Chemical Properties of Alkanes

Edited By Shivani Poonia | Updated on Jul 02, 2025 07:13 PM IST

Think about vast fields filled with natural Gas , long trunk lines to bring the fuel to the site of generation, or rail cars shuttling the fuel between homes and industry. These and many other forms of energy resources are based on alkanes, the simplest class of hydrocarbons. They hold an essential place in our daily life. These are the components of natural gas that we use as heating at home, as well as the gasoline in our cars. They are not only a fuel, but in combination with other ones, the basic material for vast amounts of synthetic products and chemicals that are important to our lives.

This Story also Contains

Main Idea of Alkanes
Halogenation
Nitration
Sulphonation
Types and Kinds of Alkanes
Combustion
Catalytic oxidation
Aromatization
Pyrolysis
Uses
Academic Industrial Interest
Some Solved Examples
Summary

Chemical Properties of Alkanes

Main Idea of Alkanes

The Alkanes are the class of Saturated Hydrocarbons containing a simple structural formula of carbon and hydrogens having connected with each other only single covalent bounds. The general formula of alkanes is CnH2n+2 where n is the number of C atoms Connected with each other. These are some of the properties in this class of Organic compounds that they are comparatively stable and shows less reactivity with other organic compounds. The single bonds in alkanes are of the sigma type. Since these are strong in nature, and since they are nonpolar, they are expected to be inert. Their boiling and melting points increase with molecular weight as a result of the increased van der Waal's force in larger molecules. Alkanes are non-polar and therefore do not dissolve in water, but they dissolve in all organic solvents. They have low reactivity because the dissociation energy between the C−C,C−H bond is very high. The high stability makes them unreactive; their unreactivity is, in fact, a major feature of their chemical behavior.

Halogenation

When alkanes are treated with halogens in the presence of light or at elevated temperatures, the hydrogen atoms of alkanes are successively replaced by halogen atoms. This process is known as halogenation. The rate of reaction of alkanes with halogen follows the following order:
F₂>C_l2>Br₂>I₂
This reaction is carried out with chlorine as fluorine is too violent to be controlled and iodine is too slow and reversible.
The mechanism of this reaction occurs by a free radical mechanism and it is done in three successive steps as follows:

Chain initiation step: The reaction is initiated by homolysis of chlorine molecules in the presence of light or heat. The Cl−Clbond is weaker than theC−C and C−Hbond and hence, is easiest to break.
Chain propagation step: Chlorine-free radical attacks the methane molecule and takes the reaction in the forward direction by breaking the C-H bond to generate methyl free radical with the formation of H-Cl.
Chain termination step: The reaction stops after some time due to the consumption of reactants and/or due to the following side reactions.
The possible chain-terminating steps are:

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The above mechanism helps us to understand the reason for the formation of ethane as a byproduct during the chlorination of methane.

Nitration

Nitration is a substitution reaction in which a hydrogen atom of an alkane is replaced by nitro(−NO₂)group. The reaction occurs as follows:

$\mathrm{R}-\mathrm{H}+\mathrm{HONO}_2 \xrightarrow[\text { Temperature }]{\text { Hign }} \mathrm{R}-\mathrm{NO}_2+\mathrm{H}_2 \mathrm{O}$
Lower members do not react with concentrated nitric acid at ordinary temperatures but long-chain members on heating with fuming nitric acid yield nitroalkanes. However, when a mixture of vapours of an alkane and nitric acid is heated at 673-773K, nitroalkane is formed readily. This is known as vapour phase nitration. By this process, lower, as well as higher alkanes, can be converted into nitroalkanes.

$\mathrm{CH}_3+\mathrm{HONO}_2 \xrightarrow{723 \mathrm{~K}} \mathrm{CH}_3-\mathrm{NO}_2+\mathrm{H}_2 \mathrm{C}$ CH₃+HONO₂→ KCH₃−NO₂+H₂C

Sulphonation

The replacement of hydrogen atom by sulphonic cid group(-SO₃H) is known as sulphonation. Lower alkanes do not undergo sulphonation but higher members (from hexane onwards) are sulphonated slowly when treated with fuming sulphuric acid. at about 673K. The reaction occurs as follows:

$\mathrm{R}-\mathrm{H}+\mathrm{HOSO}_3 \mathrm{H} \xrightarrow[\text { Prolonged heating }]{\mathrm{SO}_3} \mathrm{R}-\mathrm{SO}_3 \mathrm{H}+\mathrm{H}_2 \mathrm{O}$
For example:

$\mathrm{C}_6 \mathrm{H}_{13}+\mathrm{HOSO}_3 \mathrm{H} \xrightarrow{\mathrm{SO}_3} \mathrm{C}_6 \mathrm{H}_{12} \mathrm{SO}_3 \mathrm{H}+\mathrm{H}_2 \mathrm{O}$
However, lower members such as propane, butane, pentane, etc. react with SO₃ in vapour phase to form sulphonic acids.

Types and Kinds of Alkanes

Three types of alkanes have been identified based on the types of carbon chains that they have, such as straight-chain (normal alkanes), branched-chain (iso alkanes), and cycloalkanes. Normal alkanes hold those carbon atoms that are linked in with a consecutive chain and branched alkanes hold one or many carbon atoms that branch from the main chain. Alkanes whose atoms create a ring of their own self, like that of cyclohexane, are termed cycloalkanes.
Alkanes show mainly substitution and combustion reactions. In substitution reactions, for instance, a hydrogen atom in the alkane may be replaced by another atom or group, especially a halogen, in the presence of light or heat. Combustion reactions involve an alkane reacting with an oxygen molecule to produce carbon dioxide and water, besides the liberation of energy: alkanes are hence important fuels. Another principal reaction of alkanes is cracking, the process of breaking larger alkanes into smaller ones, significantly increasing the yield of readily useful hydrocarbons such as gasoline.

Combustion

Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat.

$\mathrm{CH}_4(\mathrm{~g})+2 \mathrm{O}_2(\mathrm{~g}) \rightarrow \mathrm{CO}_2(\mathrm{~g})+2 \mathrm{H}_2 \mathrm{O}(1), \Delta_{\mathrm{c}} \mathrm{H}^{\circ}=-890 \mathrm{~kJ} / \mathrm{mol}$
Due to the production of large amount of heat, alkanes are used as fuels.

Catalytic oxidation

Alkanes on heating with a regulated supply of dioxygen or air at high pressure and in the presence of suitable catalysts give a variety of oxidation products. The reactions occur as follows:

$2 \mathrm{CH}_4+\mathrm{O}_2 \xrightarrow[120 \mathrm{~atm} .]{475 \mathrm{KCu} .} 2 \mathrm{CH}_3 \mathrm{OH}$

Isomerization

n-Alkanes on heating in the presence of anhydrous aluminum chloride and hydrogen chloride gas isomerize to branched-chain alkanes. Major products are given below. Some minor products are also possible which you can think over. Minor products are generally not reported in organic reactions.

Aromatization

n-Alkanes having six or more carbon atoms on heating to 773K at 10-20 atmospheric pressure in the presence of oxides of vanadium, molybdenum or chromium supported over alumina get dehydrogenated and cyclized to benzene and its homologs. This reaction is known as aromatization or reforming.

Pyrolysis

The higher alkanes split into lower alkanes when heated strongly at a high temperature in the absence of air. During pyrolysis, C-C bond breaks rather than C-H bonds as bond energy of $\mathrm{C}-\mathrm{H}>\mathrm{C}-\mathrm{C}$. Here product formation depends upon the structure of alkane, the extent of temperature and pressure and the presence/absence of catalysts like$\mathrm{SiO}_2-\mathrm{Al}_2 \mathrm{O}_3$ etc. tc. Pyrolysis of alkanes is believed to be a free radical reaction. Preparation of oil gas or petrol gas from kerosene oil or petrol involves the principle of pyrolysis. For example:

$\underset{\text { Dodecane }}{\mathrm{C}_{12} \mathrm{H}_{26} \xrightarrow[\text { atak }]{\text { Ptedane }} \mathrm{C}_7 \mathrm{H}_{16}+\mathrm{C}_5 \mathrm{H}_{10}+\text { other products }}$

Uses

Alkanes are very useful in our daily lives and in a myriad of industries. Natural gas consisting mostly of methane is used to heat buildings and cook food, as well as to generate electricity. Propane and butane are marketed as liquefied petroleum gas (LPG) for domestic use and are also valuable for many industrial applications. Gasoline, primarily a mixture of alkanes, is the major fuel used for transportation. The alkanes are also raw materials for a very wide range of important chemicals, including plastics, synthetic fibers, and detergents.

Academic Industrial Interest

Academically, much support is extended through the study of alkanes in organic chemistry, and the study takes an in-depth position toward the knowledge of how behavior and reactivity in hydrocarbons might be, which will help in initiating new structures of chemical processing and material. On the other hand, with regard to industry, the refining of crude oil is a very important process for the generation of different fractions of alkanes starting from the raw material. The improvement of its efficiency due to the development in the field of catalytic reactions has increased cracking and reforming.

Some Solved Examples

Example 1

Question:
The major product obtained in the photo-catalyzed bromination of 2-methylbutane is:

1) 1-bromo-2-methylbutane
2) 1-bromo-3-methylbutane
3) 2-bromo-3-methylbutane
4) 2-bromo-2-methylbutane (correct)

Solution:
In the photo catalyzed bromination of 2-methylbutane, the bromine atom preferentially substitutes at the tertiary carbon due to stability considerations. This gives us 2-bromo-2-methylbutane as the major product. Hence, the correct answer is option (4).

Example 2

Question:
How many chiral compounds are possible on monochlorination of 2-methylbutane? (Report the number of enantiomeric pairs as the answer)

1) 8
2) 2 (correct)
3) 4
4) 6

Solution:
Monochlorination of 2-methylbutane leads to the formation of two chiral compounds, each with its enantiomer. Therefore, the total number of chiral compounds (enantiomeric pairs) is 2. Hence, the correct answer is option (2)

Example 3

Question:
How many isomers are obtained on monochlorination of isopentane? (excluding stereoisomers)

1) 2
2) 3
3) 4 (correct)
4) 5

Solution:
Monochlorination of isopentane results in four different positional isomers, excluding stereoisomers. These positional isomers arise due to the chlorine substituting at different carbon atoms within the molecule. Hence, the correct answer is option (3).

Summary

Hence, these are the simplest types of hydrocarbons, significant in an individual's basic energy needs and daily survival. Furthermore, they have the distinction of being stable; their reactivity is generally low while their diversity is extremely high since they can exist in three different forms: straight chain, branched chain, and cycloalkanes. The main reactions that they undergo are essentially only substitution and combustion, although the applications used for the alkanes span from household fuel to industrial raw materials. The understanding of alkanes augments the study of organic chemistry and helps usher in a scientific revolution in energy and material sciences.

Frequently Asked Questions (FAQs)

1. What is meant by the term "cracking" in relation to alkanes?

Cracking is a process where larger alkane molecules are broken down into smaller, more useful hydrocarbons. This process involves breaking C-C bonds using heat, pressure, or catalysts, and is important in the petroleum industry for producing more valuable products like gasoline from heavier hydrocarbons.

2. How does increasing the carbon chain length affect the reactivity of alkanes?

As the carbon chain length increases in alkanes, their reactivity generally increases slightly. This is because longer chains have more C-H bonds that can potentially react, and the electron density is more spread out, making the molecule slightly more polarizable and reactive.

3. How does pressure affect the reactivity of alkanes in industrial processes?

Increased pressure can enhance the reactivity of alkanes in certain industrial processes. Higher pressures can force reactant molecules closer together, increasing the likelihood of successful collisions. This is particularly important in reactions like hydrogenation or in some cracking processes where increased pressure can improve reaction rates and yields.

4. What role do alkanes play in the production of syngas?

Alkanes, particularly methane, are key in the production of syngas (synthesis gas). Through a process called steam reforming, methane reacts with steam at high temperatures to produce a mixture of carbon monoxide and hydrogen (syngas). This is an important industrial process, as syngas is a precursor for many chemical products and fuels.

5. What is the significance of bond dissociation energies in understanding alkane reactivity?

Bond dissociation energies provide insight into the strength of chemical bonds and, consequently, the reactivity of alkanes. Higher bond dissociation energies indicate stronger bonds that are more difficult to break. Understanding these energies helps predict the likelihood and ease of various reactions, such as combustion or radical substitutions.

6. How do alkanes participate in reforming processes in the petroleum industry?

In the petroleum industry, alkanes undergo reforming processes to produce more valuable products. Catalytic reforming converts straight-chain alkanes into branched alkanes and aromatic compounds, which have higher octane numbers. This process typically involves dehydrogenation, isomerization, and cyclization reactions, often using platinum-based catalysts.

7. What role do catalysts play in alkane reactions?

Catalysts play a crucial role in many alkane reactions by lowering the activation energy required for the reaction to occur. They can enable reactions to proceed at lower temperatures or pressures, or increase the rate of reaction without being consumed in the process.

8. How does the presence of a catalyst affect the combustion of alkanes?

Catalysts can significantly lower the activation energy required for alkane combustion, allowing the reaction to occur at lower temperatures. This can lead to more complete combustion, reducing harmful emissions and increasing fuel efficiency. Catalytic converters in vehicles use this principle to reduce pollutants from exhaust gases.

9. How do alkanes participate in oxidation reactions besides combustion?

Besides combustion, alkanes can undergo controlled oxidation reactions. For example, they can be partially oxidized to form alcohols, aldehydes, or ketones under specific conditions. These reactions often require catalysts and controlled amounts of oxygen or other oxidizing agents, and are important in the production of various industrial chemicals.

10. How do alkanes interact with transition metal catalysts?

Alkanes can interact with transition metal catalysts through a process called C-H activation. The metal can insert itself into a C-H bond of the alkane, forming a metal-carbon bond. This interaction is crucial in many catalytic processes and can lead to functionalization of otherwise unreactive alkanes.

11. How do alkanes react with oxygen in the absence of a flame?

In the absence of a flame, alkanes react very slowly with oxygen at room temperature in a process called autoxidation. This reaction produces small amounts of alcohols, aldehydes, and carboxylic acids. It's a significant concern in the storage of hydrocarbons, as it can lead to the degradation of fuels over time.

12. What type of reaction occurs when alkanes burn in excess oxygen?

When alkanes burn in excess oxygen, they undergo a combustion reaction. This is an exothermic reaction that produces carbon dioxide and water as the main products, releasing energy in the form of heat and light.

13. What is the difference between complete and incomplete combustion of alkanes?

Complete combustion of alkanes occurs in the presence of excess oxygen, producing only carbon dioxide and water. Incomplete combustion happens when there's insufficient oxygen, leading to the formation of carbon monoxide, carbon (soot), and other partially oxidized products. Complete combustion is more efficient and less polluting.

14. What is the significance of the carbon-hydrogen ratio in alkane combustion?

The carbon-hydrogen ratio in alkanes is significant for combustion because it affects the energy released and the products formed. Alkanes with a higher proportion of hydrogen (like methane) tend to produce more water and less CO2 per unit of energy released when burned, making them "cleaner" fuels in terms of greenhouse gas emissions.

15. What are the main chemical properties of alkanes?

The main chemical properties of alkanes include their low reactivity, combustibility, and ability to undergo substitution reactions. Alkanes are generally unreactive due to their strong C-C and C-H bonds, but they can undergo combustion with oxygen and substitution reactions with halogens under specific conditions.

16. What is the role of alkanes in the formation of atmospheric pollutants?

Alkanes contribute to atmospheric pollution primarily through their incomplete combustion, which can produce carbon monoxide and particulate matter. Additionally, when released into the atmosphere, larger alkanes can participate in photochemical reactions, contributing to the formation of ground-level ozone and smog.

17. How do alkanes react with halogens, and what conditions are required?

Alkanes can undergo substitution reactions with halogens (like chlorine or bromine) in a process called halogenation. This reaction typically requires UV light or heat to initiate the formation of halogen radicals, which then substitute hydrogen atoms in the alkane with halogen atoms.

18. What is the mechanism of chlorination in alkanes?

The chlorination of alkanes follows a free radical mechanism:

19. How do alkanes participate in free radical reactions?

Alkanes can participate in free radical reactions, typically initiated by heat or light. In these reactions, a radical (an atom or molecule with an unpaired electron) attacks the alkane, leading to the formation of new radicals. This process can continue as a chain reaction, resulting in various products.

20. What is the concept of "selectivity" in alkane reactions, and why is it important?

Selectivity in alkane reactions refers to the preferential formation of one product over others when multiple reaction pathways are possible. It's important because it determines the efficiency and usefulness of a reaction. For instance, in the chlorination of methane, achieving high selectivity for monochloromethane over di-, tri-, or tetrachloromethane is often desirable for specific applications.

21. What is the concept of "alkane activation" in organometallic chemistry?

In organometallic chemistry, "alkane activation" refers to processes where transition metal complexes interact with alkanes to form metal-carbon bonds. This overcomes the usual inertness of alkanes and can lead to functionalization of C-H bonds. These reactions are of great interest in developing new methods for converting alkanes into more valuable chemicals.

22. How do alkanes undergo isomerization reactions?

Alkanes can undergo isomerization reactions where the carbon skeleton is rearranged without changing the molecular formula. This typically requires high temperatures or catalysts and involves the breaking and reforming of C-C bonds. For example, n-butane can isomerize to isobutane under appropriate conditions.

23. How do alkanes interact with zeolite catalysts, and why is this important?

Zeolite catalysts, with their porous structure and acidic sites, can interact with alkanes in several ways. They can facilitate isomerization, cracking, and even alkylation reactions. The shape selectivity of zeolites allows for control over which molecules can enter the pores and react, making them crucial in petroleum refining and the production of high-octane gasoline components.

24. What is the concept of "alkane metathesis" and how does it differ from typical alkane reactions?

Alkane metathesis is a reaction where two different alkanes exchange carbon atoms to form new alkanes. Unlike typical alkane reactions that focus on functionalization or breaking down molecules, metathesis rearranges the carbon skeletons. This reaction usually requires specific metal catalysts and can be useful for upgrading lower-value hydrocarbons.

25. How do alkanes behave under extreme pressure conditions, such as in the Earth's mantle?

Under extreme pressure conditions like those in the Earth's mantle, alkanes can exhibit unusual behavior. They may undergo polymerization, form complex structures, or even decompose into their elemental components (carbon and hydrogen). This behavior is of interest in geochemistry and in understanding the deep carbon cycle.

26. Why are alkanes considered relatively unreactive?

Alkanes are considered relatively unreactive because they contain strong, nonpolar covalent bonds between carbon atoms (C-C) and between carbon and hydrogen atoms (C-H). These bonds are difficult to break, making alkanes stable and less likely to participate in many chemical reactions.

27. How does the reactivity of alkanes compare to other hydrocarbon groups?

Alkanes are generally less reactive compared to other hydrocarbon groups like alkenes and alkynes. This is because alkanes contain only single bonds, while alkenes and alkynes have double and triple bonds, respectively, which are more reactive and prone to addition reactions.

28. Why is methane considered the least reactive alkane?

Methane (CH4) is considered the least reactive alkane because it has the highest C-H bond strength among all alkanes. Its tetrahedral structure provides maximum stability, making it more difficult to break its bonds compared to larger alkanes.

29. What is the significance of the C-H bond strength in alkane reactions?

The C-H bond strength is crucial in determining the reactivity of alkanes. Stronger C-H bonds are more difficult to break, making the alkane less reactive. This bond strength decreases slightly as the carbon chain length increases, which partly explains why larger alkanes are somewhat more reactive than smaller ones.

30. How do alkanes interact with biological systems, particularly in terms of toxicity?

Alkanes generally have low toxicity in biological systems due to their chemical inertness. However, they can cause physical effects such as disrupting cell membranes or displacing oxygen in lungs if inhaled. Longer-chain alkanes tend to be more biologically active than shorter ones, potentially interfering with cellular processes.

31. How do alkanes behave under pyrolysis conditions?

Under pyrolysis conditions (high temperatures in the absence of oxygen), alkanes undergo thermal decomposition. This process can lead to the formation of smaller alkanes, alkenes, and even some alkynes. Pyrolysis is an important industrial process for producing valuable chemicals from larger, less useful hydrocarbons.

32. What is the significance of the carbon-carbon bond strength in alkane reactions?

The carbon-carbon (C-C) bond strength is crucial in alkane reactions as it determines the stability of the molecule. Strong C-C bonds make alkanes resistant to many types of reactions. However, in processes like cracking, overcoming this bond strength is key to breaking down larger alkanes into smaller, more useful hydrocarbons.

33. What is the difference between thermal cracking and catalytic cracking of alkanes?

Thermal cracking uses high temperatures (around 800°C) to break down large alkane molecules, while catalytic cracking uses lower temperatures (around 500°C) in the presence of a catalyst. Catalytic cracking is more selective and can produce a higher yield of desirable products like gasoline.

34. What is meant by the "activation" of alkanes in chemical reactions?

The "activation" of alkanes refers to overcoming their inherent chemical inertness to make them more reactive. This often involves breaking C-H or C-C bonds, which requires significant energy input. Activation can be achieved through various means such as high temperatures, catalysts, or the use of highly reactive species like radicals or transition metal complexes.

35. What is the significance of the autoignition temperature for alkanes?

The autoignition temperature is the lowest temperature at which an alkane will spontaneously ignite in air without an external source of ignition. This property is crucial for understanding the fire and explosion hazards of alkanes, as well as for designing engines and combustion processes. Generally, smaller alkanes have higher autoignition temperatures than larger ones.

36. How does branching affect the chemical properties of alkanes?

Branching in alkanes can affect their chemical properties by changing the accessibility of C-H bonds and altering the overall molecule shape. Highly branched alkanes are often more reactive in substitution reactions because they have more tertiary carbons, which form more stable carbocations during reaction mechanisms.

37. What is the role of alkanes in the formation of petroleum deposits?

Alkanes are major components of petroleum deposits, formed from the remains of ancient organisms over millions of years. Under high pressure and temperature conditions deep underground, larger organic molecules break down into a mixture of hydrocarbons, including various alkanes. The distribution of different alkanes in petroleum can provide information about its origin and maturity.

38. What is the role of alkanes in the formation of natural gas hydrates?

Alkanes, particularly methane, play a crucial role in forming natural gas hydrates. These are crystalline solids where water molecules form a cage-like structure around alkane molecules. This occurs under high pressure and low temperature conditions, typically in deep ocean sediments or permafrost regions, and is of interest for both energy resources and climate science.

39. What is meant by the "inertness" of alkanes, and why is it important?

The term "inertness" refers to the low chemical reactivity of alkanes under normal conditions. This property is important because it makes alkanes stable and suitable for use as fuels and lubricants, where chemical stability is crucial for performance and safety.

40. Why are alkanes used as fuels despite their relative inertness?

Alkanes are used as fuels because of their high energy content and controlled reactivity. While they are relatively inert under normal conditions (ensuring safe storage and transport), they readily undergo combustion when ignited, releasing a large amount of energy. This combination of stability and energy content makes them ideal fuel sources.

41. How do alkanes interact with acids, and why is this interaction generally limited?

Alkanes generally have very limited interactions with acids due to their non-polar nature and lack of reactive sites. However, under extreme conditions (like with superacids), alkanes can undergo protonation to form carbonium ions, which can lead to various rearrangement and cracking reactions. This limited acid reactivity is part of what makes alkanes useful as lubricants and fuels.

42. How do alkanes behave in supercritical conditions?

In supercritical conditions (beyond the critical point where distinct liquid and gas phases do not exist), alkanes can exhibit unique properties. They often become more reactive due to increased molecular mobility and density. This behavior is exploited in some extraction processes and in certain types of chemical reactions where traditional solvents are ineffective.

43. How do alkanes behave in radical polymerization reactions?

Alkanes themselves do not typically undergo radical polymerization due to their stable single bonds. However, they can participate in these reactions as chain transfer agents. In this role, they can terminate growing polymer chains, affecting the molecular weight and properties of the resulting polymer.

44. How do alkanes behave in photochemical reactions?

Alkanes themselves are not typically reactive in photochemical processes due to their lack of chromophores that can absorb visible or near-UV light. However, in the presence of other reactive species in the atmosphere, they can participate in complex photochemical reaction cycles, contributing to the formation of secondary pollutants like ozone in urban environments.

45. What is the significance of the heat of combustion in alkanes?

The heat of combustion is the energy released when an alkane undergoes complete combustion. This value is crucial in determining the energy content of fuels. Generally, the heat of combustion per mole increases with the number of carbon atoms in the alkane, but the heat of combustion per gram tends to decrease slightly. This property is fundamental in assessing the energy efficiency and environmental impact of alkane fuels.

46. How do alkanes behave in supercritical carbon dioxide, and what are the applications?

In supercrit