Introductory note on the mass spectrum of 1-bromo-2-methylethane

Students and teachers please note my explanation of the mass spectrum of 1-bromo-2-methylethane is designed for advanced, but pre-university, chemistry courses.

If M represents the 1-bromo-2-methylethane molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^-  ==> [M•]⁺_(g) + 2e^- and for fragmentation equations assume [M]⁺ is the start of the processes and all species are in a gaseous state.

I've not usually shown an unpaired electron on e.g. an ion or a non-ionised alkyl radical R e.g.

[M•]⁺ ==> [X]⁺  +  R•, but you should be aware this is a more accurate depiction of some processes.

I've used simplified equations to show how some of the ions that might be formed in the fragmentation pattern for the mass spectrum of 1-bromo-2-methylethane and only the formation of singly charged positive are considered for the mass spectrum of 1-bromo-2-methylethane.

I've included a stick diagram and table of m/z ions for the mass spectrum of 1-bromo-2-methylethane and doing the mass spectrum analysis under standard conditions, databases can be compiled based on complex fingerprint patterns, often involving the relative intensities of many fragment ions, and used to identify compounds including 1-bromo-2-methylethane.

In selected cases, where two different fragment ions have the same integer m/z value, I've pointed out that modern mass spectrometers can measure relative ion mass to four decimal places. So, using accurate isotopic masses, I've calculated the accurate ion masses, BUT strictly speaking, 0.0005 should be deducted for singly charged ions to account for the loss of the electron in their formation. I have NOT done this, but the mass spectrometer computer software does!

1-bromo-2-chloroethane,  C₂H₄BrCl,  BrCH₂CH₂Cl

The molecular structure and naming of haloalkanes

Interpreting the fragmentation pattern of the mass spectrum of 1-bromo-2-chloroethane

[M]⁺ is the molecular ion peak (M) with m/z values of 142, 144 and 146 corresponding to [C₂H₄BrCl]⁺, the original 1-bromo-2-chloroethane molecule minus an electron, [BrCH₂CH₂Cl]⁺

This is quite a tricky mass spectrum to interpret because of the two sets of isotopes of bromine and chlorine.

Since chlorine has two common isotopes of ³⁵Cl (~75%) and ³⁷Cl (~25%) in the ratio 3 : 1, you should observe double peaks in the intensity ratio 3 : 1, two mass units apart for molecular fragments containing a chlorine atom from the fragmentation of 1-chlorobutane (if the fragment has no bromine atom).

BUT, this is complicated by the isotopes of bromine too!

Bromine has two isotopes, ~50% ⁷⁹Br and ~50% ⁸¹Br, so any fragment carrying a bromine atom should show up as twin peaks of intensity ratio ~1:1, two mass units apart (if the fragment has no chlorine atom), but you will observe the Br-79 isotope containing fragment ion has just a bit smaller intensity.

So, what you observe in terms of abundances, is the result of two ratios, and therefore I'm keeping the interpretation as simple as I can (at pre-university level!).

However, you should be able to work out that the four possible parent molecular ions consist of:

m/z 142 [⁷⁹BrCH₂CH₂³⁵Cl]⁺, M-2 ion

m/z 144 [⁷⁹BrCH₂CH₂³⁷Cl]⁺, M ion (technically part of highest intensity peak)

m/z 144 [⁸¹BrCH₂CH₂³⁵Cl]⁺, M ion (as above, assuming integer values)

m/z 146[⁸¹BrCH₂CH₂³⁷Cl]⁺, M+2 ion

and in terms of probability the ratio would be: 3:1:3:1 (assuming 1:1 ratio for Br isotopes),

so the ratio observed for m/z values 142 : 144 : 146 will be 3 : 4 : 1 and you can see this on the mass spectrum diagram on the right for m/z values of the molecular ions 142, 144 and 146.

There is one Br and one Cl in each molecule of 1-bromo-2-chloroethane.

(Cross 79 and 81 with 35, 35, 35 and 37 in a probability chart to get the above ratios)

See Permutation ratios based on the isotopes of chlorine and/or bromine in organic halogen compounds

The most abundant ion of the molecule under mass spectrometry investigation (1-bromo-2-chloroethane) is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

The base peak ion is the m/z 63 ion [CH₂CH₂³⁵Cl]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 1-bromo-2-chloroethane.

Unless otherwise indicated, assume the carbon atoms in 1-bromo-2-chloroethane are the ¹²C isotope.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 1-bromo-2-chloroethane  (See TOF note at the end of the analysis)

Atomic masses: H = 1; C = 12 (~1% 13); Cl = 35 or 37 (~ration 3:1);  Br = 79 or 81 (ratio ~1:1);

Bond enthalpies = kJ/mol: C-C = 348;  C-H = 412; C-Cl = 338; C-Br 276

Equations to explain the most abundant ion peaks of 1-bromo-2-chloroethane

Formation of m/z 107 and 109 ions:

[BrCH₂CH₂Cl]⁺  ===>  [⁷⁹BrCH₂CH₂]⁺  or  [⁸¹BrCH₂CH₂]⁺  +  Cl

Scission of C-Cl bond, but weaker C-Br bond more likely to be broken (see below)

Where R is alkyl, the double RBr peaks of roughly 1 : 1 abundance ratio are characteristic of organo-bromine compounds (one m/z ion peak is slightly shorter than the other, technically 50.7 : 49.3).

Formation of m/z 93 and 95 ions:

[BrCH₂CH₂Cl]⁺  ===>  [CH₂⁷⁹Br]⁺  or  [CH₂⁸¹Br]⁺  +  CH₂Cl

Scission of C-C bond, strong bond, low probability.

Note the approximate 1 : 1 ratio of the ion intensities.

Again, the double RBr peaks of roughly 1 : 1 ratio are characteristic of organo-bromine compounds (one m/z ion peak is slightly shorter than the other, technically 50.6 : 49.4).

Formation of m/z 63 and 65 ions:

[BrCH₂CH₂Cl]⁺  ===>  [CH₂CH₂³⁵Cl]⁺  or  [CH₂CH₂³⁷Cl]⁺   +  Br

Scission of the C-Br bond, the weakest bond in the molecule.

The m/z 63 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Note the approximate 3 : 1 ratio of the ion intensities.

Where R is alkyl, the appearance of double RCl m/z ion peaks of roughly 3 : 1 abundance ratio are characteristic of organo-chlorine compounds i.e. caused by the 3 : 1 isotope ratio of ³⁵Cl : ³⁷Cl.

Formation of m/z 49 ion:

[BrCH₂CH₂Cl]⁺  ===>  [CH₂³⁵Cl]⁺  +  BrCH₂

Scission of C-C bond, strong bond, low probability.

Small peak at m/z 51 corresponding to  [CH₂³⁷Cl]⁺ formed by the same process and note the characteristic approximate 3:1 ratio for the two ions.

Again, note where R is alkyl, you get double RCl m/z ion peaks of roughly 3 : 1 abundance ratio are characteristic of organo-chlorine compounds i.e. caused by the 3 : 1 isotope ratio of ³⁵Cl : ³⁷Cl.

Formation of m/z 28 ion:

[C₂H₄Cl]⁺  ===>  [C₂H₄]⁺  +  Cl

[C₂H₄Br]⁺  ===>  [C₂H₄]⁺  +  Br

C-Br C-Cl bond scission of the m/z 63/65 or 107/109 pairs of ions to give an ionised ethene molecule. The C-halogen bonds are weaker than the C-C and C-H bonds.

Formation of m/z 27 and 28 ions:

[C₂H₄Cl]⁺  ===>  [C₂H₃]⁺  +  HCl

[C₂H₄Br]⁺  ===>  [C₂H₃]⁺  +  HBr

Elimination of hydrogen chloride or hydrogen bromide from the m/z 63/65 or 107/109 ions.

Formation of halogen ions

There only tiny peaks at m/z 35, 37, 79 and 81 resulting from the C-halogen bond scission of the parent molecular ion or halogen containing fragment ions.

R₃C-X ===> R3C  +  [X]⁺ (where R = H, alkyl or halogen)

Example of what high resolution TOF mass spectrometers are capable of

In the mass spectrum of 1-bromo-2-chloroethane, two of the molecular ions have the same integer m/z value of 144, because of the coincidence of the relative isotopic masses of bromine and chlorine isotopes.

BUT mass spectrometers can measure m/z values to four decimal places and you can calculate the precise relative mass value of any molecular or fragment ion using the following very accurate isotopic masses:

¹H = 1.0078  ¹²C = 12.0000  ³⁵Cl = 34.9689  ³⁷Cl = 36.9659 ⁷⁹Br = 78.9183  ⁸¹Br = 80.9163

Therefore the following molecular ions have the following relative mass

[⁷⁹BrCH₂CH₂³⁷Cl]⁺ has a relative ion mass of 143.9154

[⁸¹BrCH₂CH₂³⁵Cl]⁺ has a relative ion mass of 143.9164

The difference is only 0.001 relative mass units, but this is enough to distinguish the ions.

Note: Strictly speaking 0.0005 relative mass units should be subtracted from these values, I haven't, but the mass spectrometry software will.

 

Note on being able to differentiate between 1-bromo-2-chloroethane from 1-bromo-1-chloroethane from their mass spectra

Both molecules will have significantly different fragmentation patters overall.

BUT, 1-bromo-1-chloroethane will give many of the m/z ion peaks of 1-bromo-2-chloroethane

e.g. the four possible C₂H₄BrCl molecular ion peaks based on the permutations of bromine (79 and 81) and chlorine (35 and 37) isotopes.

However there will be some significant differences e.g. in the mass spectrum of 1-bromo-1-chloroethane you would expect to observe:

A peak from C-C bond scission of the parent molecular ion for the m/z 15 ion.

[CH₃-CHBrCl]⁺  ==> [CH₃]⁺  +  CHBrCl

This would be absent from the mass spectrum of 1-bromo-2-chloroethane with no methyl group.

There would be (traces) of m/z ions that 1-bromo-2-chloroethane cannot produce because both halogen atoms are attached to the same fragment ion.

e.g also from C-C bond scission of the parent molecular ion

[CH₃-CHBrCl]⁺  ==> [CHBrCl]⁺  +  CH₃

with m/z values of 127, 129 and 131, though this is far less likely than the formation of the m/z 15 ion..

The mass spectrum of 1-bromo-2-chloroethane (C₂H₄BrCl) is a classic example for showcasing isotopic patterns, fragmentation behavior, and interpretation strategy—especially useful for exam settings.

Prominent m/z Ions for the mass spectrum of 1-bromo-2-chloroethane and their origins (see also below)

Common Misconceptions about the mass spectrum of 1-bromo-2-chloroethane (see also above)

Exam Tips for questions involving mass spectrum 1-bromo-2-chloroethane

• Label isotope clusters: Expect M, M+2, M+4 peaks for molecules with two halogens.
• Use abundance ratios: Br (⁷⁹Br/⁸¹Br ≈ 1:1), Cl (³⁵Cl/³⁷Cl ≈ 3:1) to deduce peak identities.
• Identify base peak logic: Often corresponds to the most stable fragment, not the parent ion.
• Practice isotope combinations: For C₂H₂BrCl, calculate all molecular ion M⁺ combinations:
  • ¹²C₂ + ¹H₄ + ⁷⁹Br + ³⁵Cl → m/z 142
  • ¹²C₂ + ¹H₄ + ⁷⁹Br + ³⁷Cl and ⁸¹Br + ³⁵Cl → m/z 144
  • ¹²C₂ + ¹H₄ + ⁸¹Br + ³⁷Cl → m/z 146

Practice questions based on the mass spectrum of 1-bromo-2-chloroethane BrCH₂CH₂Cl

Two advanced-level multiple choice questions on the mass spectrum of 1-bromo-2-chloroethane (C₂H₄BrCl), complete with model answers and distractor analysis.

These are tailored for AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB, and US AP/Honors chemistry syllabi.

Question 1: Molecular Ion Peak Pattern in the mass spectrum of 1-bromo-2-chloroethane

Which of the following best explains why the molecular ion region of 1-bromo-2-chloroethane shows three peaks at m/z 142, 144, and 146 in a 3:4:1 ratio?

1. The molecule fragments into three different ions of similar mass.
2. The molecule contains carbon-13 atoms, causing isotopic splitting.
3. The molecule contains both bromine and chlorine, each with two major isotopes.
4. The molecule undergoes rearrangement during ionization, forming three isomers.

Correct Answer: C

Explanation:

• Bromine isotopes: ⁷⁹Br and ⁸¹Br (≈50:50)
• Chlorine isotopes: ³⁵Cl and ³⁷Cl (≈75:25)

These combine to give:

• ⁷⁹Br + ³⁵Cl → m/z 142
• ⁷⁹Br + ³⁷Cl or ⁸¹Br + ³⁵Cl → m/z 144
• ⁸¹Br + ³⁷Cl → m/z 146

The relative intensities reflect the statistical combinations: 3:4:1

Distractor Analysis:

Question 2: Fragment Ion Identification in the mass spectrum of 1-bromo-2-chloroethane

A prominent fragment ion in the mass spectrum of 1-bromo-2-chloroethane appears at m/z 63. Which fragment is most likely responsible for this peak?

1. CH₂CH₂⁺
2. CH₂Cl⁺
3. CH₂Br⁺
4. CH₂CH₂Cl⁺

Correct Answer: D

Explanation:

• CH₂CH₂Cl⁺ = 24 (C₂) + 4 (H₄) + 35 (³⁵Cl) = 63
• This fragment forms when the bromine atom is lost, leaving a stable ethyl chloride cation.

Distractor Analysis:

The infrared spectrum of 1-bromo-2-chloroethane

The H-1 NMR spectrum of 1-bromo-2-chloroethane

The C-13 NMR spectrum of 1-bromo-2-chloroethane

The physical properties, hazards and uses of halogenoalkanes (haloalkanes)

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

 Mass spectrometry - introduction and spectra index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

Use My Google search site box

Email doc b: chem55555@hotmail.com