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Double bonds that aere separated by two or more single bonds are called __.
When a molecule has two isolated double bonds, the heat of hydrogenation is __.
For conjugated dienes, the heat of hydrogenation is __.
close tot eh sum of the heats of hydrogenation for the individual double bonds
less than the sum for the individual double bonds
What happens if two double bonds are even closer togeter than in the conjugated case? These are called __.
The heat of hydrogenation of these is __, thus making them __.
successive double bonds with no intervening single bonds are called cumulated double bonds
larger than any other dienes
less stable than isolated and conjugated double bonds
When a molecule is more stable than another, that extra energy is __, also called __.
The pi bonds in 1,3-dienes are__.
Why are pi bonds shorter?
The electrons in the double bonds are __ over the entire molecule, creating some pi overlap and pi bonding in the C2-C3 bond. The length of this bond is __ between the normal length of a single bond and that of a double bond.
- resonance energy
- conjugation energy
because they have more s character
All four carbon atoms of buta-1,3-diene are __.
In __, the lobes that overlap in the bonding region betewen the nuclei are in phase; they have the same sign. This is __.
sp2 hybridized and in the planar conformation all have overlapping p orbitals
- bonding MO
- constructive overlap
In the pi __, lobes of opposite phase overlap in the bodning region called __, thus cancelling of the wave function in the bonding region. Midway between the nuclei, this antibonding MO has a __: a region of zero electron density where positive and negative cancel out.
- antibonding MO
- destructive overlap
Electrons have lower energy in the __ than in the original p orbitals, and higher energy in the __. Stable molecules tend to have __ and __.
- bonding MO
- antibonding MO
- filled bonding MOs and empty antibonding MOs
The number of MOs is always the same as __. These MOs have energies that are symmetrically distributed __ and __ the energy of the starting p orbitals. Half are __ and half are __.
- the number of atomic orbitals used to form the MOs
- bonding MOs
- antibonding MOs
Why is the lowest-energy orbital exceptionally stable?
- 1) There are three bonding itneractions
- 2) the electrons are delocalized over four nuclei
The second molecular orbital (pi2) of butadiene has one __ in the center of the molecule. THis MO represents the classic picture. What is the classic picture of a diene?
there are bonding interactions at the C1-C2 and C3-C4 bonds, and a (weaker) antibonding interaction betwee C2 and C3
The pi orbital has __ and __, so we expect it to be a __. It is not as strongly bonding nor as low in energy as the __. Adding and subtracting bonding and antibonding interactions is not a reliable method for calculating energies of molecular orbitals, but it is useful for predicting whether a given orbital is bonding or antibonding and for __.
- two bonding interactions and one antibonding interaction
- bonding orbital
- all-bonding pi1 orbital
- ranking orbitals in order of their energy
The third butadiene MO (pi3*) has __. There is a bonding interaction at the C2-C3 bond, and there are two antibonding interactions one between C1 and C2 and the other between C3 and C4. This is an __ and it is __.
- two nodes
- antibonding orbital (*)
- vacant in the ground state
The fourth, and last, MO (pi4*) of buta-1,3-diene has __ and is totally __. This MO has __ and is unoccupied in the molecule's ground state. This __ is typpical: For most systems, the highest-energy MO has antibonding interactions between all pairs of adjacent atoms.
- three nodes
- totally antibonding
- highest energy
- highest energy MO
Each MO can accommodate __, and the __ are filled first. Therefore, the four pi electrons go into __ and __.
- two electrons
- lowest-energy MOs
- pi1 and pi2
The partial double bond character between C2 and C3 in buta-1,3-diene explains why the molecule is most stable in the __. There are actually __ that allow overlap between C2 and C3. These conformation sarise by __ about the C2-C3 bond, and they are considered single-bond analogues of trans and cis isomers about a double bond. Thus, they are named __ and __.
- planar conformation
- two planar conformations
The __ is more stablethan the __.
What are some common intermediates of conjugated compounds? Describe how they are the intermediate.
- allylic systems, particularly allylic cations and radicals
- They are stabilized by delocalizeation
What is the allyl group?
When allyl bromide (CH2=Ch-CH2Br) is heated with a good ionizing solvent, it ionizes to the __, an allyl group with a positive charge. More-substituted analogues are called __ which are stabilized by __ with the adjacent double bond, which delocalizes the positive charge over two carbon atoms.
- allyl cation
- allylic cations
Because of its resonance stabilization the primary allyl cation is __ a simple secondary carbocation. Most subbed allylic cations have at least one __ bearing part of the positive charge. They are about as stable as __.
- about as stable as
- secondary carbon atom
- simple tertiary carbocations
What is the order of stability of carboncations?
H3C+ < primary < secondary < allyl < tertiary < subbed allylic
__ usually involve allylic cations as intermediates. Unlike simple carbocations, an allylic cation can react with a __ at __.
- electrophilc additions
- either of its positive centers
The first product results from electrophilic addition across a double bond, called __.
In the second product, the proton and the bromide ion add at the ends of the conjugated system to carbon atoms with a __, called __.
- 1,4-relationship 1,4-addition
In these additions, the __ is the electrophile, adding to the alkene to give the __, which usually occurs through __.
- most stable carbocation
- resonance delocatlization of the positive charge over two carbon atoms
Attack of the bromide (or other) in an addition reaction at the secondary carbon gives __; attack at the primary carbon gives __.
What is the mechanism for 1,2- and 1,4- addition of a conjugated diene?
1: Protonation of on eo fthe double bonds forms a resonance-stabilized allylic cation
2: a nucleophile attacks at either electrophilic carbon atom
The key to formation of these two products is the presence of __.
a double bond in position to form a stabilized allylic cation
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