Mass_Spec_Handout

Mass_Spec_Handout - 
 
 
 
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Unformatted text preview: 
 
 
 
 o Key
Terms
and
Concepts
 
 MW ionization e MW fragmentation 
 
 o When
a
compound
is
injected
into
a
mass
spec,
it
is
ionized
by
a
“source”
of
 energy
and
an
electron
is
lost.

The
ionized
compound
can
then
fragment
into
 smaller
charged,
uncharged,
or
radical
species.
 o
 
 Only
charged
species
show
a
peak
on
the
mass
spectrum.
 
 
 o The
molecular
ion
peak
is
the
molecular
weight
of
the
compound
minus
the
 mass
of
an
electron
(so,
essentially
the
molecular
weight
of
the
compound).


 o The
base
peak
is
mass
of
the
most
abundant
fragment
(i.e.
the
most
stable)
 o M+2
peaks
are
present
for
compounds
that
contain
heteroatoms
with
 isotopes
(i.e.
Cl
and
Br)
 In
these
cases,
there
will
be
an
M+.
at
the
molecular
weight
of
the
 compound
AND
an
M+2
peak
at
two
amu
greater
than
the
M+.
peak
 For
Cl,
the
M+2
will
be
1/3
as
abundant
as
the
M+.
(i.e.
a
3:1
ratio)
 For
Br,
the
M+2
will
be
as
abundant
as
the
M+.
(i.e.
a
1:1
ratio)
 
 o Ionization
and
Fragmentation
Patterns
 o The
most
labile
electrons
will
be
“ejected”
first.
 
 
 Lone
pairs
>
Π‐bonds
>
σ‐bonds
 
 
 Lone
Pairs:
 
 
 Π‐bonds:
 
 
 
 σ‐bonds:
 
 
 
 
 
 
 X IE X IE IE + 
 X = O , N, S McLafferty
Rearrangement
 o Must
contain
a
proton
on
the
γ‐carbon
 
 X H R IE X H # ! H # ! " " R X H R X H + R X = O , N, S 
 X H R IE X R X H R X H + R 
 X=C 
 
 Retro­Diels
Alder
 o Must
be
a
cyclohexene
ring
 o Will
always
yield
a
dienophile
and
a
diene
 IE + 
 
 
 α ­Cleavage
 o Cleaves
the
bond
adjacent
to
the
bond
attached
to
the
radical
(i.e.
 between
the
α
and
β‐carbons)
 o Can
cleave
by
1­electron
movement
(the
positive
charges
doesn’t
move):
 
 X ! "R X = O , N, S IE X R X + R 
 X !"R X=C IE X R X + R 
 
 
 
 o OR
can
cleave
by
2­electron
movement
(the
radical
doesn’t
move):
 X ! "R X=C IE X R X + R 
 X !"R X = O , N, S IE X R X + 
 R 
 ...
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This note was uploaded on 01/17/2011 for the course CHEM 6b taught by Professor Pettus during the Fall '07 term at UCSB.

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