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Chapter 7
Alkenes and Alkynes
• CHP6 Problems: 6.1-13, 16-34, 36.
• CHP7 Problems: 7.1-23, 25-28, 31-34, 37-39, 41-47,
49-56.
Alkenes and Alkynes
Alkene (or “olefin”)
• Hydrocarbon that contains a carbon-carbon double
bond
• Present in most organic and biological molecules
• β-Carotene
Alkyne
• Hydrocarbon that contains a carbon-carbon triple bond
• Rarely occur in biological molecules or pathways
Alkenes and Alkynes
Ethylene and propylene
are the two most
important industrially
produced organic
chemicals
• Produced by
“cracking” C2-C8
alkanes upon heating
to temperatures up to
900 oC.
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7.1 Calculating a Degree of Unsaturation
Because of its double bond an alkene has fewer
hydrogens than an alkane with the same number of
carbons and is therefore referred to as unsaturated
Alkene
CnH2n
C2H4
Alkane
CnH2n+2
C2H6
Calculating a Degree of Unsaturation
Degree of unsaturation
• Number of rings and/or multiple bonds present in the
molecule
•
•
•
Unknown hydrocarbon of formula C6H10with molecular weight
of 82 has two fewer pairs of hydrogens (H14 – H10 = H4 = 2H2)
than a saturated hydrocarbon
Degree of unsaturation is two
Possible structures for unknown
Calculating a Degree of Unsaturation
Calculations for compounds containing other elements
in addition to carbon and hydrogen:
• Organohalogen compounds (C,H,X, where X = F, Cl, Br,
or I)
•
Add number of halogens and hydrogens to arrive at an
equivalent hydrocarbon formula
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Calculating a Degree of Unsaturation
• Organooxygen compounds (C,H,O)
• Oxygen does not affect the formula of an equivalent
hydrocarbon
• Ignore the number of oxygens
Calculating a Degree of Unsaturation
• Organonitrogen compounds (C,H,N)
• Has one more hydrogen than a related hydrocarbon
• Subtract the number of nitrogens from the number of
hydrogens for equivalent hydrocarbon formula
7.2 Naming Alkenes and Alkynes
•
Naming Alkenes
Similar to the naming rules for alkanes
• Step 1 - Name the parent hydrocarbon
• Find longest carbon chain containing the double bond
•
•
Step 2 – Number the carbon atoms in the chain
• Double-bond carbons should receive lowest possible numbers
• Begin at end nearer first branch point if double bond is equidistant
from the two ends
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Naming Alkenes and Alkynes
• Step 3 - Write the full name using suffix -ene in place of -
ane
•
If more than one double bond indicate position and use
suffixes -diene, -triene, and so on
Naming Alkenes and Alkynes
•
Older naming system still in use which places the locant, or
number locating the position of the double bond, at the
beginning of the name
Naming Alkenes and Alkynes
• Cycloalkenes
• Number cycloalkene so double bond is between C1
and C2 and first substituent has lowest number
possible
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Naming Alkenes and Alkynes
Common names that are often used and are recognized
by IUPAC:
Naming Alkenes and Alkynes
Alkynes
• Named just like alkenes using suffix -yne
• Number main chain so triple bond receives as low a
number as possible
Alkyl, alkenyl, and alkynyl groups:
7.3 Cis-Trans Isomerism in Alkenes
•
•
Carbon-carbon double bond description
Valence bond language
• Carbons are sp2 hybridized
• Three equivalent hybrid orbitals that lie in a plane at angles of
120º to one another
• Carbons form a σ bond by head-on overlap of sp2 orbitals and a p
bond by sideways overlap of unhybridized p orbitals oriented
perpendicular to sp2 plane
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Cis-Trans Isomerism in Alkenes
•
Molecular orbital language
• Interaction between p orbitals leads to one bonding and one
antibonding molecular orbital
•
bonding MO contains no node between nuclei
•
antibonding MO contains a node between nuclei resulting from
combination of lobes with different algebraic signs
Cis-Trans Isomerism in Alkenes
Free rotation is not possible around a double bond
• The barrier to double bond rotation must be at least as great
as the strength of the
bond itself (~ 350 kJ/mol)
Cis-Trans Isomerism in Alkenes
Disubstituted alkene
•
•
Two substituents other than hydrogen are attached to the double-bond
carbons
But-2-ene
• Two isomers cannot interconvert spontaneously
• Methyl groups are cis- or trans- to each other
• Bond rotation cannot occur – the two but-2-enes are cis-trans
stereoisomers
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Cis-Trans Isomerism in Alkenes
Cis-trans isomerism occurs when both double-bond
carbons are bonded to two different groups
• Carbons bonded to two identical groups cannot exist as cis-
trans isomers
7.4
Alkene Stereochemistry and the E,Z
Designation
•
•
E,Z system
Sequence rules used to assign priorities to the substituent groups on
the double-bond carbons (alkenes)
• E double bond
• For German entgegen meaning “opposite”
• Higher priority groups on each carbon are on opposite sides of
•
double-bond
•
Z double bond
• For German zusammen meaning “together”
• Higher priority groups on each carbon are on same side of doublebond
Alkene Stereochemistry and the E,Z Designation
Cahn-Ingold-Prelog sequence rules
• Rule 1 – look at the two atoms directly attached to each
double-bond carbon and rank them according to atomic
number
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Alkene Stereochemistry and the E,Z Designation
• Rule 2 – If a decision cannot be reached by ranking the
first atom in the substituent, look at the second, third, and
fourth atoms away from the double-bond carbons until the
first difference is found
Alkene Stereochemistry and the E,Z Designation
• Rule 3 – multiple bonded atoms are equivalent to the same
number of single-bonded atoms
Worked Example 7.1
Assign E and Z Configurations to Substituted
Alkenes
Assign E or Z Configuration to the double bond in the
following compound:
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