Protons alpha

In the previous two chapters, we focused on the reactions that can take place when a nucleophile attacks a carhonyl group  

We first learned about nucleophilic attack on ketones and aldehydes (in Chapter 5). Then, in Chapter 6, we explored reactions of carboxylic acid derivatives. Now, we are ready to move away from the carbonyl group, and explore the chemistry that can take place at the alpha (a) carbon  

We call this the alpha carbon, because it is the carbon atom directly connected to the carbonyl group. We use the Greek alphabet to label carbon atoms, moving away from the carbonyl group, in either direction  

Notice that in this compound, there are two alpha positions. In this chapter, we will focus on the chemistry that can take place at the alpha positions. 

Before we get started, we should discuss one more piece of terminology. Any protons connected to an alpha carbon are called alpha protons  

---

Twenty five isotopes of polonium are known, with atomic masses ranging from 194 to 218. Polonium-210 is the most readily available. Isotopes of mass 209 (half-life 103 years) and mass 208 (half-life 2.9 years) can be prepared by alpha, proton, or deuteron bombardment of lead or bismuth in a cyclotron, but these are expensive to produce. 

The resultant spectrum is clearly very different from the alcohol, as the trifluoroacetic ester function is far more deshielding with respect to the alpha proton than is the -OH group. A downfield shift of > 1 ppm can be seen. This clearly distinguishes the alcohol from the analogous chloro compound which would of course give no reaction. 

TTie TOCSY 2D NMR experiment correlates all protons of a spin system, not just those directly connected via three chemical bonds. For the protein example, the alpha proton, Ft , and all the other protons are able to transfer magnetization to the beta, gamma, delta, and epsilon protons if they are connected by a continuous chain—that is, the continuous chain of protons in the side chains of the individual amino acids making up the protein. The COSY and TOCSY experiments are used to build so-called spin systems—that is, a list of resonances of the chemical shift of the peptide main chain proton, the alpha proton(s), and all other protons from each aa side chain. Which chemical shifts correspond to which nuclei in the spin system is determined by the conventional correlation spectroscopy connectivities and the fact that different types of protons have characteristic chemical shifts. To connect the different spin systems in a sequential order, the nuclear Overhauser effect spectroscopy... 

It is obvious from these experiments that the absorption spectrum of the Martian red surface can be simulated reasonably well by a non-unique variety of Fe rich phases or their mixtures as can the weak magnetism, so that a positive identification will probably only be possible, following further in situ analyses and/or sample return and analysis in the lab.Two Mars Exploration Rovers (MERs) are due to arrive at Mars in 2004 and will attempt to analyze rocks and soils on the surface using several small spectrometers, including PanCAM (an extended visible region spectrometer), MiniTES (a thermal emission spectrometer), APXS (alpha proton X-ray spectrometer measuring the major elements), Mossbauer (run at current local temperature), as well as a 5-level magnet array similar to that on-board the Pathfinder Lander. 

The Mars Pathfinder rover carried an Alpha Proton X-ray Spectrometer (APXS), and the two Mars Exploration Rovers (MER - Spirit and Opportunity) carried Alpha Particle X-ray Spectrometers (also called APXS, but in this case more precise versions of the Pathfinder instrument, though without the ability to monitor protons for light element analyses). These instruments contained radioactive curium sources (Fig. 13.16) whose decay produced a-particles, which irradiated target rocks and soils. The resulting characteristic X-rays provided measurements of major and minor element abundances. The MER rovers also carried Mossbauer spectrometers, which yielded information on iron oxidation state. 

Rieder R, Economou T, Wanke H, Turkevich A, Crisp J, Briickner J, Dreibus G, McSween HY Jr (1997) The chemical composition of Martian soil and rocks returned by the Mobile Alpha Proton X-ray Spectrometer Preliminary results from the X-ray mode. Science 278 1771-1774... 

Therefore, due to the four equivalent beta protons, a quintet is expected, which should be further split into ten lines by the alpha proton (Figure 5). This constructed spectrum can be applicable to either 3-chloropentane or PVC, the R groups being CH3 or polymer chains respectively. If this PVC radical could be observed in the liquid state, all ten lines would be visible. However, for the solid at LNT, the central eight lines will overlap, and the spectrum will appear as a symmetrical sextet with a theoretical line intensity ratio of 1 5 10 10 5 1. 

A yield optimization study of [18F]FAP, an intermediate possessing both an electrophilic carbonyl carbon and acidic alpha protons, has been carried out146,147 (equation 66) to ful-... 

Hydroxide ioo abstracts an acidic alpha proton fTom one molecule of acetaldehyde, yielding an enolaitcion. 

O The base catalyst removes an acidic alpha proton from the starting /S-kelo ester to generate a stabilized enolate ion nucleophile. 

Viking landers carried out Rutherford back-scattering and X-ray fluorescence (XRF) analyses of soils at two sites (Clark et al., 1982). Rocks and soils were analyzed by an alpha-proton X-ray spectrometer (APXS) on the Mars Pathfinder rover (Rieder et al., 1997) (Figure 1). Early APXS analyses (Rieder et al., 1997 McSween... 

Chemical composition of martian soil and rocks comparison of the results from the alpha, proton, and x-ray modes of the Mars Pathhnder Alpha-Proton-X-ray spectrometer (abstr.). Meteorit. Planet. Sci. 35 (suppl.), A55-A56. 

With respect to previous work by B. Ignasiak (14), we are in agreement that alpha proton abstraction from tetrahydrofuran is occurring under Sternberg conditions. The nature of our hydrogen evolution in the model system is different, however, for it comes from a reverse quench into D2O, not from alkylation conditions. Our results implicate hydride formation during the naphthalene-sodium-tetrahydrofuran reaction. 

Hydroxide ion abstracts an acidic alpha proton from one molecule of... 

Particle Accelerators X-rays, alphas, protons, deuterons, electrons, gammas -4 TBq to 40PBq Anywhere in an industrial area. Be aware of possible activation products. 

---