The Unsung Heroes: Introduction to organic Hydrocarbons

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 asorganic, 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 C^4- 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 CC 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.

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.

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 C_nH_2n+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 25^oC
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
eicosane	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: C_nH_2n+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).