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An unknown, foul-smelling hydrocarbon gives the mass
Chapter 20, Problem 23SP(choose chapter or problem)
An unknown, foul-smelling hydrocarbon gives the mass spectrum and infrared spectrum shown.
(a) Use the mass spectrum to propose a molecular formula. How many elements of unsaturation are there?
(b) Use the IR spectrum to determine the functional group(s), if any.
(c) Propose one or more structures for this compound. What parts of the structure are uncertain? If you knew that hydrogenation of the compound gives n-octane, would the structure still be uncertain?
(d) Propose structures for the major fragments at 39, 67, 81, and 95 in the mass spectrum. Explain why the base peak is so strong.
Equation Transcription:
Text Transcription:
M^+
m/z
({mu}m)
(cm^{-1})
Questions & Answers
QUESTION:
An unknown, foul-smelling hydrocarbon gives the mass spectrum and infrared spectrum shown.
(a) Use the mass spectrum to propose a molecular formula. How many elements of unsaturation are there?
(b) Use the IR spectrum to determine the functional group(s), if any.
(c) Propose one or more structures for this compound. What parts of the structure are uncertain? If you knew that hydrogenation of the compound gives n-octane, would the structure still be uncertain?
(d) Propose structures for the major fragments at 39, 67, 81, and 95 in the mass spectrum. Explain why the base peak is so strong.
Equation Transcription:
Text Transcription:
M^+
m/z
({mu}m)
(cm^{-1})
ANSWER:
Solution 23SP
(a)
The M+ peak is at 110. Since the compound is a hydrocarbon, the only molecular formula that fits is C8H14. Hydrocarbons normally have a molecular formula in the form CnH n+2, meaning that this compound has two elements of unsaturation.
(b)
We know that we have two elements of unsaturation in the molecule. One of them could be a ring, but cycloalkanes are not readily identifiable in an IR spectrum. A double bond would have a peak around 1625 cm-1, which is not present in this IR spectrum. An unsymmetrically-substituted alkyne (triple bond) should have a small peak at around 2200 cm