Atmospheric Chemistry of Titan Options

[Juramike]

Here is a "Benzene-O-Vision" graphic showing the amount of benzene and phenyl radicals at high altitudes on Titan. This is based on detections of benzene and phenyl radical (which recombined in the sample chamber to make benzene) using the INMS instrument during closest approach. The numbers are normalized to constant pressure altitude, roughly 1000 km.



The data was taken from Table 1 in: Vuitton et al, Journal of Geophysical Research 113 (2008) E05007. "Formation and distribution of benzene on Titan". doi: 10.1029/2007JE002997 [EDIT 5/24/10: Article freely available here] and overlaid on a map of Titan.

The authors mentioned that the errors in these measurements are 20%.

These detections are well above the detached haze layer. Most are at the same sun azimuth angle. (T23 observation had the lowest angle.) Assuming that the temporal difference is minimal (each dot is from a different flyby), there doesn't appear to be an obvious correlation with latitude.

This graphic does show that benzene is present even waaaay up in the thermosphere and ionosphere.

[Juramike]

Beyond Benzene - PAH's and Polyphenyls

Polyaromatic hydrocarbons (fused aromatics)



Benzene is actually a pretty wussy molecule: it can cleave a C-H bond with “normal” UV light at 248 nm (= 115 kcal/mol). This generates a benzene radical (.C6H6) and a hydrogen radical.
One of the things benzene radical can do is react with acetylene (or diacetylene, or vinyl acetylene) and then close the pendant chain to form a new fused aromatic ring. This process is thought to be an important route to polyaromatic hydrocarbons during soot formation. The benzene radical first attacks the triple bond of acetylene, then places the radical at the terminal end of the alkyne that just got attached. This can repeat the process and react with another acetylene molecule. But this last radical now has a pendant radical that can attack the pi-system of the parent benzene ring to make a new six-membered ring. (Five and six-membered rings form pretty easily in most chemical processes). This places the radical likely at the ring junction (most substituted). This collapses to kick out a hydrogen radical and a fully aromatized fused ring system – naphthalene. Most studies indicate this can happen only at high temperatures – places like interstellar clouds illuminated with high energy photons or in the back of your car’s exhaust pipe.

Polyphenyls (linked aromatics)



At lower temperatures, another process occurs with benzene radical, it reacts with another molecule of benzene. Attack of the radical into the pi-system gives a transient radical that quickly rearomatizes and kicks out a hydrogen radical. This creates a linked aromatic system (not fused).
Fused ring systems are usually planar. When they get really big, like C30 or so, they can become cup-shaped. Fused ring systems also can have different reactivities. In anthracene (3 benzenes fused in a line) the central carbons are very prone to oxidation. The central double bonds also are prone to reaction. (They do [4+2] cycloadditions easily). But as a general rule, fused ring carbons act more electron-deficient. PAH’s can do some funky chemistry.
In contrast, polyphenyls are about as exciting as linked benzene. The chemistry is almost exactly like that of benzene. Structurally, rings are twisted out of plane in the gas phase, but will flatten out when excited. In solid phase, the rings are planar. (All this is assuming that the hydrogens are at the ortho position. If there is a larger substituent, it will twist. For ortho-terphenyl, the two rings are oriented perpendicular to the central ring to prevent bumping each other.)



PAH’s require high temperatures to form, while polyphenyls can form at lower temperatures. It is likely that the low temperatures on Titan prefer the formation of polyphenyls such as biphenyl, terphenyl, and higher. There are many recent reports coming out that implicate the formation of biphenyls in Titan’s atmosphere.