2,3-Dimethylpentane
2,3-Dimethylpentane is an organic compound of carbon and hydrogen with formula , more precisely ––––: a molecule of pentane with methyl groups – replacing hydrogen atoms on carbon atoms 2 and 3. It is an alkane ("paraffin" in older nomenclature), a fully saturated hydrocarbon; specifically, one of the isomers of heptane. Like typical alkanes, it is a colorless flammable compound; under common ambient conditions, it is a mobile liquid, less dense than water. 2,3-Dimethylpentane is notable for being one of the two simplest alkanes with optical (enantiomeric) isomerism. The optical center is the middle carbon of the pentane backbone , which is connected to one hydrogen atom, one methyl group, one ethyl group –, and one isopropyl group –. The two enantiomers are denoted (3"R")-2,3-dimethylpentane and (3"S")-2,3-dimethylpentane. (The other simplest chiral alkane is its structural isomer 3-methylhexane.) Properties. Most properties listed in the literature refer to the racemic compound (an equimolar mixture of the two optical isomers). The boiling point of 89.7 °C is 0.3 °C higher than the value of 89.4 °C predicted by Wiener's formula, based on the structure of the molecule and the boiling point of "n"-heptane. The speed of sound at  3 MHz is 1149.5 m/s at 20 °C and 889.5 m/s at 80 °C. The racemic mixture has a glass transition temperature of about 123 K (−150 °C), but reportedly it does not crystallize—a fact that has been claimed to be a characteristic of high-purity optically active alkanes. Preparation. 2,3-Dimethylpentane is practically absent in the synthetic fuel produced from hydrogen and carbon monoxide by the Fischer–Tropsch process. The pure compound can be prepared by reacting the Grignard reagent "sec"-butyl magnesium bromide – with acetone to form 2,3-dimethyl-2-pentanol, then dehydrating this  alcohol to form 2,3-dimethyl-2-pentene, and hydrogenating this product. The isomer is present at about 2.4% by weight in the hydrocarbon mixture obtained by the condensation of methanol at 200 °C with a zinc iodide catalyst (the main component of the mixture being the isomer 2,2,3-trimethylbutane, obtained at almost 50% yield).