Download HW4

CHEM 4341/6371 Fall 2016
Homework 4 Questions
Answers must be submitted on the answer form. You may collaborate on the development of strategies
to answer these problems, but completion of your final answers should be done independently of
others.
Identify compounds A-X based on the Idata provided.
A possible approach:
If provided, consider the IR spectra – are oxygen and/or nitrogen-containing bonds present? And if
so, what type of functional group is present? Is there evidence for a benzene ring, or an alkene? If
so, what is the substitution pattern? What other common functional groups and structural features
are present?
If it is not provided, determine the molecular formula using clues from the IR spectrum (e.g.,
presence of O and N) and mass spectrum (e.g., presence of Cl, Br; molecular weight)
Consider the mass spectra. Be on the lookout for peaks with even values of m/z in the mass spectra
of compounds with even molecular masses – these are usually good clues.
If you have determined the functional group that is present, consult the appropriate section of the
textbook and the “handy dandy” appendix; consider how molecules containing that functional group
undergo fragmentation.
In most cases, once you have determined the molecular formula and the presence of specific
functional groups, considering the fragmentation that gives the base peak is a huge clue. In other
cases it will be useful to also consider:
- fragmentation that lead to a peak with the highest m/z below the molecular ion (e.g., loss of
a small radical or molecule , e.g., CH3, H2O).
- the presence of large peaks corresponding to loss of 15, 29, 43, 57, etc (e.g., loss of .R).
- fragments arising from loss of a neutral molecule (i.e., with an even m/z; e.g., loss of H2O;
C2H2, etc).
- McLafferty rearrangement.
In some cases it might not be possible to determine the exact structure of an alkyl group in a
compound. For example, you might determine that fragmentation of the molecular ion leads to loss
of C4H9•, but in some cases it will not be possible to determine whether this is an n-butyl, isobutyl,
sec-butyl or t-butyl chain. In these cases, it is appropriate to simply label a substituent on the
molecule as C4H9–. In other cases there is evidence for a specific type of alkyl group (e.g., from the
IR or NMR spectrum). Do your best to be as specific as possible, but do not “over interpret!
1. Compounds A-C, all C10H14 mono-substituted benzenes
All C10H14 mono-substituted benzenes (there are only four possibilities) – draw them and predict how
they might fragment (hint – consider how cleavage at the benzylic position might give a tropylium
ion), then look at the spectra. You should be able to identify the alkyl groups here.
2. Compounds D-F, all C6H14O alcohols
All C6H14O alcohols – consider the section in the book that deals with fragmentation of alcohols. You
might not be able to identify the exact arrangement of carbon atoms, but you should be able to
determine the position of the OH group in the molecule.
3. Compounds G-I, all MW = 70
Molecular weight is only 70, no oxygen atoms. The two of the mass spectra are almost identical, but
their IR spectra are different. What functional group is present? What is the most common mode of
fragmentation for molecules containing this functional group? Hint: Pavia.
4. Compounds J-L, all C8H19N
For these compounds, SODAR = 0, so these are all saturated amines. The IR might give clues about
the nature of the amine (1°, 2° or 3°). Then you need to determine the structure of the alkyl group on
the nitrogen. Consider how amines fragment.
5. Compounds M-O, all MW = 101
Note molecular weight of 101 (that’s odd!) Consider the base peak in the mass spectra (note that
these all have an even molecular weight, but that this compound has an odd molecular weight, so
these all result from loss of alkyl radicals – but which ones?) What functional group is present? What
is the most common mode of fragmentation for molecules containing this functional group? Consult
the appropriate section of the textbook.
6. Compounds P-Q, both C9H13N
7. Compounds R-W, all C10H14
SODAR = 4, and the infrared spectra all suggest the presence of a benzene ring (with four additional
carbon atoms; this leads to <20 possible compounds). Determine the substitution pattern of the
benzene ring. Then consider how alkylbenzenes fragment (HINT: tropylium) to identify the alkyl
substituents on the ring. There might be other clues in the IR spectra regarding the presence of i-Pr
and t-Bu groups. In some cases, the IR spectra of spectra are nearly identical, but there are
important differences in the mass spectra, and vice versa.
8. Compound X
Problem 1: compounds A-C (all C10H14 mono-substituted benzenes)
A
B
C
Problem 2: compounds D-F (all C6H14O alcohols)
D
E
F
Problem 3: compounds G-I (molecular weight = 70)
G
Problem 3: compounds G-I (molecular weight = 70)
H
Problem 3: compounds G-I (molecular weight = 70)
I
Problem 4: compounds J-L (C8H19N)
J
Problem 4: compounds J-L (C8H19N)
K
Problem 4: compounds J-L (C8H19N)
L
Problem 5: compounds M-O (molecular weight = 101)
M
Problem 5: compounds M-O (molecular weight = 101)
N
Problem 5: compounds M-O (molecular weight = 101)
O
Problem 6: compounds P-Q (C9H13N)
P
Problem 6: compounds P-Q (C9H13N)
Q
Problem 7: compounds R-W (C10H14)
R
Problem 7: compounds R-W (C10H14)
S
Problem 7: compounds R-W (C10H14)
T
Problem 7: compounds R-W (C10H14)
U
Problem 7: compounds R-W (C10H14)
V
Problem 7: compounds R-W (C10H14)
W
Problem 8: compound X
X
Formula not given
Chemical ionization mass spectrum gives (M+H)+ peak at145
1H NMR spectrum provided (bottom right)
1
13 6