An unknown compound is found to contain 22.1% carbon and 4.6% hydrogen by mass.

Question

An unknown compound is found to contain 22.1% carbon and 4.6% hydrogen by mass. The mass spectrum of this compound contains two molecular ion peaks, of equal intensity, at m/z 108 and 110. The 1H NMR spectrum of this compound is shown below.

1)What is the molecular formula of this compound?

a) C2H5Br

(b) C2H5Cl

(c) C2H4Br

(d) CH5Br

(e) Impossible to tell from the information given

2)Why are there two molecular ion peaks, of equal intensity, in the mass spectrum?

a) Some of the molecules in the sample contain 13C

(b) Some of the molecules in the sample contain 79Br, while others contain 81Br

(c) Some of the molecules in the sample contain 35Cl, while others contain 37Cl

(d) Experimental error

(e) Some of the molecules in the sample contain 2H

3)What is the index of hydrogen deficiency (IHD)? 4) How many hydrogen environments exist in this compound?

5)What is the likely structure of this compound?

Explanation / Answer

(1) The most dramatic feature of the mass spectra of alkyl chlorides and alkyl bromides is the presence of an important M + 2 peak. This peak arises because Cl and Br are present in nature in two isotopic forms.

Ratio of M:M+2 for chlorine is 3:1

Ratio of M:M+2 for bromine is 1:1.

It is given in the question two molecular ion peaks, of equal intensity are observed. This shows the presence of bromine atom.

The correct option is a) C2H5Br

(CH5 does not exist)

(2) The correct option is (b) Some of the molecules in the sample contain 79Br, while others contain 81Br

81Br is the isotope of 79Br. Both the them are present in almost same abundance in nature.

(3) Index of hydrogen deficiency (IHD) is the count of how many multiple bond and rings are present in struture.

For the given compound, CaHbNcOdXe

IHD = 0.5 x [2a+2-b-e+c]

For C2H5Br, IHD = 1/2[(2x2)+2- 5- 1] = 0

(4) Two hydrogen environment exist according to the structure

CH3-CH2Br

(5) Structure - CH3-CH2Br

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