Introduction to Unsaturated Hydrocarbons
Alkenes & Alkynes
Hydrocarbons
• The simplest
organic molecules
containing only
carbon and hydrogen
atoms
• Four basic types:
Alkanes
Alkenes
Alkynes
Aromatic
hydrocarbons
Alkenes
- Contain at least one carbon–carbon double bond.
- Unsaturated.
Have fewer than maximum number of hydrogens.
CnH2n
Properties Similar to alkanes
• Non-polar
• Insoluble in water
• Soluble in non-polar solvents
• Less dense than water
Table 12-2, p.388
Properties of Alkenes
• Notice that geometric isomerism exists for alkenes when
the double bond is not on a terminal carbon.
• To determine the cis-/trans- isomerism, draw a line through
the bond and identify same or different
“same”
“different”
• Structure also affects physical properties of alkenes.
•Which
isomer
would you
predict to
be more
stable, and
why?
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Naming Alkenes
– Step 1: Name the longest chain that contains the
C=C bond. Use the IUPAC root and the –ene
ending.
– Step 2: Number the longest chain so the C=C bond
gets the lowest number possible.
– Step 3: Locate the C=C bond with the lowestnumbered carbon.
•cis- alkenes have carbons in chain on same side of
molecule.
•trans- alkenes have carbons in chain on opposite
side of molecule.
– Step 4: Locate and name attached groups.
– Step 5: Combine all the names as you did with
alkanes.
2.
1.
3.
–
The Geometry of Alkenes
–In C=C bonds, carbon atoms undergo sp2
hybridization in which the s orbital and two
of the p orbitals mix to form bonding orbitals
with one electron left in an unhybridized 2p
orbital.
4.
The result is a bonding arrangement with 3
σ bonds having 120o bond angles and an
unhybridized p orbital that can form a Π
bond.
–This gives a planar shape for the sp2 bonding
orbitals with the unhybridized p orbital
perpendicular to the plane of the three sp2
hybridized orbitals.
Sp2 hybridized
orbitals
1/3 s character
2/3 p character
2
–The planar geometry of the sp2 hybrid
orbitals and the ability of the 2p electron to
form a “pi bond” bridge locks the C=C bond
firmly in place.
Alkynes
5. Draw the hybridization for the first three
carbons of the pentene molecule.
Alkynes
– Ethyne (commonly called acetylene) is the
simplest alkyne and is used as a fuel for torches
and in making plastics.
• Contain at least one carbon–carbon triple
bond.
• Carbons in triple bond sp-hybridized and have
linear geometry.
• Also unsaturated.
• CnH2n-2
– Alkynes are named in exactly the same ways as
alkenes, except the ending –yne is used.
8.
Examples:
3
The Geometry of Alkynes
–In C≡
≡C bonds, carbon atoms undergo sp
hybridization in which the s orbital and one
of the p orbitals mix to form bonding orbitals
with one electron left in two unhybridized 2p
orbitals.
–
The result is a bonding arrangement with 2
σ bonds having 180o bond angles and two
unhybridized p orbitals that can form a Π
bonds.
Sp hybridized
orbitals
1/2 s character
1/2 p character
–This gives a linear shape for the sp bonding
orbitals with the unhybridized p orbitals
perpendicular to the plane of the two sp
hybridized orbitals.
–Notice, no rotation along the sigma bond
and no possible geometric isomerism.
4