Neopentane
Encyclopedia
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 (pentane
s), the other two being n-pentane
and isopentane
.
nomenclature retains the trivial name neopentane. 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 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.
. As a result, 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.
Alkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
with five carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
atoms. Neopentane is an extremely flammable gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
at room temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
and pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
which can condense into a highly volatile
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...
liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
on a cold day, in an ice bath, or when compressed to a higher pressure.
Neopentane is the simplest alkane with a quaternary carbon
Carbon-carbon bond
A carbon–carbon bond is a covalent bond between two carbon atoms. The most common form is the single bond: a bond composed of two electrons, one from each of the two atoms. The carbon–carbon single bond is a sigma bond and is said to be formed between one hybridized orbital from each...
. It is one of the three structural isomers
Structural isomerism
Structural isomerism, or constitutional isomerism , is a form of isomerism in which molecules with the same molecular formula have bonded together in different orders, as opposed to stereoisomerism. There are multiple synonyms for constitutional isomers.Three categories of constitutional isomers...
with the molecular formula C5H12 (pentane
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 n-pentane isomer; the other two being called...
s), the other two being n-pentane
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 n-pentane isomer; the other two being called...
and isopentane
Isopentane
Isopentane, C5H12, also called methylbutane or 2-methylbutane, is a branched-chain alkane with five carbon atoms. Isopentane is an extremely volatile and extremely flammable liquid at room temperature and pressure. The normal boiling point is just a few degrees above room temperature and...
.
Nomenclature
IUPACInternational Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science . The international headquarters of IUPAC is located in Zürich,...
nomenclature retains the trivial name neopentane. The systematic name is 2,2-dimethylpropane, but the substituent numbers are unnecessary because it is the only possible "dimethylpropane".
A neopentyl substituent
Substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...
, often symbolized by Np, has the structure Me3C-CH2- for instance neopentyl alcohol
Neopentyl alcohol
Neopentyl alcohol is a compound with formula C5H12O or 3C-CH2OH. It is an alcohol derived from neopentane, or C4....
(Me3CCH2OH or NpOH).
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.
NMR spectrum
Neopentane has Td symmetrySymmetry group
The symmetry group of an object is the group of all isometries under which it is invariant with composition as the operation...
. As a result, all protons are chemically equivalent leading to a single NMR chemical shift
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
δ = 0.902 when dissolved in carbon tetrachloride
Carbon tetrachloride
Carbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent...
. In this respect, neopentane is similar to its silane
Silane
Silane is a toxic, extremely flammable chemical compound with chemical formula SiH4. In 1857, the German chemists and Friedrich Woehler discovered silane among the products formed by the action of hydrochloric acid on aluminum silicide, which they had previously prepared...
analog, tetramethylsilane
Tetramethylsilane
Tetramethylsilane is the chemical compound with the formula Si4. It is the simplest tetraorganosilane. Like all silanes, the TMS framework is tetrahedral...
, 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
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
atoms. In particular, if each methyl group has a different number of substituted atoms (0, 1, 2, and 3), one obtains a chiral
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
molecule. The chirality in this case arises solely by the mass distribution of its nuclei, while the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
distribution is still essentially achiral.
External links
- IUPAC Nomenclature of Organic Chemistry (online version of the "Blue Book")