Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: gangara1 on December 15, 2004, 09:01:03 AM
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III. Write the structure of the product(s) formed when 1-methylcyclohexanol is reacted
with 85% H3PO4 at 150 C. Write a detailed mechanism for the reaction (l think that it can be (CH2=C-CH2-CH2-CH2-CH2-CH2 (cyclo)AND CH3-CH=CH-CH-CH2-CH2-CH2 (cyclo) BUT WHY WE USE 85% H3PO4 at 150 C
CAN YOU EXPLAIN AND SHOW TH?S PLEAS ???
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I think that you have predicted the correct products. Phosphoric acid is used just because it is a strong acid. I suppose something like sulfuric acid could have been used as well, although an acid that strong could promote the reverse reaction by protonating the alkene.
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I think that you have predicted the correct products. Phosphoric acid is used just because it is a strong acid. I suppose something like sulfuric acid could have been used as well, although an acid that strong could promote the reverse reaction by protonating the alkene.
This is strange. A few people here on the boards have stated that Phosphoric Acid is a 'strong acid', but EVERYWHERE I've looked and read has stated that phosphoric acid is a weak acid. Did you mean strong for a 'weak acid'?
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I guess I hadn't thought of it that much, but I looked it up. Phosphoric acid has a pKa of 2.12 (first proton) which makes it stronger than acetic acid (pKa = 4.76), formic acid (pKa = 3.77), and carbonic acid (pKa = 3.6, first proton). It's acidity is about the same as o-nitrobenzoic acid (pKa =2.17). I suppose that would put it in the range of "weak" acids, but it's a pretty strong weak acid. ;D
It's all relative though, right? Certainly phosphoric acid is much less acidic than triflic acid, but it's apparantly strong enough to be useful in elimination reactions. I think it is especially useful for this purpose because it is high boiling and the conjugate base in not very nucleophilic, thereby minimizing SN1 type products you would expect from acids like HCl.
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pretty strong weak acid
Classification "strong-weak" acids (or base) allows terms: strong acids (Ka .GE. 10) or weak acids (Ka .LT. 10). When compare acidity of two acids we should rather use term: stronger or weaker then reference acid (eg. acetic acid)
Concerning use of 85 % H3PO4 (not 100 %) - Pure H3PO4 has m.p. 43 C and is glassy solid
difficult to handle becuse of its hygroscopicity and very high viscosity at concentration close to 100 %. Pure H3PO4 starts to decompose (partial dehydration) at 165 C. Commercially avaiable 85 % H3PO4 is a liquid with boiling point 162 C (a few centidegree before decomposition of H3PO4).
And at last, concerning the mechanism of dehydration by phosphoric acid - I think H3PO4 forms esters (under acidic catalysis of its own protons) and at 150 C these esters pyrolyse to alkene and H3PO4.
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Isn't hydrochloric acid also a strong acid? So, it's conjugate base wouldn't be that nucleophilic either, right? Well, it also depends wether it's protic or aprotic...
arghhhhh there's just too much possibilities in Organic x(
I've been reading the Francis A Carey Organic Chemistry 5th Ed. book, and I'm finding sooooo many contradictions in it. It just gets confusing after a while.
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Cl- isn't a great nucleophile, but when it's the only thing around it can do nucleophile type reactions.
I found that Carey book very difficult to read, so I finally just stopped reading it and went off my lecture notes alone. It's tough to find a good physical organic textbook. One of the professors at my school has written a new textbook that is due out in March. I'm excited to see it; hopefully it will avoid the problems of a lot of the other texts.
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Yeah, all of us in the physical organic community have been waiting for it. Bergman has givin it a glowing review. Lowry and Richardson is a fine book, but sometimes overly complicated. I read the parts of the watered down Carey book to make the nonsense of Lowry and Richardson make sense. It worked fine, just took forever to finish reading a single chapter.
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I haven't seen the Lowry book. Is it worth it or should I just wait for Dougherty's to come out? He's teaching a physical organic class next quarter that I am definitely going to sit in on. I assume he will have advance copies of the text available for the students. I don't really want to take the class for credit though, so I hope I can get it at the bookstore without any problems.
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Lowry and Richardson is a good book, I like it. I would suggest you just wait for the new physical-organic book though.