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  Preparation of 1º - Amines Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Preparation of 1º- Amines

Even though direct alkylation of ammonia by alkyl halides leads to 1º-amines, unusual procedures are preferred in many cases. These techniques need two steps, but they provide pure product, generally in good yield. The usual approach is to first form a carbon-nitrogen bond by reacting a nitrogen nucleophile with a carbon electrophile. The table lists various general illustrations of this strategy in the rough order of decreasing nucleophilicity of the nitrogen reagent. In the 2nd step, extraneous nitrogen substituents that might have facilitated this bonding are removed to give the amine product.

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A particular instance of each general class is shown in the picture below. In the first two, an anionic nitrogen species goes through an SN2 reaction with a modestly electrophilic alkyl halide reactant. For instance #2 an acidic phthalimide derivative of ammonia has been substituted for the sulfonamide analog listed in the table. The principle is similar for the two examples, as will be noted later. instance #3 is identical in nature, but extends the carbon system by a methylene group (CH2). In all 3 of these methods 3º-alkyl halides cannot be employed because the main reaction path is an E2 elimination.

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The techniques demonstrate by instances #4 and #5 proceed by attack of ammonia, or equal nitrogen nucleophiles, at electrophilic carbon of a carbonyl group. A complete discussion of carbonyl chemistry is presented later, but for current purposes it is enough to recognize that the C=O double bond is polarized so that the carbon atom is electrophilic. The Nucleophile addition to aldehydes and ketones is frequently catalyzed by acids. Anhydrides and Acid halides are even more electrophilic, and do not generally need catalysts to react with nucleophiles. The reaction of ammonia with ketones ketones or aldehydes takes place by a reversible addition-elimination pathway to give imines (compounds having a C=N function). These intermediates are not generally isolated, but are reduced as they are formed (that is in situ). Acid chlorides react with ammonia to provied amides, also by an addition-elimination path and these are reduced to amines by LiAlH4.
The 6th instance is a specialized procedure for bonding an amino group to a 3º-alkyl group (none of the earlier methods accomplishes this). Because a carbocation is the electrophilic species rather poorly nucleophilic nitrogen reactants can be employed. Urea, the diamide of carbonic acid, fits this necessity nicely. The resultant 3º-alkyl-substituted urea is then hydrolyzed to give the amine.
One significant method of preparing 1º-amines, especially aryl amines, uses a opposite technique. Now a strongly electrophilic nitrogen species (NO2(+)) bonds to a nucleophilic carbon compound. This nitration reaction provides a nitro group that can be reduced to a 1º-amine by any of various reduction procedures.

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