Hexane is a hydrocarbon compound with a chemical formula of C6H14. Hexane is classified as an alkane and is composed of a chain of 6 central carbon atoms each saturated with hydrogen. The “hex-” means that it has a backbone of 6 atoms, and the “-ane” means that all the atoms have only single bonds. Hexane is one of the main ingredients in modern gasoline as it is cheap to manufacture and readily combustible. It is also commonly used in the lab as a solvent.
Hexane is non-polar on account of its C–H bonds and symmetrical geometric structure. Carbon and hydrogen have an electronegativity difference of 0.35, which classifies the molecule as non-polar. Even were C–H bonds polar, hexane would still be non-polar on account of its symmetric geometric structure. If C–H bonds were polar, the symmetric structure of hexane would ensure that the partial charge of each C–H bond is exactly canceled out by another.
Hexane is a colorless liquid with a boiling point of about 50-70°C. It is mostly produced by the refinement of crude oil and has applications in agriculture, food processing, leather products, and chromatography.
A Quick Primer On Polarity
First a quick review on polarity. The polarity of a molecule is a measure of how evenly electric charge is distributed across the molecule. Every element has an electronegativity value that represents how much individual atoms of the element pull on electrons. The greater the EN value, the more that atoms of that element pull on electrons. For instance, fluorine (F) is the most electronegative element and is assigned an EN value of 4. All other EN values are calculated relative to fluorine.
Atoms form covalent bonds by sharing their valence electrons. When two atoms with a large difference in electronegativities share electrons, the more electronegative element will pull harder on the shared electrons. This causes the shared electrons to move closer to the more electronegative element. Because the more electronegative atom of the compound has an abundance of electrons, it picks up a partial negative charge. Conversely, the less electronegative elements pick up a partial positive charge. This is the essence of polarity: polarity is a measure of how evenly spatially distributed electrons are in a compound.
Whether or not two atoms will form a polar bond depends on the difference between their EN values. If the EN difference falls between 0.5-2.0, the bond is classified as polar. If the EN difference is less than 0.5, then the bond is considered non-polar. If the difference is greater than 2, then the bond is considered completely polar and is more appropriately called an ionic bond.
Whether or not an entire molecule is considered polar depends on 2 things; the polarity of its constituent bonds and its geometric structure. A molecule with non-polar bonds could still be overall polar is the molecule has an asymmetric geometry. A molecule with polar bonds could still be overall non-polar if it has a spatially symmetric geometric structure. The symmetry of the molecular geometry ensures that the partial charge of each polar bond is exactly canceled out by an antipodal polar bond.
Polarity Of Hexane
Using our above lesson on polarity, we can determine if hexane is polar or non-polar. Hexane is composed of mostly C–H bonds. The EN value of carbon is 2.55 and hydrogen is 2.2. The difference between these two EN values is 0.35, so C–H bonds are considered nonpolar. What’s more, hexane has a very symmetrical molecular geometry, so even if C–H bonds were considered polar, the entire molecule would still be non-polar. The spatial placement of the bonds would ensure that any opposing charges get canceled out, so overall the molecule would not be polar.
Technically, C–H bonds are not completely non-polar. Carbon has a higher EN value than hydrogen, so the carbon atoms do pull slightly harder on electrons than the hydrogen atoms do. This amount of pull is very small and negligible, so in normal circumstances, it is safe to treat C–H bonds as if they were completely non-polar. At very small scales and minute levels of precision, the slight polarity of C–H bonds would have a noticeable effect, so whether or not C–H bonds will be considered polar or non-polar depends on the context.
Most chemistry textbooks will consider a bond that has an EN difference of less than 0.5 as non-polar, as any polar action is small enough to be ignored. The only truly non-polar bonds are formed between atoms that identical EN values (for instance, the diatomic elements)
Isomers Of Hexane
Strictly speaking, the name “hexane” can refer to one of any 5 structural isomers with the chemical formula C6H14. A structural isomer of a compound is one that has the same chemical formula, but a different molecular structure. The most common form of hexane is called n-hexane, and consists of a linear chain of 4 methylene (CH2) functional groups sandwiched between 2 terminal methyl groups (CH3). The linear nature of the molecule gives n-hexane a relatively inert character and n-hexane is often used in the lab as a non-polar solvent for highly reactive chemicals.
Depending on the exact system of chemical nomenclature, the structural isomers of hexane are sometimes referred to as derivatives of pentane (C5H12) and butane (C4H10). Most of the isomers have similar physical properties, although they show an unusually wide variation in melting points. For instance, isohexane (sometimes called 2-methylpentane) has a melting point of -153.7 °C while n-hexane has a melting point of only -95.3 °C.
Like n-hexane, the other isomers of hexane tend to be colorless liquids at room temperature, non-polar, relatively chemically inert, and combustible.
Hexane As A Compound
N-hexane is a linear hydrocarbon that is made of a central chain of 6 single-bonded carbon atoms. Each carbon atom is bonded to enough hydrogens that all of its valence slots are taken up. Each carbon atom is “saturated” with hydrogens because they have no unbonded electron pairs.
The linear geometry of hexane and its fully saturated carbon chain makes it a stable compound that is relatively inert. Hexane generally will not react with other compounds and will not combust unless exposed to a sufficient amount of heat. Once exposed to that heat though, it will combust violently, generating large amounts of heat and energy. The precise amount of energy required to combust hexane is one of the reasons why it is used in gasoline. Gasoline manufacturers do not want their gas to combust too readily, as that reduces efficiency, but they also do not want it to be too difficult to combust. Hexane is rather inert until a specific temperature, after which it will combust energetically. The combustion reaction for hexane in oxygen is:
2C6H14 + 19O2 → 12CO2 + 14H2O
In cases where there is a limited supply of oxygen, the combustion of hexane looks like:
C6H14 + 😯2 → 3CO + 3CO2 +7H2O
In general, hexane is relatively non-toxic and not a significant risk for humans. Acute inhalation of large quantities can cause blurred vision, headaches, and muscle weakness, but one would have to ingest a very large amount for it to be fatal. Though it is mostly non-toxic, Inhalation of hexane can agitate lung tissue and cause breathing problems or an allergic reaction.
Uses Of Hexane
Hexane is one of the primary ingredients in commercial gas and petrol. Most commercial kinds of gasoline consist of a mixture of various 4 to 12 carbon alkanes, including hexane and its isomers, along with other additives. Most of the hexane that composes gasoline is produced via the refinement of crude oil. The chemical constituents of crude oil are separated via a technique called fractional distillation that removes impurities and separates constituents by their chemical structure.
Hexane is often used to extract lipids from other food items. For instance, cooking oil manufacturers use hexane to extract vegetable oils from soy and seeds. In fact, most soy products produced in the U.S. are treated using hexane. This has led to some controversy due to a lack of regulation of the use of hexane by the FDA. The lipid extraction capacities of hexane have also seen its use as a cleaning product and industrial degreaser.
Hexane is also used in the manufacture of glues, roofing tiles, leather products, and as a laboratory solvent. All of these uses are due to the fact that hexane is relatively non-reactive to a wide range of environmental factors. Roofing tiles are treated with hexane to prevent their corrosion and leather is treated to protect it from UV damage and chemical degradation. Shoe glue needs to be stable and maintain is adhesivity and hexane prevents the breakdown of the adhesive substances in shoe glue.
In the lab, hexane is favored as a non-polar solvent because it can dissolve a wide range on non-polar organic compounds and is non-reactive. Hexane solvent is often used to purify mixtures of compounds and isolate the constituents of a mixture.