[psychoNOT]

Why is TiCl4 a Lewis acid and not a base?  In my O Chem book it says Lewis acids have an empty low energy orbital and thus can accept an electron pair, or can donate a H⁺, while bases can donate a nonbonding electron pair.  Ti and Cl both have complete octets so wouldn't the Cl's be able to "donate" any one of their three nonbonding e^- pairs and then share it with i.e. H⁺? Ti wouldn't have any empty orbitals either (right?) because with 8 e^- it would have a filled 4s and then four of the 3d orbitals would be half filled and one would be filled... or is it because Cl is so electronegative, the bonds have a lot of ionic character and thus the electrons are more or less owned by only Cl?  Please let me know if any of my logic is off... and also why this is the case.

[Markovnikov]

Ti has empty d-orbitals which are a lot lower in energy than the chlorides.

[psychoNOT]

Why are they empty?

[stewie griffin]

Well I'm an organic chemist and I won't lie that I don't have the firmest grasp on transition metals. But here goes my explanation.
Remember that all elements have d orbitals. Hydrogen, lithium, carbon, etc all have d orbitals... it's just that the d orbitals in those elements are so high in energy that they never get used. This changes for sulfur and phosphorous where the d orbitals are low enough in energy that the d orbitals can be populated.
So titanium is 4s²3d²for it's valence electrons, right. The d orbitals in the 3rd shell can fit up to 10 electrons (there's 5 d orbitals in each shell, each orbital can hold 2 electrons). We've only filled them up with 2 electrons as is. Now bring in the four chlorine atoms, each with an electron to form a bond with titanium. Where can these electrons go? They go into the d orbitals. So now we've got in 6 electrons in the d orbitals of titanium, so that leaves room for 4 more. These empty d orbitals are what allow TiCl₄ to serve as a Lewis acid.
Hope that makes some sense.

[psychoNOT]

Ooooh.  Ok I get it now, I thought he meant empty as in empty, not half-filled.  I just figured it would be more difficult to fill the rest of the orbitals as they already have electrons but I guess that doesn't really make sense now that I think about it as they would be getting closer to a pseudo-noble gas configuration.  Thanks for the explanation... also thanks for telling me that all elements have d orbitals, they just aren't used... apparently my chemistry professor forgot to mention that when we were discussing general chemistry?  Don't know how I made it through that entire course without being told that.

[Rathal0s]

I actually think I may have the same book and came across the same problem thanks for the info but I'm fuming that this problem was given and no explanation was given to why this is a lewis acid. This book hasn't even gone into d orbitals. I don't know how they'd imagine anyone would have gotten this.

[orgopete]

I don't have that reaction in my book either, so I don't have any description about how it reacts. I can sympathize with anyone trying to understand a problem if the prior information is missing or too abstract to understand. I have seen a few examples of this so if your book has committed this error, you are probably not alone.

This was a straw that broke the camels back for me with a textbook (though I later returned to it as I learned it actually was a lot better than the alternatives). I had assigned some problems, but the real explanation was only present in the solutions manual. I have gotten over that now and consider the solutions manual an extension of the textbook itself.

In my book, it was my objective to render all reactions understandable by simple logic. I may have gone too far in the opposite direction, but I didn't expect a student to solve a problem unless they could write at least one mechanism of that type. 

[psychoNOT]

I actually think I may have the same book and came across the same problem thanks for the info but I'm fuming that this problem was given and no explanation was given to why this is a lewis acid. This book hasn't even gone into d orbitals. I don't know how they'd imagine anyone would have gotten this.

Was it McMurry's Organic Chemistry 7th Edition?  I just started reading it in my spare time a week or two ago... I really really liked how many exercises they had.. throughout the chapters and then at the end of each... however this problem made me doubt it a little... nowhere could I find the answer, and it just seems like in random places things just arent explained... at all, and they expect you to know how they work.  For that reason I have switched over to Clayden's Organic Chemistry book... it is much more well put together (thus far, only in the 2nd chapter), and it definitely seems like concepts are explained in much better detail.  The only drawback I foresee is that it seems to have much less exercises... but obviously I can supplement that with an organic chemistry workbook of some sort...

[Rathal0s]

Hahah yes, that's the book I'm using. I don't like to go forward in anything until I feel I have a full understanding for it and I literally spent like 2 hours back tracking through my reading and found nothing on it. And yes I agree I like how many exercises but throwing in problems like that are frustrating without an explanation. I have the study guide as well that gives the answers to compare with afterwards and even there it doesn't give a proper explanation of why it's considered a Lewis Acid.

[Marvinthefish]

Was it McMurry's Organic Chemistry 7th Edition? 

I can't stand that book! Buying it was a big mistake. And it was not even prescribed for my course!

Clayden, Greeves, Warren, Wothers for the win