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Amines

Amines. The organic bases. Categorizing Amines. Amines are categorized by the number of alkyl groups attached to nitrogen : 1º (primary amine) R N H 2 2º (secondary amine) R 2 N H 3º (tertiary amine) R 3 N 4º (quaternary amine salt) R 4 N +. Naming simple amines.

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Amines

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  1. Amines The organic bases

  2. Categorizing Amines • Amines are categorized by the number of alkyl groups attached to nitrogen: 1º (primary amine) RNH2 2º (secondary amine) R2NH 3º (tertiary amine) R3N 4º (quaternary amine salt) R4N+

  3. Naming simple amines • Simple 1º amines are named as “alkylamine” • Examples: • methylamine CH3NH2 • ethylamine CH3CH2NH2 • butylamine CH3CH2CH2CH2NH2 • Symmetrical 2º or 3º amines are named as “dialkylamine” or “trialkyamine” • Examples: • diethylamine (CH3CH2)2NH; trimethylamine (CH3)3N

  4. Naming more complex amines • Amines with more than one type of alkyl group may be named as N-substituted primary amines. The longer alkyl chain determines the base name. • Examples • N-methylpropylamine CH3NHCH2CH2CH3 • N,N-dimethylethylamine (CH3)2NCH2CH3

  5. Naming more complex amines • Amines that have more than one functional group may be named using “amino” as a substituent on the parent molecule. • Examples: • 2-aminoethanol H2NCH2CH2OH • 4-aminobutanoic acid H2NCH2CH2CH2CO2H

  6. Heterocyclic amines • Some amines have a nitrogen as part of a ring. These generally have common (non-systematic) names, which should be memorized: aniline pyridine pyrimidine pyrrole quinoline indole imidazole benzimidazole

  7. Structure of amines • Amines have an sp3 hybridized nitrogen • In principle, tertiary amines with three different R groups should be chiral(i.e., have a stereocenter). • However, rapid pyramidal inversion of the amine nitrogen prevents isolation of the enantiomers except where the nitrogen is part of a ring or has other geometrical constraint. fast

  8. Properties of amines • Amines are moderately polar and are capable of hydrogen bonding. • Low MW amines (up to about C5) are soluble in water; higher MW amines will dissolve in acidic solution (as their conjugate acid). • Many amines have foul odors. • Amines are weak bases.

  9. Example of biologically active amines

  10. More biologically active amines…

  11. More biologically active amines…

  12. More biologically active amines…

  13. More biologically active amines…

  14. Basicity of amines • Amines are slightly basic. This because they have a lone pair of electrons to donate to a proton. This same feature makes them nucleophiles. • Typical amines have Kb values = 10-3 to 10-4

  15. Basicity of amines… • However, instead of measuring an amine’s basicity using the above equilibrium, chemists usually refer to the acidity of the conjugate acid of the amine. The weaker the conjugate acid, the stronger the base strength of the amine. • Typical amines have Ka values (of their conjugate acids) of 10-10 to 10-11 (pKa values of 10 to 11.)

  16. pKa values of conjugate acids of amines CH3NH2 10.7 CH3CH2NH2 10.8 (CH3)2NH 10.7 (CH3CH2)2NH 10.5 (CH3)3N 9.8 (CH3CH2)3N 11.0 All are about the same value. The same substituent effects that stabilize carbocations that are more highly substituted by alkyl groups are offset by diminished stabilization by solvent due to crowding.

  17. Some amines that are weaker bases (conj. acid) • Aniline is a weaker base because the lp of electrons is delocalized by resonance into the aromatic ring. • Pyridine is weaker because it is an imine (C=N). • Pyrrole is much weaker because the lp of electrons is delocalized with the other p electrons to make 6 p e-. Therefore, the lp is unavailable to act as a base. aniline pyridine pyrrole pKa = 4.6 5.2 0.4

  18. Substituent Effects on Basicity of Aniline 2 explanations of weaker basicity of nitroaniline: Resonance stabilizes free base, destabilizes its protonated form (see next slide)

  19. Substituent Effects on Basicity of Anilines

  20. An amine that is a stronger base • There is one type of amine that is a stronger base: • Guanidine is a strong base because its conjugate acid isstabilized by resonance: guanidine pKa = 13.6 (conj. acid) A guanidine group is part of the structure of the amino acid arginine.

  21. Amines are Protonated at Physiological pH • Recall the Henderson-Hasselbalch equation: • Consider the neurotransmitter dopamine, a typical amine (having a pKa of its conjugate acid = 10.6) in a living cell (buffered at pH = 7.3): • That is, the concentration of the protonated amine is 2000x that of the neutral amine! Typical amines are >99.9% protonated at physiological pH.

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