organic compunds

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What is an Organic Compound?
When you drive up to the pump at some gas stations you are faced with a variety of choices.
You can buy "leaded" gas or different forms of "unleaded" gas that have different octane numbers. As you filled the tank, you might wonder, "What is 'leaded' gas, and why do they add lead to gas?" Or, "What would I get for my money if I bought premium gas, with a higher octane number?"
You then stop to buy drugs for a sore back that has been bothering you since you helped a friend move into a new apartment. Once again, you are faced with choices (see the figure below). You could buy aspirin, which has been used for almost a hundred years. Or Tylenol, which contains acetaminophen. Or a more modern pain-killer, such as ibuprofen. While you are deciding which drug to buy, you might wonder, "What is the difference between these drugs?," and even, "How do they work?"

You then drive to campus, where you sit in a "plastic" chair to eat a sandwich that has been wrapped in "plastic," without worrying about why one of these plastics is flexibile while the other is rigid. While you're eating, a friend stops by and starts to tease you about the effect of your diet on the level of cholesterol in your blood, which brings up the questions, "What is cholesterol?" and "Why do so many people worry about it?"
Answers to each of these questions fall within the realm of a field known as organic chemistry. For more than 200 years, chemists have divided materials into two categories. Those isolated from plants and animals were classified as organic, while those that trace back to minerals were inorganic. At one time, chemists believed that organic compounds were fundamentally different from those that were inorganic because organic compounds contained a vital force that was only found in living systems.
The first step in the decline of the vital force theory occurred in 1828, when Friederich Wohler synthesized urea from inorganic starting materials. Wohler was trying to make ammonium cyanate (NH4OCN) from silver cyanate (AgOCN) and ammonium chloride (NH4Cl). What he expected is described by the following equation.
AgOCN(aq) + NH4Cl(aq) AgCl(s) + NH4OCN(aq)
The product he isolated from this reaction had none of the properties of cyanate compounds. It was a white, crystalline material that was identical to urea, H2NCONH2, which could be isolated from urine.

Neither Wohler nor his contemporaries claimed that his results disproved the vital force theory. But his results set in motion a series of experiments that led to the synthesis of a variety of organic compounds from inorganic starting materials. This inevitably led to the disappearance of "vital force" from the list of theories that had any relevance to chemistry, although it did not lead to the death of the theory, which still had proponents more than 90 years later.
If the difference between organic and inorganic compounds isn't the presence of some mysterious vital force required for their synthesis, what is the basis for distinguishing between these classes of compounds? Most compounds extracted from living organisms contain carbon. It is therefore tempting to identify organic chemistry as the chemistry of carbon. But this definition would include compounds such as calcium carbonate (CaCO3), as well as the elemental forms of carbon diamond and graphite that are clearly inorganic. We will therefore define organic chemistry as the chemistry of compounds that contain both carbon and hydrogen.
Even though organic chemistry focuses on compounds that contain carbon and hydrogen, more than 95% of the compounds that have isolated from natural sources or synthesized in the laboratory are organic. The special role of carbon in the chemistry of the elements is the result of a combination of factors, including the number of valence electrons on a neutral carbon atom, the electronegativity of carbon, and the atomic radius of carbon atoms (see the table below).
The Physical Properties of Carbon
Electronic configuration 1s2 2s2 2p2
Electronegativity 2.55
Covalent radius 0.077 nm
Carbon has four valence electrons 2s2 2p2 and it must either gain four electrons or lose four electrons to reach a rare-gas configuration. The electronegativity of carbon is too small for carbon to gain electrons from most elements to form C4- ions, and too large for carbon to lose electrons to form C4+ ions. Carbon therefore forms covalent bonds with a large number of other elements, including the hydrogen, nitrogen, oxygen, phosphorus, and sulfur found in living systems.
Because they are relatively small, carbon atoms can come close enough together to form strong C=C double bonds or even C C triple bonds. Carbon also forms strong double and triple bonds to nitrogen and oxygen. It can even form double bonds to elements such as phosphorus or sulfur that do not form double bonds to themselves.
Several years ago, the unmanned Viking spacecraft carried out experiments designed to search for evidence of life on Mars. These experiments were based on the assumption that living systems contain carbon, and the absence of any evidence for carbon-based life on that planet was presumed to mean that no life existed. Several factors make carbon essential to life.
• The ease with which carbon atoms form bonds to other carbon atoms.
• The strength of C C single bonds and the covalent bonds carbon forms to other nonmetals, such as N, O, P, and S.
• The ability of carbon to form multiple bonds to other nonmetals, including C, N, O, P, and S atoms.
These factors provide an almost infinite variety of potential structures for organic compounds, such as vitamin C shown in the figure below.

No other element can provide the variety of combinations and permutations necessary for life to exist.

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The Saturated Hydrocarbons or Alkanes
Compounds that contain only carbon and hydrogen are known as hydrocarbons. Those that contain as many hydrogen atoms as possible are said to be saturated. The saturated hydrocarbons are also known as alkanes.
The simplest alkane is methane: CH4. The Lewis structure of methane can be generated by combining the four electrons in the valence shell of a neutral carbon atom with four hydrogen atoms to form a compound in which the carbon atom shares a total of eight valence electrons with the four hydrogen atoms.

Methane is an example of a general rule that carbon is tetravalent; it forms a total of four bonds in almost all of its compounds. To minimize the repulsion between pairs of electrons in the four C H bonds, the geometry around the carbon atom is tetrahedral, as shown in the figure below.

Use the fact that carbon is usually tetravalent to predict the formula of ethane, the alkane that contains two carbon atoms.

The alkane that contains three carbon atoms is known as propane, which has the formula C3H8 and the following skeleton structure.

The four-carbon alkane is butane, with the formula C4H10.

The names, formulas, and physical properties for a variety of alkanes with the generic formula CnH2n+2 are given in the table below. The boiling points of the alkanes gradually increase with the molecular weight of these compounds. At room temperature, the lighter alkanes are gases; the midweight alkanes are liquids; and the heavier alkanes are solids, or tars.
The Saturated Hydrocarbons or Alkanes
Name Molecular
Formula Melting
Point (oC) Boiling
Point (oC) State
at 25oC
methane CH4 -182.5 -164 gas
ethane C2H6 -183.3 -88.6 gas
propane C3H8 -189.7 -42.1 gas
butane C4H10 -138.4 -0.5 gas
pentane C5H12 -129.7 36.1 liquid
hexane C6H14 -95 68.9 liquid
heptane C7H16 -90.6 98.4 liquid
octane C8H18 -56.8 124.7 liquid
nonane C9H20 -51 150.8 liquid
decane C10H22 -29.7 174.1 liquid
undecane C11H24 -24.6 195.9 liquid
dodecane C12H26 -9.6 216.3 liquid
excusing C20H42 36.8 343 solid
triacontane C30H62 65.8 449.7 solid
The alkanes in the table above are all straight-chain hydrocarbons, in which the carbon atoms form a chain that runs from one end of the molecule to the other. The generic formula for these compounds can be understood by assuming that they contain chains of CH2 groups with an additional hydrogen atom capping either end of the chain. Thus, for every n carbon atoms there must be 2n + 2 hydrogen atoms: CnH2n+2.
Because two points define a line, the carbon skeleton of the ethane molecule is linear, as shown in the figure below.

Because the bond angle in a tetrahedron is 109.5, alkanes molecules that contain three or four carbon atoms can no longer be thought of as "linear," as shown in the figure below.



Propane Butane
In addition to the straight-chain examples considered so far, alkanes also form branched structures. The smallest hydrocarbon in which a branch can occur has four carbon atoms. This compound has the same formula as butane (C4H10), but a different structure. Compounds with the same formula and different structures are known as isomers (from the Greek isos, "equal," and meros, "parts"). When it was first discovered, the branched isomer with the formula C4H10 was therefore given the name isobutane.
Isobutane
The best way to understand the difference between the structures of butane and isobutane is to compare the ball-and-stick models of these compounds shown in the figure below.



Butane Isobutane
Butane and isobutane are called constitutional isomers because they literally differ in their constitution. One contains two CH3 groups and two CH2 groups; the other contains three CH3 groups and one CH group.
There are three constitutional isomers of pentane, C5H12. The first is "normal" pentane, or n-pentane.

A branched isomer is also possible, which was originally named isopentane. When a more highly branched isomer was discovered, it was named neopentane (the new isomer of pentane).


Ball-and-stick models of the three isomers of pentane are shown in the figure below.



n-Pentane Isopentane


Neopentane

The following structures all have the same molecular formula: C6H14. Which of these structures represent the same molecule?


Determine the number of constitutional isomers of hexane, C6H14.

There are two constitutional isomers with the formula C4H10, three isomers of C5H12, and five isomers of C6H14. The number of isomers of a compound increases rapidly with additional carbon atoms. There are over 4 billion isomers for C30H62, for example.

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The Cycloalkanes
If the carbon chain that forms the backbone of a straight-chain hydrocarbon is long enough, we can envision the two ends coming together to form a cycloalkane. One hydrogen atom has to be removed from each end of the hydrocarbon chain to form the C C bond that closes the ring. Cycloalkanes therefore have two less hydrogen atoms than the parent alkane and a generic formula of CnH2n.
The smallest alkane that can form a ring is cyclopropane, C3H6, in which the three carbon atoms lie in the same plane. The angle between adjacent C C bonds is only 60, which is very much smaller than the 109.5 angle in a tetrahedron, as shown in the figure below.

Cyclopropane is therefore susceptible to chemical reactions that can open up the three-membered ring.
Any attempt to force the four carbons that form a cyclobutane ring into a plane of atoms would produce the structure shown in the figure below, in which the angle between adjacent C C bonds would be 90.

One of the four carbon atoms in the cyclobutane ring is therefore displaced from the plane of the other three to form a "puckered" structure that is vaguely reminiscent of the wings of a butterfly.
The angle between adjacent C C bonds in a planar cyclopentane molecule would be 108, which is close to the ideal angle around a tetrahedral carbon atom. Cyclopentane is not a planar molecule, as shown in the figure below, because displacing two of the carbon atoms from the plane of the other three produces a puckered structure that relieves some of the repulsion between the hydrogen atoms on adjacent carbon atoms in the ring.

By the time we get to the six-membered ring in cyclohexane, a puckered structure can be formed by displacing a pair of carbon atoms at either end of the ring from the plane of the other four members of the ring. One of these carbon atoms is tilted up, out of the ring, whereas the other is tilted down to form the "chair" structure shown in the figure below.


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Rotation Around C C Bonds
As one looks at the structure of the ethane molecule, it is easy to fall into the trap of thinking about this molecule as if it was static. Nothing could be further from the truth. At room temperature, the average velocity of an ethane molecule is about 500 m/s more than twice the speed of a Boeing 747. While it moves through space, the molecule is tumbling around its center of gravity like an airplane out of control. At the same time, the C H and C C bonds are vibrating like a spring at rates as fast as 9 x 1013 s-1.
There is another way in which the ethane molecule can move. The CH3 groups at either end of the molecule can rotate with respect to each around the C C bond. When this happens, the molecule passes through an infinite number of conformations that have slightly different energies. The highest energy conformation corresponds to a structure in which the hydrogen atoms are "eclipsed." If we view the molecule along the C C bond, the hydrogen atoms on one CH3 group would obscure those on the other, as shown in the figure below.

The lowest energy conformation is a structure in which the hydrogen atoms are "staggered," as shown in the figure below.

The difference between the eclipsed and staggered conformations of ethane is best illustrated by viewing these molecules along the C C bond, as shown in the figure below.



Eclipsed Staggered
The difference between the energies of these conformations is relatively small, only about 12 kJ/mol. But it is large enough that rotation around the C C bond is not smooth. Although the frequency of this rotation is on the order of 1010 revolutions per second, the ethane molecule spends a slightly larger percentage of the time in the staggered conformation.
The different conformations of a molecule are often described in terms of Newman projections. These line drawings show the six substituents on the C C bond as if the structure of the molecule was projected onto a piece of paper by shining a bright light along the C C bond in a ball-and-stick model of the molecule. Newman projections for the different staggered conformations of butane are shown in the figure below.

Because of the ease of rotation around C C bonds, there are several conformations of some of the cycloalkanes described in the previous section. Cyclohexane, for example, forms both the "chair" and "boat" conformations shown in the figure below.



Chair Boat
The difference between the energies of the chair conformation, in which the hydrogen atoms are staggered, and the boat conformation, in which they are eclipsed, is about 30 kJ/mol. As a result, even though the rate at which these two conformations interchange is about 1 x 105 s-1, we can assume that most cyclohexane molecules at any moment in time are in the chair conformation.

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The Nomenclature of Alkanes
Common names such as pentane, isopentane, and neopentane are sufficient to differentiate between the three isomers with the formula C5H12. They become less useful, however, as the size of the hydrocarbon chain increases.
The International Union of Pure and Applied Chemistry (IUPAC) has developed a systematic approach to naming alkanes and cycloalkanes based on the following steps.
• Find the longest continuous chain of carbon atoms in the skeleton structure. Name the compound as a derivative of the alkane with this number of carbon atoms. The following compound, for example, is a derivative of pentane because the longest chain contains five carbon atoms.

• Name the substituents on the chain. Substituents derived from alkanes are named by replacing the -ane ending with -yl. This compound contains a methyl (CH3-) substituent.

• Number the chain starting at the end nearest the first substituent and specify the carbon atoms on which the substituents are located. Use the lowest possible numbers. This compound, for example, is 2-methylpentane, not 4-methylpentane.

• Use the prefixes di-, tri-, and tetra- to describe substituents that are found two, three, or four times on the same chain of carbon atoms.
• Arrange the names of the substituents in alphabetical order.
Name the following compound.





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The Unsaturated Hydrocarbons: Alkenes and Alkynes
Carbon not only forms the strong C C single bonds found in alkanes, it also forms strong C=C double bonds. Compounds that contain C=C double bonds were once known as olefins (literally, "to make an oil") because they were hard to crystallize. (They tend to remain oily liquids when cooled.) These compounds are now called alkenes. The simplest alkenes have the formula C2H4 and the following Lewis structure.

The relationship between alkanes and alkenes can be understood by thinking about the following hypothetical reaction. We start by breaking the bond in an H2 molecule so that one of the electrons ends up on each of hydrogen atoms. We do the same thing to one of the bonds between the carbon atoms in an alkene. We then allow the unpaired electron on each hydrogen atom to interact with the unpaired electron on a carbon atom to form a new C H bond.

Thus, in theory, we can transform an alkene into the parent alkane by adding an H2 molecule across a C=C double bond. In practice, this reaction only occurs at high pressures in the presence of a suitable catalyst, such as piece of nickel metal.

Because an alkene can be thought of as a derivative of an alkane from which an H2 molecule has been removed, the generic formula for an alkene with one C=C double bond is CnH2n.
Alkenes are examples of unsaturated hydrocarbons because they have fewer hydrogen atoms than the corresponding alkanes. They were once named by adding the suffix -ene to the name of the substituent that carried the same number of carbon atoms.

The IUPAC nomenclature for alkenes names these compounds as derivatives of the parent alkanes. The presence of the C=C double bond is indicated by changing the -ane ending on the name of the parent alkane to -ene.

The location of the C=C double bond in the skeleton structure of the compound is indicated by specifying the number of the carbon atom at which the C=C bond starts.

The names of substituents are then added as prefixes to the name of the alkene.


Compounds that contain C C triple bonds are called alkynes. These compounds have four less hydrogen atoms than the parent alkanes, so the generic formula for an alkyne with a single C C triple bond is CnH2n-2. The simplest alkyne has the formula C2H2 and is known by the common name acetylene.

The IUPAC nomenclature for alkynes names these compounds as derivatives of the parent alkane, with the ending -yne replacing -ane.

In addition to compounds that contain one double bond (alkenes) or one triple bond (alkynes), we can also envision compounds with two double bonds (dienes), three double bonds (trienes), or a combination of double and triple bonds.










Chemical structure of methane, the simplest alkane
Alkanes, also known as paraffins are chemical compounds that consist only of the elements carbon (C) and hydrogen (H) (i.e., hydrocarbons), wherein these atoms are linked together exclusively by single bonds (i.e., they are saturated compounds) without any cyclic structure (i.e. loops). Alkanes belong to a homologous series of organic compounds in which the members differ by a constant relative atomic mass of 14.
Each carbon atom must have 4 bonds (either C-H or C-C bonds), and each hydrogen atom must be joined to a carbon atom (H-C bonds). A series of linked carbon atoms is known as the carbon skeleton or carbon backbone. In general, the number of carbon atoms is often used to define the size of the alkane (e.g., C2-alkane).
An alkyl group is a functional group or side-chain that, like an alkane, consists solely of singly-bonded carbon and hydrogen atoms, for example a methyl or ethyl group.
Saturated hydrocarbons can be linear (general formula CnH2n+2) wherein the carbon atoms are joined in a snake-like structure, branched (general formula CnH2n+2, n>3) wherein the carbon backbone splits off in one or more directions, or cyclic (general formula CnH2n, n>2) wherein the carbon backbone is linked so as to form a loop. According to the definition by IUPAC, the former two are alkanes, whereas the third group is called cycloalkanes. In other words, saturated hydrocarbons are divided into alkanes and cycloalkanes, depending on whether or not they have cyclic structures, and, in the technical sense, cycloalkanes are not alkanes. However, cycloalkanes are sometimes called cyclic alkanes, which can be confusing when "real" alkanes are called acyclic alkanes. Saturated hydrocarbons can also combine any of the linear, cyclic (e.g., polycyclic) and branching structures, and they are still alkanes (no general formula) as long as they are acyclic (i.e., having no loops).
The simplest possible alkane (the parent molecule) is methane, CH4. There is no limit to the number of carbon atoms that can be linked together, the only limitation being that the molecule is acyclic, is saturated, and is a hydrocarbon. Saturated oils and waxes are examples of larger alkanes where the number of carbons in the carbon backbone tends to be greater than 10.
Alkanes are not very reactive and have little biological activity. Alkanes can be viewed as a molecular scaffold upon which can be hung the interesting biologically-active/reactive portions (functional groups) of the molecule.


Different C4-alkanes and -cycloalkanes (left to right) n-butane and isobutane are the two C4H10 isomers; cyclobutane and methylcyclopropane are the two C4H8 isomers; bicyclo[1.1.0]butane is the only C4H6 isomer; tetrahedrane (not shown) is the only C4H4 isomer.
Alkanes with more than three carbon atoms can be arranged in a multiple number of ways, forming different structural isomers. An isomer is like a chemical anagram, in which the atoms of a chemical compound are arranged or joined together in a different order. The simplest isomer of an alkane is the one in which the carbon atoms are arranged in a single chain with no branches. This isomer is sometimes called the n-isomer (n for "normal", although it is not necessarily the most common). However the chain of carbon atoms may also be branched at one or more points. The number of possible isomers increases rapidly with the number of carbon atoms (sequence A000602 in OEIS). For example:
• C1: 1 isomer—methane
• C2: 1 isomer—ethane
• C3: 1 isomer—propane
• C4: 2 isomers—, n-butane isobutane
• C12: 355 isomers
• C32: 27,711,253,769 isomers
• C60: 22,158,734,535,770,411,074,184 isomers, many of which are not stable.
Branched alkanes can be chiral: 3-Methylhexane and its higher homologues are chiral due to their stereogenic center at carbon atom number 3. Chiral alkanes are of certain importance in biochemistry, as they occur as sidechains in chlorophyll and tocopherol (vitamin E). Chiral alkanes can be resolved into their enantiomers by enantioselective chromatography.
In addition to these isomers, the chain of carbon atoms may form one or more loops. Such compounds are called cycloalkanes.
Nomenclature
Main article: Organic nomenclature
The IUPAC nomenclature (systematic way of naming compounds) for alkanes is based on identifying hydrocarbon chains. Unbranched, saturated hydrocarbon chains are named systematically with a Greek numerical prefix denoting the number of carbons and the suffix "-ane".August Wilhelm von Hofmann suggested systematizing nomenclature by using the whole sequence of vowels a, e, i, o and u to create suffixes -ane, -ene, -ine (or -yne), -one, -une, for the hydrocarbons. The first three name hydrocarbons with single, double and triple bonds; "-one" represents a ketone; "-ol" represents an alcohol or OH group; "-oxy-" means an ether and refers to oxygen between two carbons, so that methoxy-methane is the IUPAC name for dimethyl ether.
It is difficult or impossible to find compounds with more than one IUPAC name. This is because shorter chains attached to longer chains are prefixes and the convention includes brackets. Numbers in the name, referring to which carbon a group is attached to, should be as low as possible, so that 1- is implied and usually omitted from names of organic compounds with only one side-group; "1-" is implied in Nitro-octane. Symmetric compou will have two ways of arriving at the same name.
Straight-chain alkanes are sometimes indicated by the prefix n- (for normal) where a non-linear isomer exists. Although this is not strictly necessary, the usage is still common in cases where there is an important difference in properties between the straight-chain and branched-chain isomers, e.g., n-hexane or 2- or 3-methylpentane.
The first four members of the series (in terms of number of carbon atoms) are named as follows:
methane, CH4
ethane, C2H6
propane, C3H8
butane, C4H10
Alkanes with five or more carbon atoms are named by adding the suffix -ane to the appropriate Greek-language prefix numerical multiplier with elision of any terminal vowel (-a or -o) from the basic numerical term. Hence, pentane, C5H12; hexane, C6H14; heptane, C7H16; octane, C8H18; etc. For a more complete list, see List of alkanes.
Branched alkanes

Ball-and-stick model of isopentane (common name) or 2-methylbutane (IUPAC systematic name)
Simple branched alkanes often have a common name using a prefix to distinguish them from linear alkanes, for example n-pentane, isopentane, and neopentane.
IUPAC naming conventions can be used to produce a systematic name.
The key steps in the naming of more complicated branched alkanes are as follows: Identify the longest continuous chain of carbon atoms
• Name this longest root chain using standard naming rules
• Name each side chain by changing the suffix of the name of the alkane from "-ane" to "-yl"
• Number the root chain so that sum of the numbers assigned to each side group will be as low as possible
• Number and name the side chains before the name of the root chain
• If there are multiple side chains of the same type, use prefixes such as "di-" and "tri-" to indicate it as such, and number each one.
Comparison of nomenclatures for three isomers of C5H12
Common name n-pentane isopentane neopentane
IUPAC name pentane 2-methylbutane 2,2-dimethylpropane
Structure



Main article: Cycloalkane
So-called cyclic alkanes are, in the technical sense, not alkanes, but cycloalkanes. They are hydrocarbons just like alkanes, but contain one or more rings.
Simple cycloalkanes have a prefix "cyclo-" to distinguish them from alkanes. Cycloalkanes are named as per their acyclic counterparts with respect to the number of carbon atoms, e.g., cyclopentane (C5H10) is a cycloalkane with 5 carbon atoms just like pentane (C5H12), but they are joined up in a five-membered ring. In a similar manner, propane and cyclopropane, butane and cyclobutane, etc.
Substituted cycloalkanes are named similar to substituted alkanes — the cycloalkane ring is stated, and the substituents are according to their position on the ring, with the numbering decided by Cahn-Ingold-Prelog rules.
Trivial names
The trivial (non-systematic) name for alkanes is "paraffins." Together, alkanes are known as the paraffin series. Trivial names for compounds are usually historical artifacts. They were coined before the development of systematic names, and have been retained due to familiar usage in industry. Cycloalkanes are also called naphthenes.
It is almost certain that the term paraffin stems from the petrochemical industry. Branched-chain alkanes are called isoparaffins . The use of the term "paraffin" is a general term and often does not distinguish between a pure compounds and mixtures of isomers with the same chemical formula (i.e., like a chemical anagram), e.g., pentane and isopentane.
Examples
The following trivial names are retained in the IUPAC system:
• isobutane for 2-methylpropane
• isopentane for 2-methylbutane
• neopentane for 2,2-dimethylpropane
Occurrence
Occurrence of alkanes in the Universe




Methane and ethane make up a large proportion of Jupiter's atmosphere


Extraction of oil, which contains many different hydrocarbons including alkanes
Alkanes form a significant portion of the atmospheres of the outer gas planets such as Jupiter (0.1% methane, 0.0002% ethane), Saturn (0.2% methane, 0.0005% ethane), Uranus (1.99% methane, 0.00025% ethane) and Neptune (1.5% methane, 1.5 ppm ethane). Titan (1.6% methane), a satellite of Saturn, was examined by the Huygens probe, which indicate that Titan's atmosphere periodically rains liquid methane onto the moon's surface. Also on Titan, a methane-spewing volcano was spotted and this volcanism is believed to be a significant source of the methane in the atmosphere. There also appear to be Methane/Ethane lakes near the north polar regions of Titan, as discovered by Cassini's radar imaging. Methane and ethane have also been detected in the tail of the comet Hyakutake. Chemical analysis showed that the abundances of ethane and methane were roughly equal, which is thought to imply that its ices formed in interstellar space, away from the Sun, which would have evaporated these volatile molecules. Alkanes have also been detected in meteorites such as carbonaceous chondrites.
Occurrence of alkanes on Earth
Traces of methane gas (about 0.0001% or 1 ppm) occur in the Earth's atmosphere, produced primarily by organisms such as Archaea, found for example in the gut of cows.
The most important commercial sources for alkanes are natural gas and oil. Natural gas contains primarily methane and ethane, with some propane and butane: oil is a mixture of liquid alkanes and other hydrocarbons. These hydrocarbons were formed when dead marine animals and plants (zooplankton and phytoplankton) died and sank to the bottom of ancient seas and were covered with sediments in an anoxic environment and converted over many millions of years at high temperatures and high pressure to their current form. Natural gas resulted thereby for example from the following reaction:
C6H12O6 → 3CH4 + 3CO2
These hydrocarbons collected in porous rocks, located beneath an impermeable cap rock and so are trapped. Unlike methane, which is constantly reformed in large quantities, higher alkanes (alkanes with 9 or more carbon atoms) rarely develop to a considerable extent in nature. These deposits, e.g., oil fields, have formed over millions of years and once exhausted cannot be readily replaced. The depletion of these hydrocarbons is the basis for what is known as the energy crisis.
Solid alkanes are known as tars and are formed when more volatile alkanes such as gases and oil evaporate from hydrocarbon deposits. One of the largest natural deposits of solid alkanes is in the asphalt lake known as the Pitch Lake in Trinidad and Tobago.
Methane is also present in what is called biogas, produced by animals and decaying matter, which is a possible renewable energy source.
Alkanes have a low solubility in water, so the content in the oceans is negligible; however, at high pressures and low temperatures (such as at the bottom of the oceans), methane can co-crystallize with water to form a solid methane hydrate.Although this cannot be commercially exploited at the present time, the amount of combustible energy of the known methane hydrate fields exceeds the energy content of all the natural gas and oil deposits put together;methane extracted from methane hydrate is considered therefore a candidate for future fuels.