|Preferred IUPAC name
3D model (JSmol)
|Molar mass||72.15 g·mol−1|
|Density||3.255 kg/m3 (gas, 9.5 °C)
601.172 kg/m3 (liquid, 9.5 °C)
|Melting point||−16.5 °C (2.3 °F; 256.6 K)|
|Boiling point||9.5 °C (49.1 °F; 282.6 K)|
|Vapor pressure||146 kPa (at 20 °C)|
|4.7 nmol Pa−1 kg−1|
|121.07–120.57 J K−1 mol−1|
|217 J K−1 mol−1|
Std enthalpy of
|−168.5–−167.3 kJ mol−1|
Std enthalpy of
|−3.51506–−3.51314 MJ mol−1|
|R-phrases (outdated)||R12, R51/53|
|S-phrases (outdated)||(S2), S16, S33|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Neopentane, also called 2,2-dimethylpropane, is a double-branched-chain alkane with five carbon atoms. Neopentane is a flammable gas at room temperature and pressure which can condense into a highly volatile liquid on a cold day, in an ice bath, or when compressed to a higher pressure.
Neopentane is the simplest alkane with a quaternary carbon, and has achiral tetrahedral symmetry. It is one of the three structural isomers with the molecular formula C5H12 (pentanes), the other two being n-pentane and isopentane. Out of these three, it is the only one to be a gas at standard conditions; the other being liquids.
The traditional name neopentane was still retained in the 1993 IUPAC recommendations, but is no longer recommended according to the 2013 recommendations. The preferred IUPAC name is the systematic name 2,2-dimethylpropane, but the substituent numbers are superfluous because it is the only possible “dimethylpropane.”
A neopentyl substituent, often symbolized by "Np", has the structure Me3C-CH2- for instance neopentyl alcohol (Me3CCH2OH or NpOH). As Np also symbolises the element neptunium (atomic number 93) one should use this abbreviation with care.
Boiling and melting points
The boiling point of neopentane is only 9.5 °C, significantly lower than those of isopentane (27.7 °C) and normal pentane (36.0 °C). Therefore, neopentane is a gas at room temperature and atmospheric pressure, while the other two isomers are (barely) liquids.
The melting point of neopentane (−16.6 °C), on the other hand, is 140 degrees higher than that of isopentane (−159.9 °C) and 110 degrees higher than that of n-pentane (−129.8 °C). This anomaly has been attributed to the better solid-state packing assumed to be possible with the tetrahedral neopentane molecule; but this explanation has been challenged on account of it having a lower density than the other two isomers. Moreover, its enthalpy of fusion is lower than the enthalpies of fusion of both n-pentane and isopentane, thus indicating that its high melting point is due to an entropy effect resulting from higher molecular symmetry. Indeed, the entropy of fusion of neopentane is about 4 times lower than that of n-pentane and isopentane.
Because of neopentane's full tetrahedral symmetry, all protons are chemically equivalent, leading to a single NMR chemical shift δ = 0.902 when dissolved in carbon tetrachloride. In this respect, neopentane is similar to its silane analog, tetramethylsilane, whose single chemical shift is zero by convention.
The symmetry of the neopentane molecule can be broken if some hydrogen atoms are replaced by deuterium atoms. In particular, if each methyl group has a different number of substituted atoms (0, 1, 2, and 3), one obtains a chiral molecule. The chirality in this case arises solely by the mass distribution of its nuclei, while the electron distribution is still essentially achiral.
- Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 652. ISBN 978-0-85404-182-4. doi:10.1039/9781849733069-FP001.
- Aston, J.G.; Messerly, G.H., Heat Capacities and Entropies of Organic Compounds II. Thermal and Vapor Pressure Data for Tetramethylmethane from 13.22K to the Boiling Point. The Entropy from its Raman Spectrum, J. Am. Chem. Soc., 1936, 58, 2354.
- Table 19(a) Acyclic and monocyclic hydrocarbons. Parent hydrocarbons
- Panico, R.; & Powell, W. H. (Eds.) (1994). A Guide to IUPAC Nomenclature of Organic Compounds 1993. Oxford: Blackwell Science. ISBN 0-632-03488-2.
- Whitmore, Frank C.; Fleming, Geo. H. (1934-09-01). "Preparation of Tetramethylmethane (Neopentane) and Determination of its Physical Constants1". Journal of the American Chemical Society. 55 (9): 3803–3806. ISSN 0002-7863. doi:10.1021/ja01336a058.
- LaCoste, Lucien J. B. (1934-10-15). "The Rotational Wave Equation of Tetramethylmethane for Zero Potential and a Generalization". Physical Review. 46 (8): 718–724. doi:10.1103/PhysRev.46.718.
- James Wei (1999), Molecular Symmetry, Rotational Entropy, and Elevated Melting Points. Ind. Eng. Chem. Res., volume 38 issue 12, pp. 5019–5027 doi:10.1021/ie990588m
- Spectral Database for Organic Compounds, Proton NMR spectrum of neopentane, accessed 19 Nov 2006.
- Nature. 2007 Mar 29. Absolute configuration of chirally deuterated neopentane. Haesler Jacques, Schindelholz Ivan, Riguet Emmanuel, Bochet Christian G, Hug Werner.