Aromatic- Electrophilic substitution Flashcards
(37 cards)
1
Q
How does benzene react with strong electrophiles
A
- In substitution rather than addition
2
Q
What takes place in an electrophilic substitution reaction
A
- An atom or group of atoms replaces a hydrogen atom on the benzene ring so that the product retains the stable aromatic ring
3
Q
Describe the mechanism for an S(E)Ar reaction
A
- In situ generation of electrophile
- The aromatic ring breaks aromaticity and attacks the very reactive electrophile
- Forms an Wheland intermediate
- Stabilisation of Wheland intermediate by resonance
- The proton at the site of electrophilic attack is lost from this cation intermediate to restore aromaticity
4
Q
Which is the slow, rate determining step in an SEAr
A
- The breaking of the aromaticity requires a lot of energy- slow
5
Q
What position does the charge take in Wheland intermediate resonance
A
- Either ortho or para never meta
6
Q
What are the 5 most common substitution reactions of benzene
A
- Halogenation
- Nitration
- Sulfonation
- Alkylation
- Acylation
7
Q
Which halogens are accessible and why
A
- Chlorine
- Bromine
- NOT Fluorine- too reactive/ difficult to handle
- NOT iodine- too unreactive
8
Q
What is formed when chlorine/bromine reacts with benzene
A
- Chlorobenzene
2. Bromobenzene
9
Q
What catalyst is used in bromination/ chlorination of benzene
A
- FeBr3
2. AlCl3
10
Q
What are the steps in bromination of benzene
A
- Bromine atom donates a lone pair of electrons to the Lewis acidic iron centre to give a zwitterion
- This causes the Br-Br bond to become polarised- stronger electrophile
- Benzene acts as a nucleophile and attacks the partially positive bromine atom releasing FeBr4-
- The Wheland intermediate, stabilised by resonance, then rapidly loses a proton to restore aromaticity
- By-products FeBr4- + H+ combine to restore FeBr3 catalyst and HBr
11
Q
What is needed for the nitration of benzene
A
- Mixture of concentrated nitric acid and concentrated sulfuric acid
12
Q
What is the first step in the nitration of benzene
A
- Generation of NO2+ electrophile
- Sulfuric acid protonates nitric acid to give OH2+ - an excellent leaving group
- Dehydration then occurs to give reactive nitronium ion
13
Q
What are the uses of nitration
A
- Synthesis of explosives
- Nitro group can be easily reduced to an amino group to form anilines- found in many pharmaceuticals and natural products
14
Q
What is used to reduce the nitro group of nitrobenzene
A
- Small scale Bechamp reduction- Tin in dilute hydrochloric acid
- In industry- hydrogenation using a palladium on carbon catalyst- avoids cost of large amounts of toxic tin waste
15
Q
What is the product of the sulfonation of benzene
A
- Benzenesulfonic acid
16
Q
What is needed for the sulfonation of benzene
A
- Concentrated sulfuric acid which protonates itself to produce protonated sulfur trioxide HSO3+
17
Q
Describe the steps of sulfonation of benzene
A
- H3SO4- produced by H2SO4 protonating itself is dehydrated to produce protonated HSO3+
- HSO3+ is attacked by benzene to form a nonaromatic carbocation Wheland intermediate
- Two electrons move from benzene ring to form new C-S bond and at same time a pair of electrons in S=+OH bond moves to positively charged oxygen
- -OSO3H ion deprotonates the carbocation to form benzene-sulfonic acid and regenerate sulfuric acid.
18
Q
Describe energy of product in sulfonation of benzene
A
- Similar energy of product to starting material
- So sulfonation is reversible, the side of the equilibrium the reaction favours depends on the concentration of sulfuric acid used.
19
Q
What can the reversibility of sulfonation be used for
A
- Improving regioselectivity of SEAr reactions
20
Q
What else can be used for sulfonation of benzene
A
- Oleum- SO3 in H2SO4
21
Q
How can benzenesulfonic acids be purified
A
- Easily purified by recrystallisation of their corresponding sodium salts made by treating the sulfonic acid with NaCl
22
Q
What can chlorosulfuric acid be used for
A
- Directly synthesise benzenesulfonyl chlorides
2. Valuable intermediates en route to the synthesis of sulfonamides
23
Q
What is the name for alkylation of benzene using alkyl halides
A
- Friedel-Crafts alkylation
24
Q
What is used in Friedel-Crafts alkylation
A
- Bromoalkane
2. Lewis acid catalyst (AlCl3, FeBr3, FeCl3)
25
What is the initial step of Friedel-Crafts alkylation e.g.R-Cl + AlCl3
1. The chloro or bromoalkane reacts with the Lewis acid to form a strong electrophile
2. e.g. Cl joins onto AlCl3 to form AlCl4- and produces a carbocation
26
What happens once the carbocation electrophile has been formed e.g. R-Cl + AlCl3
1. Benzene attacks the C+ to produce a nonaromatic carbocation
2. AlCl4- removes a proton from the carbocation to five an alkylbenzene with HCl and AlCl3
27
Which alkyl halides are most reactive in Friedel-Crafts alkylation
1. tertiary>secondary>primary
| 2. In line with carbocation stability
28
What else can be used as alkylating agents
1. Alkenes and alcohols
| 2. Bronsted acid has to be used instead of Lewis acid
29
What are two major limitations of Friedel-Crafts alkylation
1. The intermediate carbocation can rearrange to a more stable isomer, if possible- primary--> secondary etc
2. The product is typically more reactive than the starting material- as generally more nucleophilic- which can lead to over-alkylation- especially Br
3. Lead to production of unwanted and difficult to separate by-products which lowers the efficiency of the reaction and limits its use in synthesis
30
What did Friedel and Crafts come up with to address the problem with their alkylation reaction
1. Developed acylation reaction
| 2. Using acid chlorides rather than alkyl halides as electrophile
31
Describe Friedel-Crafts alkylation
1. Aluminium trichloride is used as a Lewis acid to help remove chloride to leave behind a cation- acylium ion
2. The cation is stabilised by a lone pair on the adjacent oxygen atom- resonance
3. Benzene attacks the acylium ion to produce a nonaromatic carbocation
4. AlCl4- removes a proton from the nonaromatic carbocation to give an acylbenzene together with HCl and AlCl3
32
Why does the reaction stop after acylation compared to alkylation
1. The addition of the electrophile makes the aromatic ring electron deficient so the reaction stops after a single substitution
33
What can be formed from the acylbenzene
1. Can be reduced so just R chain present
34
What are the 3 ways an acylbenzene can be reduced
1. Wolff-Kishner Reduction
2. Clemmensen reduction
3. Mozingo Reduction
35
What is involved in a Wolff-Kishner reduction
1. Basic conditions
2. H2N-NH2, KOH, EtOH forms intermediate
3. Heat added removes N2 to form reduced product
36
What is involved in the Clemmensen reduction
1. Acidic conditions
| 2. Zn(Hg), HCl reduces straight to product
37
What is involved in Mozingo reduction
1. First add SHCH2CH2CH2SH to form intermediate
| 2. Then Raney Ni, H2 to form reduced product
