16 16-1 © 2003 Thomson Learning, Inc. All rights reserved General, Organic, and Biochemistry, 7e Bettelheim, Brown, and March

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16-1© 2003 Thomson Learning, Inc.All rights reserved

General, Organic, and General, Organic, and Biochemistry, 7eBiochemistry, 7e

Bettelheim,Bettelheim,

Brown, and MarchBrown, and March

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Chapter 16Chapter 16

AminesAmines

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Structure & ClassificationStructure & Classification• 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

• aromatic amine:aromatic amine: one or more of the groups bonded to nitrogen are aryl groups

CH3-NH2 CH3-N-CH3

H

CH3-N-CH3

CH3

Methylamine(a 1° amine)

Dimethylamine(a 2° amine)

Trimethylamine(a 3° amine)

NH2 N-CH3

HCH2-N-CH3

CH3

Aniline(a 1° aromatic amine)

N-Methylaniline(a 2° aromatic amine)

Benzyldimethylamine(a 3° aliphatic amine)

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Structure & ClassificationStructure & Classification• 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

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NomenclatureNomenclature• 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 --amineamine

• use a number to locate the amino group on the parent chain

CH3CHCH3

NH2

NH2

H2NNH2

1,6-HexanediamineCyclohexanamine2-Propanamine

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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

NH2

CH3

NH2

NO2

NH2

3-Methylaniline (m-Toluidine)

Aniline 4-Nitroaniline(p-Nitroaniline)

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NomenclatureNomenclature• IUPAC 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)

NCH3

CH3

NHCH3

N,N-Dimethyl- cyclopentanamine

N-Methylaniline

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NomenclatureNomenclature• Common names

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

Diethylmethylaminesec-Butylamine

NNH2

NH2

Cyclohexylamine

NH2

Propylamine

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NomenclatureNomenclature• 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 -ammonium-ammonium (or anilinium or pyridinium or the like) and add the name of the anion

(CH3CH2)3NH+Cl-

Triethylammonium chloride

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Physical PropertiesPhysical 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 PropertiesPhysical Properties• 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 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

CH3OHCH3CH3 CH3NH2

-6.3 65.032.031.1MW (amu)

bp (°C)

30.1

-88.6

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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

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Basicity of AminesBasicity 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

CH3-NH

HH-O-H CH3-N-H

H

HO-H

Methylammoniumhydroxide

Methylamine(a base)

++

-: :

::

::

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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

[CH3NH3+][OH

- ]

[CH3NH2]Kb = = 4.37 x 10-4

pKb = - log 4.37 x 10-4 = 3.360

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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

C6H5NH2

CH3CH2NH2Aliphatic

Ammonia

Aromatic

3.0 - 4.0

4.74

8.5 - 9.5

Class pKb Example Name

Ethanamine

Aniline

Stronger base

Weaker base

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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) formHO

HO

NH2

Dopamine

HO

HO

NH3+

Conjugate acid of dopamine(the major form present

in blood plasma)

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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

RNH2 H2O RNH3+ OH-+ +

[RNH3+][OH-]

[RNH2]Kb =

[RNH3+]

[RNH2][OH-]Kb =

[RNH3+]

[RNH2]

4.0 x 10-4

2.5 x 10-7 = = 1600

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Reactions of AminesReactions 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

HO H

NH2HO

HOHCl

H2O

HO

HO NH3+Cl-

HHO

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

+

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

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Reactions of AminesReactions of Amines• example:example: complete each acid-base reaction and name

the salt formed

(CH3CH2)2NH HCl

NCH3COOH

+(a)

(b) +

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Reactions of AminesReactions of 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) +

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End End Chapter 16Chapter 16

AminesAmines

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16 16-1 © 2003 Thomson Learning, Inc. All rights reserved General, Organic, and Biochemistry, 7e Bettelheim, Brown, and March