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UK CHE 232 - Aromatic amines

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Chapter 25a: Aromatic aminesSuggested Problems: 3, 4, 5, 6, 7, 8, 21, 27, 28 (Problems in bold face are HIGHLYrecommended).Aromatic amines are certainly related in their behavior to the aliphatic amines we have justdiscussed. However, because of the highly conjugated benzene rings attached to them, the lonepair on aromatic amines is far less “active” than the lone pair on aliphatic amines (you should beable to draw resonance structures showing the delocalization of the nitrogen lone pair over thering). This means that they are less basic, and less nucleophilic. However, they are fantasticsynthetic “handles” on a benzene ring, and thus quite useful synthetically.PreparationThere is basically one method commonly used to prepare aromatic amines, and you havealready seen it. The reduction of aromatic nitro compounds (WHICH YOU SHOULD KNOWHOW TO PREPARE) leads to the aromatic amine under quite mild conditions. While there are anumber of methods that can be used, the one I would remember is SnCl2 / HCl followed by NaOHin water:ClHNO3H2SO4ClNO21) SnCl2 / HCl2) NaOH / H2OClNH2And there you have it – this is how you make an aryl amine.ReactionsOne of the main reasons we organikers are so fond of aryl amines is that they are VERYreactive. It is frequently possible to substitute the aromatic ring with an electrophile in the absenceof catalyst!!!! Just to give you an example, I’ll babble about some work I did as a graduatestudent. I was interested in investigating some materials used as X-ray contrast agents forbiological imaging. The common thread among most contrast agents is the presence of LOTS ofheavy atoms, in particular, Iodine. My goal was to prepare 1,3,5-triiodo-2,4,6-benzenetricarboxylic acid. Triiodobenzene is commercially available, but difficult to work with (besides,how would you stick 3 deactivating carboxyl groups on a ring that’s already deactivated?)Benzene tricarboxylic acid is also available, but it is impossible to halogenate. The trick turned outto be using the amino dicarboxylic acid!!! The amino group is so incredibly activating, it was veryeasy to halogenate the ring. What we did once the ring was halogenated will be described in thenext section...COOHCOOHIIHOOCIWe Wanted:COOHCOOHHOOCIIIOCl OMeAlCl3XBZZZT! WRONG! Too deactivated...I2 / H2SO4Periodic acidXBZZZT! WRONG! WAY Too deactivated...NH2COOHHOOCI2NH2COOHIIHOOCIOkay, NOW we're getting somewhereSo, an amino group is powerfully activating – can this cause problems? Of course. It isfrequently necessary to “tone down” the reactivity of an aromatic amine, in order to get onlymonosubstitution, or for Friedel-Crafts reactions to work at all(!). The absolute best way to dothis is to turn the amine into an amide (usually by allowing it to react with acetic anhydride /pyridine). The lone pair on nitrogen becomes a bit more strongly delocalized out the amide end,thus leaving the ring a bit less activated. After the reaction is finished, the amide can be cleaved togive back the amine with NaOH / water (note: an amide is a bulky group, therefore additionsusually go para first):NH2ClAlCl3No reaction (actually, a foul mess)NH2OOOPyridineHNOClAlCl3HNONaOHH2ONH2The next cool reaction you can do with aromatic amines it the Sandmeyer. This reactionis probably the neatest thing about aromatic compounds in general. The Sandmeyer proceeds intwo stages. First, reaction with sodium nitrite to give the diazo compound. Basically, you’renow looking at a benzene ring with nitrogen gas stuck onto it. Who could ask for a better leavinggroup? Next, you add your favorite nucleophile as a copper salt (for example, if you wanted toadd bromide, you’d dump in some CuBr. If you wanted to add cyanide, dump in CuCN, etc).Warm that puppy up for a few minutes, and voila! Instant substitution! First, a simpler example:NH2NaNO2aq. HClNNCuBrBrThere are a couple of fairly cool modifications that you can do here. First, if you addhypophosphorous acid (H3PO2) to a diazonium salt, the aromatic ring just gets “protonated.” Thisis usually the best way to make 1,3,5-trihalobenzenes (remember - halogens are o,p-directing!):NH2I2NH2IIINaNO2aq. HClNIIINH3PO2IIIThis is one of the best (and most common) ways to “abuse” an aromatic amine - you justuse it for its activating and directing effects, then you throw it away.Another modification can be used to make phenols: If you just add water to the diazoniumsalt, you add an OH group to the ring:NNH2OOHAnd now, back to the X-ray contrast agent thingie. I had everything in place except that Ihad an amine where I wanted a carboxylic acid. This looks like a job for Super Sandmeyer!NH2COOHIIHOOCINaNO2aq. HClNCOOHIIHOOCINCuCNNaCNCCOOHIIHOOCINH2SO4I just diazotized the amine, then stirred it with a lovely toxic mixture of CuCN and NaCN.After I had isolated the product, I cooked it up with conc. H2SO4 to give the desired triacid.Chemistry sometimes works...Read the section on diazonium coupling reactions, be familiar with it. It is pretty clear, so Iwon’t go over it much in class...And that’s aromatic


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UK CHE 232 - Aromatic amines

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