email doc brown  Re-edit mass spectrum of CH₃CH(CH₃)CH(CH₃)CH₂CH₃

2,3-dimethylpentane C₇H₁₆

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

Interpreting the fragmentation pattern of the mass spectrum of 2,3-dimethylpentane

[M]⁺ is the molecular ion peak (M) with an m/z of 100 corresponding to [C₇H₁₆]⁺, the original 2,3-dimethylpentane molecule minus an electron, [(CH₃)₂CHCH(CH₃)CH₂CH₃]⁺.

The tiny M+1 peak at m/z 101, corresponds to an ionised 2,3-dimethylpentane molecule with one ¹³C atom in it i.e. an ionised 2,3-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.

2,3-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 2,3-dimethylpentane - though the ratio will be greater:

e.g. m/z 44 ion could be [¹³C¹²C₂H₇]⁺

the m/z 58 ion could be [¹³C¹²C₃H₉]⁺

and the m/z 72 ion [¹³C¹²C₄H₁₁]⁺

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

i.e. [C₃H₈]⁺, [C₄H₁₀]⁺ and [C₅H₁₂]⁺

Either way, for identification purposes, all these peaks add uniqueness to the fragmentation pattern of the mass spectrum of 2,3-dimethylpentane.

The most abundant ion of the molecule under mass spectrometry investigation (2,3-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 2,3-dimethylpentane is the m/z ion 43 [C₃H₇]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 2,3-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 2,3-dimethylpentane.

The parent molecular ion of 2,3-dimethylpentane m/z of 100: [C₇H₁₆]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 2,3-dimethylpentane.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2,3-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.

Suggested examples of equations to explain some of the most abundant ion peaks in the mass spectrum of 2,3-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)₂CHCH(CH₃)CH₂CH₃]⁺  ===>  [C₆H₁₃]⁺  +  CH₃

C-C bond fission in the parent molecular ion of 2,3-dimethylpentane.

Here a methyl group is broken off,

mass change = 100 - 15 = 81 (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 71 ion:

[(CH₃)₂CHCH(CH₃)CH₂CH₃]⁺  ===>  [(CH₃)₂CHCHCH₃]⁺  +  CH₂CH₃

C-C bond fission in the parent molecular ion of 2,3-dimethylpentane.

Here an end ethyl group is broken off,

mass change = 100 - 29 = 71 (M-29 ion peak)

The m/z 72 ion is probably formed in the same way but containing a ¹³C atom rather than the alternative ion [C₅H₁₂]⁺

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 72: [C₅H₁₂]⁺ = 72.0936 *  [¹³C¹²C₄H₁₁]⁺ = 72.0892, a difference of 0.0044 in relative ion mass.

Formation of m/z 57 ion:

[(CH₃)₂CHCH(CH₃)CH₂CH₃]⁺  ===>  [CH₃CHCH₂CH₃]⁺  +  CH₃CHCH₃

C-C bond fission in the parent molecular ion of 2,3-dimethylpentane.

Loss of a CH₃CHCH₃ group,

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

The m/z 57 ion intensity is not as high as the base peak ion of m/z 43 and the almost equally high m/z 56 ion.

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:

[(CH₃)₂CHCHCH₃]⁺  ===>  [C₄H₈]⁺  +  CH₃

C-C bond fission in the m/z 71 ion, loss of methyl group, mass change = 71 - 15 = 56.

The m/z 56 ion intensity is nearly as high as the base peak ion of m/z 43 ion.

Formation of m/z 43 ion:

[(CH₃)₂CHCH(CH₃)CH₂CH₃]⁺  ===>  [CH₃CHCH₃]⁺  +  C₄H₉

C-C bond fission in the parent molecular ion of 2,3-dimethylpentane.

Loss of a C₄H₉ group, mass change = 100 - 57 = 43 (M-57 ion), a secondary carbocation.

Ethene is often eliminated to give a smaller fragment e.g. the m/z 71 ion gives the m/z 43 ion.

[C₅H₁₁]⁺  ===>  [C₃H₇]⁺  +  C₂H₄

Mass change 71 - 28 = 43.

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

C-C bond fission in the parent molecular ion of 2,3-dimethylpentane.

Here an end ethyl group is broken off, 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).

2,3-Dimethylpentane (C₇H₁₆) is a branched alkane with no functional groups, so its fragmentation pattern is dominated by cleavage of C–C bonds and formation of stable carbocations.

The mass spectrum is useful for understanding alkane fragmentation and identifying isomeric structures based on peak intensity and stability.

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Key m/z Peaks and Fragment Origins for the mass spectrum of 2,3-Dimethylpentane

m/z	Ion Formula	Fragment Origin	Notes
100	C7H16⁺ (M⁺)	Molecular ion	Often weak for alkanes due to instability
57	C4H9⁺ (tert-butyl or sec-butyl)	Cleavage near central carbon (C–C bond break)	Base peak; highly stable carbocation
43	C3H7⁺ (propyl or isopropyl)	Further fragmentation of larger alkyl chains	Common in alkane spectra
29	C2H5⁺ (ethyl)	Minor cleavage product	Often seen in saturated hydrocarbons
15	CH3⁺ (methyl)	Terminal methyl group loss	Weak; diagnostic for simple alkanes

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Common Misconceptions in Exams

• Assuming the molecular ion is always the base peak: In alkanes, the molecular ion (M⁺) is often weak or absent due to fragmentation, but if present, useful for determining M_r of the molecule.
• Confusing fragment ions with neutral fragments: Only charged species appear in the spectrum; neutral losses are inferred.
• Misidentifying isomers: Isomers like 2,3-dimethylpentane and 3-methylhexane may share m/z peaks but differ in intensity due to fragmentation preferences.
• Overlooking rearrangements: Some fragments may arise from hydride shifts or methyl migrations, not just simple bond cleavage.

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

• Know your base peak logic: For branched alkanes, the most stable carbocation (e.g., tert-butyl or sec-butyl) often gives the base peak.

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

• Comparing isomers: Mass spectra help distinguish between e.g. heptane, 2-methylhexane or  2,2-dimethylpentane etc.
• Practice fragmentation pathways: Draw possible cleavage routes and identify resulting cations.
• Use molecular ion cautiously: Don’t rely solely on M⁺ for identification in alkanes—focus on fragment patterns.
• Compare isomers: Learn how branching affects fragmentation and peak intensity.
• Link to structure: Use m/z values to deduce possible alkyl groups and their positions.
• Ion structure: Be ready to propose structures for ions at given m/z values and justify their stability.

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 2,3-dimethylpentane

The H-1 NMR spectrum of 2,3-dimethylpentane

The C-13 NMR spectrum of 2,3-dimethylpentane

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