Amines aromatic 257

Aromatic amines may be divided into three classes  

Primary aromatic amines are prepared by the following procedures. 

The replacement of a phenolic hydroxyl group by an amino group (Expt 6.55). Secondary and tertiary amines are prepared by  

Methods for the protection of the amino and imino groups are discussed in Section 6.5.6. 

SUMMARY OF RETROSYNTHETIC STRATEGIES Functional group interconversion (FGI) (method 1) 

Aromatic amines are widely used as curing agents for epoxy resins. However, they are not used as widely in adhesive formulations as they are in composites, molding compounds, and castings. They offer cured epoxy structures with good heat and acid resistance. 

The lower reactivity of the aromatic amines in adhesive formulations is an advantage in that epoxy resin mixtures can be B-staged at room temperature (react to a glassy but fusible and thermoplastic intermediate structure) and will not fully cure for months. In this way, dry films and solid powders can be formulated as elevated-temperature curing, one-component adhesives with long shelf life. 

The color of aromatic amines is poor (dark), and they stain easily. They are generally solid materials that require some formulating at elevated temperatures to produce a product that can be easily handled. The vapors resulting from elevated temperatures can cause staining, and their irritancy can be a problem. Certain aromatic amines such as diamin-odiphenylmethane are carcinogenic. 

When compared to aliphatic amines, aromatic amines generally have reduced exotherm and reactivity. Elevated temperatures are required to achieve optimum properties. In certain cases aromatic amines can be cured at room temperature with catalysts such as phenols, BF3 complexes, and anhydrides. 

Examples of aromatic amines are shown in Fig. 5.5. Of these compounds the most common are mefa-phenylene diamine (MPDA), methylene dianiline (MDA), and eutectics of the two. 

Secondary aromatic amines and, in part, aromatic diamines are very efficient hydrogen donors (H donors). The reaction products can react further with the radicals, similar to phenols. The use of amine stabilizers can lead to discoloration of the plastic due to the formation of azo compounds (chinonimines). Therefore, the use of aromatic amines is limited to carbon-black filled elastomers as well as other substrates where discoloration is not an issue. 

The sequence of important reactions forming amines from the essential amino-acids phenylalanine and tryptophan is worthy of attention because of the physiological role of these amines in many animals. Little is known of their effects in insects but they have been shown to be present in insects as neurotransmitters. 

Aniline, the first representative of the aromatic amines, is highly important in the industrial context as the basis for the synthesis of a large number of dyestuffs. Aniline and 

Aniline C6H5NH2 and the most common aromatic anilines  

Close to their boiling points of approximately 200 °C (aniline boils at 184 °C), certain amines, if totally anhydrous, can severely attack aluminium. As in organic acids, traces of humidity prevent this attack. 

Varney and Preston [42] discussed the measiuement of trace aromatic amines in estuary and sea water using high performance liquid chromatography. Aniline, methyl aniline, 1-naphthylamine and diphenylamine at trace levels were determined using this technique and electrochemical detection. Two electrochemical detectors (a thin layer, dual glassy carbon electrode cell and a dual porous electrode system) were compared. The electrochemical behaviour of the compounds was investigated using hydrodynamic and cyclic voltammetry. Detection limits of 15 and 1.5nM 

In view of the industrial importance of this class of compounds, several studies to evaluate methods of degradation of some aromatic amines in aqueous wastes and spillage have been performed and published in the literature. 

Ozonation has been proved to be effident for degrading benzidine and 2-naphthylamine in aqueous media (173-174), and loss of mutagenic effect was observed. In other oxidation methods that have been evaluated (175-182), the amines are oxidized to quin-one imines, which can easily be reduced back to the original amines by, for example, ascorbic acid (176). The most promissing oxidation technique seems to be use of potassium permanganate/sulfuric acid, and this method was investigated further (183) for a number of amines, including 4-nitrobiphenyl, a metabolite of 4-aminobiphenyl. 

Oxidation of aromatic amines by hydrogen peroxide, catalysed by horseradish peroxidase (184), has also been investigated and is outlined below. 

Pliss (185) showed that bromination could degrade 3,3 -dichlorobenzidine, and this method might be useful for decontamination of other aromatic amines. 

Waste categories Recommended destruction method no. (in order of preference) Aromatic 4-Nitro-amines biphenyl  

Secondary amines. The NH deformation absorption, which is a strong to medium intensity band in primary amines, is usually extremely weak in secondary aliphatic amines, so that it frequently cannot be detected at all with the film thicknesses normally employed. With secondary aromatic amines the position is confused to some extent by the presence of the ring C=C stretching absorptions in the same region, coupled with the fact that in certain cases these become intensified when a nitrogen atom is directly attached to the ring [31]. 

In ahphatic secondary amines this band is too weak to be detected readily. In cyclic bases, for example, only the normal aromatic absorptions are found in this region unless salt formation occurs [58]. Similarly, Barr and Haszeldine [74] could not identify this band in dimethyl or diethylamine, although a band near 1500 cm was traced in some fluorinated derivatives. Imines with the structure —C=NH are similar to other secondary amides in that any NH deformation absorption shown is extremely weak, but of course they also show an absorption in the same region corresponding to the terminal C=N linkage. 

Aromatic amines. Colthup[21] has given the following correlation for aromatic amines. A strong band appears as follows  

These are based on his own work, and no publications have dealt in detail with these particular correlations. However, its persistence in 

The intensity of these bands appears to be rather variable, some being relatively strong and others of not more than medium intensity. This may ako be associated with some differences in substitution or other structural features. Lieber et al. [22] have also found these bands in a series of thirteen tetrazoles of various types. More recently Hadzi [70] has studied the effects of deuteration upon this absorption in aniline derivatives. The band near 1260 cm moves to near 1350 cm . This identifies it with a C—N stretching mode, which in the hydrogen compounds is lowered in frequency by coupling with the NH deformation frequency. 

N-Ethylaniline was alkylated by reaction with benzyl chloride under liquid-liquid PTC conditions in the presence of 30% sodium hydroxide solution and CTAB as a catalyst. Microwave irradiation (25 min) of the reaction mixture in a sealed vessel afforded N-benzyl-N-ethylaniline in 90% yield, compared with 16 h of conventional heating (oil bath) (Eq. 30) [24]. 

A facile synthesis of a series of N-alkylpyrrolidino[60]fullerenes by solvent-free PTC under the action of microwaves has been described (Eq. 31). 

The synergy between the dry media and microwave irradiation was convincingly demonstrated in this work. For instance, with the allyl compound, the yield is only 16% after 24 h in toluene under reflux, and no reaction occurs after 10 min at 100 °C (classical heating), thus revealing an important specific microwave effect. 

It was found that the obtained adducts accelerated the cure of UPRs. The efficiency of the tested accelerators was characterized by the gelation time and peak exotherm temperature. The shortest gelation time was obtained for the adduct of p-toluidine and bisphenol A or p,p -dihydroxydiphenylmethane based low-molecular weight epoxy resins. Adducts remained in the polyester resin as permanently bonded fragments and could not be extracted with methylene chloride from the cured UPR. Moreover, products extracted from the crossHnked resins were determined by the chromatographic and gravimetric methods. 

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C. A typical aromatic amine. Best prepared by the prolonged action of concentrated ammonia solution at a high temperature upon anthraquinone-l-sulphonic acid in the presence of BaClj and by reduction of the corresponding nitro compound or by amination of the chloroanthraquinone. 

They are prepared by the action of HNO2 on aromatic amines. The amine is dissolved in excess of mineral acid and sodium nitrite is added slowly until a slight excess of HNO2 is present. The reaction is usually carried out in ice-cold solution. The solution then contains the diazonium salt of the mineral acid used, anhydrous diazonium salts of unpredictable stability may be precipitated with complex anions like PF , SnCl6 BF4 . 

The most important reaction of the diazonium salts is the condensation with phenols or aromatic amines to form the intensely coloured azo compounds. The phenol or amine is called the secondary component, and the process of coupling with a diazonium salt is the basis of manufacture of all the azo dyestuffs. The entering azo group goes into the p-position of the benzene ring if this is free, otherwise it takes up the o-position, e.g. diazotized aniline coupled with phenol gives benzeneazophenol. When only half a molecular proportion of nitrous acid is used in the diazotization of an aromatic amine a diazo-amino compound is formed. 

Doebner-von Miller reaction Condensation of an aromatic amine with an aldehyde or ketone in the presence of hydrochloric acid to form a quinoline derivative. A general method, thus aniline and ethanal give 2-methyl-quinoline (quinaldine) and p-phenetidine. 

SchifT s bases A -Arylimides, Ar-N = CR2, prepared by reaction of aromatic amines with aliphatic or aromatic aldehydes and ketones. They are crystalline, weakly basic compounds which give hydrochlorides in non-aqueous solvents. With dilute aqueous acids the parent amine and carbonyl compounds are regenerated. Reduction with sodium and alcohol gives... 

Antioxidant and deactivation additives substituted phenols, dithiophosphates, dithiocarbamates, alkylated aromatic amines. 

A halogen atom directly attached to a benzene ring is usually unreactive, unless it is activated by the nature and position of certain other substituent groups. It has been show n by Ullmann, however, that halogen atoms normally of low reactivity will condense with aromatic amines in the presence of an alkali carbonate (to absorb the hydrogen halide formed) and a trace of copper powder or oxide to act as a catalyst. This reaction, known as the Ullmant Condensation, is frequently used to prepare substituted diphenylamines it is exemplified... 

B) Secondary amines, (i) Aromatic amines. Monomethyl and monoethylaniline, diphenylamine. (ii) Aliphatic and other amines. Diethyhmine, di-n-propylamine, di-isopropylamine. Also piperidine piperazine diethylene-diamine). 

A) PRIMARY AROMATIC AMINES. RNH. Aniline, o-, m-, and p-toluidine (and other nuclear-substitiited anilines) 1- and 2-naphthylamines. (For note on Aliphatic Amines, cf. p. 375 )... 

Almost insoluble in cold water. Higher alcohols (including benzyl alcohol), higher phenols (e.g., naphthols), metaformaldehyde, paraldehyde, aromatic aldehydes, higher ketones (including acetophenone), aromatic acids, most esters, ethers, oxamide and domatic amides, sulphonamides, aromatic imides, aromatic nitriles, aromatic acid anhydrides, aromatic acid chlorides, sulphonyl chlorides, starch, aromatic amines, anilides, tyrosine, cystine, nitrocompounds, uric acid, halogeno-hydrocarbons, hydrocarbons. 

Note. Useful information can often be obtained by adding (i) dilute H,SO or (ii) dilute NaOH solution to an aqueous solution of the substance under investigation. A precipitate with (i) usually indicates an aromatic carboxylic acid from a metallic or from an ammonium salt. A precipitate or oil with (ii) usually indicates an aromatic amine from an amine salt. 

Aromatic amine produced,. mine salts anilides aromatic amino-carboxylic acids sulphanilic acid, sulphanilamide. 

Various basic substances, such as aromatic amines (naphthyl-amines dissolve with difficulty in dil. HCl, diphenylamine only in cone. HCl, triphenylamine insoluble) nitro-anilines some amino-carboxylic acids. 

The experimental technique is similar to that given under Aromatic Amines, Section IV,100,2. The following alternative method may also be used. Mix together 0 -5-0 - 8 ml. of the polyhydroxy compound, 5 ml. of pyridine and 2 -5 ml. of redistilled benzoyl chloride in a 50 ml. flask. 

The above is a general procedure for preparing trialkyl orthophosphates. Similar yields are obtained for trimethyl phosphate, b.p. 62°/5 mm. triethyl phosphate, b.p. 75-5°/5 mm. tri-n-propyl phosphate, b.p. 107-5°/5 mm. tri-Mo-propyl phosphate, b.p. 83-5°/5 mm. tri-wo-butyl phosphate, b.p. 117°/5-5 mm. and tri- -amyl phosphate, b.p. 167-5°/5 mm. The alkyl phosphates are excellent alkylating agents for primary aromatic amines (see Section IV,41) they can also be ua for alkylating phenols (compare Sections IV,104-105). Trimethyl phosphate also finds application as a methylating agent for aliphatie alcohols (compare Section 111,58). 

To 5 ml. of water add 1-2 drops of the amine if the amine does not dissolve, add a drop or two of concentrated hydrochloric acid. Add 0-5-1 ml. of this amine solution to 2-3 ml. of the reagent an almost immediate precipitate indicates the presence of a primary amine. A slight turbidity indicates the presence of a primary amine as an impurity. (Primary aromatic amines generally require 2-3 minutes for the test. Urea and other amides, as well as amino acids, do not react.)... 

Benzenesulphonyl or p-toluenesulphonyl derivatives. These are generally very satisfactory. For experimental details, see under Aromatic Amines, Section IV,100,d. 

AHOMATIC AMINES AND THEIR SIMPLE DERIVATIVES AROMATIC AMINES... 

Tertiary aliphatic - aromatic amines, unlike those of the aliphatic series, react with nitrous acid with the formation of G-nitroso compounds the nitroso group enters almost exclusively in the para position if available, otherwise in the ortho position. Thus dimethylaniline yields />-nitrosodiniethylaniline ... 

It is convenient to include under Aromatic Amines the preparation of m-nitroaniline as an example of the selective reduction of one group in a polynitro compound. When wt-dinitrobenzene is allowed to react with sodium polysulphide (or ammonium sulphide) solution, only one of the nitro groups is reduced and m-nitroanUine results. Some sulphur separates, but the main reaction is represented by ... 

Acetyl derivatives of aromatic amines may be prepared either witli acetic anhydride or acetic acid or with a mixture of both reagents. Primary amines react readily upon warming with acetic anhydride to yield, in the first instance, the mono-acetyl derivative, for example ... 

In general, benzoylation of aromatic amines finds less application than acetylation in preparative work, but the process is often employed for the identification and characterisation of aromatic amines (and also of hydroxy compounds). Benzoyl chloride (Section IV, 185) is the reagent commonly used. This reagent is so slowly hydrolysed by water that benzoylation can be carried out in an aqueous medium. In the Schotten-Baumann method of benzoylation the amino compound or its salt is dissolved or suspended in a slight excess of 8-15 per cent, sodium hydroxide solution, a small excess (about 10-15 per cent, more than the theoretical quantity) of benzoyl chloride is then added and the mixture vigorously shaken in a stoppered vessel (or else the mixture is stirred mechanically). Benzoylation proceeds smoothly and the sparingly soluble benzoyl derivative usually separates as a solid. The sodium hydroxide hydrolyses the excess of benzoyl chloride, yielding sodium benzoate and sodium chloride, which remain in solution ... 

SULPHONATION OF AROMATIC AMINES If aniline is treated with excess of concentrated sulphuric acid and the resulting mixture, which contains aniline sulphate, is heated at 180° until a test portion when mixed with sodium hydroxide solution no longer liberates aniline, p-aminobenzenesulphonic acid or sulphanilic acid is formed this separates as the dihydrate upon pouring the cooled mixture into water. The reaction prohahly proceeds as follows ... 

Primary aromatic amines differ from primary aliphatic amines in their reaction with nitrous acid. Whereas the latter yield the corresponding alcohols (RNHj — ROH) without formation of intermediate products see Section 111,123, test (i), primary aromatic amines 3neld diazonium salts. Thus aniline gives phcnyldiazonium chloride (sometimes termed benzene-diazonium chloride) CjHbNj- +C1 the exact mode of formation is not known, but a possible route is through the phenjdnitrosoammonium ion tlius ... 

Salts of primary aromatic amines react with solutions of alkali cyanates to yield first the amine cyanate, which then undergoes molecular rearrangement to the arylurea, for example ... 

This preparation illustrates the direct iodination of a primary aromatic amine by iodine the sodium bicarbonate removes the hydrogen iodide as formed ... 

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