Ch. 18: Reactions of Benzene and Substituted Benzenes Flashcards
When benzene is a substituent, what is it called?
A phenyl group
Explain an electrophilic aromatic substitution reaction.
Aromatic compounds such as benzene undergo them. An electrophile substitutes for a hydrogen that is attached to a ring carbon of an aromatic compound.
What are the five most common electrophilic aromatic substitution reactions?
What is the general mechanism for an electrophilic aromatic substitution reaction?
What is a lewis acid?
a compound that accepts a share in an electron pair
What is requirement of the chlorination and bromination of benzene?
Requires a lewis acid catalyst such as FeBr3 or FeCl3.
How do you generate an electrophile in the chlorination and bromination of benzene?
Why does the reaction of benzene with Br2 or Cl2 require a catalyst when the reaction of an alkene with these reagents does not require a catalyst?
Benzene’s aromaticity makes is much more stable and, therefore, much less reactive than an alkene, so, benzene requires a better electrophile.
What is the mechanism for bromination of a benzene?
What is the mechanism for chlorination of a benzene?
What reagents are used for the iodination of benzene?
Hydrogen peroxide, H2O2 and H2SO4
What is the mechanism for iodination of a benzene?
What catalyst does benzene require for nitration?
H2SO4
What is the mechanism for the nitration of a benzene?
What is the mechanism for sulfonation of a benzene?
What is the mechanism for desulfonation of a benzene?
What is an acyl group?
What is the mechanism for Friedal-Crafts acylation of benzene?
Places an acyl group on a benzene ring.
What is the mechanism for Friedal-Crafts alkylation of a benzene ring?
Places an alkyl group on a benzene ring.
What is a 1,2-hydride shift?
A carbocation will rearrange if the rearrangement leads to a more stable carbocation.
What is a 1,2-methyl shift?
A carbocation will rearrange if the rearrangement leads to a more stable carbocation.
How can a straight chain alkyl group be added to a benzene?
- Friedal-Crafts acylation
- reduction of the carbonyl group to a methylene group.
What is a Wolff-Kishner reduction?
It converts the carbonyl group of a ketone to a methylene group. This method reduces all ketone carbonyl groups, not just those adjacent to benzene rings.
What is the mechanism for the Wolff-Kishner reduction?
Outside of the basic five substitution reactions for a benzene, name a few others and how they’re made.
- add a halogen to the benzylic position
- it can now be replaced by a nucleophile via an SN2 or SN1 reaction
Can a substituted benzene undergo an elimination reaction?
Yes.
How can you reduce a substituted benzene? (both the substitutions and the benzene itself)
Substitutions with double and triple bonds can be reduced via catalytic hydrogenation.
How can a substituted benzene be oxidized?
How can a nitro substituent be reduced to an amino substituent?
What does ortho mean when naming disubstituted benzenes?
ortho substituents are adjacent to each other
What does meta mean when naming disubstituted benzenes?
meta substituents are separated by one carbon
What does para mean when naming disubstituted benzenes?
para substituents are opposite each other
What is this?
toluene
What is this?
phenol
What is this?
aniline
What is this?
benzosulfonic acid
What is this?
anisole
What is this?
styrene
What is this?
benzaldehyde
What is this?
benzoic acid
What is this?
benzonitrile
What is this?
phenyl group
What is this?
benzyl group
Does electron donation increase or decrease reactivity?
Increases reactivity. The slow step of an electrophilic aromatic substitution reaction is the addition of an electrophile to the nucleophilic aromatic ring to form a carbocation intermediate (positive). A substituent that makes benzene a better nucleophile makes it more attractive to nucleophiles and vice versa. As a result, substituents that donate electrons to the benzene ring not only increase benzene’s nucleophilicity, but stabilize the partially positively charged transition state, thereby increasing the rate of electrophilic aromatic substitution; these are called activating substituents.
Does electron withdrawal increase or decrease reactivity?
Decreases reactivity. The slow step of an electrophilic aromatic substitution reaction is the addition of an electrophile to the nucleophilic aromatic ring to form a carbocation intermediate (positive). A substituent that makes benzene a poorer nucleophile makes it less attractive to nucleophiles and vice versa. As a result, substituents that withdraw electrons to the benzene ring decrease benzene’s nucleophilicity and destabilize the transition state, thereby decreasing the rate of electrophilic aromatic substitution; these are called deactivating substituents.
What are the strongly activating substituents?
NH2, NHR, NR2, OH, OR
What are the moderately activating substituents?
NH(C=O)R, O(C=O)R, R, Ar, CH=CHR
What are the deactivating substituents?
F, Cl, Br, I (the halogens)
What are the moderately deactivating substituents?
(C=O)H, (C=O)R, (C=O)OR, (C=O)OH, (C=O)Cl
What are the strongly deactivating substituents?
C=-N, SO3H, (N+)H2R, (N+)H3, (N+)H2R, (N+)HR2, (N+)R3, NO2
Name the characteristics of moderately activating substituents?
They both donate electrons to the ring and withdraw electrons inductively. Overall, they donate electrons by resonance more strongly than they withdraw electrons inductively.
Name the characteristics of weakly activating substituents?
They are slightly more electron donating than they are electron withdrawing.
Name the characteristics of weakly deactivating substituents?
The halogens are weakly deactivating unlike OH and OCH3 because their orbitals are all 3p+ when bonding with the 2p orbital or C on benzene but O bonds with 2p. (except F, but it’s outweighed by F electronegativity). All halogens withdraw electrons inductively more strongly than they donate electrons by resonance.
Name the characteristics of moderately deactivating substituents?
The moderately deactivating substituents all have a carbonyl group directly attached to the benzene ring. A carbonyl group withdraws electrons from a benzene ring both inductively and by resonance.
Name the characteristics of strongly deactivating substituents?
The ammonium ions have no resonance effect, but the positive charge on the nitrogen atom causes them to strongly withdraw electrons inductively.
Can a halogen attached to a methyl group donate electrons by resonance?
No, it can only withdraw electrons inductively.
Where do activating and weakly deactivating substituents bond on a halogen ring? (o, p, m)
They direct an incoming electrophile to the ortho and para positions.
Where do deactivating substituents bond on a halogen ring? (o, p, m)
They direct an incoming electrophile to the ortho and para positions.
Where do moderately and strongly deactivating substituents bond on a halogen ring? (o, p, m)
They direct an incoming electrophile to the meta position.
When a benzene ring with an ortho-para directing substituent undergoes an electrophilic aromatic substitution reaction, what percentage of the product is the ortho isomer and what percentage is the para isomer?
The ortho positions are sterically hindered by the substituent on the ring, whereas the para position is not. Consequently, the para isomer will be formed preferentially if either the substituent on the ring or the incoming electrophile is large.
Is halogenation of a substituted benzene ring carried out with or without a Lewis acid catalyst (FeBr3 or FeCl3)?
Halogenation is the fastest of the electrophilic aromatic substitution reactions, so carried out without the Lewis acid catalyst IF the ring has a strongly activating substituent. If the lewis acid catalyst and excess bromine are used, the tribromo compound is obtained.
If a benzene ring has been moderately or strongly deactivated (has a meta-directing substituent), can it undergo a Friedel-Crafts acylation or alkylation?
No, it’ll be too unreactive.
Can aniline and N-substituted anilines undergo Friedel-Crafts reactions?
No
If two subtituents direct the new substituent to different positions, what happens?
The strongly activating substituent will win out over weakly activating or deactivating sub. If the two subs have similar activating properties, neither will dominate and a mixture of products will be obtained.
What is this?
An arenediazonium salt.
How can aniline be converted to an arenediazonium salt?
By treatment with nitrous acid (HNO2). Because nitrous acid is unstable, it must be formed in situ, using an aqueous solution of sodium nitrite and an acid. N2 is such a good leaving group that the diazonium salt must be synthesized at 0C.
What is a sandmeyer reaction?
The reaction of an arenediazonium salt with a copper(I) salt.
How do you replace a diazonium group with an Iodo substituent?
What is a Schiemann reaction?
How do you synthesize a phenol?
How do you replace a diazonium group with a hydrogen.
What happens in the reaction of a highly activated benzene ring with arenediazonium ion electrophiles?
An azobenzene. The N=N linkage is an azo linkage. It goes to the para position preferentially.
What’s the mechanism for electrophilic aromatic substitution with an arenediazonium ion electrophile?
What’s the mechanism for the formation of a diazonium ion from aniline?
