Chlorine and bromine give rise to characteristic isotopic patterns in the mass s
ID: 1061220 • Letter: C
Question
Chlorine and bromine give rise to characteristic isotopic patterns in the mass spectrum. Chlorine has two stable isotopes, 35Cl and 37Cl in a ratio of 3:1. Hence, if a mass spectrum shows two ions separated by 2 mass units in a 3:1 ratio, it is likely to contain one chlorine.
Bromine has two stable isotopes, 79Br and 81Br in a 1:1 ratio. If a mass spectrum shows two ions separated by 2 mass units in a 1:1 ratio, it is likely to contain one bromine. An ion that contains 2 bromines will show a 1:2:1 pattern, so on an so forth.
. An optically active hydrocarbon gives the electron ionization mass spectrum reproduced to the left below. It absorbs only one mole of H2 upon catalytic hydrogenation to give a single, achiral product. The EI mass spec of the hydrogenation product is reproduced to the right below.
A. What is the molecular formula of the optically active hydrocarbon? How many degrees of unsaturation does that formula represent?
B. What is the molecular formula of the achiral hydrogenation product? How many degrees of unsaturation does that formula represent?
100 30 A 40 0.0 t 0.0 15 30 45 75 10 80 60 20 90 0.0 20 40 60 m/z 80 100Explanation / Answer
Electron ionization mass spectrum of optically active hydrocarbon shown molecular weight is 82 (m/z = 82). Molecular weight of achiral hydrogenated product is 84 (m/z = 84).
Since, hydrocarbon contains only carbon and hydrogen atoms, molecular formula should be according to general formula CnH(2n+2). Based on degree on unsaturation, number of hydrogen atoms will reduce by 2 for each degree of unsaturation.
Thus, when n = 6, we get molecular weight of fully saturated molecule as 86 (for C6H14). Given chiral molecule has molecular weight of 82, thus, molecular formula should be C6H10.
Degree of unsaturation of chiral molecule = (No. of C atoms ) - (No. of H atoms/2) +1
Degree of unsaturation of chiral molecule = 6 - 5 + 1 = 2
Molecular formula of unsaturated achiral molecule should be C6H12
Degree of unsaturation of achiral molecule = (No. of C atoms ) - (No. of H atoms/2) +1
Degree of unsaturation of achiral molecule = 6 - 6 + 1 = 1
Therefore, molecular formula of unsaturated chiral molecule is C6H10, with degree of unsaturation 2.
Molecular formula of saturated achiral molecule is C6H12, with degree of unsaturation 1.
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