Hydrocarbons
Aromatic
Aliphatic
Straight chain
Saturated
Alkanes
Cyclic
Unsaturated
Alkenes
Alkynes
Cyclic
Representation Of Molecular Formulae
4
 Alkanes are the hydrocarbons of aliphatic row.
General formula of Alkanes CnH2n+2
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 alkanes in which all or some of
the carbon atoms are arranged in a ring
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).
The Structure Of Alkanes
In alkanes, the four sp3 orbitals of
carbon repel each other into a
TETRAHEDRAL arrangement
with bond angles of 109.5º.
Each sp3 orbital in
carbon overlaps with
the 1s orbital of a
hydrogen atom to
form a C-H bond.
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109.5º
ATOMIC ORBITALS
‫المدارات الذرية‬
S-Orbital
P-Orbital
The three 2p orbitals are orientated perpendicular to each other
d-Orbital
The length of the band: 1.54 A
Hybridization
3
sp
sp2
sp
• Tetrahedral
• Trigonal Planar
• Linear
sp3 hybridization in Methane
sp3 Hybrid Orbitals
• 4 VSEPR pairs – 1 s and 3 p orbitals
• Tetrahedral e- pair geometry
• 109.5° bond angle
2
sp
hybridization in Ethene
sp2 Hybrid Orbitals
3 VSEPR pairs – 1 s and 2 p orbitals
Trigonal planar e- pair geometry
120° bond angle
sp hybridization in Ethyne
sp Hybrid Orbitals
2-VSEPR pair -1S and 1P orbital –2sp hybridized
orbital
Linear electron pair geometry
180° bond angle
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 two or more molecules with
the same molecular formula but …..
…differ in how the atoms are connected (linkages )
or
…how the atoms are arranged in space (spatial) .
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 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.
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
30
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?
2
pent-2-ene
trans
(2E)-pent-2-ene
Geometrical Isomerism
cis
(2Z)-pent-2-ene
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
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