Chapter 16: Amines

-N--N---

Amines are classified as 1°, 2°, or 3° depending on the number of carbon groups bonded to nitrogen. CH3-NH2 CH3-N-CH3

H

CH3-N-CH3

CH3

Methylamine(a 1° amine)

Dimethylamine(a 2° amine)

Trimethylamine(a 3° amine)

Low molecular weight amines are toxic and some are easily absorbed through the skin. Many higher molecular weight amines are highly active biologically.

AmphetamineNicotine

Serotonin

Adenine

Chapter 16: Amines

Example:Example: Mark each of the following amines as primary, secondary, or tertiary:

a)a)

b)b)

c)c)

d)d)

Chapter 16: Amines

-N--N---

Amines are classified as 1°, 2°, or 3° depending on the number of carbon groups bonded to nitrogen.

Aliphatic amine:Aliphatic amine: All carbons bonded to nitrogen are derived from alkyl groups. See the three above.

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

NH2 N-CH3

HCH2-N-CH3

CH3

Aniline(a 1° aromatic amine)

N-Methylaniline(a 2° aromatic amine)

Benzyldimethylamine(a 3° aliphatic amine)

CH3-NH2 CH3-N-CH3

H

CH3-N-CH3

CH3

Methylamine(a 1° amine)

Dimethylamine(a 2° amine)

Trimethylamine(a 3° amine)

Chapter 16: Amines

• Heterocyclic amine:Heterocyclic amine: An amine in which the nitrogen atom is part of a ring.

• Heterocyclic aliphatic amine:Heterocyclic aliphatic amine: A heterocyclic amine in which the ring is saturated (has no C=C bonds).

• Heterocyclic aromatic amine:Heterocyclic aromatic amine: The amine nitrogen is part of an aromatic ring.

PiperidinePyrrolidine Pyridine(heterocyclic aliphatic amines) (heterocyclic aromatic amines)

NH

NH

N N

N

Pyrimidine

N

N

Imidazole

N

N

N

N

HPurine

H

Nomenclature

• Drop the final -e-e of the parent alkane and replace it by -amine.-amine.• Use a number to locate the amino group on the parent chain.

CH3CHCH3

NH2

NH2

H2NNH2

1,6-HexanediamineCyclohexanamine2-Propanamine

• 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).

NCH3

CH3

NHCH3

N,N-Dimethyl- cyclopentanamine

N-Methylaniline

Nomenclature

Common names• For most aliphatic amines, list the groups bonded to nitrogen in

alphabetical order in one word ending in the suffix -amineamine.

• We already spoke about how to name aniline and its derivatives:

Diethylmethylaminesec-Butylamine

NNH2

NH2

Cyclohexylamine

NH2

Propylamine

NH2

CH3

NH2

NO2

NH2

3-Methylaniline (m-Toluidine)

Aniline 4-Nitroaniline(p-Nitroaniline)

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-amine (or aniline or pyridine or the like) by --

ammoniumammonium (or anilinium or pyridinium or the like) and add the name of the anion.

(CH3CH2)3NH+Cl-

Triethylammonium chloride

Physical Properties

• Low-molecular-weight amines have very sharp, penetrating odors.• Amines are Polar compounds.• 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.

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).

CH3OHCH3CH3 CH3NH2

-6.3 65.032.031.1MW (amu)

bp (°C)

30.1

-88.6

• Most low-molecular-weight amines are completely soluble in water.

• Higher-molecular-weight amines are only moderately soluble in water or are insoluble.

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.

CH3-NH

HH-O-H CH3-N-H

H

HO-H

Methylammoniumhydroxide

Methylamine(a base)

++

-: :

::

::

Chemical Properties

[CH3NH3+][OH

- ]

[CH3NH2]Kb = = 4.37 x 10-4

pKb = - log 4.37 x 10-4 = 3.360

Chemical Properties

Amines, whether soluble or insoluble in water, react quantitatively with strong acids to form water-soluble salts.

HO H

NH2HO

HOHCl

H2O

HO

HO NH3+Cl-

HHO

(R)-Norepinephrine hydrochloride(a water-soluble salt)

+

(R)-Norepinephrine(only slightly soluble in water)

It is possible to predict which form of the amine is prevalent given the pKb of the amine and the pH of the environment: (see example in the next slide)

Chapter 16: 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:

• That means that if an aliphatic amine is dissolved in blood, it is present predominantly as its protonated (conjugated acid) form.

RNH2 H2O RNH3+ OH-+ +

[RNH3+]

[RNH2]3.2 x 10-4

2.5 x 10-7= = 1300

[RNH3+][OH-]

[RNH2]Kb =

[RNH3+]

[RNH2][OH-]Kb =

Chapter 16: Amines

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

(CH3CH2)2NH HCl

NCH3COOH

+(a)

(b) +

Chapter 16: Amines

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

(CH3CH2)2NH HCl

NCH3COOH

NH

(CH3CH2)2NH2+Cl-

CH3COO-

Diethylammoniumchloride

+Pyridinium acetate

+(a)

(b) +