Neopentane
Neopentane, also called 2,2-dimethylpropane, is a double-branched-chain alkane with five carbon atoms. Neopentane is an extremely 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. It is one of the three structural isomers with the molecular formula C5H12 (pentanes), the other two being n-pentane and isopentane.
Contents
Nomenclature
IUPAC nomenclature retains the trivial name neopentane.[3][4] The systematic name is 2,2-dimethylpropane, but the substituent numbers are unnecessary 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 symbolises the element neptunium (atomic number 93) one should not use this abbreviation.
Physical properties
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 some 140 degrees higher than that of isopentane (-159.9°C) and some 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. Indeed, the entropy of fusion of neopentane is about 4 times lower than that of n-pentane and isopentane.[2]
NMR spectrum
Neopentane has Td symmetry. As a result, all protons are chemically equivalent leading to a single NMR chemical shift δ = 0.902 when dissolved in carbon tetrachloride.[5] 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.
References
- ↑ "neopentane - Compound Summary". PubChem Compound. USA: National Center for Biotechnology Information. 16 September 2004. Identification and Related Records. http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=10041&loc=ec_rcs#x291. Retrieved 8 March 2012.
- ↑ 2.0 2.1 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
- ↑ Table 19(a) Acyclic and monocyclic hydrocarbons. Parent hydrocarbons
- ↑ Script error
- ↑ Spectral Database for Organic Compounds, Proton NMR spectrum of neopentane, accessed 19 Nov 2006.
External links
- Linstrom, P.J.; Mallard, W.G. (eds.) NIST Chemistry WebBook, NIST Standard Reference Database Number 69. National Institute of Standards and Technology, Gaithersburg MD. http://webbook.nist.gov
- IUPAC Nomenclature of Organic Chemistry (online version of the "Blue Book")
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