Take Home Lessons from Lecture 5
REACTIONS (CONT.)
1. Roadmap 2: Second semester roadmap part 1. Use this template to make your own roadmap of the reactions from the first part of the second semester. These reactions involve all the carbonyl chemistry up until enolates. There are many carbon-carbon bond forming reactions in this group, along with the reactions of aldehydes, ketones and carboxylic acid derivatives. There are no reaction summaries to go on, so you should use the reaction summaries at the end of each chapter in your Brown, Foote, Iverson and Anslyn text.
2. Roadmap 3: Second semester roadmap part 2. Use this template to make your own roadmap of the reactions from the second part of the second semester. These reactions involve enolates. There are many carbon-carbon bond forming reactions in this group. There are no reaction summaries to go on, so you should use the reaction summaries at the end of each chapter in your Brown, Foote, Iverson and Anslyn text.
3. Roadmap 4: Second semester roadmap part 3. Use this template to make your own roadmap of the reactions from the third part of the second semester. These reactions involve aromatic compounds as well as amines. There are no reaction summaries to go on, so you should use the reaction summaries at the end of each chapter in your Brown, Foote, Iverson and Anslyn text.
4. Absorption of light
A. Electromagnetic radiation in the ultraviolet and visible regions occurs when electrons are excited from filled to unfilled molecular orbitals.
B. A material appears to our eyes as the combination of reflected wavelength (that are not absorbed).
5. Mass spectral analysis involves making molecules into ions, then accelerating them into a magnetic field in a vacuum. Their trajectory can be used to determine the mass of the ion, thereby determining a molecules molecular formula.
6. Nuclear Magnetic Resonance (NMR) The three part NMR handout covers most of the important aspects of NMR
A. Physics: Moving charge generates a magnetic field, and a magnetic field causes charges to move.
B. Atomic nuclei, like electrons, have a quantum mechanical property of "spin". Spin can be thought of as a small magnetic field around the nucleus created as if the positive charge of the nucleas were circulating.
C. The location of a given signal with respect to a standard, TMS, is called chemical shift and this has the units ppm (parts per million). The moreelectron density around a nucleus (the moreshielded it is), the smaller the chemical shift. Different functional groups have characteristic chemical shifts.
D. NMR spectra record the energy (plotted as frequency) necessary for the nuclei to be excited from the lower energy spin state to the higher energy spin state in the presence of a strong external magnetic field. Different atoms in a molecule take different amounts of energy to accomplish this, and the different energies can be correlated to structure of the molecule.
E. Equivalent hydrogen atoms in a molecule give the same NMR signal. Equivalent hydrogen atoms in a molecule have an identical relationship to all the other atoms in the molecule, and are found on the same sp3 atom (bond rotation makes them equivalent) or entire groups are equivalent due to symmetry in the molecule (i.e. the six equivalent hydrogens on the two methyls of an isopropyl group). Determining how many equivalent hydrogens are in a molecule can be very tricky (Skull and Crossbones!) so PRACTICE.
F. The size (integration) of the signal in an 1H NMR spectrum is proportional to the number of equivalent hydrogens in each signal.
G. Adjacent nuclei have magnetic fields associated with their spins. The spins of equivalent adjacent nuclei can be either +1/2 or -1/2, and at room temperature they are found in about a 50:50 mixture at any given nucleus (very slight excess of lower energy +1/2). These can add to give n+1 different spin combinations in the proportions predicted by Pascal's triangle. Each different spin combination produces a different magetic field, which leads to n+1 splittings in the peaks of the NMR spectra of the adjacent (no more than three bonds away) nuclei.
H. The distance between peaks split in this way is called the coupling constant ("J").
7. For infrared (IR) spectroscopy, all you have to know is that bonds absorb energy in the infrared region corresponding to bond vibrations. The more polar the bond, the larger the absorption, and the stronger the bond/lighter the atoms, the higher the frequency of absorbed energy. Entirely symmetric bonds do not absorb. Important peaks in an infrared spectrum are associated with NH, OH, CO, and CN bonds. -OH bonds and Carbonyls, C=O, give very strong absorptions and are particularly useful for identifying the different functional groups in molecules.
SUMMARY OF SKILLS YOU MUST MASTER FOR THE MCAT:
1) Be familiar with second semester reactions by using the roadmaps as a study aid and organizational tool. Focus on aldehydes/ketones, carboxylic acids, esters, the aldol reaction, Claisen reaction, Michael reaction, electrophilic aromatic substitution (directing effects) and reactions of aryl diazoniums.
2) Understand that absorbtion of ultraviolet and visible light involves exciting electrons from filled to unfilled orbitals.
3) Mass spectral analysis is the primary means of determining molecular weight.
3) Be able to derive structure from NMR spectra by understanding how chemical shift identifies functional groups, integration quantifies the amount of equivalent hydrogens corresponding to each signal and splitting provides information concerning which atoms are next to each other.
4) Be able to identify the presence of OH, NH, CO and especiallyC=O groups in molecules by IR (infrared spectroscopy).