This set of Tricky Organic Chemistry Questions and Answers focuses on “Electrophilic Aromatic Substitution”.
1. What will be the product formed when phenol reacts with Br2 in CCl4 medium?
b) 4- Bromophenol
Explanation: Because the non-polar solvent (i.e. CCl4) induces a weaker dipole in Br-Br as compared to the polar solvent (i.e. water), a weaker electrophile (Br+) is produced that undergoes electrophilic aromatic substitution with the benzene ring. Consequently, we get mono-bromination with Br (in CCl4) with compared with tri-bromination Br (in aq.).
2. What is the electrophile in the electrophilic substitution reaction of benzene using oleum and conc. H2SO4?
Explanation: This reaction is sulphonation of benzene ring in which SO3 is the real attacking electrophile and oleum will increase the concentration of SO3.
Explanation: In halogenation of benzene ring halonium ion (Cl+) will attack at benzene ring.
4. Which of the following aromatic compounds undergo Friedel–Crafts alkylation with methyl chloride and aluminum chloride?
a) Benzoic acid
Explanation: Toluene will undergo Friedel–Crafts alkylation with methyl chloride and aluminum chloride as because as weak electron withdrawing functional group — will not make the ring less electron dense, hence more reactive in an aromatic electrophilic substitution as compared to electron withdrawing group (benzoic acid, nitrobenzene, aniline).
Explanation: -OH is a ortho and para directing group so attack of electrophile (carbocation- CH3) will attack there, as shown below:
6. What is the electrophile in the acylation of benzene?
Explanation: The electrophile in the electrophilic substitution reaction of acetyl chloride (CH3COCl) and AlCl3 reacting with benzene is R-CO+.
Explanation: This reaction is nitration of benzene and conc. HNO3 and conc. H2SO4 is used to carry out this synthesis although HNO2 + HNO3 can also be used.
Explanation: This is Gattermann Koch Reaction in which HCl + CO + AlCl3 are used as reactant. H-CO+ is the real attacking electrophile.
Explanation: Here no reaction will take place because CCl4 + Cl2 show addition reaction and benzene don’t give addition reaction.
10. Which of the following is the most activating in electrophilic aromatic substitution?
Explanation: Groups with unshared pairs of electrons, such as the amino group of aniline, are strongly activating and ortho/para-directing by resonance. Such activating groups donate those unshared electrons to the pi system, creating a negative charge on the ortho and para positions. These positions are thus the most reactive towards an electron-poor electrophile. The highest electron density is located on both ortho and para positions, although this increased reactivity might be offset by steric hindrance between substituent and electrophile.
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