Flashcards in Aromatics. Deck (68):
What kind of hybridisation do aromatic carbons have?
All of them have SP2 hybridisation.
What is the structure of benzene?
Each sp2 hybridised C in the ring has an unhybridised p orbital perpendicular to the ring which overlaps around the ring.
The six pi electrons are delocalised over the six carbons.
When bromine adds to benzene, what catalyst is needed?
When bromine adds to benzene, a catalyst such as FeBr3 is needed.
When bromine adds to benzene, what reaction takes place?
The reaction that occurs is the substitution of a hydrogen by bromine.
Is Br2 ever added to benzene?
No. Only 1 bromine is added.
What are annulenes?
Annulenes are hydrocarbons with alternating single and double bonds.
Benzene is a six-membered annulene, so it can be named -annulene. Cylobutadiene is -annulene, cyclooctatetraene is -annulene.
What are the 4 aromatic requirements?
Structure must be cyclic with conjugated
Each atom in the ring must have an unhybridized p orbital (sp2 or sp).
The p orbitals must overlap continuously around the ring. Structure must be planar (or close to planar for effective overlap to occur).
Delocalization of the pi electrons over the ring must lower the electronic energy.
What are antiaromatic compounds?
Antiaromatic compounds are cyclic, conjugated, with overlapping p orbitals around the ring, but electron delocalization increases its electronic energy.
What are non aromatic compounds?
Nonaromatic compounds do not have a continuous ring of overlapping p orbitals and may be nonplanar.
When does Huckels rule apply?
Once the aromatic criteria is met, Huckel’s rule applies.
What is Huckels rule?
If the number of pi electrons is (4N + 2) the compound is aromatic (where N is an integer).
If the number of pi electrons is (4N) the compound is antiaromatic.
How can cyclopentadiene become aromatic?
By deprotonating the sp3 carbon of cyclopentadiene, the electrons in the p orbitals can be delocalized over all five carbon atoms and the compound would be aromatic.
How is a dianion formed?
Cyclooctatetraene reacts with potassium metal to form an aromatic dianion.
What is a phenol?
An aromatic with an alcohol attached.
What is a toluene?
An aromatic with a methyl group attached.
What is anilene?
An aromatic with an NH2 group attached.
What is anisole?
An aromatic with an OCH3 group attached.
What is styrene?
An aromatic with a HC=CH2 group attached.
What is acetophone?
An aromatic with a C-CH3 group attached.
What is benzaldehyde?
An aromatic with a C-H group attached.
What is benzoic acid?
An aromatic with a C-OH group attached.
On what carbons are the substituents if the benzene is ortho?
1,2. The 2 groups are next to each other.
On what carbons are the substituents if the benzene is para?
1,4. The 2 groups are directly across the benzene group from each other.
On what carbons are the substituents if the benzene is meta?
1,3. The 2 groups have a C in-between them.
When naming an aromatic with 3 or more substituents, which group take primary position?
Use the smallest possible numbers, but the carbon with a functional group is #1.
What is the difference between a phenyl bromide and a benzyl bromide?
Phenyl indicates the benzene ring attachment. The benzyl group has an additional carbon.
What are the 2 steps of an electrophilic aromatic substitution?
Step 1. Attack on the electrophile forms the sigma complex.
Step 2. Loss of the proton gives the substitution product.
What is the mechanism for the bromination of benzene?
Before the electrophilic aromatic substitution can take place, the electrophile must be activated. A strong Lewis acid catalyst, such as FeBr3, should be used.
Electrophilic attack and formation of the sigma complex.
Step 3: Loss of a proton to give the products.
What catalyst is used for chlorination and iodination of benzene?
AlCl3 is most often used as catalyst, but FeCl3 will also work.
Is the mechanism of chlorination and iodination of benzene similar to bromination?
What does iodination of benzene require?
Iodination requires an acidic oxidising agent, like nitric acid, to produce iodide cation.
H+ + HNO3 + ½ I2 I+ + NO2 + H2O.
What acts as the catalyst for the nitration of benzene and how is the nitronium ion formed?
Sulfuric acid acts as a catalyst, allowing the reaction to be faster and at lower temperatures.
HNO3 and H2SO4 react together to form the electrophile of the reaction: nitronium ion (NO2+).
What is the nitro group?
How is the nitro group reduced?
Treatment with zinc, tin, or iron in dilute acid will reduce the nitro to an amino group.
This is the best method for adding an amino group to the ring.
What is the electrophile in the sulfonation of benzene?
Sulfur trioxide (SO3) is the electrophile in the reaction.
What is fuming sulfuric acid?
A 7% mixture of SO3 and H2SO4 is commonly referred to as “fuming sulfuric acid”.
What are the SO3H groups?
The —SO3H groups is called a sulfonic acid.
Is sulfonation reversible?
How can the sulfonic acid group be removed from an aromatic ring?
The sulfonic acid group may be removed from an aromatic ring by heating in dilute sulfuric acid.
What is the sulfonic acid group?
What reacts faster, toluene or benzene?
Toluene reacts 25 times faster than benzene.
The methyl group on toluene can be thought of as what?
When toluene undergoes nitration the product is made up of what kind of substituted molecules?
The product mix contains mostly ortho and para substituted molecules.
Why are ortho and para attacks preferred?
Ortho and para attacks are preferred because their resonance structures include one tertiary carbocation.
When substitution occurs at the meta position, what happens to the positive charge?
When substitution occurs at the meta position, the positive charge is not delocalized onto the tertiary carbon, and the methyl groups has a smaller effect on the stability of the sigma complex.
Are alkyl groups activating or deactivating substituents and what kind of directors are they?
Alkyl groups are activating substituents and ortho, para-directors.
What is inductive effect?
This effect is called the inductive effect because alkyl groups can donate electron density to the ring through the sigma bond, making them more active.
When anisole is attacked in the meta position can the methoxy group stabilise the sigma complex?
Resonance forms show that the methoxy group cannot stabilise the sigma complex in the meta substitution.
When anisole is attacked in the ortho and para positions can the methoxy group stabilise the sigma complex?
Resonance stabilization is provided by a pi bond between the —OCH3 substituent and the ring.
The activating strength of a methoxy group affects anisole in what way?
A methoxy group is so strongly activating that anisole is quickly tribrominated without a catalyst.
What is the amino group?
Aniline reacts with bromine water (without a catalyst) to yield the tribromoaniline.
Sodium bicarbonate is added to neutralize the HBr that is also formed.
List the activating groups from most reactive to least reactive?
-R (No lone pairs.)
List the activating compounds from most reactive to least reactive?
List the deactivating groups?
-SO3H. (Sulfonic acid.)
-C TRIPLE BOND N. (Cyano.)
Ketone or aldehyde.
When chlorobenzene is nitrated the main substitution products are what?
When chlorobenzene is nitrated the main substitution products are ortho and para. The meta substitution product is only obtained in 1% yield.
Why are ortho and para the main substitution products of chlorobenzene nitration?
Because there are 2 para positions which is why para is the most formed. And there is alot of steric hindrance on the meta position so ortho is formed more.
What are the percentages of forming ortho, para and meta in chlorobenzene nitration?
When toluene is nitrated, what are the percentages of forming ortho, para and meta?
During alkyl group stabilisation, what are the percentages of forming ortho, para and meta?
Why are halogens deactivators?
Inductive Effect: Halogens are deactivating because they are electronegative and can withdraw electron density from the ring along the sigma bond.
What is the resonance effect of halogens?
Resonance Effect: The lone pairs on the halogen can be used to stabilize the sigma complex by resonance.
What does the Friedel crafts reaction give?
Synthesis of alkyl benzenes from alkyl halides and a Lewis acid, usually AlCl3.
What is the reaction in the Friedel Crafts reaction?
Reactions of alkyl halide with Lewis acid produces a carbocation, which is the electrophile.
What are the 3 limitations of the Friedel crafts reaction?
Reaction fails if benzene has a substituent that is more deactivating than halogens.
Rearrangements are possible.
The alkylbenzene product is more reactive than benzene, so polyalkylation occurs.
What is used in the Friedel crafts acylation?
Acyl chloride is used in place of alkyl chloride.
What is the product of the Friedel crafts acylation?
The product is a phenyl ketone that is less reactive than benzene.
What is the Clemmensen reaction used for?
The Clemmensen reduction is a way to convert acylbenzenes to alkylbenzenes by treatment with aqueous HCl and amalgamated zinc.