Q.12-21E - Organic Chemistry (Mcmurry)

Question

Question: Halogenated compounds are particularly easy to identify by their mass

spectra because both chlorine and bromine occur naturally as mixtures

of two abundant isotopes. Recall that chlorine occurs as ³⁵Cl (75.8%)

and ³⁷C (24.2%); and bromine occurs as ⁷⁹Br (50.7%) and ⁸¹Br (49.3%).

At what masses do the molecular ions occur for the following formulas?

What are the relative percentages of each molecular ion?

(a) Bromomethane, CH₃Br

(b) 1-Chlorohexane, C₆H₁₃Cl

Step-by-Step Solution

Answer

Answer

(a) Bromoethane, occurs at 94 and 96 with relative intensity 1:1

The relative percentages of isotopes are 51:49

CH₃Br⁷⁹  CH₃Br⁸¹

(M⁺ = 94)        (M⁺ =96)

(b) 1-Chlorohexane, occur at 120 and 122 with relative intensity 3:1

C₆H₁₃Cl³⁵ C₆H₁₃Cl³⁷ 

(M⁺ = 120)        (M⁺ =122)

The relative percentages of isotopes are 76:24

Step 1: Isotopes

An isotope is one of two or more species of atoms of a chemical element with the same atomic number and position in the periodic table, and nearly identical chemical behavior but with different physical properties and atomic masses. For example, carbon having isotopes and here, ¹²C , ¹³C and  ¹⁴C  here,12,13, and 14  represents the number of neutrons.

Step 2: Calcuate the relative percentage andmasses at which molecular ions occurs

In order to simplify the problem neglect the ¹³C  and ²H isotopes in determining the molecular ion of these compound.

(a) The formula weight of -CH₃ is 15, and the atomic masses of two bromine isotopes are 79 and 81. The two molecular ions of bromomethane occur at M⁺ = 94(50.7%), and M⁺ =96 (49.3%). The relative percentages of isotopes are 51:49

 (b) The formula weight of -C₆H₃ is 85, and the atomic masses of the two chlorine isotopes are 35 and 37. The two molecular ions of 1-chlorohexane occur at M⁺ = 120 (75.8%) and M⁺ = 122 (24.2%). The relative percentage of isotopes are 76:24.