| Neopentane | |
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neopentane |
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| Identifiers | |
| CAS number | 463-82-1 |
| PubChem | 10041 |
| ChemSpider | 9646 |
| EC number | 207-343-7 |
| MeSH | Neopentane |
| ChEBI | CHEBI:30358 |
| Jmol-3D images | Image 1 |
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| Properties | |
| Molecular formula | C5H12 |
| Molar mass | 72.15 g mol−1 |
| Exact mass | 72.093900384 g mol−1 |
| Appearance | Colorless gas |
| Density | 0.627 g cm-3 |
| Melting point |
−17 °C, 256 K, 1 °F |
| Boiling point |
10 °C, 283 K, 50 °F |
| Thermochemistry | |
| Std enthalpy of formation ΔfH |
−168 kJ mol-1 |
| Std enthalpy of combustion ΔcH |
−3514 kJ mol-1 |
| Standard molar entropy S |
217 J K−1 mol−1 |
| Hazards | |
| EU Index | 601-005-00-6 |
| EU classification | F+ N |
| R-phrases | R12, R51/53 |
| S-phrases | (S2), S9, S16, S33, S61 |
| NFPA 704 |
4
1
0
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| Related compounds | |
| Related Tetraalkylcarbons | Tetra-tert-butylmethane |
| Related compounds | Tetramethylsilane |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
Neopentane, also called 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.
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IUPAC nomenclature retains the trivial name neopentane.[1][2] 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).
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.[3]
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.[4] 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.
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