explain ?

Since halogens are more electronegative than carbon and also possesses lone pair electrons , therefore they exert both -I and +R effects . Now in F , the lone pair of electrons are present in 2p-orbitals but in Cl , they are present in 3p-orbitals . Since 2p- orbitals of fluorine and carbon are almost  equal size , therefore , the +R effect is more pronounced in p- flurobenzoic acid than p-chlorobenzoic acid.

 
Thus in p- flurobenzoic acid +R effect is out weight the -I effect but in p-chlorobenzoic acid, it is the -I effect which is out weight  the +R effect. Hence p- flurobenzoic acid is weaker acid than p-chlorobenzoic acid.

Trifluoroethyl carbocation is less stable than trifluromethyl carbocation why ?

In case of trifluroethylcarbocation the highly electronegative fluorine atom , withdraws shared pair electrons between C-F bonds towards itself to great extent and intensifies positive charge on carbon. Greater the intensification of charge more is the instability.theirfore this carbocation is destabilised.

In case of trifluroethylcarbocation carbocation, the unshared paired of electrons in P - Orbital of each of the fluorine can be shifted into vacant Orbital of carbon atom of carbocation via P-P overlap.

It's leads to dispersal of charge and provide stability to the carbonation.

Stability of Heteroatomic carbocations:

The carbocaton contaning hetero atoms adjacent to positive carbon of cation such as oxygen, nitrogen and sulphur etc which are more electronegative than carbon, you might expect that they would by definition be electron withdrawing groups that destabilize carbocations.

But there is opposite effect, if the oxygen, nitrogen or sulphur atom is present at adjacent to carbocation, the overall effect is carbocation stabilization.

This is due to the fact that although these heteroatoms are electron withdrawing groups by induction, they are electron donating groups by resonance, and it is this resonance effect which is more powerful.

ILLUSTRATIVE EXAMPLE(1): Give the correct Stability of given carbocation .

SOLUTION: Sulphur containing carbocation is more stable because lone pair of sulphur atom show more resonating effect due to least electronegativity as compared to nitrogen as well as oxygen atom similarly  nitrogen show more resonance than oxygen atom

Conceptual Facts:

(1) If number of conjugation increases stability of carbocation increases ...

(2) We known that on increasing conjugation stability increases. their is exception in option (A) their is three nitrogen may involving in resonance but  actually not, 

 Because positive carbon does not involved in resonance due to bridge head carbon. we known according to Bredt,s rule bridge carbon cannot for duble bond.

Hence correct stability order is  (B) > (C) >> (A): 

(3)  Similarly 

Hence correct stability order is  (B) > (C) >> (A): 

ILLUSTRATIVE EXAMPLE(3): Give the correct Stability of given carbocation.

SOLUTION: (1):  (C) > (B) > (A)     and    (2):   (C) > (B) > (A)  

ILLUSTRATIVE EXAMPLE(4): Give the correct Stability of given carbocation.

SOLUTION: (1):  (D) > (B) > (C) > (A)     and    (2):  (D) > (B) > (C) > (A)  

Other important examples:

IIT UPDATE:

QUESTION(1)

SOLUTION: (D)