Structure and

Nomenclature of

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 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.

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.

AgOCN(aq) + NH4Cl(aq) - AgCl(s) + NH4OCN(aq)

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.

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).

Electronic configuration 1s2 2s2 2p2

Electronegativity 2.55

Covalent radius 0.077 nm

The Physical Properties of Carbon

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.

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 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

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: 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.

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-PentaneIsopentane

Neopentane

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.

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

are 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.

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.

Chair Boat

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.

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 alkene has 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.