Video Transcript
In this video, we will learn what a
homologous series or family of organic chemicals is. We will learn how to write the
general formula for a homologous series, and we will also investigate the trends in
physical and chemical properties within a series.
What do we mean when we talk about
an homologous series? Firstly, in general, an organic
compound is a chemical compound that contains carbon and hydrogen covalently bonded
together. Many organic compounds also contain
other covalently bonded atoms such as oxygen, sulfur, or nitrogen. Every organic compound belongs to a
specific family of organic compounds. A family of organic compounds is
called an homologous series. Each member or each compound within
the same family or same homologous series are identical in terms of structure except
that each successive member differs from each other by a simple repeating unit. Member one and two differ from each
other only by this simple repeating unit.
In the same way, member two and
three differ from each other by the same simple repeating unit. And again, members three and four
differ from each other by that same simple repeating unit. We could go on and add more members
to the series in the same manner. We can define an homologous series
as a family of compounds with the same functional group and each compound in the
series differing from the previous by a simple structural unit. We will see that the members of an
homologous series or family of compounds have the same general formula in much the
same way as members of a family tend to have the same surname and that compounds in
the same family tend to have the same or similar properties in much the same way as
members of a family tend to look similar to each other.
Let’s now have a look at the most
basic homologous series, the alkanes. The alkane homologous series or
alkane family has methane as the first and simplest member. Methane’s molecular formula is CH4,
and its structural formula is shown here. Methane has one carbon atom and
four hydrogen atoms. Ethane, the next alkane in this
series, has two carbon atoms and six hydrogen atoms. Ethane differs from methane by one
carbon and two hydrogens, which is a simple CH2 unit. The next compound in the alkane
homologous series is propane, whose molecular formula is C3H8, with the following
structural formula. Propane differs from ethane by one
carbon atom and two hydrogen atoms, which is again a simple CH2 structural unit.
All the compounds in the alkane
homologous series follow this pattern. Each compound differs from the
previous or from the next by the same simple structural unit, a CH2 unit. We call compounds that differ from
each other by a repeating unit homologs. Methane, ethane, and propane are
homologs in the same homologous series. We can write a general formula for
all the alkanes since they differ by the same unit. The general formula for the alkanes
is C𝑛H2𝑛+2, where 𝑛 is a natural number such as one, two, three, four, et
cetera.
Many homologous series, such as the
alkenes, have one or more functional groups. Ethene, propene, and butene all
have one carbon-carbon double bond. Ethene is C2H4, propene C3H6, and
butene C4H8. As with the alkanes, the homologs,
ethene, propene, and butene successively differ from each other by one carbon and
two hydrogens or a CH2 unit. As a side note, the two isomers of
butene, but-1-ene and but-2-ene, both also differ from propene by a simple CH2 unit,
even though the double-double carbon bond is in a different place in each
compound. The general formula of alkenes is
C𝑛H2𝑛.
Now, there are many homologous
series. Let’s look at two more examples
briefly, and those are the alcohols and carboxylic acids. The simple straight-chain alcohols
all contain the hydroxy or hydroxyl functional group. The homologs, methanol, ethanol,
propanol, butanol, and so on all differ from each other by a simple repeating unit
and all have the same general formula C𝑛H(2𝑛+1)OH. Even those compounds in the series,
such as propanol and butanol, which have more than one isomer, still differ from
each other by the same simple repeating unit and have the same general formula.
The simple straight-chain
carboxylic acids all have the same functional group, the carboxyl group or
carboxylic acid group, COOH. Methanoic acid, ethanoic acid,
propanoic acid, and butanoic acid again all differ from each other by the successive
addition of a CH2 unit. And they all share the same general
formula C𝑛H(2𝑛+1)COOH. Now we know that compounds within
an homologous series all have a similar structure except that successive structures
differ from the previous or next by a simple repeating unit. We also know that all compounds
within the same family have the same functional group and the same general
formula. But what about their
properties? How do properties compare?
Let’s start with the chemical
properties within an homologous series. The alkanes all tend to react in
the same manner because they have similar structures and they are all fully
saturated with no other functional groups. For example, when ethane reacts
with chlorine gas in the presence of UV light, one of the chlorine atoms is
substituted for a hydrogen atom to give a monosubstituted product,
1-chloroethane. Other alkanes react in the same way
with halogens. In other words, they undergo
substitution reactions.
Now, the alkenes tend to react in a
similar manner to each other. For example, ethene, which is
unsaturated with a carbon-carbon double bond, can react with chlorine gas to give a
disubstituted product, 1,2-dichloroethane. This is correctly called an
addition product. And alkenes all tend to undergo
addition reactions because extra atoms are added on to the double bond. We have seen here how members of
the same homologous series tend to react in a similar manner.
Now, let’s investigate physical
properties within an homologous series. The physical properties within a
series are also similar between compounds except that they tend to follow a
recognizable trend. This rough graph shows the boiling
points of some straight-chain alkanes. The short molecules, which have few
carbon atoms, have weak Van der Waals attractive forces between them. And so their boiling points are low
as little energy is required to overcome the intermolecular attraction during
boiling. However, the longer the carbon
chain becomes, the stronger the Van der Waals attractive forces between the
molecules are, and so the higher the boiling points are because more energy is
needed to overcome these attractive forces to separate the molecules when
boiling. We can see this general trend. An increasing number of carbon
atoms corresponds with an increasing boiling point.
Viscosity, which is the resistance
to flow, is another physical property with a gradually changing recognizable trend
within a homologous series. The graph shows the viscosity of
some straight-chain alkanes with five carbons or more. Shorter carbon chains have low
viscosity and flow easily. But the longer the chains are or
the more carbon atoms they are, the higher the viscosity. We say these substances are more
viscous or sticky and they flow very slowly. This is because of increased or
stronger Van der Waals forces of attraction between these larger molecules. We can see this general increasing
trend. An increasing number of carbon
atoms corresponds with an increase in viscosity.
Let’s look at the physical property
flammability. A flammable substance catches fire
immediately when it is exposed to a flame. Flammability in the alkanes tends
to follow an opposite trend to boiling point and viscosity. That is, with an increasing number
of carbon atoms in the compound chain, there is a decreasing flammability. In other words, the larger the
molecule, the less flammable or the less easily it burns. Large alkane molecules tend to
undergo incomplete combustion, giving off a smoky flame.
Lastly, let’s look at the density
of the straight-chain alkanes and their trends. The table shows the density of some
alkanes at 20 degrees Celsius and one atmosphere. Notice that as the number of carbon
atoms increases, so the density increases. In general, with an increasing
number of carbon atoms, in other words, an increasing carbon chain length or
increasing molecular mass, there is a corresponding increase in density. It’s interesting to note there is a
sudden big increase in density going from butane to pentane. That is because the compounds up to
butane are all gaseous at 20 degrees Celsius. But pentane and those following in
the table are liquid and thus are much more dense.
Now, it’s time to practice.
Which of the following is the
general formula of haloalkanes that contain one halogen atom? (A) C𝑛H2𝑛+1X, (B) C𝑛H2𝑛+2X,
(C) C𝑛H2𝑛-1X, (D) C𝑛H2𝑛X, or (E) C𝑛H2𝑛-2X.
The question asks for the general
formula of alkane compounds containing one halogen atom. Alkanes belong to the alkane
homologous series, in other words, the alkane family, whereas the haloalkanes belong
to the haloalkane homologous series or the haloalkane family. A homologous series is a family of
compounds with the same functional group with each compound in the series differing
from the previous or next compound by a simple structural unit. Because each compound in the series
differs from the next or previous by the same unit, every compound within the same
homologous series has the same general formula. The alkanes all have the same
general formula C𝑛H2𝑛+2, where 𝑛 is a natural number such as one, two, three,
four, et cetera.
A haloalkane is an alkane where one
of the hydrogen atoms has been removed and replaced with a halogen. Let’s call the halogen atom X. So if we remove one hydrogen atom
from an alkane, the formula becomes C𝑛H2𝑛+1. But we need to add in a halogen
atom. So we can put X into the formula
in place of the missing hydrogen atom. This formula corresponds with
answer option (A). Therefore, the general formula for
a haloalkane that contains one halogen atom is C𝑛H2𝑛+1X.
Before we end this question, let’s
have a look at two quick examples of haloalkanes. If 𝑛 is, say, two and X is, say,
chlorine, we can write the general formula as C2H2(2)+1Cl, which is C2H5Cl or
chloroethane. Here is another example. If 𝑛 is, say, four and X is, say,
bromine, then the general formula is C4H2 multiplied by four plus one Br, which is
C4H9Br, which is 1-bromobutane or any other isomer of bromobutane.
Let’s summarize what we have
learnt. We learnt that an homologous series
is a family of compounds that have the same functional group, where each compound in
the series differs from the previous compound by a simple repeating structural
unit. Compounds in an homologous series
have the same general formula, similar chemical properties, and physical properties
that gradually change in a recognizable trend within the series.
