Neopentane, also called 2,2-dimethylpropane, is a double-branched-chain

alkane In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in wh ...

with five

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

atoms. Neopentane is a flammable gas at room

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

and

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...

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 In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct bonds between them. The term meta ...

with the molecular formula C₅H₁₂ (

pentane Pentane is an organic compound with the formula C5H12—that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the ...

s), the other two being ''n''-pentane and

isopentane Isopentane, also called methylbutane or 2-methylbutane, is a branched-chain saturated hydrocarbon (an alkane) with five carbon atoms, with formula or . Isopentane is an extremely volatile and extremely flammable liquid at room temperature and p ...

. Out of these three, it is the only one to be a gas at standard conditions; the others are liquids.

Nomenclature

The traditional name neopentane was still retained in the 1993

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

recommendations, but is no longer recommended according to the 2013 recommendations. The

preferred IUPAC name In chemical nomenclature, a preferred IUPAC name (PIN) is a unique name, assigned to a chemical substance and preferred among the possible names generated by IUPAC nomenclature. The "preferred IUPAC nomenclature" provides a set of rules for choo ...

is the systematic name 2,2-dimethylpropane, but the substituent numbers are superfluous because it is the only possible “dimethylpropane”. A neopentyl

substituent A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as '' side ...

, 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.

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 resulting from higher molecular symmetry. Indeed, the entropy of fusion of neopentane is about four times lower than that of ''n''-pentane and isopentane.

¹H 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. 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 Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one ...

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 The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...

distribution is still essentially achiral.

References

External links