HOME PAGE * KS3 SCIENCES * GCSE BIOLOGY CHEMISTRY PHYSICS * ADVANCED LEVEL CHEMISTRY

SPECTROSCOPY INDEXES  *  All Advanced Organic Chemistry Notes  *  [SEARCH BOX]

Advanced Organic Chemistry: Mass spectrum of 2-iodo-2-methylpropane

Interpreting the mass spectrum of 2-iodo-2-methylpropane (tert-butyl iodide)

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing mass spectra of 2-iodo-2-methylpropane

See also comparison of the infrared, mass, 1H NMR and 13C NMR spectra of the four isomers of C4H9I

mass spectrum of 2-iodo-2-methylpropane (CH3)3CI fragmentation pattern of m/z m/e ions for analysis and identification of tert-butyl iodide image diagram doc brown's advanced organic chemistry revision notes 

2-iodo-2-methylpropane (tert-butyl iodide), C4H9I, (CH3)3C-I

Interpreting the fragmentation pattern of the mass spectrum of 2-iodo-2-methylpropane

[M]+ is the parent molecular ion peak (M) with an m/z of 184 corresponding to [C4H9I]+, the original 2-iodo-2-methylpropane molecule minus an electron, [(CH3)3C-I]+.

Iodine consists of 100% of the 127I isotope, so there are no double peak complexities with 2-iodo-2-methylpropane that you get with the mass spectra of organic chlorine and bromine compounds, where you get M+2 peaks due to two isotopes of the halogen of mass difference 2 units and other double peaks to m/z units apart.

See Mass spectroscopy index

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-iodo-2-methylpropane has 4 carbon atoms, so on average, ~1 in 25 molecules will contain a 13C atom.

You may find two peaks one unit apart with a height ratio of 25:1 e.g. m/z ions 57 and 58.

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

You may find two peaks one unit apart with a height ratio of 25:1 e.g. m/z ions 57/58 and 184/185.

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 2-iodo-2-methylpropane.

Unless otherwise indicated, assume the carbon atoms in 2-iodo-2-methylpropane are the 12C isotope.

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

The parent molecular ion of 2-iodo-2-methylpropane m/z 184: [(CH3)3C-I]+

Data table of some of the ions formed in the fragmentation pattern of the mass spectrum of 2-iodo-2-methylpropane

m/z value of [fragment]+ 184 169 156 128 127 58
[molecular fragment]+ [C4H9I]+ [C3H6I]+ [C2H5I]+ [HI]+ [I]+ [13C12C3H9]+
m/z value of [fragment]+ 57 56 55 41 39 29 28 27
[molecular fragment]+ [C4H9]+ [C4H8]+ [C4H7]+ [C3H5]+ [C3H3]+ [C2H5]+ [C2H4]+ [C2H3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2-iodo-2-methylpropane

Atomic masses: H = 1;  C = 12;  I = 127

Bond enthalpies = kJ/mol: C-C = 348;  C-H = 412; C-I = 238

Possible equations to explain some of the most abundant ion peaks of 2-iodo-2-methylpropane (tabulated above)

Formation of m/z 169 ion:

[(CH3)3C-I]+  ===>  [C3H6I]+  +  CH3

C-C bond scission in parent molecular ion, methyl group lost, mass change 184 - 15 = 169.

Small peak, low probability due to the high C-C bond enthalpy compared to the C-I bond.

Formation of m/z 156 ion:

[(CH3)3C-I]+  ===>  [C2H5I]+  +  C2H4

C-C bond scission in parent molecular ion, elimination of ethene molecule, mass change 184 - 15 = 169.

Small peak, low probability due to the high C-C bond enthalpy compared to the C-I bond.

Formation of m/z 127 and 128 ions:

[(CH3)3C-I]+  ===>  [I]+  +  C4H9

C-I bond scission, iodine atom freed and ionised, mass change 184 - 57 = 127, this is the weakest bond in the molecule, but the alkyl fragment is much likely to retain the positive charge, especially as it has a tertiary carbocation structure.

The m/z of 127 is indicative of an iodine compound.

[(CH3)3C-I]+  ===>  [HI]+  +  C4H8

Elimination of a hydrogen iodide molecule, mass change 184 - 56 = 128.

Both reactions have a low probability judging from the small abundances - small peaks.

Formation of m/z 57 ion:

[(CH3)3C-I]+  ===>  [C4H9]+  +  I

Scission of the weakest bond in the molecule, breakage of the C-I bond releases an iodine atom and forms the base peak ion.

The most probable bond scission (see bond enthalpies above for 2-bromobutane) and the mass change is 184 - 127 = 57.

The alkyl carbocation is much more likely to carry the positive charge than the iodine atom, especially as it has a tertiary carbocation structure [(CH3)3C]+.

The m/z 57 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The m/z 58 ion is probably,  [13C12C3H9]+, i.e. the base peak ion containing a 13C isotope.

Formation of m/z 56 ion:

[(CH3)3C-I]+  ===>  [C4H8]+  +  HI

Elimination of hydrogen bromide molecule from the parent molecular ion, mass change 184 - 128 = 56.

Formation of m/z 55 ion:

[C4H9]+  ===>  [C4H7]+  +  H2

Elimination of a hydrogen molecule from the m/z 57 ion.

Loss of a H atom from the same ion can also form the m/z 55 ion.

Low probability for these fragmentation reactions due to the high C-H bond enthalpy.

Formation of m/z 41 and 39 ions:

Possible reactions include:

m/z 41: [C4H8]+  ===>  [C3H5]+  +  CH3

C-C bond scission of fragment ion.

m/z 39: [C3H5]+  ===>  [C3H3]+  +  H2

Formation of m/z 29 and 27 ions:

Possible reactions include:

m/z 29: [(CH3)2CHCH2I]+  ===>  [CH3CH2]+  +  CH2CH2I

From bond scission in the parent molecular ion (above) or C-C bond scission of the fragmentation ions.

m/z 27: [C4H8]+  ===>  [C2H3]+  +  C2H5

m/z 29: [C4H8]+  ===>  [C2H5]+  +  C2H3

m/z 29: [C4H9]+  ===>  [C2H5]+  +  C2H4

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 4 halogenoalkane isomers of C4H9I

NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original 1-iodobutane, 2-iodobutane, 1-iodo-2-methylpropane and 2-iodo-2-methylpropane image sizes.  These four molecules are structural isomers of molecular formula C4H9I and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic halogenoalkanes (haloalkanes, alkyl halides, iodoalkanes, alkyl iodides).

INFRARED SPECTRA (above): Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, there are no other great striking differences, but each could be identified from its infrared spectrum.

MASS SPECTRA (above): All four give the parent molecular ion of m/z 184, but it is only a relatively tiny peak for 2-iodo-2-methylpropane. All four give the base ion peak of m/z 57. All four give prominent peaks for m/z ions 29 and 41 and all give a tiny peak from an ionised iodine atom at m/z 127. They look quite similar to me and lack a clear fingerprint fragmentation pattern.

1H NMR SPECTRA (above): The 1H NMR spectra of all three molecules give different proton ratios i.e.1-iodobutane four peaks 3:2:2:2, 2-iodobutane four peaks 3:3:2:1, 1-iodo-2-methylpropane three peaks 6:2:1 and 2-iodo-2-methylpropane one peak '1' (effectively no ratio involved), so all four molecular structures can be distinguished from each other by their 1H NMR spectra proton ratios, numbers of peaks and (n+1) rule splitting patterns.

13C NMR SPECTRA (above): The 13C NMR spectra of the four molecules show various numbers of carbon-13 chemical environments i.e 1-iodobutane and 2-iodobutane show four 13C NMR resonances, 1-iodo-2-methylpropane three 13C NMR resonances and 2-iodo-2-methylpropane only two 13C resonances. Therefore 1-iodo-2-methylpropane and 2-iodo-2-methylpropane can be distinguished from the other three by their number of resonances in their 13C NMR spectra, but 1-iodobutane and 2-iodobutane cannot be distinguished from each other from their number of 13C NMR resonance lines - other data would be required.

Key words & phrases: isomer of molecular formula (CH3)3CI image diagram on how to interpret and explain the mass spectrum of 2-iodo-2-methylpropane m/z m/e base peaks, image and diagram of the mass spectrum of 2-iodo-2-methylpropane, details of the mass spectroscopy of 2-iodo-2-methylpropane,  low and high resolution mass spectrum of 2-iodo-2-methylpropane, prominent m/z peaks in the mass spectrum of 2-iodo-2-methylpropane, comparative mass spectra of 2-iodo-2-methylpropane, the molecular ion peak in the mass spectrum of 2-iodo-2-methylpropane, analysing and understanding the fragmentation pattern of the mass spectrum of 2-iodo-2-methylpropane, characteristic pattern of peaks in the mass spectrum of 2-iodo-2-methylpropane, relative abundance of mass ion peaks in the mass spectrum of 2-iodo-2-methylpropane, revising the mass spectrum of 2-iodo-2-methylpropane, revision of mass spectroscopy of 2-iodo-2-methylpropane, most abundant ions in the mass spectrum of 2-iodo-2-methylpropane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of 2-iodo-2-methylpropane, how to analyse the mass spectrum of 2-iodo-2-methylpropane, how to describe explain the formation of fragmented ions in the mass spectra of 2-iodo-2-methylpropane equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of 2-iodo-2-methylpropane recognising the base ion peak of 2-iodo-2-methylpropane interpreting interpretation the mass spectrum of 2-iodo-2-methylpropane  alkyl iodide haloalkane halogenoalkane tert-butyl iodide functional group

Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of 2-iodo-2-methylpropane. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of 2-iodo-2-methylpropane. The m/e m/z value of the molecular ion peak in the mass spectrum of 2-iodo-2-methylpropane.  The m/e m/z value of the base ion peak in the mass spectrum of 2-iodo-2-methylpropane. Possible examples of equations showing the formation of the ionised fragments in 2-iodo-2-methylpropane. Revision notes on the mass spectrum of 2-iodo-2-methylpropane. Matching and deducing the structure of the 2-iodo-2-methylpropane molecule from its mass spectrum. Mass spectroscopy of aliphatic halogenoalkanes iodoalkanes, mass spectra of 2-iodo-2-methylpropane, an isomer of molecular formula C4H9I


Links associated with 2-iodo-2-methylpropane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

The infrared spectrum of 2-iodo-2-methylpropane (tert-butyl iodide)

The H-1 NMR spectrum of 2-iodo-2-methylpropane (tert-butyl iodide)

The C-13 NMR spectrum of 2-iodo-2-methylpropane (tert-butyl iodide)

Mass spectroscopy index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

Use My Google search site box

Email doc b: chem55555@hotmail.com

TOP OF PAGE

 Doc Brown's Chemistry 

*

TOP OF PAGE