Chapter 2 HYDROCARBON FRAMEWORKS. ALKANES

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Chapter 2HYDROCARBON FRAMEWORKS. ALKANES
Chapter 2 Hydrocarbon Frameworks. Alkanes
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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

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s- and s- molecular orbitals
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s-BONDS AND p-BONDS

• s-bonds can form by the combination of
• Two s-orbitals

b. Two p-orbitals
c. One s- and one p-orbital
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s-BONDS AND p-BONDS
2. p-bonds form via interaction of p-orbitals
only.
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s-BONDS AND p-BONDS

• IMPORTANT NOTES
• s-orbitals, due to their spherical symmetry,
  cannot participate in the formation of p-orbitals.

• 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.

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PREDICTIONS BASED ON ELECTRONIC STRUCTURE OF
CARBON

• 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.
• 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!!!!!

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HYBRIDIZATION AND MOLECULAR SHAPES
121.7o
109.5o
116.6o
ethylene planar
methane tetrahedral
180.0o
acetylene linear
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SEVERAL WAYS TO REPRESENT METHANE
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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.
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RULE 1
RULE 2
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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.
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IUPAC RULES (Contd.)

1. 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!!
2. 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