Amines Help (page 2)
Amines are alkyl or aryl derivatives of NH3. Replacing one, two, or three H's of NH3 with alkyl groups gives primary (1°), secondary (2°), or tertiary (3°) amines, respectively. Examples are shown below. All four H's of NH4+ can be replaced with alkyl groups to give quaternary (4°) ammonium salts. Amines are named by adding the suffix -amine to the name of the alkyl group attached to N or to the longest alkane chain attached to N.
Aromatic and cyclic amines often have common names such as aniline, pyridine, and piperidine.
Alkylation of NH3, RNH2, and R2NH with RX. Amines are nucleophiles, and hence they can be alkylated with alkyl halides. This reaction tends to produce a mixture of products, as each of the products are also reactive.
Reduction of N-Containing Compounds. Nitro compounds can be reduced with LiAlH4, Zn/HCl, or H2/Pt to produce the corresponding amine.
Nitriles and amides are most effectively reduced LiAlH4. Other functional groups, notably oximes and azides, can also be reduced to amines.
Reductive Amination of Carbonyl Compounds. Ketones and aldehydes can be converted to amines in a 2 step procedure that involves an imine (sometimes called a Schiff base) intermediate. Imines are formed by addition of a 1° or 2° amine to the carbonyl, usually with removal of water to drive the reaction to completion. The imine can then be reduced, usually with sodium cyanoborohydride (NaCNBH3) to produce the new amine
Alkylation of Imides; Gabriel Synthesis of 1° Amines. To circumvent the difficulty of monalkylation of amines, the Gabriel synthesis is often used. Phthalimide is deprotonated with base and alkylated with R-X, then the imide is hydrolyzed to produce the monoalkyl amine.
This method, even though it requires 2 steps, results in good yields of monoalkylamine
Hofmann Degradation of Amides. Amides can be degraded by one carbon to produce amines by the Hofmann degradation. This procedure involves the treatment of the amide with Br2, which results in an N-bromoamide. Deprotonation at nitrogen triggers a rearrangement in which a highly reactive, electron-deficient nitrene is formed, The alkyl R group migrates to nitrogen, resulting in an isocyanate. Hydrolysis and decarboxylation of the isocyanate produces the amine.
Stereochemistry. Amines with three different substituents and an unshared pair of electrons are chiral. However, in most cases, these chiral amines cannot be resolved. The amine undergoes a very rapid nitrogen inversion similar to that for a C undergoing an Sn2 reaction. This inversion of stereochemistry, essentially the rapid racemization of a chiral amine, is shown below.
Acid-Base Chemistry. The lone pair of electrons on amines make them both nucleophiles and bases. Thus, amines will react with acids or with electrophiles.
Acylation. 1° and 2° amines react with acid halides or esters to form amides. Overall this reaction results in transfer of the acyl group to the amine.
Reaction of 1° and 2° amines with benzenesulfonyl chloride (the Hinsberg Reaction) is quite similar to the reaction of these amines with carboxylic acid chlorides. The resulting sulfonamides are soluble in base if the amine was ammonia or a 1° amine [the remaining proton(s) are acidic]; but they are not soluble in base if the amine was 2°. Tertiary amines do not form stable products with benzenesulfonyl chloride.
Reaction with Isocyanates. Isocyanates are reactive electrophiles and are readily attacked by NH3 and 1° and 2° amines. The product of this addition is a urea. Again, 3° amines do not form stable products.
Reactions of Quaternary Ammonium Salts
Quaternary ammonium salts are not nucleophilic or basic, since there is no lone pair of electrons on nitrogen as there is in amines. Upon heating in the presence of strong base, quaternary ammonium salts undergo the Hofmann elimination, an E2 elimination in which an alkene and a 3° amine are formed.
Amines react with nitrous acid (HONO) to form diazonium ions. Alkyl diazonium ions decompose too quickly to be useful, but aryl diazonium ions have rich and highly useful chemistry. The N2 molecule is an extremely good leaving group, so it can be substituted by a number of different nucleophiles. Aryl halides can be prepared by adding CuCl or CuBr to the diazonium ion (the Sandmeyer reaction). The N2 can be replaced with a hydrogen by treatment of a diazonium ion with hypophosphorus acid (H3PO2). Examples of these substitution reactions of aryl diazonium ions are shown below.
The N–H stretching and NH bending frequencies occur in the IR spectrum at 3050–3550 cm–1 and 1600–1640 cm–1 respectively. In the N–H stretching region, 1° amines and unsubstituted amides show a pair of peaks for a symmetric and an antisymmetric vibration. In the 1H NMR spectrum, N–H proton signals of amines fall in a wide range (1 – 5 ppm) and are often very broad to due chemical exchange. The signals of N–H protons of amides, appearing at 5 – 8 ppm, are even broader. The mass spectra of amines show an odd mass for parent ions if there is an odd number of nitrogens, and an even mass if there is an even number of nitrogens. For example, Et3N has a molecular weight of 101.
Practice problems for these concepts can be found at: Amines Practice Problems
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