2.5 Drawing Three-Dimensional Molecules
The three-dimensional structure of ethane C2H6, has the shape of two
tetrahedra joined together. Each carbon atom is SP3 hybridized, with
four sigma bonds formed by the four SP3 hybrid orbitals. Dashed lines
represent bonds that go away from the viewer, wedges represent
bonds that come out toward the viewer, and other bond lines are in
the plane of the page. All the bond angles are close to 109.5°.
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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2.6 General rules of hybridization and geometry
Rule 1:
Both sigma bonding electrons and lone pairs can occupy hybrid
orbitals. The number of hybrid orbitals on an atom is computed by
adding the number of sigma bonds and the number of lone pairs of
electrons on that atom.
Rule 2:
Use the hybridization and geometry that give the widest possible
separation of the calculated number of bonds and lone pairs.
Summary of Hybridization and Geometry
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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Rule 3:
If two or three pairs of electrons form a multiple bond between two
atoms, the first bond is a sigma bond formed by a hybrid orbital. The
second bond is a pi bond, consisting of two lobes above and below the
sigma bond, formed by two unhybridized p orbitals. The third bond of
a triple bond is another pi bond, perpendicular to the first pi bond
Example 1:
Planar geometry of ethylene CH2=CH2. The carbon atoms in ethylene
are sp2 hybridized, with trigonal bond angles of about 120°.
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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Example:
Linear geometry of acetylene. The carbon atoms in acetylene are sp
hybridized, with linear (180°) bond angles. The triple bond contains
one sigma bond and two perpendicular pi bonds
Problem 1:
Predict the hybridization of the nitrogen atom in ammonia, Draw a picture of the three
dimensional structure of ammonia NH3, and predict the bond angles.
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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2.7 Bond Rotation
2.71 Rotation of Single Bonds
We can draw many structures for ethane CH3CH3, structures, differing only in rotations about
a single bond, are called conformations.
Rotation of single bonds
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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2.72 Rigidity of Double Bonds
Rotation about single bonds is allowed, but double bonds are rigid
and cannot be twisted
Because double bonds are rigid, we can separate and isolate
compounds that differ only in how their substituents are arranged
on a double bond.
Example: but-2-ene (CH3-CH=CH-CH3)
Two different compounds are possible, and they have different
physical properties
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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2.8 Isomerism
Isomers are different compounds with the same molecular formula.
2.81 Constitutional Isomerism
are isomers that differ in their bonding sequence
Example 1:
Example 2:
Example 3:
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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2.82 Stereoisomers
Are isomers that differ only in how their atoms are oriented in space
Cis-trans isomers are also called geometric isomers because they differ
in the geometry of the groups on a double bond
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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