Resonance Flashcards
Where is resonance usually observed
electron density is dynamically shared in organic compounds, especially in pi-systems and atoms that possess p-orbitals
What is resonance
it essentially is the delocalization (movement) of pi-electron density across a consecutive number of atoms that all possess a p-orbital
- the p orbs act as a conduit for the flow of electron density and this delocalizability affords increased stability to the the molecule
How does electron density flow
from areas of high electron density to areas of electron deficiency
Concentrated electron density tends to be more ___ and delocalizing electron density enables multiple atoms to accommodate the density
concentrated E density is more reactive (higher in energy and thus less stable)
Resonance energy is
- the amount of stability (lowering of energy) afforded to a molecule through electron destabilization
- it depends on how many distinct contributors a compound has and how similar in energy resonance contributors are to one another
Compounds that have a greater number of resonance contributors tend to have a ____ resonance energy (exist at a lower energy)
more resonance contributors = greater resonance energy (aka more stable)
The more similar resonance contributors are in energy to one another, the ___ the resonance energy of the compound
the greater the resonance energy of the compound
___ of the atoms bearing charges can also impact the stability of contributors
The electronegativity
ex. O atom accommodates a negative charge much better than a C atom
Two factors that affect resonance stability
1) full octets - full octet species are more stable (lower in energy) than incomplete octet species
2) electronegativity
*** octets > electroneg
Positive and negative atoms are:
Positive atoms are stabilized by electron donation and destabilized by electron withdrawal
Negative atoms are stabilized by electron withdrawal and destabilized by electron donation
Carbocations are ____ species
incomplete octet species
You can delocalize lone pairs or pi-electrons towards:
1 - pi systems
2 - atoms with an empty p-orbital (eg. carbocations, boron, aluminum)
- ** NEVER delocalize towards an sp3 atom - they do not possess p-orbitals
- ** NEVER break sigma bonds
- ** overall charge should be conserved between contributors
Electrons resonante in a ___ fashion for neutral and overall negative compounds
1,3 fashion - aka when drawing individual contributors if the electrons are in the 1 location they would move to the 3 location
A positive charge will resonate in a ___ fashion when a heteroatom resonates to stabilize a carbocation but a carbocation resonates in a ____ fashion along a carbon chain
1,2 fashion for positive charge that is stabilizing a carbocation
1,3 fashion for carbocations along a carbon chain
use ___ arrow for overall positive and heteroatoms resonating towards a carbocat
ONE arrow (two for negative and neutral)
Radicals resonate in a ____ fashion and ___ arrows that are __ __ are used to show next contributor
radicals resonate in a 1,3 fashion and need THREE single barbed arrows to show next contributor
Delocalized vs. localized
delocalized lone pairs are located in p-orbitals
localized lone pairs will exist in hybrid orbitals - they are isolated and cannot be delocalized
**only electrons residing in p-orbs or in pi-bonds are delocalizable
Pyrrole vs pyridine
pyrrole’s lone pair exists in a p-orbital while pyridine’s lone pair is in a sp2 hybrid orbital and will NOT be delocalized into the pi-system
explanation: N is already making 3 sigma bonds in pyrrole so all the sp2 orbitals are being utilized so lone pair will be in p orb, but N in pyridine is making 2 sigma bonds and 1 pi bond so the pi orb is used by the pi bond, leaving an sp2 orb available for the lone pair
Furan and thiophene each have a ___ pair and a ____ pair
a localized lone pair and a delocalized lone pair
General notes on N and P atoms:
** if the N atom is already visually making a pi bond, then its lone pair will reside in a hybrid orbital and will NOT be delocalizable
** if the N atoms is NOT visually pi-bonded, then the lone pair will reside in a p orb and will delocalize into the pi-system
Note on O and S atoms
** O and S when adjacent to a pi-system will delocalize one lone pair and the other lone pair will not be delocalizable
** when classifying compounds as aromatic/non or anti, do NOT count the pi-electrons of a carbonyl that is coming off the ring - these electrons are NOT part of the ring’s ___
pi cloud
conjugated system
alternating double and single bonds (each consecutive atom possesses a p orb)
- pi bonds in conjugated systems can be delocalized
Each pi bond is formed by ___ electrons aka ___ pair, but in conjugated systems, the electrons in the pi bonds can be delocalized
2 electrons aka one pair of electrons
what is a heteroatom
- a non carbon or hydrogen atom (O, N, Cl, S etc)
- one lone pair of a heteroatom can be delocalized into a pi-system
- if the heteroatom has 2 lone pairs, only one can be delocalized
Carbocat- heteroatom resonance
- heteroatoms that are bonded to a carbocat may resonate and form a pi bond
- the heteroatom donates its lone pair into the carbocat’s empty p orb
- carbocats are incomplete octet structures but the resonance contributor with the positive on the heteroatom is a full-octet contributor and is notably more stable than the carbocat
Boron and aluminum heteroatom resonance
- like carbocats, B and Al have empty p orbs when neutral so when B or Al are bonded to a heteroatom, the heteroatom can resonate into the empty p orb and form a pi bond
- B and Al take on a formal negative charge when they make 4 bonds
in negative charge resonance, two arrows are necessary to describe the flow of pi-electrons between contributors because
one pi bond is formed and one is broken
in positive charge resonance, only one arrow is necessary to describe the flow of pi electrons between contributors because
all of the pi-density in one pi-bond is moved to the adjacent empty p-orbital to form a new pi-bond
