Lack of rotation leads to __. Same side: __. Different side: __.
cis-trans isomerism (geometric isomerism)
cis
trans
True or False:
Not at all alkenes are capable of showing cis-trans isomerism.
True
If either carbon of the double bond holds two identical groups, it is __.
not cis trans
__ are unstable unless the ring is large enough (at least eight atoms) to accommodate the __. Therefore, all __ are assumed to be __ unles they are specifically named __. The __ name is rarely used with cycloalkenes, except to distinguish them.
trans cycloalkenes
trans double bond
cycloalkenes
cis
trans
cis
Cis- trans is too __. SO, we use __.
ambiguious
E-Z system
Steps of E-Z nomenclature:
1) mentally separate the double bonds
2) assign first and second priorities to the two groups
What is E and what is Z?
E: two first-priority are on opposite sides
Z: two first-priority atoms are together on the same side
Alkenes can be used as __(3)__ and other valuable chemicals.
polymers
drugs
pesticides
A __ is a molecule made up of many __ molecules. An alkene monomer can __ by a chain reaction where additional alkene molecules add to the end of hte growing polymer chain. Becuase these polymers result from addition of many individual alkene units, they are called __. __ are polymers made from monofunctional alkenes such as ethylene and propylene.
polymer
monomer
polymerize
addition polymers
polyolefins
How can stabilities be compared?
Alkene energies are often compared by measuring the __: the heat given off during catalytic hydrogenation
by converting different compounds to a common product and comparing the heat given off
heat of hydrogenation
When an alkene is treated with H in the presence of a __, H adds to the double bond, doing what?
Hydrogenation is __.
The difference in stability of cis and trans is the difference in the __.
platinum catalyst
reducing the alkene to an alkanemildly exothermic
heat of hydrogenation
True or Mast: most alkenes with similar substitution patterns give similar heats of hydrogenation.
true
In practice, we can use __ to compare the stabilities of different alkenes as long as they hydrogenate to give __.
heats of hydrogenation
alkanes of similar energies
The most stable bonds are those with __. This is called __. In other words, the __ attached to the double-bonded carbons stabilize the alkene.
the most alkyl groups attached
Zaitsev's rule
alkyl groups
Two factors are probably responsible for hte stabilizing effect of alkyl groups on a double bond. What is the first?
alkyl groups are electron-donating, and they contribute electron density to the pi bond.
Two factors are probably responsible for hte stabilizing effect of alkyl groups on a double bond. What is the second?
bulky substituents like alkyl groups are best situated as far apart as possible.
In an alkane, the bulky substitutensts are separated by the __. A double bond increases this separation to about __.
tetrahedral bond angle, about 109.5
120
In general, __ are separated best by the most highly substituted double bond. This steric effect is illustrated for two __ (isomers that differ only in the position of the double bond). The isomer with the monosubbed double bond separates the alkyl groups by only __, while the trisubbed double bond separates them by about __.
alkyl groups
double-bond isomers
109.5
120
What do heats of hydrohalogenation show? Why is this reasonable?
trans are most stable than cis
alkyl substituents are separaetd farther in trans than they are in cis
Most cycloalkenes react like __. The presence of a ring makes a major difference only if there is __, either because of a small ring or because of a trans double bond. rings that are __ or larger can easily accommodate double bonds, and these cycloalkenes react much like straight- chain alkenes. __ and __ show evidence of ring strain, however.
acyclic (noncyclic) alkenes
ring strain
5-membered
3 and 4-membered rings
Which has a higher heat of dehydrogenation: cyclobutene or cyclopentene
Why?
cyclobutene
extra ting strain by virtue of the small ring
Another difference between cyclic and acyclic alkenes is the relatinship between __ and __. In acyclic alkenes, the __ are usually more stable; but the __ of small cycloalkenes are rare, and those with fewer than 8 are usually unstable at room temp.
cis and trans isomers
trans
trans isomers
What is the problem with making a trans cycloalkene?
lies in the geometry of the trans double bond; the alkyl groups are so far apart that several carbon atoms are needed to complete the ring
Cyclohexene is too strained to be isolated, but __ can be isolated at low temps. __ is stable at room temp, although its cis isomer is still more stable.
trans-cyclohexene
trans cyclooctene
Bredt's Rule
a bridged bicyclic compound cannot have a double at a bridgehead position unless one of the rings contains at least eight carbon atoms
bicyclic
bridged carbon atoms
bridged bicylic
one that contains two rings
part of both rings, with three links connecting them
at least one C atom is in each of the three links between teh bridgehead carbons
If there is a double bond at the bridgehead carbon of a bridged bicyclic system, then what?
one of the two rings contains a cis double bond adn the other must contain a trans double bond
If the larger ring contains a t least eight carbon atoms, then what?
it can contain a trans double bond and the bridgehead double bond is stable
In general, compounds that violate Bredt's rule are __.
