TiCl4 lewis acidity

[uclaLabrat]

Titanium tetrachloride (TiCl4) is more lewis acidic than titanium tetraalkoxides, like titanium tetraisopropoxide. Why? Chlorine is less electronegative than oxygen, is larger so its electrons should donate more easily since the nucleus has a weaker hold on the valence electrons.

Thoughts?

 

[TecHNooB]

Gah, chemistry :|

 

[William Gaatjes]

How are the atoms organized ?
I have no chemical background, but i would think it is because of the way the atoms are organized(distances apart). The electrons move around over the titanium atom and the chlorine atom in a different way when compared to titanium tetraaloxides. I think it is not a good idea to see the atoms as separate. When combined as in your example, they form a single entity and the electron distribution will also behave that way. If you read about the latest findings about proteins, it may explain what you seek...

The "distance" of electrons from a nucleus means something totally different from your perception of distance.

Perhaps this can help :

http://en.wikipedia.org/wiki/Quantum_mirage

The mirage occurs at the foci of a quantum corral, a ring of atoms arranged in an arbitrary shape on a substrate. The quantum corral was demonstrated in 1993 by Lutz, Eigler, and Crommie[2] using an ellipitical ring of cobalt atoms on a copper surface. The ferromagnetic cobalt atoms reflected the surface electrons of the copper inside the ring into a wave pattern, as predicted by the theory of quantum mechanics.

The size and shape of the corral determine its quantum states, including the energy and distribution of the electrons. To make conditions suitable for the mirage the team at Almaden chose a configuration of the corral which concentrated the electrons at the foci of the ellipse.

When scientists placed a magnetic cobalt atom at one focus of the corral, a mirage of the atom appeared at the other focus. Specifically the same electronic properties were present in the electrons surrounding both foci, even though the cobalt atom was only present at one focus.

 

[Golgatha]

http://en.wikipedia.org/wiki/Inductive_effect

The net electronegativity of something like an isopropoxide is still less than that of chlorine. Same reason the pKa of fatty acids increases as your fatty acid chain gets longer. e.g. Acetic acid is more acidic than propionic acid.

 

[Murloc]

I think it's because isopropoxide as a whole is less electronegative than Cl, because the Hs around "screen" the O at the center.

 

[Turtle.Man]

Alkyl groups are electron- donating: this effect lowers the electron withdrawing potential of the alkoxy groiups with respect to Cl.

 

[996GT2]

Alkyl groups are electron- donating: this effect lowers the electron withdrawing potential of the alkoxy groiups with respect to Cl.

Yep. This is the inductive effect.

 

[Georgeisdead]

I think you have started to hit it on the head. Going back to the definition of a Lewis Acid as a substance that can donate a pair of electrons to another atom, I believe the TiCl4 can more readily donate electrons than the Ti{OCH(CH3)2}4 simply because TiCl4 has more free electrons available, and they are much more readily accessible.

Trust me, I make TiCl4 everyday at a titanium dioxide production facility.

 

[Turtle.Man]

George: Lewis acids are electron acceptors, not donors. The above replies supplied by myself and 996GT2 are the correct answer to the OP's question.

 

[Slammy1]

Sounds like a steric hinderance effect.

Acids accept, bases donate electrons. Think in terms of a proton.

 

[Turtle.Man]

The acidity of a Lewis acid is primarily determined by the magnitude of the electron withdrawing effect of the ligands. Other contributing factors are comparitively small.

 

[Slammy1]

The acidity of a Lewis acid is primarily determined by the magnitude of the electron withdrawing effect of the ligands. Other contributing factors are comparitively small.

I was talking about what the poster you were talking to was saying (great sentence structure there on my part). The original question is academic, really, if you substitute H- for Ti- you're asking why hydrochloric acid is stronger than acetic acid.

 

[Georgeisdead]

George: Lewis acids are electron acceptors, not donors. The above replies supplied by myself and 996GT2 are the correct answer to the OP's question.

You are correct Turtle, I mis-spoke. Had Lewis acid/base backwards. I still agree with you that the answers supplied are correct.

 

[Turtle.Man]

I was talking about what the poster you were talking to was saying (great sentence structure there on my part). The original question is academic, really, if you substitute H- for Ti- you're asking why hydrochloric acid is stronger than acetic acid.

Well, all of the atoms are important, in the sense that if you change any one of them, you're dealing with a different compound with altered electronic properties. I was trying to stay away from there... But, as far as the OP question, if you hold the central atom (Ti) fixed and start swapping out the ligands, the pKa of the resulting acid will change very predictably. Let's take some TiCl4, and replace one of the chlorines with a stronger electron withdrawing group. The resulting acid is stronger than TiCl4. Replace one of the chlorines with a weaker electron withdrawing group? The resulting acid is weaker than TiCl4. We get a whole new family of compounds if we swap out the central atom, but the ligand effect on pKa is still in force.

 

[Slammy1]

Perhaps I'm misunderstanding, I'm thinking something's strength as a Lewis Acid is different than pKa,

 

[Turtle.Man]

In aqueous solutions, it's pKa. It is defined differently in anhydrous media.