What Is The Difference Between A Primary And Tertiary Amine

The world of organic chemistry can seem daunting at first, but breaking it down into manageable components helps to simplify the learning process. Amines, organic compounds derived from ammonia, play a crucial role in various chemical and biological processes. Understanding the difference between primary and tertiary amines is fundamental to grasping organic chemistry's complexities. This article explores these differences in detail, covering their structure, properties, reactivity, and applications.

Understanding Amines: The Basics

Amines are derivatives of ammonia (NH3), where one or more hydrogen atoms are replaced by alkyl or aryl groups. This substitution leads to a wide variety of amines with different properties and reactivities. Amines are classified based on the number of alkyl or aryl groups attached to the nitrogen atom:

• Primary Amine (1°): One hydrogen atom in ammonia is replaced by an alkyl or aryl group (R-NH2).
• Secondary Amine (2°): Two hydrogen atoms in ammonia are replaced by alkyl or aryl groups (R2-NH).
• Tertiary Amine (3°): All three hydrogen atoms in ammonia are replaced by alkyl or aryl groups (R3-N).
• Quaternary Ammonium Ion (4°): The nitrogen atom is bonded to four alkyl or aryl groups and carries a positive charge (R4-N+). This is not an amine in the strictest sense but is closely related.

Focusing on primary and tertiary amines, the structural differences directly influence their chemical behavior and applications.

Structural Differences: A Closer Look

The most significant difference between primary and tertiary amines lies in their molecular structure. This structural variance dictates their properties and reactivity.

Primary Amines (R-NH2)

In a primary amine, the nitrogen atom is bonded to one alkyl or aryl group and two hydrogen atoms. The general formula is R-NH2.

• Nitrogen Atom: The nitrogen atom in a primary amine is bonded to one carbon atom and two hydrogen atoms.
• Hydrogen Bonding: Primary amines have two hydrogen atoms directly bonded to the nitrogen atom, allowing them to form intermolecular hydrogen bonds. This capability significantly affects their physical properties, such as boiling point and solubility.
• Examples: Common examples include methylamine (CH3NH2), ethylamine (CH3CH2NH2), and aniline (C6H5NH2).

Tertiary Amines (R3-N)

In a tertiary amine, the nitrogen atom is bonded to three alkyl or aryl groups. The general formula is R3-N.

• Nitrogen Atom: The nitrogen atom in a tertiary amine is bonded to three carbon atoms.
• Hydrogen Bonding: Tertiary amines do not have hydrogen atoms directly bonded to the nitrogen atom. Consequently, they cannot form intermolecular hydrogen bonds with themselves. This absence of hydrogen bonding significantly affects their physical properties.
• Examples: Common examples include trimethylamine ((CH3)3N), triethylamine ((CH3CH2)3N), and N,N-dimethylaniline (C6H5N(CH3)2).

Physical Properties: How Structure Affects Behavior

The structural differences between primary and tertiary amines lead to significant variations in their physical properties, including boiling point, solubility, and odor.

Boiling Point

• Primary Amines: Due to their ability to form intermolecular hydrogen bonds, primary amines generally have higher boiling points than tertiary amines with similar molecular weights. Hydrogen bonding requires more energy to overcome, resulting in higher boiling points.
• Tertiary Amines: Tertiary amines cannot form intermolecular hydrogen bonds with themselves, so they have lower boiling points compared to primary amines. Their boiling points are primarily influenced by Van der Waals forces, which are weaker than hydrogen bonds.

Solubility

• Primary Amines: Lower molecular weight primary amines are soluble in water because they can form hydrogen bonds with water molecules. As the alkyl or aryl group becomes larger, the solubility decreases due to the increasing hydrophobic character.
• Tertiary Amines: Similar to primary amines, lower molecular weight tertiary amines are somewhat soluble in water. However, their solubility is generally lower than that of primary amines with comparable molecular weights because they can only act as hydrogen bond acceptors, not donors.

Odor

• Primary Amines: Primary amines often have strong, fishy odors. For example, lower molecular weight primary amines like methylamine and ethylamine have pungent smells.
• Tertiary Amines: Tertiary amines also have fishy odors, but they are generally less intense than those of primary amines. Trimethylamine, for instance, is known for its fishy smell and is associated with decaying fish.

Chemical Properties and Reactivity

The reactivity of amines is primarily determined by the availability of the lone pair of electrons on the nitrogen atom. The steric hindrance and electronic effects of the substituents attached to the nitrogen atom also play a crucial role.

Basicity

• Primary Amines: Primary amines are basic due to the lone pair of electrons on the nitrogen atom, which can accept a proton. Their basicity is influenced by the electron-donating or electron-withdrawing nature of the attached alkyl or aryl group. Alkyl groups generally increase basicity, while aryl groups decrease it.
• Tertiary Amines: Tertiary amines are also basic, but their basicity can be affected by steric hindrance. The three substituents around the nitrogen atom can make it more difficult for a proton to approach and bond with the lone pair of electrons. Additionally, solvation effects can play a role in determining their basicity in aqueous solutions.

Acylation

• Primary Amines: Primary amines readily undergo acylation reactions with acyl chlorides or anhydrides to form amides. This reaction involves the nucleophilic attack of the amine nitrogen on the carbonyl carbon of the acyl compound.
  • Reaction: R-NH2 + R'COCl → R-NHCOR' + HCl
• Tertiary Amines: Tertiary amines do not undergo acylation reactions in the same way as primary amines because they do not have a hydrogen atom to lose after the nucleophilic attack. Instead, they can act as catalysts in acylation reactions by reacting with the acyl chloride to form an acylammonium intermediate, which then reacts with another nucleophile.

Reaction with Nitrous Acid

• Primary Amines: Primary amines react with nitrous acid (HNO2) to form unstable diazonium salts, which decompose to give a mixture of products, including alcohols, alkenes, and alkyl halides.
  • Reaction: R-NH2 + HNO2 → [R-N≡N]+ + H2O → products
• Tertiary Amines: Tertiary amines react with nitrous acid to form N-nitroso compounds. These compounds are often yellow oils and can be carcinogenic.
  • Reaction: R3N + HNO2 → R3N-NO + H2O

Hofmann Elimination

• Primary Amines: Primary amines can undergo Hofmann elimination after exhaustive methylation to form a quaternary ammonium hydroxide, which then decomposes upon heating to yield an alkene.
• Tertiary Amines: Tertiary amines can also undergo Hofmann elimination, similar to primary amines, after being converted to quaternary ammonium salts.

Applications of Primary and Tertiary Amines

Both primary and tertiary amines have a wide range of applications in various fields, including pharmaceuticals, polymers, dyes, and chemical synthesis.

Primary Amines

• Pharmaceuticals: Many drugs contain primary amine functional groups. For example, amino acids, the building blocks of proteins, are primary amines. Certain antibiotics and analgesics also contain primary amine groups.
• Polymers: Primary amines are used as monomers or curing agents in polymer synthesis. For instance, ethylenediamine is used in the production of polyamides and epoxy resins.
• Dyes: Aromatic primary amines, such as aniline, are used in the production of azo dyes, which are widely used in the textile industry.
• Chemical Synthesis: Primary amines are versatile building blocks in organic synthesis, used in the preparation of various compounds through reactions like reductive amination, acylation, and diazotization.

Tertiary Amines

• Catalysis: Tertiary amines, such as triethylamine and pyridine, are commonly used as catalysts in organic reactions. They can act as bases to neutralize acids formed during the reaction or facilitate nucleophilic reactions.
• Solvents: Certain tertiary amines, like pyridine, are used as solvents in chemical reactions due to their ability to dissolve a wide range of organic compounds.
• Pharmaceuticals: Some drugs contain tertiary amine functional groups. For example, antihistamines like diphenhydramine contain a tertiary amine moiety.
• Polymer Chemistry: Tertiary amines are used as catalysts in the polymerization of certain monomers, such as epoxides.
• Chemical Synthesis: Tertiary amines are used as ligands in metal-catalyzed reactions and as protecting groups for alcohols and amines.

Key Differences Summarized

To provide a clear overview, here's a summary of the key differences between primary and tertiary amines:

Illustrative Examples

To further clarify the differences, let's look at some specific examples:

1. Methylamine (CH3NH2) vs. Trimethylamine ((CH3)3N)

   • Methylamine is a primary amine with one methyl group attached to the nitrogen atom. It can form hydrogen bonds and has a higher boiling point compared to trimethylamine.
   • Trimethylamine is a tertiary amine with three methyl groups attached to the nitrogen atom. It cannot form hydrogen bonds with itself and has a lower boiling point.

2. Aniline (C6H5NH2) vs. N,N-Dimethylaniline (C6H5N(CH3)2)

   • Aniline is an aromatic primary amine. It is used in the production of dyes and pharmaceuticals.
   • N,N-Dimethylaniline is an aromatic tertiary amine. It is used as a precursor in the synthesis of dyes and as a catalyst in certain reactions.

Common Misconceptions

• All Amines are Strong Bases: While amines are basic, their basicity varies depending on the substituents attached to the nitrogen atom. Aromatic amines, for example, are significantly less basic than aliphatic amines due to the delocalization of the lone pair of electrons into the aromatic ring.
• Tertiary Amines are Always More Reactive: The reactivity of amines depends on the specific reaction. In some cases, primary amines are more reactive due to less steric hindrance, while in other cases, tertiary amines may be preferred as catalysts or ligands.
• Hydrogen Bonding is the Only Factor Affecting Boiling Point: While hydrogen bonding is a significant factor, other intermolecular forces, such as Van der Waals forces, also contribute to the boiling points of amines. Molecular weight and shape also play a role.

Advanced Concepts

For those looking to delve deeper into the subject, here are some advanced concepts related to primary and tertiary amines:

• Chirality in Amines: Amines with three different substituents attached to the nitrogen atom are chiral. However, due to nitrogen inversion, the enantiomers rapidly interconvert at room temperature, making it difficult to isolate them.
• Phase-Transfer Catalysis: Quaternary ammonium salts, derived from tertiary amines, are used as phase-transfer catalysts to facilitate reactions between reactants in immiscible phases.
• Applications in Material Science: Amines are used in the synthesis of various functional materials, including polymers, surfactants, and nanoparticles. Their ability to modify surface properties makes them valuable in material science applications.
• Amine Protecting Groups: In complex organic syntheses, it is often necessary to protect amine functional groups to prevent unwanted reactions. Common amine protecting groups include tert-butoxycarbonyl (Boc) and benzyloxycarbonyl (Cbz) groups.

Conclusion

Understanding the difference between primary and tertiary amines is essential for anyone studying organic chemistry or related fields. The structural variations between these two types of amines lead to significant differences in their physical properties, reactivity, and applications. Primary amines, with their ability to form hydrogen bonds and undergo acylation, are widely used in pharmaceuticals, polymers, and dyes. Tertiary amines, with their catalytic properties and use as solvents, play crucial roles in chemical synthesis and polymer chemistry. By grasping these fundamental differences, one can better appreciate the complexities and versatility of amines in various chemical and biological processes.