Topic 7 Modern Analytical Techniques Flashcards
Molecular ion peak
Indicates the species formed from the molecule by the loss of one electron.
Fragmentation
Occurs when the molecular ion breaks into smaller pieces.
Base peak
Indicates the peak with the greatest abundance. The tallest peak. Relative intensity of 100%. It represents the most stable fragment.
In a mass spectrum of an element, what does a peak represent?
An isotope of an element.
The peak furthest right on an m/z spectrum
- Has the greatest m/z value.
- Is the molecular ion peak.
- Indicates the relative molecular mass of the organic compound.
Equation for the formation of a molecular ion peak (example: butane):
C4H10 + e- –> C4H10 + 2e-
How does the molecular ion peak form?
An electron collides with the molecule, and knocks out an electron to form a positive ion.
M + 1 peak
Small peak just right of the molecular ion peak due to the presence of the 13C isotope of carbon.
13C
1% of all naturally-occurring carbon atoms.
14C
Used in radiocarbon dating. The proportion of 14C atoms in a sample of an organic compound is negligible.
What causes peaks with smaller m/z values?
Fragmentation, rearrangement reactions and the loss of more than one electron.
Rearrangement
Beyond the spec. Often unpredictable. Explains the high number of peaks in some spectra.
When a C-C bond breaks by fragmentation, what is produced?
- Another positive ion.
- A neutral species, usually a radical.
y-axis on a mass spectrum
Relative intensity (%) 0%-100%
x-axis on a mass spectrum
m/z (no units) Usually 0 to just past the molecular ion peak.
What causes all the peaks formed by fragmentation?
Positive ions as radicals are not detected in a mass spectrometer.
The breaking of a carbon-hydrogen bond
Can occur, but is not considered fragmentation.
Fragmentation of the molecular ion of ethane (example)
(CH3–CH3)+dot –> CH3 dot + CH3+
Fragments to form a methyl cation + a methyl radical, as there is only one C-C bond. Both methyls are equally likely to form the cation, so the products are identical. Thus, the spectrum does not depend on how the bond breaks. There is a peak at m/z= 15.
Fragmentation of the molecular ion of propane (example)
There are two equivalent C-C bonds, so there are two possible peaks that form: the methyl m/z= 15 cation & the ethyl m/z= 29 cation.
What is the possible ion for an m/z value of 15?
CH3+
What is the possible ion for an m/z value of 17?
OH+
What is the possible ion for an m/z value of 28?
CO+
What are the possible ions for an m/z value of 29?
CH3CH2+ & CHO+
What is the possible ion for an m/z value of 31?
CH2OH+
What are the possible ions for an m/z value of 43?
CH3CH2CH2+
CH3CHCH3+
CH3CO+
What are the possible ions for an m/z value of 45?
COOH+
CH3CHOH+
What are the possible ions for an m/z value of 57?
C4H9+ (This represents 4 possible structures.)
Compare the m/z values of the fragment ion & the molecular ion. What does the difference between the two indicate?
What has been lost during fragmentation.
What happens when some molecules absorb infrared radiation?
Stretching vibrations: where the bond length increases & decreases.
Bending vibrations: where the bond angle increases & decreases.
When a molecule absorbs infrared radiation, upon what factors does the amount absorbed depend?
- The length of the bond.
- The strength of the bond.
- The mass of each atom involved in the bond.
Why do simple, non-polar molecules not absorb infrared radiation?
The absorption of infrared radiation is linked to changes in the polarity of the molecule.
Infrared radiation
The part of the electromagnetic spectrum with frequencies below that of red light.
Stretching
Occurs when absorbs infrared radiation and uses it to alter the length of the bond.
Transmittance value
Value in an infrared spectrum that represents the amount of radiation absorbed at a particular wavenumber.
Wavenumber of an infrared absorption
Represents the frequency of infrared radiation absorbed by a particular bond in a molecule. Reciprocal of the wavelength.
Intensity of an infrared absorption
The amount of infrared radiation absorbed.
y-axis on an IR spectrum.
Transmittance 0% to 100%
x-axis on an IR spectrum.
Wavenumber (cm-1). 4000-500 cm-1. The scale changes at 2000 cm-1.
Weak intensities
High transmittance value/weak absorption
Strong intensities
Low transmittance value/strong absorption
Fingerprint region
1500-500 cm-1. Unique to each molecule, and can be used to identify a molecule by comparing it with known samples in a database. Results from vibrations produced by the whole molecule.
Why are ranges of values given for each bond?
A functional group is affected by other functional groups nearby. A functional group will give one value in one compound & another value in another compound.
IR spectrum exam technique: what should be stated?
The bond responsible for the peak & the wavenumber range.
What causes the broad peak for O-H bond in a carboxylic acid?
The formation of dimers: two carboxylic acids forming hydrogen bonds with each other.
How does mass spectrometry work?
The gaseous sample is injected into the mass spectrometer. The molecules are ionised, & then fragmented.
Exam technique for mass spectrometry
Always identify fragments as + ions.
The fragmentation pattern
Unique to each compound. The presence or absence of peaks can be used to identify compounds.
High-resolution mass spectrometry
Uses more precise isotopic masses to measure the mass of a molecular ion.
m/z
Mass-to-charge ratio. If charge= +1, then m/z value = mass of the ion.
Molecules with polar bonds absorb IR. What happens when this occurs?
The polarity of the molecule changes. The vibrational energy of the molecule increases, so stretching and bending of the bonds occurs. Each bond vibrates at a particular frequency & absorbs IR at a particular frequency.
Method to find Mr from the molecular ion m/z
Start with the hydrocarbon with the maximum number of C atoms that has an Mr less than the m/z of the molecular ion. Then add O atoms.
Why does the OH bond absorb over a wide range of frequencies?
Hydrogen bonding. Produces a broad band.
How can IR spectra show whether a reaction has been successful?
It can show if a functional group has changed.
Greenhouse gases, specifically CO2
H2O, CO2, CH4 & NO all absorb IR radiation.
In CO2, the centres of partial + & - charge are in the centre. Symmetric stretching CO2 does not absorb in the infrared region, as the centres of partial + & - are unchanged. Asymmetric stretching in CO2 does absorb in the IR spectrum as the centre of partial - is unaltered.
Why don’t molecules such as H2, and atmospheric gases such as O2 & N2, absorb infrared?
The atoms are identical, so there is no change in dipole when they vibrate.
C-H aldehyde
2700-2900 cm-1 medium-strong
C-H alkene
3095-3010 cm-1 (just above 3000) medium-strong
C-H alkane
2962-2853 cm-1 (just below 3000) medium-strong
C=C alkene
1669-1645 cm-1 medium
C=O aldehyde
1740-1720 cm-1 strong
C=O ketone
1720-1700 cm-1 strong
C=O carboxylic acid
1725-1700 cm-1
C-O alcohols
1000-1300 cm-1 strong
C-Cl
700-800 cm-1 strong
O-H carboxylic acid
3300-2500 cm-1 medium/broad
N-H amine
3500-3300 cm-1 strong
O-H alcohols
3750-3200 cm-1 strong/broad
Pros of chemical tests
Rapid result.
Tests are readily available.
Cons of chemical tests
Not as sensitive.
Uses some of the sample, which cannot be recovered.
What must be included when giving the formula of a fragment in mass spectrometry?
+ charge.
DO NOT include the bond to the fragment.
When deducing the Mr of a compound from a mass spectrum, what must be included?
State the m/z of the molecular ion peak. State that the molecular ion peak has the highest m/z.
What factor determines the size of the titre in titration?
The number of moles of substance being titrated.
Typical ion responsible for a peak at 69 m/z.
[C5H9]+
Greenhouse gases.
Absorb IR radiation.
81Br79Br & 79Br81Br
Appear as the same peak in the mass spectrum.
Pros of chemical tests:
Rapid result, less expensive; tests are readily available.
Cons of chemical tests:
Not as sensitive. Use some of the sample, which cannot be recovered.
IR OH stretch of a carboxylic acid.
Very broad. Less intense.
When does an alkene have E/Z isomers?
There is restricted rotation around the double bond. Each carbon in the double bond is attached to 2 different groups.
