aromatic chemistry Flashcards
Huckel’s rule
4n + 2 pi electrons
Anti-aromatic conditions
4n pi electrons
Pi system of pyridine description
six membered ring with a nitrogen atom in the ring. The nitrogen lone pair of electrons do not contribute towards the pi system and as a result is a weak base
Pyrrole description
Five membered cyclic ring with one nitrogen. The sp2 hybridized nitrogen donates its lone pair of electrons in a p orbital to the pi system. Therefore not a base
Mechanism of electrophilic aromatic substitution
See notes
Which is the rate determining step of electrophilic aromatic substitution and why
The initial attack on the electrophile. Aromaticity is hard to lose, easy to gain so the second step in which aromaticity is regained is rapid by comparison
Lewis acids used for electrophilic aromatic substitution
AlCl3, AlBr3, H2SO4
Electrophiles used to electrophilic aromatic substitution
Cl2, Br2, HNO3, SO3, RX, RCOX
Limitations of Friedel-Crafts alkylations
They are prone to carbocation rearrangements, deactivated benzene rings are not reactive to it, over alkylation is possible as the product is more reactive than the starting material, the Lewis acid catalyst AlCl3 complexes aryl amines making them unreactive.
With what substituents does Friedel-Crafts acylation not occur
Benzene rings with amine substituents
Benefits of acylation as supposed to alkylation
The final product is a deactivated benzene ring, inability of the acylium ion intermediate to rearrange
Stability of the acylium ion
Resonance
Product of acylation
Benzene with a ketone substituent
Reduction of acylation products to theoretical alkylation product
Zn(Hg)/ HCl, heat
Describe the reason for ortho, para and meta directing aromatic substitutents
See notes
Product of the bromination of anilines
Tri-bromo substituted aniline
Control over the bromination of anilines
Oxidation to an amide allows for steric hinderance of the ortho positions and so a single bromine is substituted at the para position
Which are more reactive to electrophilic aromatic substitution, phenols or anilines
Anilines, phenols have some control over the level of substitution
Halides activation and directing characteristics
Halides are deactivating as a result of their high electronegativity however, their accessible lone pairs make then ortho and para directing as supposed to meta
Reduction of a nitro group to diazonium salts
H2, Pd/C to give the amine. NaNO2/HCl to give the diazonium salt
Pros and cons of using deactivated benzene reagents
They react slower but react for form less of a mixture of products
Nitro group to amine reagents
H2, Pd/C, EtOH
OR
Sn/HCl
Ketone to alkyl reagents
Zn(Hg) / HCl, heat
Alcohol to ether reagents
NaH, CH3I
Amine to amide reagents
CH3COCL, pyridine
Description of nucleophilic aromatic substitution
A nucleophile displaces a good leaving group on an aromatic ring. Needs a strong nucleophile e.g. oxide salt
Nucleophilic aromatic substitution mechanism
See notes
Why does the location of the electron withdrawing group on the aromatic ring undergoing nucleophilic substitution matter
The EWG must be para. The delocalisation of charge in the intermediate means that the attack by the nucleophile can only be ortho or para (para being where the LG is)
Which is the RDS of nucleophilic aromatic substitution
Addition of the nucleophile (first step)
Meisenheimer intermediate
Nucleophilic substitution intermediate. NO2 acts as the EWG. Stabilised through delocalisation and is not aromatic
Benzylic substitution
The substitution of a LG on the carbon substituent of a benzene ring
Benzylic substitution mechanism
See notes
How many products are their following a benzylic substitution
One
Stabilisation of the benzylic intermediate
Carbocation resonance with the benzene ring
Removal of an alcohol or amine on a carbon adjacent to an aromatic ring
H2, Pd/C
