Chapter 8 Amines

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Chapter 8Amines
2
Structure and Classification

• Amines are classified as 1, 2, or 3 depending
  on the number of carbon groups bonded to
  nitrogen.
• Aliphatic amine All carbons bonded to nitrogen
  are derived from alkyl groups. See the three
  above.

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Structure and Classification

• Aromatic amine One or more of the groups bonded
  to nitrogen are aryl groups.

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Structure and Classification

• Heterocyclic amine An amine in which the
  nitrogen atom is part of a ring.
• Heterocyclic aliphatic amine A heterocyclic
  amine in which the ring is saturated (has no CC
  bonds).
• Heterocyclic aromatic amine The amine nitrogen
  is part of an aromatic ring.

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Nomenclature

• IUPAC names
• We derive IUPAC names for aliphatic amines just
  as we did for alcohols.
• Drop the final -e of the parent alkane and
  replace it by -amine.
• Use a number to locate the amino group on the
  parent chain.

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Nomenclature

• IUPAC names (contd)
• IUPAC nomenclature retains the common name
  aniline for C6H5NH2, the simplest aromatic amine.
• Name simple derivatives of aniline by using
  numbers to locate substituents or, alternatively,
  use the prefixes ortho (o), meta (m), and para
  (p).
• Several derivatives of aniline have common names
  that are still widely used among them is
  toluidine

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Nomenclature

• IUPAC names (contd)
• Name unsymmetrical secondary and tertiary amines
  as N-substituted primary amines.
• Take the largest group bonded to nitrogen as the
  parent amine.
• Name the smaller group(s) bonded to nitrogen, and
  show their location on nitrogen by using the
  prefix N- (indicating that they are bonded to
  nitrogen).

8
Nomenclature

• Common names
• For most aliphatic amines, list the groups bonded
  to nitrogen in alphabetical order in one word
  ending in the suffix -amine.

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Nomenclature

• Amine salts
• When four atoms or groups of atoms are bonded to
  a nitrogen atom, as for example CH3NH3, nitrogen
  bears a positive charge and is associated with an
  anion as a salt.
• Name the compound as a salt of the corresponding
  amine.
• Replace the ending amine (or aniline or pyridine
  or the like) by -ammonium (or anilinium or
  pyridinium or the like) and add the name of the
  anion.

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Examples

• Draw the structures of the following compounds
• t-butylamine 3-methyl-1-butanamine
• 2,4,N,N-tetramehtyl-3-hexanamine

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Physical Properties

• Like ammonia, low-molecular-weight amines have
  very sharp, penetrating odors.
• Trimethylamine, for example, is the pungent
  principle in the smell of rotting fish.
• Two other particularly pungent amines are
  1,4-butanediamine (putrescine) and
  1,5-pentanediamine (cadaverine).

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Physical Properties

• Figure 11.1 Amines are polar compounds
• Both 1 and 2 amines have N-H bonds, and can
  form hydrogen bonds with one another.
• 3 Amines have no N-H bond and cannot form
  hydrogen bonds with one another.

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Physical Properties

• An N-H---N hydrogen bond is weaker than an
  O-H---O hydrogen bond, because the difference in
  electronegativity between N and H (3.0 - 2.1
  0.9) is less than that between O and H (3.5 - 2.1
  1.4).
• We see the effect of hydrogen bonding between
  molecules of comparable molecular weight by
  comparing the boiling points of ethane,
  methanamine, and methanol.

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Physical Properties

• All classes of amines form hydrogen bonds with
  water and are more soluble in water than are
  hydrocarbons of comparable molecular weight.
• Most low-molecular-weight amines are completely
  soluble in water.
• Higher-molecular-weight amines are only
  moderately soluble in water or are insoluble.

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Basicity of Amines

• Like ammonia, amines are weak bases, and aqueous
  solutions of amines are basic.
• The acid-base reaction between an amine and water
  involves transfer of a proton from water to the
  amine.

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Basicity of Amines

• The base dissociation constant, Kb, for the
  reaction of an amine with water has the following
  form, illustrated for the reaction of methylamine
  with water to give methylammonium hydroxide.
• pKb is defined as the negative logarithm of Kb.

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Basicity of Amines
18
Basicity of Amines

• Aliphatic amines have about the same base
  strength, and are slightly stronger bases than
  NH3.
• Aromatic and heterocyclic aromatic.
• amines are considerably weaker bases than
  aliphatic amines.
• Note that while aliphatic amines are weak bases
  by comparison with inorganic bases such as NaOH,
  they are strong bases among organic compounds.

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Basicity of Amines

• Given the basicities of amines, we can determine
  which form of an amine exists in body fluids, say
  blood.
• In a normal, healthy person, the pH of blood is
  approximately 7.40, which is slightly basic.
• If an aliphatic amine is dissolved in blood, it
  is present predominantly as its protonated
  (conjugated acid) form.

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Basicity of Amines

• Assume that the amine, RNH2, has a pKb of 3.50
  and that it is dissolved in blood, pH 7.40 (pOH
  6.60).
• We first write the base dissociation constant for
  the amine and then solve for the ratio of RNH3
  to RNH2.
• Substituting values for Kb and OH- gives

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Basicity of Amines
22
Select the stronger base from each pair of amines
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Reactions of Amines

• The most important chemical property of amines
  is their basicity.
• Amines, whether soluble or insoluble in water,
  react quantitatively with strong acids to form
  water-soluble salts.

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Basicity of Amines

• Example Complete each acid-base reaction and
  name the salt formed.