Chapter 8Chapter 8AminesAmines

Structure and ClassificationStructure and ClassificationAmines 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.

Structure and ClassificationStructure and Classification Aromatic amine:Aromatic amine: One or more of the groups bonded to

nitrogen are aryl groups.

Structure and Structure and ClassificationClassification• 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.

NomenclatureNomenclature

IUPAC names

• We derive IUPAC names for aliphatic amines just as we did for alcohols.

• 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.

NomenclatureNomenclature IUPAC names (cont’d)

◦ 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:

NomenclatureNomenclatureIUPAC names (cont’d)

◦ 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).

NomenclatureNomenclatureCommon names

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

NomenclatureNomenclatureAmine 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 -ammonium-ammonium (or anilinium or pyridinium or the like) and add the name of the anion.

ExamplesExamples Draw the structures of the following compounds

t-butylamine 3-methyl-1-butanamine

2,4,N,N-tetramehtyl-3-hexanamine

Physical PropertiesPhysical PropertiesLike 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).

Physical PropertiesPhysical PropertiesFigure 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.

N

R

RR

HO

H

H-bond acceptor only

Physical PropertiesPhysical 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.

Physical PropertiesPhysical 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.

Basicity of AminesBasicity of AminesLike 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.

Basicity of AminesBasicity 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.

Basicity of AminesBasicity of Amines

N

H

HR

H O

H

N

HH

R

H

ammonium ionamine

+ OH

R NH2 + H2O

R NH3 + OHenergy

Basicity of AminesBasicity 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.

Basicity of AminesBasicity 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.

Basicity of AminesBasicity 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:

Basicity of AminesBasicity of Amines

Select the stronger base Select the stronger base from each pair of aminesfrom each pair of amines

N or NH2

NH3 or CH3NH2

Reactions of AminesReactions of AminesThe 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.

Basicity of AminesBasicity of AminesExample:Example: Complete each acid-base reaction and name the salt formed.