IR Spectra Qs from TPR

[capn jazz]

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Question from TPR Science Workbook (pg 708, Q 2)

When concentrated sulfuric acid is added to ethanol, two different reactions are possible:

CH3CH2OH + H2SO4 ---> CH3CH2OSO3H + H2O

CH3CH2OH + H2SO4 ----> CH2-(double bond)--CH2 + H2O

Which of the following observations about the infrared spectrum would indicate that Rxn 1 (the top one) had occurred?

a) The disappearance of an O-H stretch.
b) The lack of an alkene stretch
c) an increase in the intensity of the C-H stretch
d) the appearance of a sulfonic group stretch.

The answer is B: "In both cases, there will be O-H stretches and sulfonic group stretches. The easiest way to distinguish the rxns is by the presence of absence of the alkene stretch."

Now, call me crazy, but I do NOT see any sulfonic group in rxn 2. I, naturally, chose D because a sulfonic group means rxn 1 has occurred. I thought the O-H stretch disappeared in both, and the C-H stretch wouldn't really get any more intense.

The lack of an alkene stretch does NOT mean producr one has formed - it could also mean the rxn simply didn't proceed! Anyway, I don't se why D isn't the answer.

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Also, TPR is giving me some worries about the alcohol stretch. I've always thought of O-H as a 3300-3600 broad peak. In one passage, they state there's a 3000 stretch, and that this indicates and alcohol, but in the next passage, they state that a peak of 3100 does NOT indicate an alcohol, and in the answers, they say "an alcohol is 3300-3600."

So what's the deal for the MCAT? 3300-3600? Or is the lower bound actually lower than 3300? And why the hell isn't TPR internally consistent?

 

[sv3]

i use TPR and remember that question. Remember, TPR explanations are a good way to not just to check, but to learn. I find their explanations as good if not better than their content review

The sulfonic group is also a reactant. So IR spec will show it before and after as its a product as well.

Regarding the alkene thing, its not wrong. The others are all wrong. Sometimes if I can't say for sure something is right, i just disprove the other 3 choices.

 

[chemtopper]

D choice is not good at all because sulfuric acid is present in reaction 2 also.We have to choose the best answer and that is B.
B clearly indicates that reaction 2 had not occurred.
IR spectrum bands get shifted due to other groups present with alcohol.Like aromatic alcohol.aliphatic or some different group can shift the stretch ,but basically band will be broad and in the range of 3000cm-1 to 3500cm-1