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Chapter 7-7 - Stability of Alkenes
Organic Chemistry 307: Chapter 7 - Structure and Synthesis of Alkenes; Section 7 - Stability of Alkenes
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Organic Chemistry
Undergraduate 3
11/16/2011

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Term
Alkene energies are often compared by measuring the __________ __________ __________: the heat given off (delta H) during catalytic hydrogenation.
Heats of hydrogenation can be measured about as easily as __________ __________ __________, yet they are (smaller/larger) numbers and provide more accurate energy differences.
Definition
1) Heat of hydrogenation
2) Heats of combustion
3) Smaller
Term
When an alkene is treated with __________ in the presence of a platinum catalyst, __________ adds to the double bond, reducing the alkene to an __________.
Definition
1) Hydrogen
2) Hydrogen
3) Alkane
Term
Hydrogenation is mildly __________, evolving about 80 to 120kJ of heat per mole of __________ consumed.
Definition
1) Exothermic
2) Hydrogen
Term
The heat of hydrogenation is a measure of the energy content of the __________ bond.
Definition
1) Pi
Term
We can use __________ __________ __________ to compare the stabilities of different alkenes as long as they hydrogenate to give __________ of similar energies.
Definition
1) Heats of hydrogenation
2) Alkanes
Term
The __________ the heat of hydrogenation, the more stable the alkene.
Definition
1) Lower
Term
The __________ the heat of hydrogenation, the more unstable the alkene.
Definition
1) Higher
Term
The most stable double bonds in alkenes are those with the most __________ groups attached.
Double bonds with __________ alkyl groups hydrogenate to produce about 116-120kJ/mol.
Double bonds with __________ or __________ alkyl substituents further stabilize the double bond.
Definition
1) Alkyl
2) Two
3) Three
4) Four
Term
__________ __________ states that the more substituted double bonds are usually more stable.
Definition
1) Zaitsev's Rule
Term
__________ groups attached to the double-bonded carbons stabilize an __________.
Definition
1) Alkyl
2) Alkene
Term
Alkyl groups are __________-donating, and they contribute to __________ __________ to the __________ bond.
In addition, bulky substituents like __________ groups are best situated as far apart as possible; in alkenes, they are often separated by __________.
Definition
1) Electron
2) Electron density
3) Pi
4) Alkyl
5) 120 degrees
Term
In general, alkyl groups are separated best by the (least/most) substituted double bonds.
Definition
1) Most
Term
__________-__________ isomers are those that differ only in the position of the double bond.
Definition
1) Double-bond
Term
(Cis/trans) isomers are generally more stable than their corresponding (cis/trans) isomers.
This trend is reasonable because the alkyl substituents are separated farther in (cis/trans) isomers than they are in (cis/trans) isomers.
Definition
1) Trans
2) Cis
3) Trans
4) Cis
Term
A __________ difference between cis and trans isomers is typical for __________ alkenes.
Definition
1) 4kJ/mol
2) Disubstituted
Term
__________ is the least stable of the simple alkenes.
Definition
1) Ethylene
Term
Most cycloalkenes react like __________ alkenes.
The presence of a ring makes a major difference only if there is __________ __________, either because of a small __________ or because of a __________ double bond.
Definition
1) Acylic
2) Ring strain
3) Ring
4) Trans
Term
Rings that are __________-membered or larger can easily accommodate double bonds, and these cycloalkenes react much like __________-__________ alkenes.
__________ and __________ membered rings show evidence of ring strain, however.
Definition
1) Five
2) Straight-chain
3) Three
4) Four
Term
The double bond in cyclobutene has about __________ of extra ring strain by virtue of the __________ ring.
The __________ bond angles in cyclobutene compress the angles of the __________ hybrid carbons (normally 120 degrees) more than they compress the __________ hybrid angles (normally 109.5 degrees) in cyclobutane.
The extra __________ __________ in cyclobutene makes its double bond more reactive than a typical double bond.
Definition
1) 17kJ/mol
2) Small
3) 90 degree
4) sp2
5) sp3
6) Ring strain
Term
Cyclopropene has bond angles of about __________, compressing the bond angles of the carbon-carbon double bond to half their usual value __________.
The double bond in cyclopropene is highly __________.
Definition
1) 60 degrees
2) 120 degrees
3) Strained
Term
In acyclic alkenes, the __________ isomers are usually more stable; but the __________ isomers of small cycloalkenes are rare, and those with fewer than __________ carbon atoms are unstable at room temperature.
The problem with making a trans cycloalkene lies in the geometry of the __________ double bond; the __________ alkyl groups on a trans double bond are so far apart that several carbon atoms are needed to complete the ring.
Definition
1) Trans
2) Trans
3) Eight
4) Trans
5) Two
Term
*__________ __________ states that a bridged bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms.
Definition
1) Bredt's Rule
Term
Bredt's rule states that a bridged __________ compound cannot have a double bond at a bridgehead position unless one of the rings contains at least __________ carbon atoms.
Definition
1) Bicyclic
2) Eight
Term
A __________ compound is one that contains two rings.
The __________ __________ __________ are part of both rings, with three links connecting them.
A __________ __________ compound has at least one carbon atom in each of the three links between the bridgehead carbons.
Definition
1) Bicyclic
2) Bridgehead carbon atoms
3) Bridged bicyclic
Term
If there is a double bond at the __________ carbon of a bridged bicyclic system, one of the two rings contains a __________ double bond and the other must contain a __________ double bond.
Definition
1) Bridgehead
2) Cis
3) Trans
Term
In general, compounds that violate __________ __________ are not stable at room temperature.
Definition
1) Bredt's rule
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