Chapter 16: Amines
Fundamentals of General, Organic, and Biological Chemistry: Amines (Chapter 16)
Concepts to Review
Lewis Structures
Polar Covalent Bonds
Acids and Bases
Hydrogen Bonds
Acid Dissociation Constants
Functional Groups
Naming Alkanes
16.1 Classifying Amines
Definition: Amines contain one or more organic groups bonded to a nitrogen atom.
Learning Objective: Identify and classify an amine as primary, secondary, or tertiary.
Structure and Classification:
Amines are organic derivatives of ammonia (NH_3).
They are classified as primary, secondary, or tertiary based on the number of organic (R) alkyl groups directly bonded to the nitrogen atom.
Primary Amines: Contain one organic alkyl group bonded to nitrogen. General formula: RNH_2 (one R group).
Secondary Amines: Contain two organic alkyl groups bonded to nitrogen. General formula: R_2NH (two R groups).
Tertiary Amines: Contain three organic alkyl groups bonded to nitrogen. General formula: R_3N (three R groups).
Nitrogen Valence Electrons:
Nitrogen has five (5) valence electrons in its outer P-orbital.
It typically forms three (3) bonds by donating electrons to atoms like carbon or hydrogen.
Consequently, nitrogen usually possesses two (2) unpaired electrons, which form one (1) lone pair of valence electrons.
Ring-Type Structures: The organic groups (R) bound to the nitrogen atom can connect to one another, forming various ring-type structures.
Chemistry of Amines: The lone pair of valence electrons ( : ) on the amine nitrogen is crucial for its chemical reactivity.
Quaternary Ammonium Ions/Cations:
When a fourth organic group forms a bond with the nitrogen atom by utilizing the lone pair of valence electrons, the product is a quaternary ammonium ion or cation.
These ions possess a permanent positive charge and are involved in the formation of ionic compounds.
Groups Bonded to Amine Nitrogen: The atoms or groups bonded to the amine nitrogen can be:
Hydrogen (a proton).
Alkyl (hydrocarbon).
Aryl (aromatic).
Example: A hydrogen atom can leave its single electron for a chlorine atom and bind to the lone pair of nitrogen electrons.
16.2 Naming and Drawing Amines
Learning Objective: Name a simple amine given the structure and draw an amine structure given its name.
Naming Primary Alkyl Amines (RNH_2):
Identify the alkyl group attached to the nitrogen.
Add the suffix "-amine" to the name of the primary alkyl group.
Naming Non-Heterocyclic Secondary (2º) and Tertiary (3º) Amines:
Add "di-" or "tri-" to the alkyl group name (if the groups are identical).
Add the ending suffix "-amine."
Naming Amines with Different R Groups (N-substituted Derivatives):
When R groups in secondary or tertiary amines are different, they are named as N-substituted derivatives of a primary amine.
The parent compound (main chain) is the alkyl group with the largest number of carbons attached to the nitrogen.
The other groups are designated as N-substituents, indicating they are attached to the nitrogen atom.
Nomenclature follows alphabetical order for substituents (e.g., ethyl before propyl, dimethyl before propyl). N-ethyl-N-propylamine.
Amino Group as a Substituent:
The amine group (-NH_2) is also known as an amino group.
When the amino group is a substituent on a compound with a higher-priority functional group (like a carboxylic acid or double/triple bonds), "amino-" is used as a prefix.
Aromatic Amines:
Aromatic amines are primarily known by their common names.
The simplest aromatic amine is aniline.
Amino Acids:
Proteins are polymers of alpha (\alpha) amino acids.
General condensed structure: H_2N-CHR-COOH.
It contains an amino (base) group, a carboxyl (acid) group, and an alpha (\alpha) or carbon 2 position.
Worked Example 16.1: N,N-diethylbutylamine
Analysis: The name contains "butylamine" (indicating a 4-carbon parent alkyl amine) and "N,N-diethyl" (indicating two ethyl groups bonded to the nitrogen).
Structure: (CH3CH2)2N(CH2)3CH3
Classification: Since three alkyl groups (two ethyl, one butyl) are bonded to the nitrogen atom, this is a tertiary amine.
16.3 Properties of Amines
Learning Objective: Describe amine properties such as hydrogen bonding, solubility, boiling point, and basicity.
Basicity: The lone pair of electrons on the amine nitrogen is responsible for amines acting as weak Brønsted-Lowry bases or Lewis bases.
A Lewis base is a compound that contains an unshared or lone pair of electrons available for donation.
Hydrogen Bonding:
Primary (RNH2) and secondary (R2NH) amines can form hydrogen bonds with water and other amine molecules.
Definition of Hydrogen Bond: An electrostatic force between a hydrogen (H) atom covalently bonded to an electronegative donor atom (like N or O) and another electronegative acceptor atom (like N or O) that possesses a lone pair of electrons.
Hydrogen bonds can be intermolecular (between separate molecules) or intramolecular (within the same molecule).
The strength of a hydrogen bond depends on geometry, environment, and the nature of the donor and acceptor atoms.
Hydrogen bonds are stronger than van der Waals interactions but weaker than covalent or ionic bonds.
Boiling Points:
The ability to form hydrogen bonds means primary and secondary amines have higher boiling points compared to alkanes of similar size.
However, alcohols (containing oxygen) can form more hydrogen bonds per molecule than amines (containing nitrogen).
Consequently, alcohols have even higher boiling points than amines of similar size.
16.4 Heterocyclic Nitrogen Compounds
Learning Objective: Identify a heterocyclic amine.
Definition:
A heterocycle is a ring structure that contains nitrogen or another atom (e.g., O, S) in addition to carbon atoms within the ring.
In many nitrogen-containing compounds, the nitrogen atom is part of a ring with carbon atoms.
Types: Heterocyclic nitrogen compounds can be:
Non-aromatic: e.g., piperidine.
Aromatic: e.g., pyridine.
Examples of Heterocyclic Nitrogen Compounds (Table 16.1 - omitted the list but the concept is covered):
Pyrrole (aromatic)
Imidazole (aromatic)
Pyridine (aromatic)
Pyrimidine (aromatic)
Indole (aromatic)
Quinoline (aromatic)
Purine (aromatic)
Piperidine (non-aromatic)
16.5 Basicity of Amines
Learning Objective: Identify and draw the products formed when an amine reacts with an acid.
Amine Basicity in Water:
Aqueous solutions of amines are weakly basic.
In water, amines accept a proton from water, forming amine cations (e.g., R_3NH^+) and hydroxide anions (OH^-).
Formation of Ammonium Ions:
From Water: Amines react with water (H2O) where water acts as an acid (proton donor) to form ammonium cations (RNH3^+). For example:
CH3CH2NH2 (aq) + H2O (l) \rightleftharpoons CH3CH2NH_3^+ (aq) + OH^- (aq)From Hydronium Cation: Amines react with hydronium cations (H3O^+) to form ammonium cations. For example: CH3CH2NH2 (aq) + H3O^+ (aq) \rightleftharpoons CH3CH2NH3^+ (aq) + H_2O (l)
Naming Alkylamine Cations:
The ending "-amine" in the original amine name is replaced with "-ammonium."
Examples:
ethylamine \rightarrow ethylammonium cation
dipropylamine \rightarrow dipropylammonium cation
Reactions of Ammonium Cations (Weakly Acidic):
If an ammonium cation has at least one substituent group (like an alkyl group, e.g., ethyl) attached to the nitrogen along with a hydrogen, these cations are weakly acidic.
They can react with strong bases, such as hydroxide anion (OH^-), to regenerate the original amine.
Example:
CH3CH2NH3^+ (aq) + OH^- (aq) \rightleftharpoons CH3CH2NH2 (aq) + H_2O (l)
Overview of Amine Acid and Base Chemistry: Water often acts as an acid in reactions with amines, donating a hydrogen cation (proton, H^+) to the nitrogen.
16.6 Amine Salts
Learning Objective: Identify a quaternary ammonium cation and describe its properties.
Definition: An ammonium cation or ammonium salt is an ionic compound composed of an ammonium cation and some type of anion.
Properties of Ammonium Salts:
Generally odorless, white, crystalline solids.
More soluble in water compared to their neutral amine counterparts due to their ionic nature.
Naming Ammonium Salts:
Named by combining the name of the cation and the anion.
Example: Ammonium cation (NH4^+) + chloride anion (Cl^-) = ammonium chloride (NH4Cl).
Benzalkonium Chlorides:
These are examples of quaternary ammonium salts.
They possess antimicrobial and detergent properties.
Used in dilute solutions for applications such as surgical scrubs and sterile storage of instruments.
16.7 Amines in Plants: Alkaloids
Learning Objective: Describe the sources of alkaloids, name some examples, and explain how their properties are typical of amines.
Definition: An alkaloid is a naturally occurring, nitrogen-containing compound isolated from plants.
Typical properties: usually basic, bitter, and often poisonous.
Purpose in Plants: The bitterness and poisonous nature of alkaloids are believed to protect plants from being consumed by animals.
Examples of Alkaloids:
Coniine:
Extracted from poison hemlock.
Famously used by Socrates to end his life after being convicted.
Atropine:
The toxic substance found in the herb deadly nightshade or belladonna (Atropa belladonna).
Acts on the central nervous system.
Applied in medications to reduce cramping of the digestive tract.
Also used as an antidote against nerve gases like Sarin.
Solanine:
Found in potatoes and tomatoes, which belong to the Solanaceae botanical family (the same family as deadly nightshade).
Production increases in potatoes when exposed to sunlight or stored under very cold or very warm conditions, potentially reaching dangerous levels.
Alkaloids are not destroyed by cooking but can be removed by peeling.
Sunlight also stimulates chlorophyll formation under the potato skin, leading to a green color, which serves as a visible warning signal (though the chlorophyll itself is not toxic).
Morphine and Codeine:
About 20 different alkaloids are present in the poppy plant.
The free alkaloids are typically oily liquids.
Morphine has been used medicinally for pain relief.
Its medicinal use was expanded by German physician Paracelsus, who extracted opium into brandy to create laudanum.
Similar extracts are still prescribed, as is paregoric (a more dilute solution of opium combined with anise oil, glycerin, benzoic acid, and camphor).
Heroin can be synthesized from morphine.
In the body, the removal of two acetyl-groups from heroin converts it back into morphine.
Codeine differs from morphine and heroin by possessing two methoxy-groups instead of two acetyl-groups or hydroxyl groups.
Functional Group Concept Map
A concept map illustrating the various organic functional groups and their relationships, including amines, based on the presence of single bonds only, multiple bonds, and specific atoms like O, N, S, or halogens. Amines are categorized under N-attached groups within the "O, N, S present" branch of organic functional groups. This map helps visualize the structural context of amines relative to other organic compounds.
(Figure 16.2)