Aromatic Chemistry Flashcards
In benzene, what four orbitals are found on each carbon?
- 3 sp2 hybridised orbitals
- 1 p orbital
What is the bond angle in benzene?
In benzene, all bond angles are 120 degrees
What makes up the cloud of delocalised electrons above and below a benzene ring?
Pi bonds formed from laterally overlapping p orbitals
What four features makes a compound aromatic?
- Cyclic
- Fully conjugated
- Planar
- Huckel’s rule
What does it mean for a compound to be fully conjugated?
Every atom in the cycle must have one unhybridised p orbital.
There can be no sp3 atoms in the ring
Why must a compound be planar to be aromatic?
Planarity allows continual overlap between p orbitals on a ring
At what point does a cyclic compound no longer be planar?
A cyclic compound is typically no longer planar once you reach 8 carbons in the ring
What is Huckel’s Rule, and what does it tell you?
The equation for Huckel’s rule is 4n+2, where n represents any integer.
Compounds must have the 4n+2 number of pi electrons in order to be considered aromatic
What are the first four number of pi electrons that abide by Huckel’s rule?
2, 6, 10, 14
How do you count the number of pi electrons?
- 2 pi electrons per double
- 2 pi electrons per lone pair
If all criteria of aromaticity are met except Huckel’s rule, what is the compound considered?
Anti-aromatic
How do you halogenate a benzene ring?
- Halogen
- Heat
- Lewis acid (that matches the halogens
What are the possible Lewis acids for halogenation?
FeX3 or AlX3. The halogens on the Lewis acid must match the target halogen (e.g. AlCl3 for Cl2)
How do you nitrate a benzene ring?
- HNO3
- Heat
- Conc H2SO4
How does the Lewis acid make for a stronger electrophile in halogenation?
Curly arrow from one of the halides to the Al, which makes one of the bromines have a positive charge.
This increases the polarity of the X-X bond, making it a stronger electrophile
How does H2SO4 make HNO3 a stronger electrophile is nitration?
It dehydrates it to nitronium cation (NO2+)
How do sulfonate a benzene ring?
- HSO4
- Heat
- Conc H2SO4
How do you alkylate a benzene ring?
- Alkyl halide
- Heat
- Lewis acid (matches halide on alkyl halid)
How does the Lewis acid make the alkyl halide a stronger electrophile?
It reacts with the halide to form a carbocation
What will the product of alkylation of a benzene ring be and why?
There will be a mixture of di-substituted products, as the presence of the alkyl group activates the otho and para positions on the ring, allowing further substitution
How do you acylate a benzene ring?
- Acyl chloride
- Heat
- Lewis acid (AlCl3)
How does the Lewis acid make the acyl chloride a stronger electrophile?
It reacts with the halide to form an acylium cation, which is O triple bonded to C-R
What is the product of acylation of benzene?
A mono substituted aryl ketone
How can you form an acylium cation without using an acyl chloride to begin with?
If you use an acetic anhydride and a Lewis acid, an acylium cation will also be formed
What kind species activate a ring, and how?
Electron donating groups (either via induction or resonance) increase reaction rate (activate), as they enrich e-density in the ring
What kind species deactivate a ring, and how?
Electron withdrawing groups (either via induction or resonance) decrease reaction rate (deactivate), as they decrease e-density in the ring
What is the induction mechanism?
Substituents donate/withdraw e-density via sigma bond network (no curly arrows)
What is the resonance mechanism?
Substituents donate/withdraw e-density via pi bond network (curly arrows)
Which has a greater impact, resonance or induction?
Resonance
What positions do electron donating groups direct?
In the ortho & para positions
What positions are electron withdrawing groups direct?
In the meta position, except for halogens which are o- and p-
Are halogens inductive or resonance?
Halogens are both inductive and resonance, however overall they are deactivating due to their inductive withdrawal of e-density from the ring
How do you get from phenol to phenoxide?
Basic conditions (e.g. NaOH)
Is the OH group on phenol activating or deactivating?
The OH is very activating through resonance
Is the phenoxide more or less reactive than benzene?
Phenoxide is much more reactive than benzene, as the O- group is very activating through resonance.
Phenoxide is even more reactive than phenol
What happens when you brominate phenol?
It requires milder conditions than with benzene, and tribromination occurs, with bromines at both ortho positions, and the para position
What happens when you nitrate phenol?
Conc HNO3 will produce a tri-nitrated product
Dilute HNO3 will produce a mono nitrated product
Is phenoxide a base or acid?
Phenoxide is the conjugate base to phenol (the acid)
What happens when you acylate phenol?
The product will be an ester.
It can act to ‘protect’ the phenol from further reactions, as the activating effect is reduced due to competing withdrawal by the carbonyl group of the ester.
Despite this it is still activating, but will be mainly mono-substitution rather than tri
What is aniline?
Benzene with an NH2 group attached
Where does aniline rank in reactivity compared to phenol and phenoxide?
It is more reactive than phenol, but less reactive than phenoxides
Why must substitution reactions using aniline not be performed in acidic conditions?
In acidic conditions, aniline is converted into an anilinium cation, which is highly unreactive
How do anilines react with electrophiles?
Anilines react rapidly with electrophiles under mild conditions, typically forming polysubstituted products (e.g. tribromination)
How do you synthesise aniline from benzene?
HNO3 and H2SO4 to form nitrobenzene
Reduce nitrobenzene (must benon-acidic) with H2 and Pd/C
How do you synthesise aniline from nitrobenzene, while ensuring no other substitution takes place?
Nitrobenzene to anilinium salt by HCl and Sn
Then neutralise anilinium salt with NaOH
Is aniline a base or acid?
Aniline is a base, and anilinium is its conjugate acid
What happens when you acylate aniline?
It will form an amide, which will ‘protect’ it due to competing withdrawal by amide group
What does converting aniline to an amide allow you to do?
As become an amide will lower its reactivity, it will allow for the molecule to undergo mono-substitution (rather than poly-).
Once this has occurred, acidic conditions can be used to convert it back to an aniline, with the new group now attached
How do you convert aniline to diazonium attached, and what does this allow for?
NaNO2 and HCl will convert aniline to diazonium, which is a compound that can undergo nucleophilic substitution
What will reaction with a Lewis acid do to aniline?
A Lewis acid will make aniline highly unreactive
Why can aromatic rings not react via sn2?
Sn2 reactions take place via a backside attack; the aromatic ring structure blocks this
What leaving group is typically required for nucleophilic substitution?
A halogen leaving group (in particular F and Cl)
Why are halogens the best leaving group for nucleophilic attack?
Due to their electronegativity, which creates a partial positive charge for the nucleophile to attack.
The more electronegative, the faster the rate
What is the best leaving group for nucleophilic substitution?
Fluorine by a large amount
F»_space; Cl > Br > I
Other than a halogen LG, what else must be on the ring for NAS to take place?
An electron withdrawing group ortho and or para to the leaving group
Why is a good electron withdrawing group important for NAS?
They stabilise the intermediate by withdrawing electron density out of the ring
What are the best EWG for NAS?
Nitro (NO2) groups are the best EWG for NAS
Other good ones include:
Sulfones
Nitriles (C triple bond N)
Carbonyls
Where must the EWG be positioned for NAS reactions?
Ortho or para to the leaving group
Outside of the ring, what else is required for NAS?
A good nucleophile
Such as (in order):
Alkoxide (-OR)
Hydroxide (-OH)
Amine (-NH4)
Cyanide (-CN)
What are diazonium salts?
Benzene ring with an N triple bond N attached
Why are diazonium salts useful?
N2 is a very good leaving group, so it is readily eliminated by the salt as N2 gas, to form an aryl cation
It can easily undergo nucleophilic substitution
How do you synthesise a diazonium salt?
Benzene to nitrobenzene via HNO3 and H2SO4
Nitrobenzene to aniline via H2 & Pd/C
Aniline to diazonium salt via HCl & NaNO2
This must be kept at a low temperature
Diazonium salt -> Phenol
H2O & H2SO4
Diazonium salt -> Iodobenzene
Add KI and keep temp less than 25 degrees
Diazonium salt -> Fluorobenzene
NaBF4- & heat
Diazonium salt -> Benzene (N2 -> H)
H3PO2 & heat
Diazonium salt -> Chlorobenzene
HCl & CuCl
Diazonium salt -> Bromobenzene
HBr & CuBr
Diazonium salt -> Cyanobenzene
KCN & CuCN
How can diazonium salts act as an electrophile?
Diazonium salt can form a N-C bond with another ring, as long as the other ring has an EDG ortho or para to the bonding carbon
What is the product of diazonium acting as an electrophile?
It forms a two ring structure, with an N=N bridging them
What are heterocycles?
Compounds that involve atoms other than carbon within the ring (e.g. N, O or S)
How do you determine how many electrons a heteroatom gives to aromaticity?
- If heteroatom has only single bonds to itself, it has 2 electrons in p-orbital that contributes to pi cloud
- If heteroatom has a single and double to itself, it has 1 electrons in p-orbital that contributes to pi cloud
Aniline -> Aryl amide
NaOAc & Ac2O
