Benzene mass spectrum

The diborocine 99 is formed on treatment of (/-C3H7)2N-B(Cl)CeH4CH2Br-2 with sodium/po-tassium alloy in benzene mass spectrum and NMR spectra ( H, B with 6 B = 46.4 ppm) are given [25]. 

Modern mass spectrometers are interfaced with computerized data handling sys terns capable of displaying the mass spectrum according to a number of different for mats Bar graphs on which relative intensity is plotted versus m z are the most common Figure 13 40 shows the mass spectrum of benzene m bar graph form... 

FIGURE 13 40 The mass spectrum of benzene The peak at miz = 78 corresponds to the CgHg molecular ion... 

The mass spectrum of benzene is relatively simple and illustrates some of the mfor matron that mass spectrometry provides The most intense peak m the mass spectrum is called the base peak and is assigned a relative intensity of 100 Ion abundances are pro portional to peak intensities and are reported as intensities relative to the base peak The base peak m the mass spectrum of benzene corresponds to the molecular ion (M" ) at miz = 78... 

Benzene does not undergo extensive fragmentation none of the fragment ions m its mass spectrum are as abundant as the molecular ion... 

There is a small peak one mass unit higher than M m the mass spectrum of ben zene What is the origin of this peak d What we see m Figure 13 40 as a single mass spectrum is actually a superposition of the spectra of three isotopically distinct benzenes Most of the benzene molecules contain only and H and have a molecular mass of 78 Smaller proportions of benzene molecules contain m place of one of the atoms or m place of one of the protons Both these species have a molecular mass of 79... 

Not only the molecular ion peak but all the peaks m the mass spectrum of benzene are accompanied by a smaller peak one mass unit higher Indeed because all organic com pounds contain carbon and most contain hydrogen similar isotopic clusters will appear m the mass spectra of all organic compounds... 

B By knowing the natural abundances of minor isotopes, it s possible to calculate the relative heights of M+ and M+l peaks. If 33C has a natural abundance of 1.10%, what are the relative heights of theM+ and M+l peaks in the mass spectrum of benzene, C6H6 ... 

If the MW is not m/z 166 and there is an intense ion at m/z. 149, this suggests an ester of a benzene dicarboxylic acid. If m/z 149 is the most intense ion, the mass spectrum represents a phthalate, where R is an ethyl group or larger (see Figure 28.1). The most intense ions in the mass spectra of isophthalates and terephthalates are [M - OR]+. 

The ability of the Si=P bond to serve as a dienophile for a [2+4]-cycloaddition (hetero-Diels-Alder reaction) has been verified by the reaction of 15a with cyclopentadiene (Eq. 10).14 The components react at 60°C in benzene to give a quantitative yield of 25, which was isolated as a colorless solid. Interestingly, a Br0nsted acid (C—H)/base (Si=P) reaction, as in the case of 15a and phenylacetylene, was not observed. In the mass spectrum of25, the molecular peak but also the free phosphasilene 15a+ have been detected. Similar behavior was observed for related ad-... 

Mass Spectrum of Benzene, of 2,2,4 Trimethyl Pentane Some Binding Principles for finding the Base Peaks... 

Example The base peak in the El mass spectrum of (3-methylpentyl)-benzene is formed by McLafferty rearrangement of the molecular ion (Fig. 6.28). As long as pentene loss may occur, there is not much difference to spectra of other isomers such as 2-methylpentyl-, 4-methylpentyl, or n-hexyl. Reference spectra are needed to distinguish those isomers, because differences are mainly due to peak intensities, whereas only minor peaks might appear or vanish depending on the isomer. 

Fig. 6.28. El mass spectrum of (3-methylpentyl)-benzene. McLafferty rearrangement and benzylic cleavage are clearly dominating. In the low-mass range carbenium ions and the aromatic fragments are present. Spectmm used by permission of NIST. NIST 2002.

Fig. 6.48. El mass spectrum of l,2-bis(trimethylsiloxy)benzene. The isotopic pattern of...

Some aspects of the chemistry of helicenes require still more attention. Since the interpretation of the mass spectrum of hexahelicene by Dougherty 159) no further systematic work has been done on the mass spectroscopy of helicenes, to verify the concept of an intramolecular Diels-Alder reaction in the molecular ion. Though the optical rotation of a number of helicenes is known and the regular increase of the optical rotation with increasing number of benzene rings has been shown, the dependence of the rotation on the helicity is still unknown. The asymmetric induction in the synthesis of helicenes by chiral solvents, or in liquid crystals, though small, deserves still more attention because application to other organic compounds will be promoted when the explanation of observed effects is more improved. 

Dimethyl bromosuccinate was prepared from bromosuccinic acid by the diazomethane method (26) using the procedure of Eisenbraun, Morris, and Adolphen (27). It was distilled under vacuum (0.08-0.1 Torr) at 45°-49°C to yield a clear colorless oil. Thin layer chromatography with benzene as the solvent on SiC>2 yielded a symmetrical single spot, indicating either a pure compound or no separation with this particular solvent. Its mass spectrum had a very small peak corresponding to the parent compound, but none to a dibromo compound. The mass spectrum for dimethyl bromosuccinate was not found in the literature, but that for dimethyl succinate also has a small peak corresponding to the parent compound (25). 

Pure daco-free amine is a pale yellow low-melting solid that is soluble in variety of solvents including ether, chloroform, benzene, and methanol. The HNMR spectrum in CDC13 shows a quintet at 1.65, 4H, broad multiplet at 1.985, 2H, and a triplet at 2.95, 8H mp = —23 to — 18°C. Density = 0.978 g/mL. Mass spectrum m/z = 115. 

Analysis. The infrared C ) ultraviolet (uv), and nuclear magnetic resonance (nmr) spectra are distinct and characteristic for benzene and are widely used in analysis (78—80). Benzene also produces diagnostic ions in the mass spectrum (81,82) (see Analytical METHODS). 

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