Neopentane

Neopentane
Names
Preferred IUPAC name
2,2-Dimethylpropane
Other names
neopentane (no longer recommended[1])
tetramethylmethane
Identifiers
463-82-1 YesY
3D model (Jmol) Interactive image
1730722
ChEBI CHEBI:30358 YesY
ChemSpider 9646 YesY
ECHA InfoCard 100.006.677
EC Number 207-343-7
1850
MeSH neopentane
PubChem 10041
UNII M863R1J0BP N
Properties
C5H12
Molar mass 72.15 g·mol−1
Appearance Colorless gas
Odor Odorless
Density 586 mg mL−1[2]
Melting point −21 to −15 °C; −6 to 5 °F; 252 to 258 K
Boiling point 9.0 to 10.0 °C; 48.1 to 49.9 °F; 282.1 to 283.1 K
4.7 nmol Pa−1 kg−1
Thermochemistry
121.07–120.57 J K−1 mol−1
217 J K−1 mol−1
−168.5–−167.3 kJ mol−1
−3.51506–−3.51314 MJ mol−1
Hazards
F+ N
R-phrases R12, R51/53
S-phrases (S2), S16, S33
NFPA 704
Flammability code 4: Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily. Flash point below 23 °C (73 °F). E.g., propane Health code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentine Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
4
1
0
Related compounds
Related alkanes
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YesYN ?)
Infobox references

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. It is one of the three structural isomers with the molecular formula C5H12 (pentanes), the other two being n-pentane and isopentane.

Nomenclature

The traditional name neopentane was still retained in the 1993 IUPAC recommendations,[3][4] but is no longer recommended according to the 2013 recommendations.[1] 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 group attached to a generic group R.

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.

The obsolete name tetramethylmethane is also used, especially in older sources.[5][6]

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 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. Indeed, the entropy of fusion of neopentane is about 4 times lower than that of n-pentane and isopentane.[2]

NMR spectrum

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.[7] 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.[8]

References

  1. 1 2 Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 652. doi:10.1039/9781849733069-FP001. ISBN 978-0-85404-182-4.
  2. 1 2 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
  3. ↑ Table 19(a) Acyclic and monocyclic hydrocarbons. Parent hydrocarbons
  4. ↑ Panico, R.; & Powell, W. H. (Eds.) (1994). A Guide to IUPAC Nomenclature of Organic Compounds 1993. Oxford: Blackwell Science. ISBN 0-632-03488-2.
  5. ↑ 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. doi:10.1021/ja01336a058. ISSN 0002-7863.
  6. ↑ 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.
  7. ↑ Spectral Database for Organic Compounds, Proton NMR spectrum of neopentane, accessed 19 Nov 2006.
  8. ↑ Nature. 2007 Mar 29. Absolute configuration of chirally deuterated neopentane. Haesler Jacques, Schindelholz Ivan, Riguet Emmanuel, Bochet Christian G, Hug Werner.
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