Side Wall Functionalization of Carbon Nanotubes

As for the fullerenes, the development of methods for nanotube functionalization began very soon after their discovery. After first successes in opening the tubes and attaching functional groups to their ends, the nejct attempts were made in applying the common reactions of fullerene modification to the side-waU functionalization of the structurally related carbon nanotubes. Many reactions performed on fullerenes can indeed be applied to nanotubes as expected. However, the latter are generally observed to be less reactive, which has already been discussed in Section 3.5.1. 

Suitable reactions for side-wall functionalization are mainly those that attack the TT-system of the nanotube. Primarily, these are transformations like the addition or cycloaddition known from the chemistry of double bonds. These transformations shall be discussed in the following. The direct attack to the n-system of the tube has yet another interesting aspect Unlike the functionalization of the ends it enables a control over the electronic structure of the entire tube. A suitable modification thus allows for the construction of complex electronic systems based on carbon nanotubes. At first, however, the more simple side-wall functionalizations wiU be discussed. 

Hydrogenation The simplest modification conceivable for the side wall of a carbon nanotube is the hydrogenation. Taking it to the extreme, it would yield a tubular, annealed hydrocarbon. The character of a nanotube, however, would most likely be destroyed upon an exhaustive hydrogenation. Actually, experiments like 

In analogy to the chemistry of fullerenes, the reaction of carbon nanotubes with boranes could be assumed to provide hydroborated products that might be converted into a number of derivatives, for example, into partially hydrogenated structures by treatment with carbonic acids. It turned out, however, that the reactivity of double bonds in nanotubes does not suffice for a hydroboration. According to calculations, the process should be thermodynamically neutral when performed on a typical SWNT, which means it is anything but a preferred reaction. 

Halogenation The fluorination of carbon nanotubes is an important primary functionalization because it may be conducted even as heterogeneous process between gaseous and solid phase. The reaction of fluorine with SWNT, for example, can be performed in a tube furnace at about 150°C. It yields perfluori-nated nanotubes with a degree of fluorination of up to 100%. 

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