Benzene interstellar

PAHs are believed to be a major class of carbon-bearing molecules in the interstellar medium 1138], They are found in carbonaceous chondrites tliat have fallen to Earth (see section 4.2.1) and in interplanetary dust particles [28]. Shock and Schulte [139] suggested that amino acids could be syntliesized by aqueous alteration of precursor PAHs in carbonaceous chondrites. We directed attention to shock reaction of PAHs [135,140,141], and conducted shock reactions using benzene, tire simplest aromatic hydrocarbon, as a starting material to simulate possible reactions occurring in interstellar space. Furtliermore, we examined the mechanism of shock reaction on the basis of quantum chemistry and discussed the implication for cosmocheniistiy. 

Before carbonaceous chondrites arrive on the Eartli, tire carbon-bearing materials in tliem may imdergo shock events in at least tire following tliree stages die fonnation of parent bodies by accretion of interstellar medimn particles, the break-up of the parent bodies by their mutual collisions, and die fall of meteorites on the Earth traversing the atmosphere. Through these shock events, primitive carbonaceous materials diat had been present in the interstellar medium would become more complex compounds and they would be detected in meteorites. Shock reacdons may have promoted the secondaiy production of heavier and more complicated PAHs such as the insoluble polymers of muldple benzene rings detected in meteorites. 

Precursors of membrane components, carbohydrates, amino acids, and other biogenic compounds formed either directly or indirectly from those chiral and non-chiral molecules delivered to Earth. Benzene, for instance, is one of the aromatic compounds of interstellar dust from which a precursor of the membrane lipid inositol-1-phosphate apparently derived by oxidation in a long series of chemical reactions. The structure of the important lipid, a derivative of inositol-phosphate in Fig. 6.1, displays two different fatty acid side chains (marked in blue and green) that frequently occur in membranes. 

Interstellar solid-state chemistry can occur within these ices. Laboratory experiments have shown that ices of simple species such as H2O, CO, or NHj can be stimulated by ultraviolet radiation or fast particles (protons, electrons) to form complex molecules, including polycyclic aromatic hydrocarbons (PAHs) containing several benzene-type rings. The detection by astronomers of free interstellar benzene (CsHg) in at least one interstellar region suggests that this solid-state chemistry may be the route by which these molecules are made. 

Carbon chemistry occurs most efficiently in circumstellar and diffuse interstellar clouds. The circumstellar envelopes of carbon-rich stars are the heart of the most complex carbon chemistry that is analogous to soot formation in candle flames or industrial smoke stacks (26). There is evidence that chemical pathways, similar to combustion processes on Earth, form benzene, polycyclic aromatic hydrocarbons (PAHs) and subsequently soot and complex aromatic networks under high temperature conditions in circumstellar regions (27,28). Molecular synthesis occurs in the circumstellar environment on timescales as short as several hundred years (29). Acetylene (C2H2) appears to be the... 

A recent count puts the number of distinct molecules detected in interstellar space at around 150. The list includes small diatomic molecules that are common on Earth (e.g., CO, N2, O2), high-energy diatomic radicals that have exceedingly short lifetimes on Earth (e.g., HO-, HC-), and all the way up to organic compounds like acetone, ethylene glycol, and benzene. 

McEwan MJ, Scott GBI, Adams NG, Babcock LM, Terzieva R, Herbst E. New H and H reactions with small hydrocarbon ions and their roles in benzene synthesis in dense interstellar clouds, Astrophys J. 1999 513 287-93. 

Reactions of Si" " ( P) with small hydrocarbons gives C-H and C-C insertion, and adducts with benzene and naphthalene have implications for such reactions in interstellar environs, and the affinity of phenylsilanes for protons assessed. It is suggested that silyl perchlorates are... 

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