CHEM 39, Spring 2007
Previous News and Updates:
Friday, February 9, 2007 (Lecture 11)
Today we started discussing Chapter 20 (Carboxylic acids). We talked about how inductive effects and resonance effects influence the acidity of these compounds. Remember that electron withdrawing groups stabilize the carboxylate to increase the acidity and electron donating groups destabilize the carboxylate to decrease the acidity.
Wednesday, February 7, 2007 (Lecture 10)
We finished discussing aldehydes and ketones. We covered conjugate additions to alpha, beta-unsaturated aldehydes and ketones. "Hard" nucleophiles (RMgX, RLi, H- from LiAlH4) prefer 1,2-addition. "Soft" nucleophiles (R2CuLi, RNH2, R2NH, H- from NaBH4) prefer 1,4-addition. We talked in some detail about soft cuprate nucleophiles and how to prepare them. The Wolff-Kishner reaction was also discussed. This reaction allows one to convert an aldehyde or ketone to a hydrocarbon by reaction with hydrazine (H2N-NH2) and hydroxide. We also discussed the use of acetals as protecting groups. This material will be on the exam on Monday (2/12/07).
Monday, February 5, 2007 (Lecture 9)
We discussed the addition of amines to both saturated and conjugated ketones and aldehydes. Saturated ketones/aldehydes react with primary amines to yield imines and seconday amines to yield enamines. We also talked about the Wittig reaction in which a ylide is used to generate C=C double bonds. The Wittig reaction allows one to create a new carbon-carbon bond where an aldehyde or ketone carbonyl used to be.
Friday, February 2, 2007 (Lecture 8)
More reactions of aldehydes and ketones. We talked about organometallic
grignard reagents and organolithium reagents that add irreversibly to aldehydes
and ketones to yield alcohols after quenching with acid. We also reviewed
the irreversible addition of hydride reagents such as LiAlH4 and NaBH4. We
talked about the reversible reactions of hydration and acetal formation. The
rooms for EXAM1 on Monday 2/12 have been posted. Please sign up for the conflict
exam with Mike in 210 Whitmore if you have another scheduled activity during
the normal exam hours.
The exam will cover all spectroscopic methods (MS, IR, UV-VIS, NMR), spectral
problems, and reactions of aldehydes and ketones (CH. 19). There will be 25
multiple-choice questions on the exam. Two old exams are available for practice
on the web. Tables of spectroscopic data that you will have available during
the exam are on the web. Make sure that you study the combined spectroscopic
problems. There will be several of these on the exam.
Wednesday, January 31, 2007 (Lecture 7)
We started talking about the structure and reactivity of carbonyl
compounds. We will spend several weeks discussing the chemistry of the carbonyl
group, and we will cover a large number of reactions. It is important for
you to understand both general trends and reaction mechanisms. A reaction
list from CHEM 39 is available on the web.
Keep in mind that the electron deficiency of the carbonyl carbon is the reason
for facile nucleophilic attack on carbonyls. This deficiency can be increased
by protonation of the carbonyl oxygen. The nucleophile forms a new bond to
carbon to generate the tetrahedral intermediate. The fate of that intermediate
depends on the nature of the carbonyl compound and the nucleophile.
We started with Chapter 19, which covers aldehydes and ketones. Material from
this chapter will be on the first exam, which is Monday (2/12) from 8:15-10:15
p.m. in 108 Forum (last name A-K) and 111 Forum (last name L-Z).
Monday, January 29, 2007 (Lecture 6)
Today we finished discussing proton NMR, and we discussed spectral
problem A, which you can download from this web site (go to the testing center).
Please keep in mind the following "trade secrets": (1) Multiplets
of hydrogens coupled with each other show a "roof effect" in the
proton spectrum (they lean toward each other). (2) Hydrogens on O, N, and
S exchange with D2O and disappear from the proton spectrum. (3) The nitrogen
rule states that stable organic compounds with odd molecular masses must have
an odd number of nitrogen atoms. Molecules with even molecular masses must
have an even number of nitrogens (or zero).
On Wednesday we will take a break from spectroscopy and start talking about
aldehydes and ketones (Ch. 19). We will cover many carbonyl reactions within
the next week and they will be on the first exam. You should try to become
an expert in structure determination methods, and go over all of the assigned
book problems and all of the assigned spectral problems (A-J). You should
download the first exam from last semester and look at the spectral tables
at the end of the exam. These tables will be provided as part of Exam 1.
Friday, January 26, 2007 (Lecture 5)
Today, we finished our discussion of 13C NMR by talking about
how peak size is related not only to the number of carbons, but also to the
number of attached hydrogens, so integration is not commonly used in 13C NMR.
The 13C technique of DEPT spectroscopy can be used to determine the number
of attached hydrogen atoms.
We also talked in detail about 1H NMR spectroscopy and how functional groups
influence chemical shifts similarly to 13C spectroscopy. In 1H NMR, integration
can be used to determine the relative number of attached hydrogens. Coupling
is observed in 1H NMR, which tells us about the neighborhood that protons
reside in. Here we are talking about the coupling between hydrogens on carbons
that are directly bound to each other. Remember that chemically equivalent
hydrogens do not couple to each other, and (in this class) we are only interested
in vicinal coupling (i.e. coupling between hydrogens separated by three bonds
H-C1-C2-H). Other hydrogens may couple, but we will not analyze such situations
in CHEM 39.
On Monday, we will start talking about how to solve spectral puzzles, and
we will practice with some "real" spectra. Bring the first five
sets (A-E) to class.
Wednesday, January 24, 2007 (Lecture 4)
Today we began discussing the most powerful technique of all
for structure determination: Nuclear Magnetic Resonance (NMR) spectroscopy.
Proton (1H) and carbon-13 (13C) nuclei are magnetically active. Their spins
align both parallel and antiparallel to strong magnetic fields. By irradiating
a sample in the magnet with radio frequencies, the NMR instrument causes the
spins to flip to the higher-energy antiparallel state. The frequency required
to make this transition is measured as parts per million (ppm) with respect
to the standard tetramethylsilane (TMS).
We talked primarily about 13C-NMR and how carbons in different chemical environments
exhibit different chemical shifts. The chemical shift tells us about the chemical
environment of the carbon atom (shielding, deshielding). Nuclei with high
electron density will appear in the high field region of the spectrum (low
ppm values, shielded) and nuclei with diminished electron density will give
peaks in the low field region (high ppm values, deshielded). The number of
peaks in a 13C spectrum tells us the number of chemically different carbon
atoms in the molecule.
For 13C the scale used is from 0 ppm (TMS) to just above 200 ppm (for carbonyl
carbons).
You should start studying Chapter 13. We will also be discussing several of
the spectral problems (A-E) in class, so you should print out copies and bring
them to class.
Monday, January 22, 2007 (Lecture 3)
Today we briefly reviewed UV spectroscopy. UV spectroscopy is used to determine
the extent of conjugation of organic molecules We also taked in much more
detail about infrared (IR) spectroscopy, which is a more powerful technique
because it can be used to identify specific functional groups in molecules.
Different functional groups have characteristic frequencies (expressed in
wavenumbers, i.e. cm-1) that can be observed in the IR spectrum. You should
practice using IR spectra (there are several in the book) to identify functional
groups in molecules.
On Wednesday we will move on to the most powerful structure elucidation technique
of all: Nuclear magnetic resonance (NMR) spectroscopy.
Friday, January 19, 2007 (Lecture 2)
Today we started a discussion of methods used to determine the
structures of organic molecules. Mass spectrometery was introduced as a method
to determine the molecular mass of compounds. We talked about the mechanism
of ionization, parent ions (M+), the base peak, and how fragmentation can
yield clues about the presence of substituents such as methyl or ethyl groups
on organic molecules. The most stable cations are usually formed during fragmentation.
We also talked about isotopes, and how they affect the M+1 and M+2 peaks in
the spectrum. These peaks can be used to determine the number of carbon atoms
or detect the presence of chlorine or bromine atoms in a molecule.
Mass spectrometry provides crucial information about the molecular formula,
especially if the mass is determined very precisely. We explored a couple
of examples of mass spectra and you should do more of that on your own in
Chapter 12.
After talking today about blowing up molecules with electron beams, on Monday
we will finish Chapter 12 by discussing how molecular gymnastics (IR spectroscopy)
gives additional information about molecular structures.
Wednesday, January 17, 2007 (Lecture 1)
Our first class.We went over the syllabus and website today. Be sure to get
a copy of the syllabus! You can download the syllabus from the course webpage.
There are also copies in the undergraduate office (210 Whitmore).
Make sure that you have the 6th edition of McMurray, the study guide and the
book on nomenclature. I will not spend much time on nomenclature, so you should
consult the nomenclature text whenever we discuss a new functional group.
Please dig out your molecular model set and use it as needed.
Read the webpage section on "How to Study".
Download transparencies for the first section of the syllabus (under the Class
Packet). Get copies of the first five spectra (A-E) from the Testing Center.