Hydrocarbons contain only carbon and hydrogen and can be subdivided into the two classes:

1

Hydrocarbons contain only carbon and hydrogen and
can be subdivided into the two classes
2

Alkanes consist entirely of C-C and C-H single
bonds. Alkanes have the general formula CnH2n2,
where n is an integer greater than 0.
3

The first four alkanes are gases and the others
are liquids near 20o C.
The ball-and-stick representations on the left
allow a clear view through the molecule. All of
the atoms and bonds can usually be
seen. Space-filling representations on the right
more accurately represent the atomic sizes and
bond lengths in the molecule.
4

Carbon Bonding in Alkanes

• Carbon atoms in alkanes have the following
  characteristics
• Carbon is tetravalent each carbon has four
  bonds
• The four bonds possess tetrahedral geometry
• The four bonds are equivalent and have similar
  properties.
• The angle between any two bonds of a tetrahedral
  (sp3) carbon in any carbon compound is 109.5o,
  the tetrahedral bond angle.

5

The tetrahedral bond angles and equivalence of
all four bonds to an alkane carbon atom is
rationalized by using hybridization of the 2s and
three 2p atomic orbitals of the carbon atom to 4
equivalent sp3 orbitals
The single bonds formed between the sp3 orbitals
of the carbon atom and the s orbitals of the
hydrogen atoms are called sigma (?) bonds.
6

7

Conformations and Single-Bond Free Rotation
A single bond between two atoms possesses free
rotation. Free rotation allows molecules to have
different conformations, or different
orientations resulting only from rotations about
its single bonds.
Two of the many possible conformations of butane
are shown above. All of the possible
conformations are constantly being converted into
each other through rotations about the single
bonds. Several conformations of butane viewed as
ball-and-stick models
8

Each of the following structural formulas
represents the same compound, butane
I, II, and III represent the same conformation,
shown in different orientations. IV is a
different conformation. V does not specify a
conformation. VI is an expanded structural
formula and VII is a condensed structural formula.
9

Two additional types of structural
representations are also sometimes used skeleton
and line structural formulas
In a line formula, a carbon atom is assumed to be
present at every intersection and at the terminal
ends of lines.
10

11

Constitutional isomers are different compounds
with the same chemical formulas since they differ
from one another in connectivity.
In addition to normal butane (n-butane), an
isomer, isobutane exists which has the same
molecular formula as butane, C4H10 but has a
different set of chemical properties.
12

The basic difference between the structures of
constitutional isomers is the length of the
longest successive chain of carbon atoms. This
chain is called the longest continuous carbon
chain or the longest chain. In butane, the
longest chain is 4. In isobutane, the longest
chain is 3. Butane is an example of an unbranched
alkane. All of the carbon atoms are in the
longest chain. Isobutane is an example of a
branched alkane. Carbons exist in the branches
that are not part of the longest chain. The
greater the number of carbon atoms that are in a
compound, the larger the number of possible
constitutional isomers.
13

14

It is important in working out the answer to this
problem not to incorrectly show two different
structural formulas of the same isomer. For
example, including structure 4 along with
structures 1, 2, and 3 is a mistake because
structures 2 and 4 represent the same compound.
15

Common Nomenclature System
The common nomenclature system and the IUPAC
system name unbranched alkanes using the suffix
ane. The first ten alkanes are named methane,
ethane, propane, butane, penane, hexane, heptane,
octane, nonane, and decane. Alkanes having more
than 3 carbons have constitutional isomers.
Prefixes are added to the name of the
corresponding straight chain alkane to form the
name of the branched isomers.
Pentane Isopentane
Neopentane
16

Primary, Secondary, Tertiary, and Quaternary
Classification
A carbon atom is classified as primary (1o),
secondary (2o), tertiary (3o), or quaternary (4o)
depending upon whether or not it is directly
bonded to a total of one, two, three, or four
other carbon atoms, respectively.
Some functional groups, such as OH, are
categorized as primary, secondary, or tertiary if
they are attached to a carbon atom with the same
categorization. Reactivity of most compounds
changes regularly, either up or down, in the
order primary, secondary, tertiary.
17

18

The IUPAC system uses a short list of the alkyl
groups normally found in organic molecules
19

In organic chemistry, molecules are often written
in the generalized form R-X, where R represents
an alkyl group and X represents an attached
substituent -H, -OH, -Cl, etc.
20

As the number of carbon atoms increases, the
number of possible constitutional isomers rapidly
increases. C4 2 isomers C5 3 isomers C6 5
isomers C10 75 isomers To avoid an unwieldy list
of prefixes to identify isomers, the IUPAC
nomenclature system has been devised to simplify
and standardize the naming of chemical compounds.
21

22

IUPAC Nomenclature System
23

Two different-looking structural formulas of the
same compound will yield the same name if the
nomenclature rules are correctly applied.
24

25

26

27

Alkanes are acyclic or open-chain
molecules. Cycloalkanes differ only in that they
possess a cyclic chain of 3 or more carbon atoms
and are often represented by a condensed
structural formula in the form of a geometrical
shape.
28

The general molecular formula for a cycloalkane
is CnH2n. This formula contains two fewer
hydrogen atoms than an ordinary alkane, CnH2n2.
29

30

These two structures are geometric, or cis-trans
isomers of each other.
Cis stereoisomers have two similar substituents
on the same side of the ring. Trans stereoisomers
have two similar substituents on opposite sides
of the ring. Cis-trans isomers are often called
diastereomers to distinguish them from another
type of stereoisomers called enantiomers. Cis and
trans isomers are different compounds with
different chemical and physical properties. Cis
and trans isomers are not easily interconvertible
because the C-C bonds within the ring are not
free to rotate.
Stereoisomerism results from differences in
configuration, not connectivity.
31

32

The cis-trans nomenclature can only be used for
cycloalkanes in which each of the two carbons of
the ring has one hydrogen and one other
substituent. IUPAC uses the E,Z nomenclature in
this case, which will not be covered here.
The cis-trans notation cannot be used for this
molecule or the ones below.
33

34

Melting and Boiling Points
Melting is the transition from the solid to the
liquid state. Boiling is the transition from the
liquid to the gaseous state. Melting and boiling
points are directly related to the secondary
forces holding the substance in question
together. The larger the secondary forces, the
higher the melting and boiling point.
35

In organic compounds, the strength of the
secondary forces between molecules depend
upon Family (type of secondary force) Molecular
mass Molecular shape Family The family
determines what chemical bonds are present,
whether the bonds are polar or non-polar, and
whether there is hydrogen bonding. (Chapter
6.2-6.3) The order of secondary forces
is Hydrogen bonding gt dipole-dipole gt London All
molecules possess London forces. Polar molecules
possess dipole-dipole forces as well. Hydrogen
bonds are only present in organic molecules when
O-H or N-H bonds are present.
36

Molecular mass For any series of compounds in
the same organic family, London forces increase
with increasing molar mass. (Table
11.3) Molecular shape For compounds within the
same family and having the same or close
molecular masses, the order of boiling points
is Cycloalkane gt straight-chain alkane gt
branched alkane
37

Molecules that are able to pack closely together
experience much larger London forces than
unsymmetrical molecules.
38

39

Density
Molecules having large secondary forces pack
together tightly, resulting in greater
densities. Water (hydrogen bonding) has a density
of 1.0 g/ml. Liquid alkanes have densities of
0.7-0.8 g/ml, depending upon molar mass.
Solubility
A particular solute will dissolve in a solvent
only if the solute-solute secondary forces are
similar to the solvent-solvent secondary
forces. Like dissolves like.
40

Combustion
All hydrocarbons undergo combustion, or burning
in air.
Combustion is an example of an oxidation
reaction. When hydrocarbons are oxidized, the
oxidation state of the carbon atoms
increases. When the oxidation state of a carbon
atom increases, the number of bonds between
carbon and oxygen increases or the number of
bonds between carbon and hydrogen decreases, or
both. Similar combustion reactions are carried
out in the body to produce energy. The substrates
are carbohydrates, lipids, and proteins rather
than hydrocarbons. Biological oxidation involves
a complicated multistep mechanism that allows
energy to be captured and stored.
41

Halogenation (Halogen Substitution)
Molecular halogens (F2, Cl2, Br2) react with
alkanes and cycloalkanes in the presence of
ultraviolet light to form alkyl halides.
In abbreviated form
In generalized form
42

Halogenation usually yields a mixture of products
due to sequential halogenation of the same
molecule or different structures are possible.
Note that the above reaction is unbalanced.
Unbalanced reactions are often used to describe
organic reactions because the major emphasis is
to describe the reactants, the reaction
conditions needed, and the types of products
formed.
Monohalogenation predominates if the hydrocarbon
is present in large excess. Polyhalogenation
predominates if the halogen is present in large
excess.
43

Naming Alkyl Halides
The prefixes fluoro-, chloro-, bromo- and iodo-
are added to an alkane to name the corresponding
halo-alkane. In the common nomenclature system,
the prefix is added to the name of the alkyl
group, followed by the suffix ide.