Advanced Organic Chemistry: Mass spectrum of methanol CH3OH

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Interpreting & explaining the mass spectrum of methanol CH3OH

[Author © Dr Phil Brown GRIC, 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 and AP honors chemistry courses: Molecular spectroscopy analysis of methanol [spectra page updated RE-EDIT]

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 Practise exam questions on the mass spectrum of methanol with answers!


Introductory note on the mass spectrum of methanol

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

If M represents the methanol 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 methanol and only the formation of singly charged positive are considered for the mass spectrum of methanol.

I've included a stick diagram and table of m/z ions for the mass spectrum of methanol 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 methanol.

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 for methanol, but the mass spectrometer software does!

CH3OH mass spectrum of methanol fragmentation pattern of m/z m/e ions for analysis and identification of methanol methyl alcohol image diagram doc brown's advanced organic chemistry revision notes 

Methanol  CH4O alcohols and ether structure and naming (c) doc b alcohols and ether structure and naming (c) doc b alcohols and ether structure and naming (c) doc b alcohols and ether structure and naming (c) doc b

The molecular structure and naming of aliphatic alcohols and ethers

Interpreting the fragmentation pattern of the mass spectrum of methanol

[M]+ is the molecular ion peak (M) with an m/z of 32 corresponding to [CH4O]+, the original methanol molecule minus an electron, [CH3OH]+

The small M+1 peak at m/z 33, corresponds to an ionised methanol molecule with a 13C atom in it i.e. an ionised methanol molecule of formula [13CH3OH]+

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.

Methanol has 1 carbon atom, so on average, ~1 in 100 molecules will contain a 13C atom.

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

In this case the base ion peak is m/z 31 ion [CH3O]+

The molecular ion is m/z 32 [CH3OH]+ which seems to readily lose an electron to give the m/z 31 ion above.

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of methanol.

m/z value of [fragment]+ 31 [CH3O]+ 30 29 28 18 ? 15 14 13
[molecular fragment]+ [CH2OH]+ [CH2O]+ [CHO]+ [CO]+ [H2O]+ [CH3]+ [CH2]+ [CH]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of methanol

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.

Atomic masses: C = 12 (~1% 13);  O = 16; H = 1

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412;  C-O = 360;  O-H = 463

Suggested equations to explain the most abundant ion peaks of methanol

Formation of m/z 31 ion:

[CH3OH]+  ===>  [CH2OH]+  +  H

The m/z 31 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Mass change 32 - 1 = 31 (M-1 ion peak)

This is formed by proton loss from the scission of a C-H bond, not the stronger O-H bond (see list above)

Formation of m/z 30 ion: (various possibilities)

(i) [CH3OH]+  ===>  [CH2O]+  +  H2

Could be formed by loss of hydrogen molecule from original molecular ion.

Mass change 32 - 2 = 30 (M-2 ion peak)

(ii) [CH2OH]+  ===>  [CH2O]+  +  H

Proton loss from the m/z 31 ion.

Mass change 31 - 1 = 30 (M-2 ion peak)

Formation of m/z 28 and 29 ions:

(i) [CH2OH]+  ===>  [CHO]+  +  H2

Elimination of hydrogen from the m/z 31 ion gives the m/z 29 ion.

mass change 31 - 2 - 29

(ii) [CH2O]+  ===>  [CO]+  +  H2

Proton loss from the m/z 30 ion.

mass change 30 - 2 = 28

There will be other possibilities too.

Formation of m/z 15 ion:

[CH3OH]+  ===>  [CH3]+  +  OH

Scission of C-O bond in the parent molecular ion,

mass change 32 - 17 = 15 (M-17 ion peak)

This can lose hydrogen atoms to give the m/z 14 and 13 ions.


QUESTIONS

Advanced A-level chemistry - practise exam questions on the mass spectrum of methanol

alcohols and ether structure and naming (c) doc bA joint AI-doc b re-edit experiment!

Jot down your responses and check out the answers:  ANSWERS

If you think there are any errors, please email me asap at chem55555@hotmail.com

I don't mind if students/teachers do a selected printout of these questions and answers.


Q1. The molecular ion peak for methanol appears at m/z = 32. What does this peak represent?

A. CH₃⁺    B. CH₃O⁺    C. CH₃OH⁺    D. OH⁺


Q2. A strong fragment peak at m/z = 31 in methanol's mass spectrum is due to:

A. Loss of H to form CH₃OH⁺

B. Loss of H₂ to form CH₂OH⁺

C. Loss of H• to form CH₃O⁺

D. Loss of OH• to form CH₃⁺


Q3. Which fragment is responsible for a peak at m/z = 29?

A. CHO⁺    B. CH₃⁺    C. OH⁺    D. CH₂OH⁺


Q4. Methanol shows a peak at m/z = 15. What fragment causes this?

A. CH₃⁺    B. OH⁺    C. CH₂⁺    D. O⁺


Q5. Why is the m/z = 31 peak often more intense than the molecular ion peak at m/z = 32?

A. CH₃OH⁺ is more stable than CH₃O⁺

B. CH₃O⁺ is more stable than CH₃OH⁺

C. CH₃OH⁺ does not form in the mass spectrometer

D. CH₃O⁺ has a higher mass


Q6. Which peak confirms the presence of oxygen in methanol?

A. m/z = 15    B. m/z = 29    C. m/z = 31    D. m/z = 32


Q7. Which fragment results from cleavage of the C–O bond in methanol?

A. CH₃⁺ and OH•    B. CH₂OH⁺ and H•    C. CH₃O⁺ and H•    D. CHO⁺ and H₂


Q8. A student sees a peak at m/z = 29 and concludes methanol is present. Why might this be incorrect?

A. Only methanol produces m/z = 29

B. Many alcohols and aldehydes produce CHO⁺

C. m/z = 29 is the molecular ion

D. m/z = 29 is due to CH₃OH⁺


Q9. Which of the following is the base peak in methanol's mass spectrum?

A. m/z = 32    B. m/z = 31    C. m/z = 29    D. m/z = 15


Q10. Methanol's mass spectrum shows a small peak at m/z = 33. What causes this?

A. A carbon‑13 isotope of the molecular ion

B. A protonated methanol molecule

C. A nitrogen‑containing fragment

D. A chlorine isotope peak


Jot down your responses and check out the answers:  ANSWERS

If you think there are any errors, please email me asap at chem55555@hotmail.com

I don't mind if students/teachers do a selected printout of these questions and answers.


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The infrared spectrum of methanol

The H-1 NMR spectrum of methanol

The C-13 NMR spectrum of methanol

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ANSWERS

Advanced A-level chemistry - practise exam questions on the mass spectrum of methanol

If you think there are any errors, please email me asap at chem55555@hotmail.com

I don't mind if students/teachers do a selected printout of these questions and answers.


Q1. The molecular ion peak for methanol appears at m/z = 32. What does this peak represent?

A. CH₃⁺    B. CH₃O⁺    C. CH₃OH⁺    D. OH⁺

Correct answer: C

Explanation: Methanol has a molecular mass of 32, so the molecular ion is CH₃OH⁺ at m/z = 32.

Common misconception: Students often think the molecular ion is CH₃O⁺ (m/z = 31), but that is a fragment, not the parent ion.


Q2. A strong fragment peak at m/z = 31 in methanol's mass spectrum is due to:

A. Loss of H to form CH₃OH⁺

B. Loss of H₂ to form CH₂OH⁺

C. Loss of H• to form CH₃O⁺

D. Loss of OH• to form CH₃⁺

Correct answer: C

Explanation: Methanol commonly loses a hydrogen radical, forming CH₃O⁺ (m/z = 31).

Common misconception: Students sometimes think m/z = 31 is CH₂OH⁺, but that fragment is also possible and weaker; the dominant peak is CH₃O⁺.


Q3. Which fragment is responsible for a peak at m/z = 29?

A. CHO⁺    B. CH₃⁺    C. OH⁺    D. CH₂OH⁺

Correct answer: A

Explanation: The CHO⁺ ion (formyl cation) has m/z = 29 and is a common fragment in alcohol mass spectra.

Common misconception: Students often misassign m/z = 29 to CH₂OH⁺, but CH₂OH⁺ is m/z = 31.


Q4. Methanol shows a peak at m/z = 15. What fragment causes this?

A. CH₃⁺    B. OH⁺    C. CH₂⁺    D. O⁺

Correct answer: A

Explanation: The methyl cation (CH₃⁺) has m/z = 15, a common low‑mass fragment.

Common misconception: Students sometimes think CH₂⁺ is m/z = 15, but CH₂⁺ is m/z = 14.


Q5. Why is the m/z = 31 peak often more intense than the molecular ion peak at m/z = 32?

A. CH₃OH⁺ is more stable than CH₃O⁺

B. CH₃O⁺ is more stable than CH₃OH⁺

C. CH₃OH⁺ does not form in the mass spectrometer

D. CH₃O⁺ has a higher mass

Correct answer: B

Explanation: The CH₃O⁺ fragment is more stable than the parent ion, so it forms readily and gives a strong peak.

Common misconception: Students sometimes think the molecular ion “should always be the tallest peak,” but the base peak is simply the most stable fragment.


Q6. Which peak confirms the presence of oxygen in methanol?

A. m/z = 15    B. m/z = 29    C. m/z = 31    D. m/z = 32

Correct answer: C

Explanation: The CH₃O⁺ fragment (m/z = 31) contains oxygen and is characteristic of alcohols.

Common misconception: Students often assume the molecular ion (m/z = 32) confirms oxygen, but many molecules have mass 32; the fragment pattern is more diagnostic.


Q7. Which fragment results from cleavage of the C–O bond in methanol?

A. CH₃⁺ and OH•    B. CH₂OH⁺ and H•    C. CH₃O⁺ and H•    D. CHO⁺ and H₂

Correct answer: A

Explanation: Breaking the C–O bond produces CH₃⁺ (m/z = 15) and an OH radical.

Common misconception: Students sometimes think CH₃O⁺ comes from C–O cleavage, but it comes from loss of H•, not C–O bond breaking.


Q8. A student sees a peak at m/z = 29 and concludes methanol is present. Why might this be incorrect?

A. Only methanol produces m/z = 29

B. Many alcohols and aldehydes produce CHO⁺

C. m/z = 29 is the molecular ion

D. m/z = 29 is due to CH₃OH⁺

Correct answer: B

Explanation: The CHO⁺ fragment is common in many oxygen‑containing molecules, not just methanol.

Common misconception: Students often assume “one peak = one molecule,” but mass spectra identify functional groups, not specific compounds.


Q9. Which of the following is the base peak in methanol's mass spectrum?

A. m/z = 32    B. m/z = 31    C. m/z = 29    D. m/z = 15

Correct answer: B

Explanation: The base peak (tallest peak) for methanol is m/z = 31 (CH₃O⁺) because it is the most stable fragment.

Common misconception: Students often think the molecular ion is always the base peak — it is not.


Q10. Methanol's mass spectrum shows a small peak at m/z = 33. What causes this?

A. A carbon‑13 isotope of the molecular ion

B. A protonated methanol molecule

C. A nitrogen‑containing fragment

D. A chlorine isotope peak

Correct answer: A

Explanation: A small peak at m/z = 33 corresponds to ¹³C‑CH₃OH⁺, the M+1 isotope peak.

Common misconception: Students sometimes think M+1 peaks indicate contamination or impurities, but they are simply due to natural isotopes.


If you think there are any errors, please email me asap at chem55555@hotmail.com

I don't mind if students/teachers do a selected printout of these questions and answers.

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