15. Analytical Techniques:IR, NMR, Mass Spect

Propose the number of carbons for a compound that exhibits the following peak in its mass spectrum: 

(M)+• at m/z = 72, relative height = 38.3% of base peak

(M+1)+• at m/z = 73, relative height = 1.7% of base peak 

Predict the approximate height of the (M + 1) peak for the molecule icosane, molecular formula C₂₀H₄₂.

1,4-dichlorobenzene is used as an insecticide and has the formula C₆H₄Cl₂. What would be the approximate relative heights of M, M+2, and M+4 peaks. For simplicity assume that all carbon atoms are ¹²C and the ³⁵Cl:³⁷Cl ratio is 75:25.

Identify which of these four mass spectra indicate the presence of sulfur, chlorine, bromine, iodine, or nitrogen. Suggest a molecular formula for each.

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A compound gives a mass spectrum with essentially only three peaks at m/z = 77 (40%), 112 (100%), and 114 (33%). Identify the compound.

Predict the relative intensities of the molecular ion peak, the M+2 peak, and the M+4 peak for a compound that contains two bromine atoms.

Besides the molecular mass, how do you know that the fragment that gives rise to the peak at m/z 111 in the mass spectrum for 1-bromo-4-chlorobenzene contains only chlorine?

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What ratio of M, M + 2 , M + 4 and would you expect for 1,6-dibromohexane?

The presence of what atom is indicated by the following mass spectrum?

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The molecule that gave the mass spectrum shown here contains a halogen. Which halogen is present?

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Consider the following four structures, followed by mass spectral data. Match each structure with its characteristic molecular ion or fragment. In each case, give a likely structure of the ion responsible for the base peak.

(a) base peak at 105

(b) base peak at 72

(c) M⁺ doublet at 198 and 200, base peak at 91

(d) base peak at 91, large peak at 43