Chemistry and Physiochemical

Properties of Alkanes
Lecture # 04
 • Hydrocarbons are compounds that contain only atoms
of carbon and hydrogen.
• Classification of hydrocarbons
• There are three main classes of hydrocarbons:
✓ Saturated

Hydrocarbons ✓ Unsaturated
✓ Aromatic
• Saturated hydrocarbons (alkanes) contain only carbon-
carbon single bonds.
• Unsaturated hydrocarbons contain carbon-carbon
multiple bonds, either double (alkenes) or triple
(alkynes) or both.
• Hydrocarbons may be cyclic or acyclic.
• Aromatic hydrocarbons are a special class of cyclic
compounds related in structure to benzene.
 Alkanes
• Alkanes can be subdivided into the following three groups:

✓ Linear straight-chain alkanes

✓ Branched alkanes
✓ Cycloalkanes
 Straight Chain Alkanes
The straight chain alkanes, methane (CH4), ethane (C2H6), and
propane (C3H8) represent the beginning of a series of compounds in
which any two members in a sequence differ by one carbon atom
and two hydrogen atoms—namely, a methylene (CH2) unit.

The Three Simplest Alkanes

• A continuous (unbranched) chain of carbon atoms is often called a straight
chain even though the tetrahedral arrangement about each carbon gives it a
zigzag shape.

• Straight-chain alkanes are sometimes called normal alkanes, and their names
are given the prefix n-. For example, butane is called n-butane.

• The names of straight-chain alkanes are given in Table 7.2. It is important that to learn
the names of at least the first 10.
Note that as we increase the
length of the carbon chain,
the number of possible
different structural isomers
also increases.
Branched Chain Alkanes
A branched chain alkane or branched alkane is an alkane which has alkyl groups bonded to its
central carbon chain. Branched alkanes contain only carbon and hydrogen (C and H) atoms, with carbons
connected to other carbons by single bonds only, but the molecules contain branches (methyl, ethyl, etc.) so
they are not linear.

We can write the structure of butane (C4H10)

by stringing four carbon atoms in a row,–C–
C–C–C– and then adding enough hydrogen
atoms to give each carbon atom four bonds.

The compound butane has this structure,

but there is another way to put 4 carbon
atoms and 10 hydrogen atoms together.
Place 3 of the carbon atoms in a row and
then branch the fourth one off the middle
carbon atom.
Cycloalkanes
• Cycloalkanes are very important in components of food,
pharmaceutical drugs, and much more. However, to use
cycloalkanes in such applications, we must know the
effects, functions, properties, and structures of
cycloalkanes.

• Cycloalkanes are alkanes that are in the form of a ring;

hence, the prefix cyclo- is used to name these alkanes.

• Stable cycloalkanes cannot be formed with carbon chains

of just any length.

• Recall that in alkanes, carbon adopts the tetrahedral

geometry in which the angles between bonds are 109.5°.
 Ring Strain
• For some cycloalkanes to form, the angle between bonds must deviate from this ideal angle,
an effect known as angle strain.

• Additionally, some hydrogen atoms may come into closer proximity with each other than is
desirable (become eclipsed), an effect called torsional strain.

• These destabilizing effects, angle strain and torsional strain are known together as ring strain.
• The smaller cycloalkanes, cyclopropane and cyclobutane, have particularly
high ring strains because their bond angles deviate substantially from 109.5°
and their hydrogens eclipse each other. Thus, both of these ring
conformations are highly unfavorable and unstable.

• Cyclopentane is a more stable molecule with a small amount of ring strain,

while cyclohexane is able to adopt the perfect geometry of a cycloalkane in
which all angles are the ideal 109.5° and no hydrogens are eclipsed; it has no
ring strain at all.

• Cycloalkanes larger than cyclohexane have ring strain and are not as
commonly encountered in organic chemistry.
Representative
Cycloalkane
Structures
Average bond angles
and strain energy
are indicated.
Homologous Series
Any family of compounds in which adjacent members differ from each other by a definite factor
(here a CH2 group) is called a homologous series. The members of such a series,
called homologs.
The family of alkanes shown in the table 2.1 is an example of a homologous series.
Propane (CH3CH2CH3) and butane (CH3CH2CH2CH3) are homologs.

In organic chemistry, homologs have properties that vary in a regular and predictable manner.
Thus, the principle of homology gives organization to organic chemistry in much the same way
that the periodic table gives organization to inorganic chemistry.
Instead of a bewildering array of individual carbon compounds, we can study a few members of
a homologous series and from them deduce some of the properties of other compounds in the
series.
 • The general molecular formula for an alkane

General is CnH2n+2 ,where n is any integer. So, if an alkane

has one carbon atom, it must have four hydrogen
atoms; if it has two carbon atoms, it must have six
Formula hydrogens.
 Naming Alkanes
• Learning about how to name alkanes makes the basis for the names of almost
all organic compounds.

• Alkanes are composed of only carbon atoms and hydrogen atoms and contain
only single bonds.
•
• We have seen that carbon forms four covalent bonds and hydrogen forms
only one covalent bond.
• This means that there is only one possible structure for an alkane with
molecular formula (methane) and only one structure for an alkane with
molecular formula (ethane). There is also only one possible structure for an
alkane with molecular formula (propane).
Constitutional/ Structural Isomers
• As the number of carbons in an alkane increases beyond
three, the number of possible structures increases.
• There are two possible structures for an alkane with
molecular formula C4H10. In addition to butane—a straight-
chain alkane—there is a branched butane called isobutane.
Both of these structures fulfill the requirement that each
carbon forms four bonds and each hydrogen forms only one
bond.
• Compounds such as butane and isobutane that have the same
molecular formula but differ in the order in which the atoms
are connected are called constitutional isomers—their
molecules have different constitutions.
• Structural isomers have different chemical and physical
properties.
• In fact, isobutane got its name because it is an “iso”mer of
butane. The structural unit—a carbon bonded to a hydrogen
and two groups—that occurs in isobutane has come to be
called “iso.”
• Thus, the name isobutane tells that the compound is a four-
carbon alkane with an iso structural unit.
Key Takeaway
• Alkanes with four or more carbon atoms can exist in
isomeric forms.
 There are three alkanes with molecular formula C5H12
Pentane is the straight chain alkane. Isopentane, as its name
Pentane indicates, has an iso structural unit and five carbon atoms. The
third isomer is called neopentane. The structural unit with a
carbon surrounded by four other carbons is called “neo.”
 • There are five constitutional isomers with molecular formula C6H14.
• We are now able to name three of them (hexane, isohexane, and
Hexane neohexane), but we cannot name the other two without defining
names for new structural units. (For now, ignore the names written in
blue).
Heptane
• There are nine alkanes with molecular
formula C7H16.
• We can name only two of them
(heptane and iso-heptane) without
defining new structural units.
• Notice that neoheptane cannot be used
as a name because three different
heptanes have a carbon that is bonded
to four other carbons and a name must
specify only one compound.
• The number of constitutional isomers
increases rapidly as the number of
carbons in an alkane increases.
• For example, there are 75 alkanes with
molecular formula C10H22 and 4347
alkanes with molecular formula
C15H32.
• To avoid having to memorize
the names of thousands of structural
units, chemists have devised rules that
name compounds on the basis of their
structures.
• That way, only the rules have to be
learned. Because the name is based on
the structure, these rules make it
possible to deduce the structure of a
compound from its name.
IUPAC Nomenclature
• This method of nomenclature is called systematic nomenclature. It is also
called IUPAC nomenclature because it was designed by a commission of the
International Union of Pure and Applied Chemistry (abbreviated IUPAC and
pronounced “eye-youpack”) at a meeting in Geneva, Switzerland, in 1892.

• The IUPAC rules have been continually revised by the commission since then.

• Names such as isobutane and neopentane—nonsystematic names—are called

common names.
• Removing a hydrogen from an alkane results in an alkyl substituent
(or an alkyl group).
• Alkyl substituents are named by replacing the “ane” ending of the
alkane with “yl.”
• The letter “R” is used to indicate any alkyl group.
 Nomenclature of Alkanes
• The systematic name of an alkane is obtained using the following rules:
1. Determine the number of carbons in the longest continuous carbon chain. This
chain is called the parent hydrocarbon.
The name that indicates the number of carbons in the parent hydrocarbon
becomes the alkane’s “last name.” For example, a parent hydrocarbon with eight
carbons would be called octane.
The longest continuous chain is not always a straight chain; sometimes you have to
“turn a corner” to obtain the longest continuous chain.
Number the chain so that the substituent gets
the lowest possible number
2. The name of any alkyl substituent that hangs off the parent hydrocarbon is
cited before the name of the parent hydrocarbon, together with a number to
designate the carbon to which the alkyl substituent is attached.
The chain is numbered in the direction that gives the substituent as low a
number as possible.
The substituent’s name and the name of the parent hydrocarbon are joined
in one word, and there is a hyphen between the number and the
substituent’s name.
 Substituents are listed in alphabetical order
3. If more than one substituent is attached
to the parent hydrocarbon, the chain is
numbered in the direction that will result in
the lowest possible number in the name of
the compound.
The substituents are listed in alphabetical
(not numerical) order, with each substituent
getting the appropriate number.
In the following example, the correct name
(5-ethyl-3-methyloctane) contains a 3 as its
lowest number, while the incorrect name (4-
ethyl-6-methyloctane) contains a 4 as its
lowest number.
A number and a word are separated by a
hyphen; numbers are separated by a comma
• If two or more substituents are the same, the prefixes “di,”“tri,” and “tetra”
are used to indicate how many identical substituents the compound has.
• The numbers indicating the locations of the identical substituents are listed
together, separated by commas.
• Notice that there must be as many numbers in a name as there are
substituents.
di, tri, tetra, sec, and tert are ignored in alphabetizing,
iso, neo, and cyclo are not ignored in alphabetizing
• The prefixes di, tri, tetra, sec, and tert are ignored in alphabetizing
substituent groups, but the prefixes iso, neo, and cyclo are not
ignored.
4. When both directions lead to the same lowest number for one of the
substituents, the direction is chosen that gives the lowest possible
number to one of the remaining substituents.
Only if the same set of numbers is obtained in both directions does the
first group cited get the lower number?

5. If the same substituent numbers are obtained in both directions, the

first group cited receives the lower number.
In the case of two hydrocarbon chains with the same number of carbons,
choose the one with the most substituents

6. If a compound has two or more chains of the same length, the

parent hydrocarbon is the chain with the greatest number of
substituents.
7. Names such as “isopropyl,”“sec-butyl,”
and “tert-butyl” are acceptable substituent
names in the IUPAC system of
nomenclature, but systematic substituent
names are preferable.
Systematic substituent names are obtained
by numbering the alkyl substituent starting
at the carbon that is attached to the parent
hydrocarbon.
This means that the carbon that is attached
to the parent hydrocarbon is always the
number 1 carbon of the substituent.
In a compound such as 4-(1-
methylethyl)octane, the substituent name
is in parentheses; the number inside the
parentheses indicates a position on the
substituent, whereas the number outside
the parentheses indicates a position on the
parent hydrocarbon.
• These rules help us to name thousands of alkanes, and there are
additional rules necessary to name many other kinds of compounds.
• The rules are important if we want to look up a compound in the
scientific literature, because it usually will be listed by its systematic
name.
Problem
• Draw the structure of each of the following compounds:
✓ 2,3-dimethylhexane
✓ 2,2-dimethyl-4-propyloctane
Nomenclature of Cycloalkanes
• Cycloalkanes are alkanes with their carbon atoms arranged in a ring.
• Because of the ring, a cycloalkane has two fewer hydrogens than an acyclic
(noncyclic) alkane with the same number of carbons.
• This means that the general molecular formula for a cycloalkane is CnH2n
• Cycloalkanes are named by adding the prefix “cyclo” to the alkane name that
signifies the number of carbon atoms in the ring.
• Cycloalkanes are almost always written as skeletal structures. Skeletal structures show the
carbon–carbon bonds as lines, but do not show the carbons or the hydrogens bonded to carbons.
Atoms other than carbon and hydrogens bonded to atoms other than carbon are shown. Each
vertex in a skeletal structure represents a carbon. It is understood that each carbon is bonded to
the appropriate number of hydrogens to give the carbon four bonds.

Acyclic molecules can also be represented by skeletal structures. In a skeletal structure of an acyclic
molecule, the carbon chains are represented by zigzag lines. Again, each vertex represents a carbon,
and carbons are assumed to be present where a line begins or ends.
 Rules for naming cycloalkanes
• The rules for naming cycloalkanes resemble the rules for naming acyclic alkanes:
1. In the case of a cycloalkane with an attached alkyl substituent, the ring is the
parent hydrocarbon unless the substituent has more carbon atoms than the
ring.
In that case, the substituent is the parent hydrocarbon and the ring is named as a
substituent. There is no need to number the position of a single substituent on a
ring.
• 2. If the ring has two different substituents, they are cited in
alphabetical order and the number 1 position is given to the
substituent cited first.
3. If there are more than two substituents on the ring, they are cited in alphabetical order.
The substituent given the number 1 position is the one that results in a second substituent getting
as low a number as possible.
If two substituents have the same low number, the ring is numbered—either clockwise or
counterclockwise—in the direction that gives the third substituent the lowest possible number.
For example, the correct name of the following compound is 4-ethyl-2-methyl1-propylcyclohexane,
not 5-ethyl-1-methyl-2-propylcyclohexane:
Problem
• Give the systematic name for each of the following compounds: