Transcript Amines
Amines Introduction to Amines • Amines are derivatives of ammonia • Amines are designated as primary, secondary, or tertiary Examples of Amines • There are hundreds of examples of amines found in natural products Introduction to Amines • Some naturally occurring amines take part in neurochemistry Introduction to Amines • The trigonal pyramidal nitrogen atom of an amine carries a partial negative charge on its lone pair • Act as base or nucleophile Relative Basicity • Amines are stronger bases than alcohols, ethers, or water • Solubility can be increased following protonation of amine. • Amines establish an equilibrium with water; amine is protonated, producing hydroxide • Higher pKa of the corresponding ammonium ion (RNH3+) → weaker acid and stronger conjugate base. Review: Nomenclature of Amines • There are multiple methods for naming 1° amines 1. Name the alkyl group ending in “yl” followed by the word, amine Review: Nomenclature of Amines • There are multiple methods for naming 1° amines 2. Find the carbon parent chain, and use the suffix, amine Review: Nomenclature of Amines • There are multiple methods for naming 1° amines 3. Use the term, amino, for the group as a side group Review: Nomenclature of Amines • There are multiple methods for naming 1° amines 4. If the amine is directly attached to an aromatic ring, it may be called aniline Review: Nomenclature of Amines • There are multiple methods for naming 2° amines 1. Name the alkyl groups ending in “yl” followed by the word, amine Review: Nomenclature of Amines • There are multiple methods for naming 2° amines 2. Find the carbon parent chain, and use the suffix, amine. Smaller groups attached to the amine should be named as alkyl groups with the N-locant Properties of Amines • The N atom in typical amines is sp3 hybridized • The angles are 108 degrees. • The C-N bond lengths are 147 pm, which is slightly shorter than C-C bonds. Chirality Is Possible (But Not Observed) • An amine with three different substituents on nitrogen is chiral (in principle but not in practice) : the lone pair of electrons is the fourth substituent • Most amines that have 3 different substituents on N cannot be resolved because the molecules interconvert by pyramidal inversion Amines Form H-Bonds • Amines with fewer than five carbons are water-soluble • Primary and secondary amines form hydrogen bonds, increasing their boiling points Properties of Amines • • • Many small molar mass amines stink!!! These are both produced from the degradation of amino acids, and responsible for the odor of putrefying flesh. This is why zombies are unpleasant party guests! Review: Preparation of Amines Preparation of Amines via substitution Selective Preparation of Primary Amines: Azide Synthesis • Azide ion, N3 displaces a halide ion from 1°or 2°alkyl halide to give an alkyl azide, RN3 • Alkyl azides are not nucleophilic (but they are explosive) • Reduction gives the primary amine Preparation of Amines via Substitution Reactions: Gabriel Synthesis of 1° amines • The Gabriel synthesis produces primary amines via potassium phthalimide Preparation of Amines via Reductive Amination • Ketones and aldehydes can be converted to imines, and if a reducing agent is present, the imine can be reduced in situ to form an amine Reductive Amination Is Versatile • Ammonia, primary amines, and secondary amines yield primary, secondary, and tertiary amines, respectively Mechanism of Reductive Amination Mechanism of Reductive Amination: Reducing Step • Sodium cyanoborohydride, NaBH3CN, reduces C=N but not C=O • Stable in water Acylation of Amines • Amines can attack acyl chlorides • Polysubstitution is not observed. • An acyl group can act as a protecting group. • An acyl group can help to limit the reactivity of amines in Electrophilic Aromatic Substitution reactions Acylation of Amines • Aromatic amines cannot undergo Friedel Crafts directly as addition of AlCl3 deactivates ring Acylation of Amines • To achieve ring alkylation, first the amine must be acylated Hofmann and Curtius Rearrangements • Carboxylic acid derivatives can be converted into primary amines with loss of one carbon atom by both the Hofmann rearrangement and the Curtius rearrangement Hofmann Elimination • Converts amines into alkenes • NH2 is very a poor leaving group so it is converted to an alkylammonium ion, which is a good leaving group • The less substituted (less stable) product is observed as the base must abstract a hydrogen from the most sterically accessible, least hindered position Reactions of Aryldiazonium Ions • Alkyl diazonium salts are extremely reactive (potentially explosive) because of the excellent N2 leaving group • Aryl diazonium salts are a bit more stable and synthetically versatile Reactions of Aryldiazonium Ions • Treatment with a copper salt yields an aryl halide or nitrile Reactions of Aryldiazonium Ions • There are many other diazonium salt substitutions • Using this synthesis, amines can be used as a directing group and subsequently replaced with a –H atom Diazonium Coupling Reactions • Arenediazonium salts undergo a coupling reaction with activated aromatic rings, such as phenols and arylamines • The electrophilic diazonium ion reacts with the electronrich ring of a phenol or arylamine • Usually occurs at the para position but goes ortho if para is blocked Azo Dyes • Azo-coupled products have extended conjugation that lead to low energy electronic transitions that occur in visible light • Produce brightly colored dyes Heterocyclic Amines • A heterocycle is a cyclic compound that contains atoms of two or more elements in its ring, usually C along with N, O, or S Pyrole and Imidazole • Pyrole is an amine and a conjugated diene • Is also aromatic Nitrogen Heterocycles • Imidazole is another simple heterocycle that is aromatic. • Histamine contains an imidazole ring Nitrogen Heterocycles • Pyridine is a six-membered heterocycle • The lone pair on N in pyridine is not necessary to achieve 4n+2 pi electrons • Pyridine is significantly less basic than typical alkyl amines. Spectroscopy of Amines • Amines generally give broad IR peaks between 33003500 cm-1 that are less intense than O-H stretching – Recall that 1° amines give two N-H stretch IR signals (symmetric and asymmetric stretching) – 2° amines give one N-H stretch IR signal – 3° amines do not give N-H stretching signals – 3° amines can give a N-H stretching signal if treated with an acid like H-Cl. • Protonated amines show an ammonium band in the range 2200 to 3000 cm1 Examples of Infrared Spectra NMR Spectroscopy of Amines • N-H protons generally appear between 0.5 and 5.0 ppm in the 1H NMR – Solvent, concentration, and temperature can affect the chemical shift • Because N-H protons are generally exchanged at a faster rate than the NMR timescale, N-H protons generally give broad signals with no splitting • If a deuterated solvent is used, N-H proton peaks disappear completely. Chemical Shift Effects • Hydrogens on C next to N absorb at lower field than alkane hydrogens • N-CH3 gives a sharp three-H singlet at 2.2 to 2.6 13C NMR • Carbons next to amine N are slightly deshielded - about 20 ppm downfield from where they would absorb in an alkane Mass Spectrometry • A compound with an odd number of nitrogen atoms has an odd-numbered molecular weight and a corresponding parent ion • Alkylamines cleave at the C–C bond nearest the nitrogen to yield an alkyl radical and a nitrogencontaining cation Mass Spectrum of Ethylpropylamine • The two main modes of a cleavage give fragment ions at m/z = 58 and m/z = 72.