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Chapter 2
HYDROCARBON FRAMEWORKS.
ALKANES
Chapter 2: Hydrocarbon Frameworks. Alkanes
LINEAR COMBINATION OF ATOMIC
ORBITALS (LCAO)
Atomic orbitals can combine via addition or subtraction
(hence, linear combination) and form:
• MOLECULAR ORBITALS, if the starting atomic orbitals
belong to different atoms
• HYBRID ORBITALS, if the starting atomic orbitals
belong to the same atom
GENERAL RULE: The number of new orbitals (molecular
or hybrid) is ALWAYS EQUAL to the number of starting
orbitals
Chapter 2: Hydrocarbon Frameworks. Alkanes
s- and s*- molecular orbitals
node
energy
antibonding
s*
1s
1s
atomic
orbital
atomic
orbital
s
bonding
molecular
orbital
Chapter 2: Hydrocarbon Frameworks. Alkanes
s-BONDS AND p-BONDS
1. s-bonds can form by the combination of:
a. Two s-orbitals:
+
s-orbital
s-orbital
s-bonding MO
b. Two p-orbitals:
+
p-orbital
p-orbital
s-bonding MO
c. One s- and one p-orbital:
+
s-orbital
Chapter 2: Hydrocarbon Frameworks. Alkanes
p-orbital
s-bonding MO
s-BONDS AND p-BONDS
2. p-bonds form via interaction of p-orbitals only.
+
p-orbital
Chapter 2: Hydrocarbon Frameworks. Alkanes
p-orbital
p-bonding MO
s-BONDS AND p-BONDS
IMPORTANT NOTES:
1. s-orbitals, due to their spherical symmetry, cannot
participate in the formation of p-orbitals.
+
equal amounts of positive
and negative overlap
+
+
+
+
-
-
2. The first bond between any pair of atoms is
ALWAYS a s-bond.
3. Only ONE s-bond can be formed between any pair of
two atoms. The rest (if any) are p-bonds.
Chapter 2: Hydrocarbon Frameworks. Alkanes
PREDICTIONS BASED ON ELECTRONIC
STRUCTURE OF CARBON
2px 2py 2pz
2s
C
2px 2py 2pz
promotion of
electron
2s
1. There is one 2s and three 2p orbitals, to form four bonds
total. It would be therefore expected that one of the bonds
would be different from the other three.
2. The 2s orbital is spherical (non-directional). The 2p
orbitals are perpendicular to each other. One would
therefore expect that the newly formed bonds would be at
a 90o angle.
BOTH PREDICTIONS ARE FALSE!!!!!
Chapter 2: Hydrocarbon Frameworks. Alkanes
HYBRIDIZATION AND MOLECULAR
SHAPES
121.7o
109.5o
116.6o
ethylene
planar
methane
tetrahedral
180.0o
acetylene
linear
Chapter 2: Hydrocarbon Frameworks. Alkanes
SEVERAL WAYS TO REPRESENT
METHANE
H
H
H
C
H
H
C
Chapter 2: Hydrocarbon Frameworks. Alkanes
H
sp3
C
H
H
H
sp3
H
sp3
H
H
IUPAC RULES
1. Form the base for the compounds’ name. This is
the name of the longest continuous carbon chain
(main chain) in the molecule.
Other groups
attached to the main chain are considered as
substituents. If two chains of equal length can be
found in a particular molecule, use the one that
gives a greater number of substituents.
2. Number the main chain, starting with the end,
which is nearest a substituent.
Chapter 2: Hydrocarbon Frameworks. Alkanes
CH3
H
C
H3C
CH2
RULE 1
CH3
H
C
CH2
HC
CH3
CH
H3C
CH3
H3C
H3C
H
C
CH2
CH2
HC
CH3
CH
CH3
CH3
7-atom main
chain (yellow
color), three
substituents incorrect!!
1
H3C
RULE 2
H3C
3
H
C
7-atom main
chain (yellow
color), four
substituents correct!!
CH3
7
2
CH2
H
C
CH
4
5
HC
6
7
CH3
CH2
CH3
CH3
5-ethyl-3,4,6-trimethylheptane
incorrect!!
Chapter 2: Hydrocarbon Frameworks. Alkanes
CH3
H
C
H3C
H3C
5
H
C
CH3
6
CH2
H
C
CH
4
3
HC
2
1
CH3
CH2
CH3
CH3
3-ethyl-2,4,5-trimethylheptane
correct!!
IUPAC RULES (Contd.)
3. The substituents names are derived in the same fashion as
the parent hydrocarbons, but instead of suffix –ane, put a
suffix –yl. Assign numbers to the substituents matching their
positions on the main chain.
The substituents also take prefixes, whenever
necessary, to differentiate between different isomeric forms.
a) n-substituents (n-alkyl groups);
b) iso-substituents;
Both a) and b) have primary (1o) carbon atoms attached to the
main chain. They are primary alkyl groups.
c) sec-substituents;
The carbon atom attached to the main chain is secondary (2o).
The alkyl group is a secondary alkyl group.
d) tert-substituents (t-substituents);
The carbon atom attached to the main chain is tertiary (3o).
The alkyl group is a tertiary alkyl group.
Chapter 2: Hydrocarbon Frameworks. Alkanes
IUPAC RULES (Contd.)
4. Write the name as a single word, using hyphens to
separate the different prefixes and using commas to
separate the numbers.
When two or more
substituents are present, list the substituents in
alphabetical order. If some of them are identical, list
them together with a prefix identifying their total
number: di- (two), tri- (three), tetra- (four), penta(five), etc. Do not alphabetize these prefixes!!
5. When a more complex substituent is encountered,
one may need, for the naming of the substituent, to
apply the entire procedure outlined above, i.e. select
a main chain, etc. But as usual, the substituent as
whole should always bear the suffix –yl, instead of –
ane. The carbon directly bound to the main chain
must be in the chain selected for the base of the name
of the substituent! It is always number 1.
Chapter 2: Hydrocarbon Frameworks. Alkanes