Chapter 13 Mass Spectroscopy and
Infrared Spectroscopy
What is mass spectroscopy?
A technique used for measuring the molecular weight and determining the molecular formula
of an organic molecule (pg. 464)
Give an overview of how a mass spectrometer works.
A sample is vaporized and then bombarded with electrons. This forms an unstable radical
carbocation, which then decomposes. The positively charged ions are accelerated toward a
negatively charged plate, and then passed through a curved analyzer tube in a magnetic
field. These are deflected a different amount based on their respective mass to charge
ratios. A mass spectrum plots the intensity of each ion versus its mass to charge ratio (pg.
464)
What is a radical cation?
A species with an unpaired electron and a positive charge (pg. 465)
What is a molecular ion?
The radical cation M⁺°. AKA parent ion. The mass of the molecular ion represents the
molecular weight of of the compound we're taking a mass spectrum of.
What is a m/z ratio?
Mass-to-charge ratio. Because the z is almost always +1, m/z actually measures the mass
(m) of the individual ions.
What does a mass spectrum plot?
The amount of each cation (its relative abundance) versus its mass (pg. 465)
What an M peak?
The peak corresponding to the molecular ion. AKA parent peak (pg. 465)
What is the base peak?
The tallest peak in a mass spectrum (pg. 465)
What is fragmentation?
The decomposition of a molecular ion into lower molecular weight fragments (pg. 465)
, In the mass spectrum of hexane, identify the M peak, the M+1 peak, and the base peak
pg. 466
What is the M+1 peak and what causes it?
It's a peak +1 m/z above the M peak. It's due to the appearance of carbon-13 isotopes (pg.
466)
Why is a mass spectrum useful?
Because the mass of the molecular ion equals the molecular weight of a compound, a mass
spectrum can be used to distinguish between compounds that have similar physical
properties but different molecular weights (pg. 466)
What does an odd molecular ion indicate?
It generally indicates the compound contains nitrogen (pg. 467)
What is the nitrogen rule?
A compound that contains an odd number of N atoms gives an odd molecular ion (pg. 467)
Sample problem 13.2
pg. 468
Give the molecular weights of C, H, N, and O
C=12
H=1
N=14
O=16
What are the two common isotopes of Cl? What is the ratio of their abundance? What are
the implications of this in terms of a mass spectrum?
³⁵Cl and ³⁷Cl in a 3:1 ratio. Thus there are two peaks in a 3:1 ratio for the molecular ion of an
alkyl chloride. The larger one corresponds to the ³⁵Cl and the M+2 peak corresponds to the
³⁷Cl. (pg. 468)
How do we know when a Cl atom is present when looking at a mass spectrum?
When the molecular ion consists of two peaks (M and M+2) in a 3:1 ratio, a Cl atom is
present (pg. 468)
Infrared Spectroscopy
What is mass spectroscopy?
A technique used for measuring the molecular weight and determining the molecular formula
of an organic molecule (pg. 464)
Give an overview of how a mass spectrometer works.
A sample is vaporized and then bombarded with electrons. This forms an unstable radical
carbocation, which then decomposes. The positively charged ions are accelerated toward a
negatively charged plate, and then passed through a curved analyzer tube in a magnetic
field. These are deflected a different amount based on their respective mass to charge
ratios. A mass spectrum plots the intensity of each ion versus its mass to charge ratio (pg.
464)
What is a radical cation?
A species with an unpaired electron and a positive charge (pg. 465)
What is a molecular ion?
The radical cation M⁺°. AKA parent ion. The mass of the molecular ion represents the
molecular weight of of the compound we're taking a mass spectrum of.
What is a m/z ratio?
Mass-to-charge ratio. Because the z is almost always +1, m/z actually measures the mass
(m) of the individual ions.
What does a mass spectrum plot?
The amount of each cation (its relative abundance) versus its mass (pg. 465)
What an M peak?
The peak corresponding to the molecular ion. AKA parent peak (pg. 465)
What is the base peak?
The tallest peak in a mass spectrum (pg. 465)
What is fragmentation?
The decomposition of a molecular ion into lower molecular weight fragments (pg. 465)
, In the mass spectrum of hexane, identify the M peak, the M+1 peak, and the base peak
pg. 466
What is the M+1 peak and what causes it?
It's a peak +1 m/z above the M peak. It's due to the appearance of carbon-13 isotopes (pg.
466)
Why is a mass spectrum useful?
Because the mass of the molecular ion equals the molecular weight of a compound, a mass
spectrum can be used to distinguish between compounds that have similar physical
properties but different molecular weights (pg. 466)
What does an odd molecular ion indicate?
It generally indicates the compound contains nitrogen (pg. 467)
What is the nitrogen rule?
A compound that contains an odd number of N atoms gives an odd molecular ion (pg. 467)
Sample problem 13.2
pg. 468
Give the molecular weights of C, H, N, and O
C=12
H=1
N=14
O=16
What are the two common isotopes of Cl? What is the ratio of their abundance? What are
the implications of this in terms of a mass spectrum?
³⁵Cl and ³⁷Cl in a 3:1 ratio. Thus there are two peaks in a 3:1 ratio for the molecular ion of an
alkyl chloride. The larger one corresponds to the ³⁵Cl and the M+2 peak corresponds to the
³⁷Cl. (pg. 468)
How do we know when a Cl atom is present when looking at a mass spectrum?
When the molecular ion consists of two peaks (M and M+2) in a 3:1 ratio, a Cl atom is
present (pg. 468)
