Organic chem lecture 7-8 aromaticity Flashcards
Aromatic compounds
State a limitation that disproved Kekule’s structure of benzene in terms of isomers of dibromo benzene.
In accordance to Kekule’s structure of benzene, it was thought that there was 4 isomers of dibromo benzene: 1-2 (where the bromine substituents exists on the same double bonds in benzene), 1-3, 1-4 and 1-6 (where the bromine substituents exists on different double bonds in benzene).
However, it was discovered that there was only 3 isomers of benzene (the 1-2 isomer was not distinguishable from the 1-6 isomer).
What criteria must be met for any structure to have an underlying frame or base of benzene (or just to be simply be benzene on its own)?
> It should have a six membered ring, with 3 additional degrees of unsaturation).
It should be planar.
All C-C bond lengths should be equally.
The last criteria is why Kekule’s structure is not the used structure for benzene now, because Kekule’s structure shows that there are alternating single and double bonds, which of course produces 2 different bond length.
What is the hybridization of each carbon in the benzene ring?
Each carbon in the benzene ring is sp2 hybridized.
This leaves a free p-orbital that exists above and below the ring. The p-orbital of each carbon overlaps to form a delocalized Pi system.
What is resonance energy?
When looking a reaction, we may predict the enthalpy change based on the structure of the reactants. The difference between the predicted value and observed value is called resonance energy.
Benzene is characterized as being quite stable (like in hydrogenation, it has a low heat of hydrogenation which makes sense when viewing on an enthalpy profile diagram). Due to its stability, what mechanism is benzene likely to undergo? What conditions is needed for a lot of its reactions?
Due to its stability, benzene is likely to electrophilic substitution.
If it is stable, it is less likely to undergo a reaction so to initiate a lot of the reactions, a catalyst is needed.
What are the criteria that needs to be followed for a compound to be aromatic?
> The compound should be cyclic.
The compound should be planar (may not be able to tell from the structure).
Structure must be completely conjugated, or every carbon in the ring should have a p-orbital.
Molecule must follow Huckel’s rule, 4n + 2
What is Huckel’s rule?
Huckel’s rule equates the number of Pi electrons in the structure to the formula 4n+2.
So Pi electrons = 4n+2
After we solve for ‘n’.
If ‘n’ is a positive integer, the structure satisfies Huckel’s rule and is an aromatic compound (if it has also fulfilled the other rules of aromaticity).
Generally compare aromatic and anti-aromatic compounds.
Aromatic and anti-aromatic compounds both have a cyclic,, planar and conjugates structure.
Anti-aromatic compounds are less stable than aromatic compounds because of their electron delocalization, which can be seen as their Pi electrons do not satisfy Huckel’s rule. Instead, the Pi electrons of anti-aromatic compounds satisfies the equation 4n, where n is a positive integer.
It is also important to note that any aromatic compound is more stable than its alicyclic compound that has the same number of Pi electrons. On the other hand, an anti-aromatic compound is less stable than its alicyclic compound with the same number of Pi electrons.
E.g. A compound is cyclic, planar and conjugated. It also has 8 electrons. We equate 8 to 4n+2 and get 1.5; this is not a positive integer so compound does not satisfy Huckel’s rule. We equate 8 to 4n and results in ‘n’ being a positive integer 2; from this, we can say the compound in anti-aromatic and hence has less stability than aromatic compounds due to its electron delocalization.
How do we know if a compound is neither aromatic or anti-aromatic (so just non-aromatic)?
If the compound does not satisfy any of the rules of aromaticity (for the Pi electrons, this includes 4n+2 and 4n), then the compound is automatically non-aromatic.
State the trend in stability from aromatic compounds, to anti-aromatic to non-aromatic compounds.
As we moves from aromatic compounds to non-aromatic compounds, there is a decrease is stability (meaning aromatic compounds are most stable).
What are annulenes?
Annulenes are cyclic hydrocarbon compounds with alternating single and double bonds.
Annulenes do not have to be planar, so annulenes are not automatically aromatic compounds (they also have to satisfy Huckel’s rule).
Describe the structure of [10]-annulene.
[10]-annulene is a cyclic hydrocarbon compounds with alternating single and double bonds. The compound has a 10 in its name because it has 10 Pi electrons in its structure (from the double bonds).
Why is [10]-annulene not an aromatic compound?
[10]-annulene satisfies being cyclic, being conjugated and satisfying Huckel’s rule. However, it is not an aromatic compound because it is non planar.
What are cyclopentadienyl ions?
Cyclopentadienyl ions are cyclic structures that has 5 carbons and 5 hydrogens. There are 2 double bonds present, and each carbon has sp2 hybridization, meaning each carbon has a p-orbital.
The cyclopentadienyl ion can be positive or negative. If it is positive, this mean the p-orbital as the positive carbon is empty; the electron that was meant to exist in that orbital had been given away to form an ionic bond, thus the cyclopentadienyl ion is an ion.
If the cyclopentadienyl ion is negative, it means the carbon with the charge is negative, and it has a full p-orbital containing 2 electrons. It first contains the electron that exists to the carbon, but it also has an electron it has taken away from a structure to form an ionic bond.
Are the cyclopentadienyl cation and anions aromatic?
The anion is aromatic, because:
> They are cyclic.
> They are planar.
> Each carbon has a p-orbital.
> It follows Huckel’s rule (4 Pi electrons from double bonds, and 2 Pi electrons in p-orbital of negative carbon).
The cation is aromatic because it only has 4 Pi electrons in total.
What is naphthalene?
Naphthalene is the simplest fused aromatic compound. It is a structure where 2 benzene rings are held together by the bonding of 2 adjacent carbon atoms in each ring. Naphthalene has 3 resonance structures.
Fused rings are when there are multiple rings that share 2 atoms and a bond in between them.
How can polycyclic aromatic hydrocarbons be formed?
When two or more six-membered rings with alternating double and single bonds are fused together.
What effect does increasing the number of fused rings in polycyclic aromatic hydrocarbons have on resonance?
With more fused rings in the structure, the number of resonance structures also increases.
Where can larger polynuclear aromatic hydrocarbons be found?
They can be formed in combustion (tobacco smoke).
Many polynuclear aromatic hydrocarbons are carcinogenic (cancer-causing).
What effect does increasing the number of aromatic rings in a polynuclear aromatic hydrocarbon have on how easily the compound reacts (like bromination)?
As the number of aromatic rings increases, the resonance energy per ring decreases, meaning the observed enthalpy value is not far away from the predicted one.
If they are not too far away, the feasibility of reactions like bromination, increases.
Describe the structure of pyridine.
Pyridine has almost the same structure as benzene (with all the alternating single and double bonds), but instead of one of the carbons, there is a nitrogen with a lone pair of electrons.
In here, nitrogen is connected to two adjacent carbons, in which it forms a double bond with one of the carbons.
The nitrogen is sp2 hybridized because it only forms 2 sigma bonds in total, so only requires hybridization between the fully occupied 2s orbital and two of the 2p orbitals (two 2p orbitals= 2 sigma bonds). The remaining p orbital lies perpendicular to the ring and contributes to the delocalized Pi system. Nitrogen has alone pair of electrons that comes from the 2s orbital, and this lone pair of electrons lies in the same plane as the nitrogen and carbon atoms, but outside the ring.
Describe the structure of the benzene compound toluene.
Toluene is the commercial name for methylbenzene. So, it has a methyl group sticking out of the benzene ring,
Describe the structure of the benzene compound phenol.
Phenol if the commercial name for hydroxybenzene. So, it has an alcohol group sticking out of the benzene ring.
Describe the structure of the benzene compound aniline.
Aniline is the commercial for aminobenzene. It is essentially a primary amine group sticking out of the benzene ring.
When there are 2 substituents on the benzene ring, what prefixes are given to show their positioning?
Ortho is for 1,2 positioning. E.g. ortho-bromochlorobenzne (1-bromo-2-chlorobenzene).
Meta is for 1,3 positioning. E.g. meta-fluronitrobenzene (2-fluro-1-nitrobenzene).
Para is for 1,4 positioning,
Remember to name the substituents in ALPHABETICAL order.
If one of the substituents is part of a common root, name the molecule as a derivative of that monosubstituted benzene). E.g. If we have a methyl and bromine substitution on benzene in positions 1 and 4, the name would be para-bromotoluene.
How do we name when the benzene ring has more than 2 substituents?
Do not use prefixes ortho, meta or para. Instead, just use numbers in the name.
What is a benzyl group?
A benzyl group is a benzene that is attached to an uncompleted methyl group:
C6H5CH2-
It is normally used when this group is a substituent of a much larger compound.
