[Author ©  Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 & AP honors chemistry courses: Mass spectrometry - analysing mass spectrum of 2,4-dimethylpentane [spectra updated Mar 19th 2026 *]

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2,4-dimethylpentane C₇H₁₆  

For more see The molecular structure, classification and naming of alkanes

Interpreting the fragmentation pattern of the mass spectrum of 2,4-dimethylpentane

[M]⁺ is the molecular ion peak (M) with an m/z of 100 corresponding to [C₇H₁₆]⁺, the original 2,4-dimethylpentane molecule minus an electron, [(CH₃)₂CHCH₂CH(CH₃)₂]⁺.

You might see a tiny M+1 peak at m/z 101, corresponding to an ionised 2,4-dimethylpentane molecule with one ¹³C atom in it i.e. an ionised 2,4-dimethylpentane molecule of formula [¹³C¹²C₆H₁₆]⁺

Carbon-13 only accounts for ~1% of all carbon atoms (¹²C ~99%), but the more carbon atoms in the molecule, the greater the probability of observing this ¹³C M+1 peak.

2,4-dimethylpentane has 7 carbon atoms, so on average, ~1 in 14 molecules will contain a ¹³C atom.

This sort of argument also applies to fragment ions from the parent molecular ion of 2,4-dimethylpentane - though the ratio will be greater:

e.g. the m/z 44 ion could be [¹³C¹²C₂H₇]⁺

the m/z 58 ion could be [¹³C¹²C₃H₉]⁺

and the m/z 86 ion could be [¹³C¹²C₅H₁₃]⁺

These ions might be more likely than those containing only ¹²C isotope atoms

i.e. [C₃H₈]⁺, [C₄H₁₀]⁺ and [C₆H₁₄]⁺

Either way, for identification purposes, all these peaks add uniqueness to the fragmentation pattern of the mass spectrum of 2,4-dimethylpentane

The most abundant ion of the molecule under mass spectrometry investigation (2,4-dimethylpentane) 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 ion peak for the mass spectrum of 2,4-dimethylpentane is the m/z ion 43 [C₃H₇]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 2,4-dimethylpentane.

Unless otherwise indicated, assume the carbon atoms in heptane are the ¹²C isotope.

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of 2,4-dimethylpentane.

The parent molecular ion of 2,4-dimethylpentane of m/z 100: [C₇H₁₆]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 2,4-dimethylpentane.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2,4-dimethylpentane

PLEASE NOTE I have found it difficult to find 'authentic' equations to explain mass spectra fragmentation patterns and it is complex chemistry! I've identified the formulae of the ionised fragments on the mass spectrum diagram, but the equations are from the internet or my conjecture as to how the ions might be formed - please take care in using the information, especially for assignments at university or pre-university level.

Examples of equations to explain some of the most abundant ion peaks in the mass spectrum of 2,4-dimethylpentane

Atomic masses: H = 1;  C = 12 (13 for ~1 in 100)

Bond enthalpies = kJ/mol: C-C = 348;  C-H = 412

Most fragmentation in alkanes arises from C-C bond scission (C-C bond weaker than C-H bond).

Formation of m/z 85 ion:

[(CH₃)₂CHCH₂CH(CH₃)₂]⁺  ===>  [C₆H₁₃]⁺  +  CH₃

C-C bond fission in the parent molecular ion of 2,4-dimethylpentane.

Here an end methyl group is broken off,

mass change = 100 - 15 = 85 (M-15 ion peak)

The m/z 86 ion is probably formed in the same way but containing a ¹³C atom rather than the alternative ion [C₆H₁₄]⁺

Note that an accurate mass spectrometer can sort out (resolve) pairs of ions with the same integer m/z value because they can measure relative fragment ion masses to four decimal places,

e.g. using accurate relative isotopic masses:

¹H = 1.0078  ¹²C = 12.0000   ¹³C = 13.0034: you can then calculate (predict) that the accurate relative ion masses are:

m/z 86: [C₆H₁₄]⁺ = 86.1092  and  [¹³C¹²C₅H₁₃]⁺ = 86.1048, a relative ion mass difference of 0.0044

Formation of m/z 57 ion:

[(CH₃)₂CHCH₂CH(CH₃)₂]⁺  ===>  [CH₃CHCH₂CH₃]⁺  +  CH₃CHCH₃

C-C bond fission in the parent molecular ion of 2,4-dimethylpentane.

Loss of a CH₃CHCH₃ group,

mass change = 100 - 43 = 57 (M-43 ion peak)

The m/z 57 ion intensity is nearly as high as the base peak ion of m/z 43.

It can also be formed by elimination of ethene from the m/z 85 ion (85 - 28 = 57).

[C₆H₁₃]⁺  ===>  [C₄H₉]⁺  +  C₂H₄

Many other fragments are formed by hydrogen atom/molecule loss, so you get m/z ion sequences like 71 ==>70, 57 ==> 56 ==> 55, 43 ==> 39 and 29 ==> 27 etc. (see examples below).

The m/z 58 ion is probably formed in the same way but containing a ¹³C atom rather than the alternative ion [C₄H₁₀]⁺

Note that an accurate mass spectrometer can sort out (resolve) pairs of ions with the same integer m/z value because they can measure relative fragment ion masses to four decimal places,

e.g. using accurate relative isotopic masses:

¹H = 1.0078  ¹²C = 12.0000   ¹³C = 13.0034: you can then calculate (predict) that the accurate relative ion masses are:

For m/z 58: [¹³C¹²C₃H₉]⁺ = 58.0736, [C₄H₁₀]⁺ = 58.0780, a difference of 0.0044 in relative ion mass.

Formation of m/z 56 ion:

Formation of m/z 43 ion:

[(CH₃)₂CHCH₂CH(CH₃)₂]⁺  ===>  [CH₃CHCH₃]⁺  +  C₄H₉

C-C bond fission in the parent molecular ion of 2,4-dimethylpentane.

Loss of a C₄H₉ group,

mass change = 100 - 57 = 43 (M-57 ion peak), a secondary carbocation.

The m/z 44 ion is probably formed in the same way but containing a ¹³C atom rather than the alternative ion [C₃H₈]⁺

Note that an accurate mass spectrometer can sort out (resolve) pairs of ions with the same integer m/z value because they can measure relative fragment ion masses to four decimal places,

e.g. using accurate relative isotopic masses:

¹H = 1.0078  ¹²C = 12.0000   ¹³C = 13.0034: you can then calculate (predict) that the accurate relative ion masses are:

For m/z 44: [C₃H₈]⁺ = 44.0624  and [¹³C¹²C₂H₇]⁺ = 44.0580, a difference of 0.0044 in relative ion mass.

The m/z 43 ion can lose hydrogen atoms to give the m/z 42, 41 and then 39 ions (see ion data table).

Formation of m/z 29 ion:

[(CH₃)₂CHCH₂CH(CH₃)₂]]⁺  ===>  [CH₂CH₃]⁺  +  C₅H₁₁

C-C bond fission in the parent molecular ion of 2,4-dimethylpentane.

Mass change = 100 - 71 = 29 (M-71 ion)

The m/z 29 ion can lose hydrogen atoms to give the m/z 28 and then 27 ions (see ion data table).

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Overview: Mass Spectrometry of 2,4-dimethylpentane

2,4-Dimethylpentane (C₇H₁₆) is a saturated, branched alkane.

Its mass spectrum is dominated by alkyl fragment ions formed via cleavage of C–C bonds.

The molecular ion peak (M⁺) is weak due to instability, but fragment peaks are strong and diagnostic.

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Key Fragment Ions in of the mass spectrum of 2,4-dimethylpentane

m/z	Fragment Ion	Origin	Notes
100	C7H16⁺ (M⁺)	Molecular ion	Often weak or absent due to instability
85	C6H13⁺	Loss of CH3	Major peak; stable secondary carbocation
71	C5H11⁺	Loss of C2H5	Common in branched alkanes
57	C4H9⁺	Butyl cation	Very stable; often intense peak
56	C4H8⁺	butene ion?	Loss of CH3 from C5H11 ion?
43	C3H7⁺	Propyl cation	Common alkyl fragment, base peak ion
29	C2H5⁺	Ethyl cation	Minor peak in alkanes

Sources: Pearson Organic Chemistry, MassBank EU

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Common Misconceptions about mass spectra

Misconception	Clarification
"The base peak is always the molecular ion."	Not true—alkanes often have weak M⁺ peaks. The base peak is the most intense, often a fragment like m/z 57 or 85.
"Mass spectra always show functional groups."	Alkanes lack functional groups; their spectra show alkyl fragmentation patterns.
"All fragments are neutral."	Only charged fragments are detected; neutral losses are inferred.
"Mass spec gives full structure."	It gives fragmentation clues, not full structure alone—used with IR/NMR for confirmation.

Exam Revision Tips (AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB, AP)

Strategy for Interpretation

• Identify M⁺ peak (if present): gives molecular mass.
• Locate base peak: often a stable carbocation (e.g. m/z 57 or 85).
• Use m/z differences to deduce lost groups (e.g. 100 → 85 = CH₃ loss).
• Check for alkyl series: m/z 29, 43, 57, 71, 85 = C₂ to C₆ fragments.

• Use peak tables: Learn common m/z values for alkyl fragments m/z 15 = CH₃⁺, 29 = C₂H₅⁺, 43 = C₃H₇⁺, 57 = C₄H₉⁺, 71 = C₅H₁₁⁺, 85 = C₆H₁₃⁺ , but not always of diagnostic use..

Typical Board-Specific Tips

• Expect questions on fragment identification and molecular ion deduction.
• May ask for comparison of spectra or fragment origin.
• Often integrates mass spec with IR/NMR for full structure.
• Focuses on pattern recognition and fragment logic.

Practice Prompt

A mass spectrum shows peaks at m/z 43, 57, 85, and a weak peak at 100. Suggest the compound type.

Correct answer: A branched alkane such as 2,4-dimethylpentane.

Comparing the 1H NMR and 13C NMR spectra of the nine alkane structural isomers of C7H16 You can distinguish all 9 isomers from a data combination of their number of 1H NMR chemical shifts, and their resulting integrated 1H proton ratios, plus, their number of 13C chemical shifts.
Name of the alkane structural isomer of molecular formula C7H16	Abbreviated structural formulae of the nine isomers of molecular formula C7H16 (interpretation complications with 3-methylhexane and 2,3-dimethylpentane because they exhibit R/S isomerism due to a chiral carbon)	Skeletal formula of the nine alkane isomers of  molecular formula C7H16	Number of 1H NMR chemical shifts (δ) and proton ratio (links to spectrum)	Number of 13C chemical shifts (δ) (links to spectrum)
heptane			4 δ: proton ratio: 3:2:2:1 (6:4:4:2 in the molecule)	4 δ shifts
2-methylhexane			6 δ: proton ratio : 6:3:2:2:2:1	6 δ shifts
3-methylhexane			7 δ: proton ratio: 3:3:3:2:2:2:1 (simplification) !	7 δ shifts
3-ethylpentane			3 δ: proton ratio: 9:6:1	3 δ shifts
2,2-dimethylpentane			4 δ: proton ratio: 9:3:2:2	5 δ shifts
2,3-dimethylpentane			6 δ: proton ratio: 6:3:3:2:1:1 (simplification) !	6 δ shifts (simplification) !!!
2,4-dimethylpentane			3 δ: proton ratio: 12:2:2	3 δ shifts
3,3-dimethylpentane			3 δ: proton ratio: 3:3:2 (6:4:4 in the molecule)	4 δ shifts
2,2,3-trimethylbutane			3 δ: proton ratio: 9:6:1	4 δ shifts

The infrared spectrum of 2,4-dimethylpentane

The H-1 NMR spectrum of 2,4-dimethylpentane

The C-13 NMR spectrum of 2,4-dimethylpentane

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