Hydrocarbons
Aromatic
Aliphatic
Straight chain
Saturated
Alkanes
Cyclic
Unsaturated
Alkenes
Alkynes
Cyclic
Bonding in Organic Compounds
In organic compounds,
• carbon has 4 valence electrons and hydrogen has 1
• carbon achieves an octet by forming four bonds, with each
line representing 2 electrons
Representation Of Molecular Formulae
5
Expanded, Condensed, and Skeletal Structures
A hexane molecule can be
represented in several ways:
• a molecular formula
• a ball-and-stick model
• an expanded structural formula
• a condensed structural formula
• a skeletal formula
ATOMIC ORBITALS
‫المدارات الذرية‬
S-Orbital
P-Orbital
The three 2p orbitals are orientated perpendicular to each other
d-Orbital
Hybridization
We can explain the tetrahedral geometry of methane by
imagining that the 2s orbital and the three 2p orbitals of
carbon hybridize to form four identical sp3 hybrid orbitals
We can divide the hybridization process into two steps.
1. First, an electron in the 2s orbital is “promoted” to a
vacant 2p orbital to produce an excited state of carbon. The
2s orbital in this state has only one electron.
2. Second, the half-filled 2s orbital and the three half-filled
2p orbitals mix to form new sp3 hybrid orbitals (pronounced
“s-p-three,” not “sp cubed”).
sp3Hybridized
Carbon Atom
(a) The original set of four atomic orbitals on carbon are mixed, or
hybridized, to give four new sp³-hybridized atomic orbitals.
(b ) We have represented the new hybrid orbitals with a new color to
emphasize the notion that the hybrid orbitals replace the original
unhybridized orbitals.
sp3-Hybridized
Carbon in Methane
Bonding and Structure of Ethane
and Conformations of Ethane
(a) The bonding molecular orbital
of the C—C bond in ethane is
cylindrically symmetrical.
(b) Rotation of the two methyl
groups about the C—C bond axis
maintains the bond, but changes the
relative positions of the C—H bonds.
sp2 hybridization of carbon in ethene
Each carbon atom in ethene is bonded to three other atoms, we need
three s bonds. We hybridize carbon by “mixing” a 2s orbital and
two 2p orbitals to obtain three sp2 hybrid orbitals (pronounced “s-ptwo”). The third 2p orbital remains unchanged. The three sp2 hybrid
orbitals have the same shapes and energies. The orbitals differ only
in their position in space. They lie in a plane and are directed to the
corners of an equilateral triangle—therefore separated by 120°—to
achieve maximum separation of the electrons
In contrast to the rotation that occurs around the C—C bond of
ethane, no rotation occurs around the C=C bond of ethylene. Rotation
around the C=C internuclear axis would not disrupt the sp2–sp2 σ
bond. However, this motion would destroy the overlap of the two 2p
orbitals and break the π bond.
sp hybridization of carbon in ethyne
We next consider the bonding in ethyne, a molecule in which each
carbon atom is bonded to two atoms by s bonds. Let’s suppose
that is composed of two methine (CH) units. Since each carbon
atom of acetylene is connected to two other atoms, a carbon atom
and a hydrogen atom, each methine unit requires two π bonds.
Hybridization
3
sp
sp2
sp
• Tetrahedral
• Trigonal Planar
• Linear
 Alkanes are the hydrocarbons of aliphatic row.
General formula of Alkanes CnH2n+2
alkane molecules only contain C—C and C—H Bonds
Alkanes are hydrocarbons in which all the bonds are single
covalent bonds (-bonds).
 Alkanes are called saturated hydrocarbons.
The simplest one, methane (CH4), is also the most
abundant.
Cyclic alkanes have the formula CnH2n
Cycloalkanes
• are cyclic alkanes
• have two less hydrogen atoms than the open chain
• are named by using the prefix cyclo before the name of
the alkane chain with the same number of carbon atoms
propane, C3H8
cyclopropane, C3H6
Practice Exercise: Circle each molecule
that can be classified as an alkane
 Alkanes can have either simple (unbranched) or
branched carbon chain.
 Alkanes with unbranched Carbon chain are called
normal or n-alkanes.
 In the molecules of alkanes all carbon atoms are in
the state of sp3-hybridization.
 The distance between two Carbon atoms is 0.154
nm, but the distance between two atoms of Carbon
and Hydrogen is 0.110 nm.
 The rotation can take place around C—C bonds.
 As the result of this rotation the molecule have
different conformations (spatial forms).
Bond lengths of Ethyne, Ethene,
and Ethane
Alkanes can be represented in several ways.
Primary through Quaternary Carbons
Hydrogens are also referred to as 1º, 2º or 3º according to the
type of carbon they are bonded to group of alkyl.
Primary through Quaternary Carbons
Alkanes and Isomers
Isomers are compounds with the same molecular
formula but different structural formula
…differ in how the atoms are connected (linkages )
or
…how the atoms are arranged in space (spatial) .
ex. 19.1 write the structural formulas for three isomers of pentane
Alkanes are characterized by structural and optical isomer.
 Structural(Constitutional) isomer is formed by different
sequence of carbon atom connections (isomer of the
carbon chain).
e.g. methane is the only alkane of molecular formula CH4,
ethane the only one that is C2H6, and propane the only one
that is C3H8.
compounds with the same molecular formula but different
structures
 Beginning with C4H10 (Butane) constitutional isomers are
possible; two alkanes have this particular molecular formula.
 In one, called n-butane, four carbons are joined in a continuous
chain.
 The second isomer has a branched carbon chain and is called
isobutane.
TYPES OF ISOMERISM
CHAIN ISOMERISM
STRUCTURAL ISOMERISM
Same molecular formula but
different structural formulae
POSITION ISOMERISM
FUNCTIONAL
GROUP ISOMERISM
GEOMETRICAL ISOMERISM
STEREOISOMERISM
Same molecular formula
but atoms occupy different
positions in space.
Occurs due to the restricted
rotation of C=C double bonds...
two forms - CIS and TRANS
OPTICAL ISOMERISM
Occurs when molecules have a
chiral centre. Get two nonsuperimposable mirror images.
Chain
isomerism
Structural
isomerism
Different
functional groups
Position
isomerism
Different
carbon skeletons
Functional groups
in different positions
Functional group
isomerism
Structural Isomers
CH3
O CH3
and
CH3
CH2
OH
CH3
and
CH3
31
Chain isomerism
Position isomers
position isomers
position isomers
CH3
O CH3
and
CH3
CH2
CH3
and
CH3
OH
1
2-methylbut-1-ene
2
2-methylbut-2-ene
2
3
1
3-methylbut-1-ene
Functional group has a higher priority than branches
Same linking order of
atoms
Geometrical
isomerism
Stereoisomerism
Different spatial
(‫)مكاني‬
arrangements of
atoms
Optical isomerism
Geometrical Isomerism
Compounds with the same covalent arrangement
but different arrangement around a carboncarbon double bond
Cis-trans isomers must be two different groups
on the sp2 carbon.
e.g. trans-but-2-ene and Cis-but-2-ene
trans
cis
Both substituent groups are
on the same side w.r.t. the
axis of the C=C double
bond
Both substituent groups are
on the opposite sides
w.r.t. the axis of the C=C
double bond
cannot be inter-converted
at lower temperatures
H
H3C
H
C C
H
No cis-trans isomers possible
maximum overlap of
pz orbitals
minimum overlap of
pz orbitals
Rotation about the axis of a double bond
through an angle of 90o results in the
breaking of the π bond
Are these isomers?
If they are isomers, then what
type of isomers are they?
E/Z notation
It is an extension of cis /trans notation (which only
describes relative stereochemistry) that can be used to
describe double bonds having three or four substituents
If there are three or four different groups
attached to the C atoms of C=C double bond
E/Z notation rather than the cis/trans notation is
used to name the stereoisomers of a molecule.
E : in opposition to  trans
Z : together (same side)  cis
2
pent-2-ene
trans
(2E)-pent-2-ene
Geometrical Isomerism
cis
(2Z)-pent-2-ene
In simple cases, such as 2-butene, Z corresponds to cis and E to
trans. However, that is not a rule
The real advantage of the E-Z system is that it will always work. In
contrast, the cis-trans system breaks down with many ambiguous
cases. Example...
1-bromo-2-chloro-2-fluoro-1-iodoethene.
Drow five constitutional isomers with the same chemical
formula as heptane C7H16. Use condensed structure
formula