Video Transcript
In this video, we will learn how to
identify and name simple hydrocarbons and represent them using different types of
formulas. We’ll classify basic hydrocarbons,
name the first 10 straight-chain alkanes, and learn how to represent organic
molecules using displayed, structural, and condensed formulas.
Before we begin to classify and
name compounds, it is useful to have a few ways to represent the compounds in
question. One representation that we may be
familiar with is the molecular formula. A molecular formula is the chemical
formula that expresses the exact number and type of atoms of each element in a
molecule. For example, the molecular formula
of methane gas, the main component of natural gas, is CH4. The molecular formula tells us that
each molecule of methane contains one carbon atom and four hydrogen atoms. Formaldehyde, which has been used
as a biological specimen preservative, has the molecular formula CH2O. This tells us that each molecule of
formaldehyde has one carbon atom, two hydrogen atoms, and one oxygen atom.
Acetylene, a common fuel used for
welding, has the molecular formula C2H2, meaning that each molecule of acetylene has
two carbon atoms and two hydrogen atoms. A molecular formula tells us the
number of each type of atom in the molecule but does not provide any information on
how these atoms are bonded together.
To get a more detailed picture of
the molecule, we can use a displayed formula. A displayed formula is a drawing
representation of a molecule that shows all of the atoms and bonds. Bonds are represented by lines that
connect the atoms together. In the displayed formula of
methane, single lines connect the hydrogen atoms to a central carbon atom. Each single line represents a
single covalent bond, a bond where two electrons are shared between two atoms.
In the displayed formula of
formaldehyde, two lines connect the oxygen atom to the carbon atom. This represents a covalent double
bond, a bond where four electrons are shared between two atoms. In the displayed formula of
acetylene, three lines are drawn between the two carbon atoms. This represents a covalent triple
bond, a bond where six electrons are shared between two atoms. Displayed formulas show how the
atoms in a molecule are connected, but they are not necessarily the most accurate
representation of a molecule.
Displayed formulas represent
molecules as flat with 90- or with 180-degree angles between the atoms. In actuality, the atoms in these
molecules are positioned so that the electron domains are at the maximum distance
from one another as explained by valence-shell-electron-pair-repulsion theory, or
VSEPR. We won’t go into VSEPR theory in
detail in this video, but it’s worth recognizing that displayed formulas only
represent the connectivity of the atoms, not the geometry of the molecule.
Now that we can represent molecules
with molecular or displayed formulas, let’s take a look at some hydrocarbons. Organic chemistry is the study of
carbon-containing compounds, the simplest of which is a hydrocarbon, a molecule
composed of only carbon and hydrogen atoms. Hydrocarbons may be classified as
saturated or unsaturated. A saturated hydrocarbon contains
only single covalent bonds. This means that each carbon atom
will form four single bonds with four different atoms, and each carbon atom will
form bonds with the maximum number of hydrogen atoms as possible. For example, a saturated
hydrocarbon that contains two carbon atoms must contain six hydrogen atoms. The molecule contains only single
bonds, and each carbon atom has formed four total bonds with four different
atoms.
Saturated hydrocarbons are also
called alkanes. Unsaturated hydrocarbons contain at
least one double or triple bond. As each carbon atom will not form
four single bonds, an unsaturated hydrocarbon will contain fewer hydrogen atoms than
a saturated molecule with the same number of carbon atoms. Shown are two examples of
unsaturated hydrocarbons that each contain two carbon atoms. The unsaturated hydrocarbon on the
left is an example of an alkene, a hydrocarbon with at least one double bond. The example on the right is an
alkyne, a hydrocarbon with at least one triple bond.
Alkanes, alkenes, and alkynes each
have a generic molecular formula. For an alkane, the generic formula
is C𝑛H2𝑛+2, where 𝑛 represents the number of carbon atoms in the molecule. If an alkane has three carbon
atoms, then it must contain two times three plus two hydrogen atoms and have the
molecular formula C3H8. The generic formula for an alkene
that contains only one double bond is C𝑛H2𝑛. If an alkene has three carbon atoms
and only contains one double bond, then it must contain two times three hydrogen
atoms and have the molecular formula C3H6. The general formula for an alkyne
that contains only one triple bond is C𝑛H2𝑛−2. Therefore, an alkyne with only one
triple bond and three carbon atoms will have the molecular formula C3H4.
Hydrocarbons can further be
classified as either aromatic or aliphatic. The term aromatic can be used to
describe a number of organic molecules. But in order for a hydrocarbon to
be considered aromatic, it must contain at least one planar, cyclic, carbon-based
structure: planar meaning that that portion of the molecule is flat or sits on one
plane and cyclic meaning that the carbon atoms connect to form a ring. The ring structure must contain
alternating single and double bonds. This structure gives aromatic
compounds unique electronic properties. Benzene, a compound used in the
production of plastics, and naphthalene, a compound used for fumigation, are
examples of aromatic hydrocarbons. The term aliphatic is used to
describe all nonaromatic hydrocarbons. This includes alkanes, alkenes,
alkynes, and cycloalkanes.
Now that we have learned how to
classify hydrocarbons, let’s focus on alkanes, the simplest hydrocarbon. Each of these displayed formulas
represents a different alkane. But as they all contain
single-bonded carbon and hydrogen atoms, how do we indicate which alkane is
which? The international union of pure and
applied chemistry, or IUPAC, established a set of rules for naming compounds,
including alkanes. When naming an alkane, we focus on
the total number of carbon atoms in a continuous chain. We mentioned earlier in the video
that a molecule which contains one carbon atom and four hydrogen atoms is called
methane. The prefix meth- means one carbon
atom, and the suffix -ane is used to indicate that the compound is an alkane
containing only single covalent bonds.
Thus simple alkanes are named by
placing a prefix that indicates the number of continuous carbon atoms in front of
the suffix -ane. The first 10 prefixes are shown in
the table. One carbon atom is meth-. Two carbon atoms in a continuous
chain is eth-. Three is prop-. Four is but-. Five is pent-. Six is hex-. Seven is hept-. Eight is oct-. Nine is non-. And 10 is dec-. Let’s use the table to name the
alkane below methane. This alkane has five carbon atoms
in a continuous chain. We use the prefix pent- for five
carbon atoms and add the suffix -ane to indicate that the molecule is an alkane. The third alkane has seven carbon
atoms in a line, and we give it the name heptane.
Notice that when using this naming
system, the number of carbon atoms is indicated by a prefix, but the number of
hydrogen atoms is not indicated in the name at all. Let’s take a look at why it is not
necessary to indicate the number of hydrogen atoms with the example hexane.
The prefix hex- means that there
are six carbon atoms in a continuous chain, and the suffix -ane indicates that the
molecule is an alkane and contains only single covalent bonds. From the name, we can gather that
the six carbon atoms are single bonded to one another. We also know that alkanes are
saturated. This means that each carbon atom
will form four single bonds and will bond with the maximum number of hydrogen atoms
as is possible. Therefore, we can complete the
displayed formula of hexane without initially knowing how many hydrogen atoms were
in the molecule.
Displayed formulas like this one
are useful for understanding the connectivity of atoms in a molecule, but they
require a lot of space because they are drawn representations of a molecular
structure. Instead of using a displayed
formula, we can represent the bonding pattern of a molecule in text format by using
a structural formula. A structural formula is a text
representation of the bonding in a molecule. In this formula, hydrogen atoms are
grouped with the atom they are bonded to, forming units. The units are then listed in the
order in which they are bonded together. Let’s take another look at the
displayed formula of hexane.
The carbon atom labeled with a one
is bonded to three hydrogen atoms. This forms the unit CH3. The second carbon atom is bonded to
two hydrogen atoms. This forms the unit CH2. The third, fourth, and fifth carbon
atoms are each bonded to two hydrogen atoms and can be represented as the unit
CH2. The sixth carbon atom is bonded to
three hydrogen atoms. This carbon atom forms a CH3
unit. Writing the units in order gives us
the structural formula of hexane CH3CH2CH2CH2CH2CH3. Structural formulas are smaller
than displayed formulas, but they can still be quite large, depending on the number
of carbon atoms in the molecule. A shortened version of a structural
formula is called a condensed formula.
In a condensed formula, like
repeating units in a structural formula are combined by placing the unit in
parentheses, followed by a subscript to indicate the number of times the unit
repeats. To see how this works, let’s once
again take a look at hexane. Notice that in the structural
formula, the unit CH2 appears four times in a row. We can combine these like,
repeating units by writing the unit CH2 in parentheses followed by a subscript four
indicating that the unit CH2 repeats four times in a row. We write the CH3 unit on either
side of this repeating unit to complete the condensed formula. Thus, the condensed formula of
hexane is CH3(CH2)4CH3.
We’ve now seen how to write a
condensed formula from a structural formula and a structural formula from a
displayed formula. But why are these formulas
necessary? Couldn’t we have just used the
molecular formula all along? We’ve seen that C6H14 is the
molecular formula of hexane. But each of these displayed
formulas also represent compounds with the molecular formula C6H14. Each of these new molecules may
have the same molecular formula as hexane, but their connectivity and therefore
their chemical properties are different. These molecules are all structural
isomers, molecules that have the same molecular formula but differ from one another
in how their atoms are connected.
We won’t go into detail on how to
determine the number of structural isomers in this video. But it’s important to understand
that many molecules can have the same molecular formula and are therefore best
represented by using displayed, structural, and condensed formulas.
Let’s finish this video by
summarizing what we’ve learned with the key points. Hydrocarbons are a type of organic
compound that only contain carbon and hydrogen atoms. Hydrocarbons can be further
classified as saturated, unsaturated, aromatic, or aliphatic. Alkanes are saturated hydrocarbons
that have the general formula C𝑛H2𝑛+2. These compounds only contain single
covalent bonds. Alkenes are a type of unsaturated
hydrocarbon. They have the general formula
C𝑛H2𝑛 and contain at least one carbon-carbon double bond. Alkynes are another type of
unsaturated hydrocarbon. They have the general formula
C𝑛H2𝑛−2 and contain at least one carbon-carbon triple bond.
Straight-chain alkanes are named by
attaching a prefix that indicates the number of carbon atoms bonded in a continuous
chain to the suffix -ane. For example, CH4 is called methane:
meth- meaning one carbon atom and -ane meaning an alkane. We can represent organic molecules
using displayed formulas, structural formulas, or condensed formulas.