Which of the following compounds will give a molecular ion having m/z = an odd n

Question

Which of the following compounds will give a molecular ion having m/z = an odd number? CH_3 CH_2 OH A. CH_2 H_3 CI B. CH_3 CO_2 H C. CH_3 CH_2 NH_2 D. (CH)_2 NCH_2 C = N Which of the following statements best describes the meaning of the following species [CH_3 CH_2 CH_3] + ? A) It is the molecular ion of propane. B) It is the parent ion of propane C) It is the radical cation of propane D) The m/z value is 44. E) all of the above. Which of the following m/z values is the molecular ion for 2-butanone? A) 15 B) 29 c) 43 D) 57 E) 72 Which of the following is true about the base peak in mass spectrometry? A) The m/z value equals the molecular weight of the compound. B) The m/z value corresponds to a very stable carbanion C) It has the largest peak height in the spectrum D) It has the highest m/z value of all the peaks in the spectrum. E) The base peak is assigned a relative abundance equal to that of the parent ion. What m/z value would you predict for the most stable fragment peak in the mass spectrum of 4-ethylheptane? A) 128 B) 127 C) 113 D) 99 E) 85 How could you distinguish the mass spectrum of 2, 2-dimethylpropane from that of isopentane? Which of the following is not a major peak in the MS of isopentane? A) 29 B) 43 C) 57 D) 60 E) 72

Explanation / Answer

3. Answer is C (molecular ion having odd no nitrogens have odd mass, CH3CH2NH2)

6. Base peak always contains largest height in the mass spectrum, so answer is C

7. Answer is 128. Since it forms stable tertiary carbocation by loss of H.

8. 2-2-Dimethylpropane (neopentane): You will observe a base peak at m/z=57 (loss of methyl). This corresponds to loss of a methyl group and formation of the stable t-butyl cation, (CH3)3C+.

Loss of CH3(M15).This will give a peak at m/z=57, corresponding to CH3CH+CH2CH3.

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