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Naming Alkenes
• Same steps as naming alkanes, with some modifications
• Here are the modifications to the steps:
1. Find the “parent chain”
a) The longest continuous chain of carbons that contains the functional group (C=C)
b) The suffix for alkenes is “-ene”
c) If there are multiple C=C, the suffix becomes -diene, -triene, -tetraene, etc.
(e.g. butadiene, heptatriene, nonatetraene)
2. Number the atoms in the parent chain
a) Begin at the end nearer to the functional group (C=C)
b) Use branching to break ties, and alphabetical order if branching does not help
3. Identify and number the substituents
a) Treat alkyl groups and halogens just as they were for alkanes
b) The position of the alkene must also be given an address, unless it is on the first
carbon - then it is optional (i.e. hexane = 1-hexene)
4. Write names as a single word (no spaces)
a) The position of the C=C is listed just before the parent (e.g. 2-methyl-2-pentene)
5. For cycloalkenes, the C=C is always between C1 and C2; the first substituent gets the
lowest possible number
6. Ethene and propene are more commonly called “ethylene” and “propylene”
• Examples:
Alkene Electronic Structure
Hybrid Orbital Model
- sigma bonds are formed from the carbon’s sp2 hybrid orbitals
- pi bonds form from aligned, unhybridized “p” orbitals
"p" Atomic Orbitals
(" bond)
H
H
H
H
Hybrid Orbitals
(! bonds)
Molecular Orbital Model
- hybrid orbitals explain the sigma bond behavior well
- molecular orbitals will be used to discuss pi bonds
- like hybrid theory, atomic orbitals are mixed to form new orbitals (in
this case, the new orbitals are molecular orbitals)
- In this case, orbitals from different atoms are mixed
- The number of MOs formed is equal to the number of atomic
orbitals mixed (just like hybrid theory)
- The atomic orbitals are mixed in an additive and subtractive fashion
(for subtractive, change the “sign” of each region in the orbital
before mixing)
pi antibonding MO (π*)
a result of subtractive mixing
+
Opposite-signed regions avoid each other
(I have no 80s refernce for this)
2p
C
C 2p
original atomic
orbital
original atomic
orbital
pi bonding MO (π)
a result of additive mixing
+
Like-signed regions "melt" into each other
(Cue Modern English song from "Valley Girl")
molecular
orbitals
Cis-Trans Stereoisomers
Because the pi bond is formed from aligned “p” orbitals, rotation about the C=C is not allowed
cis-2-butene
two groups are on
same side of C=C
trans-2-butene
two groups are on
opposite sides of C=C
E/Z Stereoisomers
Cis and trans labels only apply to situations where there are two groups attached to the C=C
(one off each carbon).
When there are more than two, a more robust naming system is necessary. Here is how you
determine if an alkene has an “E” (Entgegen = apart) or “Z” (Zussamen = together) configuration.
Use the Cahn-Ingold-Prelog priority rules to rank the two substituents on each C of the C=C
as being “High” or “Low” priority
compare
compare
Low
Low
Low
High
High
High
High
Z
High priority groups
are on the same side
a)
b)
c)
d)
Low
compare
compare
E
High priority groups
are on opposite sides
Atoms with higher atomic number are assigned a higher priority
In the case of a tie, move one atom further away from the C=C and compare again
Treat pi bonds as additional connected atoms
The E or Z label is placed in front of the name [e.g. (E)-2-butene]
Some examples are on the next page.
Example 1)
High
Cl beats H
Cl
CH3
Low
H
What is attached to this C (aside from the C=C)?
H
H
H
Low
E
CH2CH3
What is attached to this C (aside from the C=C)?
C
H
compare
H
High
This C "beats" the highest priority atom from above
(E)-1-chloro-2-methyl-1-butene
Example 2)
2
1
Attached to this C:
O
O
C
Attached to this C:
C
C
C
O
High
3
Attached to this C:
C
C
C
O beats C
Attached to this C:
O
C
H
Z
HO
High
still
tied
C beats H
3
2
Attached to this C:
C
C
C
Attached to this C:
C
C
H