Structure and Nomenclature of Hydrocarbons

1. Structure and
Nomenclature of
Hydrocarbons

2. 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?"

3.  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.

4.  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)

5.  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

6.  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

7. 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.

8. 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.

9.  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.

10.  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.

11. 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

12.  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

13.  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

14.  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).

15. Ball-and-stick models of the three isomers of
pentane are shown in the figure below.
n-Pentane
Isopentane
Neopentane

16. 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.

17.  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.

18.  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.

19. 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.

20.  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

21.  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.

22.  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

23. 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.

24.  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.

25.  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.

26. 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.

27.  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.

28.  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.

29.  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.

30.  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.