We will now turn to the sections from Chapters 10-15 that deal with
experimental methods for identifying structure for organic molecules
and deal with them as a concerted whole.
- You can use the molecular formula to determine the "degrees
of unsaturation" (the total of the number of rings plus the number of
pi bonds) for an organic compound containing C, H, N, O or halogens.
DoU = (2x#C + 2 + #N - #X - #H)/2
- You understand the physical basis of infrared (IR
spectroscopy: absorption of electromagnetic energy at frequencies just
below those of visible light that excite particular molecular
vibrations.
- You can use the IR spectrum to identify the presence of
several important functional groups: C-H bonds, O-H and N-H bonds, C=C
and C=N bonds, C=O bonds.
Visualization: Characteristic IR spectra
and Functional Groups
Visualization: IR absorptions Activate Molecular Vibrations
- You recognize the use of the fingerprint region to confirm
the identity of a given organic compound.
- You understand the physical basis of the NMR experiment:
absorption of electromagnetic energy in the radio region by nuclei
sitting in a strong magnetic field. Ref: Wikipedia.
- You understand that electron density around a nucleus
provides "magnetic shielding" and, to the extent that this varies for
specific chemical environments, provides a systematic differentuiation
of the NMR behavior of one atom (nucleus) in a molecule from another.
- You know the chemical shift regions (in ppm from Me4Si)
for the following types of protons: alkyl; allylic and benzylic; CH/CH2/CH3
next to a ketone; terminal alkyne; CH/CH2/CH3
next to an electronegative atom (O, halogen); alkene; aromatic;
aldehyde and carboxylic acid. You know that exchangeable hydrogens (OH,
NH, SH) can vary substantially in chemical shift and appearance.
Visualization: Proton
NMR Chemical Shifts
- You recognize the unique electronic behavior of aromatic
rings, and understand how that behavior leads to extra deshielding of
aromatic protons. This effect is the third experimental test for
aromaticity.
- You can identify chemical equivalence of nuclei on the
basis of symmetry.
- You know that NMR peak integration reveals the relative
number of protons at each chemically distinct position.
- You understand the magnetic interactions between nuclei of
different chemical shifts that give rise to spin-spin coupling, and can
use Pascal's Triangle to both identify how many protons are adjacent to
any one proton from its NMR coupling pattern, and predict the peak
pattern from the structure.
Visualization: 1H-1H Coupling
- You can apply the concept of unequal coupling constants to
understand peak patterns arising from coupling to nonequivalent
neighbors.
Visualization: Assigning 1H NMR Spectra
- You can apply your understanding of NMR from the 1H
NMR world to use of 13C NMR spectroscopy, understanding
that carbon spectra are typically collected with full proton decoupling
in order to simplify the spectra and enhance the signal-to-noise ratio.
Visualization: Carbon Chemical Shifts
Visualization: Use of DEPT to Identify CH3 vs. CH vs. quaternary Cs
- You know that ultraviolet and occasionally visible light
promote electrons to higher (unfilled) molecular orbitals at a
wavelength corresponding to the HOMO-LUMO gap, and that this can be
used to identify conjugated and aromatic systems.
Visualization: UV absorption vs. HOMO-LUMO gap
- You know that mass spectra can be collected by gas phase
ionization followed by passage through a magnetic field to identify
molecules by their mass/charge ratio.
Visualization: Design of Mass Spectrometers and the MS Experiment
Visualization: Interpretation of MS Data
- You know that the radical cations normally formed in
electron impact ionization fall apart in regular ways that can help
identify molecules or functional groups. Examples are α-cleavage of
alcohols and amines, formation of allylic cations in alkenes, and
formation of the benzyl cation in alkylbenzenes.
- You can integrate your knowledge of spectroscopic
techniques to propose structures based on spectra.
Important
Link: NIST Webbook (Database that includes IR, UV/visible, and mass
spectra of many organic compounds)
Spectral Database for Organic Compounds maintained by the Japanese National Institute of Advanced Industrial Science and Technology.
And if you want a rough prediction of what the 1H NMR spectrum for any compound might look like, go to this link written by the creators of the Jsmol technology.
Recommended end-of-chapter problems: 10-33, 10-34, 10-37, 10-44, 10-48, 10-50, 10-59, 11-38, 12-62, 12-63, 12-64, 13-36, 14-73, 15-43, 16-49.
In-class worksheets:
IR Worksheet(in-class, Friday, Feb. 25)
NMR Worksheet 1--1H NMR chemical shifts (In-class, Wednesday, March 2)
NMR Worksheet 2--1H-1H coupling and structure determination (Do over the weekend, we'll discuss Monday, March 7)
A worked problem that we'll do in class.
MS/UV Worksheet--Use of MS and UV data to solve structures (In-class, Friday, March 11) |