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Author: Subject: Aromatic substitution reactions
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[*] posted on 24-1-2003 at 02:13
Aromatic substitution reactions


I'd just like to know what the variables are that make different aromatic rings harder or easier to perform substitution reactions on than others, such as nitration?
For example phenol is alot easier to nitrate than benzene and can go all the way to trinitrophenol very easily, whereas trinitrobenzene is extremely difficult to obtain by direct nitration.
This is brought up by the hexanitrobenzene thread, where people have discussed nitrating different aromatics, to everntually get to hexanitrobenzene.
I'd also like to know why, nitrating, benzene aniline or whatever, and then reducing the nitro groups to amine groups makes it easier to nitrate the remaining carbons on the ring?
So why is an aromatic ring is easier to nitrate when it has, for example an amine group, rather then a methyl group, and why dichlorobenzene is so hard to nitrate (is it because of the halogen group?)

I would really appreciate any help on this topic, its really been bothering me that i don't know why this is.
Thanks for your help.
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Microtek
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[*] posted on 24-1-2003 at 07:27


This is probably best explained in textbooks on basic organic chemistry. Substitution reactions on aromatic rings are so basic that any decent beginners textbook will have the info. It is also a very neat single subject so you can likely find a chapter exclusively dedicated to it. All in all I suggest you pick up such a text at your library and read it yourself, because you're simply better off getting the full picture.
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[*] posted on 24-1-2003 at 23:14


From Morrison and Boyd, Organic Chemistry, fifth edition
Quote:

14.4 Determination of relative activity

A group is classified as activating if the ring it is attached to is more reactive than benzene, and it is classified as deactivating if the ring it is attached to is less reactive than benzene. The reactivities of benzene and a substituted benzene are compared in one of the following ways.

The time required for reactions to occur under identical reaction conditions can be measured. Thus, as we just saw, toluene is found to react with fuming sulfuric acid in about one-tenth to one-twentieth the time required by benzene. Toluene is more reactive than benzene, and -CH3 is therefore an activating group.

The severity of conditions required for comparable reaction to occur within the same period of time can be observed. For example, benzene is nitrated in less than an hour at 60 degrees Celsius by a mixture of concentrated sulfuric acid and concentrated nitric acid; comparable nitration of nitrobenzene requires treatment at 90 degrees Celsius with fuming nitric acid and concentrated sulfuric acid. Nitrobenzene is evidently less reactive than benzene, and the nitro group, -NO2, is a deactivating group.

For an exact, quantitative comparison under identical reaction conditions, competitive reactions can be carried out, in which the compounds to be compared are allowed to compete for a limited amount of a reagent. For example, if equimolar amounts of benzene and toluene are treated with a small amount of nitric acid (in a solvent like nitromethane or acetic acid, which will dissolve both organic and inorganic reactants), about 25 times as much nitrotoluene as nitrobenzene is obtained, showing that toluene is about 25 times as reactive as benzene. On the other hand, a mixture of benzene and chlorobenzene yields a product in which nitrobenzene exceeds the nitrochlorobenzenes by 30:1, showing that chlorobenzene is only one-thirtieth as reactive as benzene. The chloro group is therefore classed as deactivating, the methyl group as activating. The activation or deactivation caused by some groups is extremely powerful: aniline, C6H5NH2, is roughly one million times as reactive as benzene, and nitrobenzene, C6H5NO2, is roughly one-millionth as reactive as benzene.

14.5 Classification of substituent groups

The methods described in the last two sections have been used to determine the effects of a great number of groups on electrophilic substitution. As shown in Table 14.3, nearly all groups fall into one of two classes: activating and ortho, para-directing, or deactivating and meta-directing. The halogens are in a class by themselves, being deactivating but ortho, para-directing.

Activating: Ortho, para directors
Strongly activating
-NH2 (-NHR, -NR2)
-OH

Moderately activating
-OCH3 (-OC2H5, etc.)
-NHCOCH3

Weakly activating
-C6H5
-CH3 (-C2H5, etc.)

Deactivating: Meta directors
-NO2
-N(CH3)3+
-CN
-COOH (-COOR)
-SO3H
-CHO, -COR

Deactivating: Ortho, para directors
-F, -Cl, -Br, -I

Just by knowing the effects summarized in these short lists, we can now predict fairly accurately the course of hundreds of aromatic substitution reactions. We now know, for example, that bromination of nitrobenzene will yield chiefly the meta isomer and that the reaction will go more slowly than the bromination of benzene itself; indeed, it will probably require severe conditions to go at all. We now know that the nitration of C6H5NHCOCH3 (acetanilide) will yield chiefly the ortho and para isomers and will take place more rapidly than the nitration of benzene.

Although, as we shall see, it is possible to account for these effects in a reasonable way, it is necessary for you to memorize the calssifications in table 14.3 so that you may deal rapidly with synthetic problems involving aromatic compounds.

Whew! My fingers are tired. If you're still interested, I suggest you take Microtek's advice and grab a textbook; I heartily recommend Morrison and Boyd.
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thumbup.gif posted on 25-1-2003 at 15:40


Thanks alot for that polverone that was just what i was looking for.
Well, i do have a chemistry book, it just didn't cover what groups are activators/deactivators, and what positions on the ring they are likely to take.
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[*] posted on 27-1-2003 at 16:44
One more question?


Ok. i have read more about different groups and the aromatic ring and how they affect further substitution reactions, but i still have one more question.

I understand how different groups affect the positioning of new groups, but what if there are multiple substitutions on nitrobenzene which is a meta director?

For example: If i were to prepare nitrobenzene, and then add a methyl group to the ring (with the freidel-crafts alkylation) it would take the meta position right?
NO2
c
c c

c c - CH3
c

(Shitty drawing but i think you get the point)

Now if i were then to further nitrate that to trinitromethylbenzene where would the next to nitro groups fall? Does the methyl group become the directing group, or is it just not possible or what?

Thankyou for any help.
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[*] posted on 27-1-2003 at 16:47


Damn the diagram didn't work. Oh well as long as you can understand the question.
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[*] posted on 28-1-2003 at 03:08


The next group could attatch to any of the activated sites, in this case all the unoccupied ones. So if you did a mono-nitration of meta-nitrotoluene, you would get a mix of isomers which would be quite hard to separate ( a mess ).
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