24.2 Structure and Properties of Amines
3 min read•may 7, 2024
Amines, organic compounds with a nitrogen atom, play a crucial role in chemistry. Their unique structure, featuring a of electrons on nitrogen, gives them special properties. These properties influence their behavior in reactions and their physical characteristics.
Understanding amines is key to grasping many organic reactions. From their to their ability to undergo , amines showcase fascinating chemical behavior. Their solubility and also set them apart from other organic compounds, highlighting their importance in various applications.
Structure and Properties of Amines
Bonding and geometry of alkylamines
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- contain nitrogen atom bonded to carbon atoms
- Nitrogen is resulting in around nitrogen atom (ammonia, methylamine)
- Lone pair on nitrogen occupies one of four sp³ hybrid orbitals
- in alkylamines slightly less than ideal 109.5° due to lone pair-bond pair repulsion
- Typical bond angles range from 106° to 108° (trimethylamine, triethylamine)
- Nitrogen-carbon (N-C) bond lengths in alkylamines approximately 1.47 Å
- Slightly shorter than carbon-carbon (C-C) single bond length of 1.54 Å due to higher electronegativity of nitrogen compared to carbon
Pyramidal inversion in amines
- process where nitrogen atom in amine passes through plane formed by three substituents attached to it
- Lone pair on nitrogen changes orientation during this process
- Energy barrier for pyramidal inversion relatively low, typically around 24-26 kJ/mol (6-7 kcal/mol)
- Rapid pyramidal inversion in amines at room temperature has consequences for
- If amine has three different substituents attached to nitrogen atom, it can potentially be chiral
- However, due to fast inversion, rapidly interconvert making it impossible to isolate individual enantiomers at room temperature (methylethylamine, methylisopropylamine)
- Pyramidal inversion can be slowed down or prevented by:
- Increasing size of substituents on nitrogen atom, which raises energy barrier for inversion ()
- Lowering temperature, which reduces kinetic energy available for inversion
- Protonating amine, which eliminates lone pair and prevents inversion ()
Physical properties of amines vs organics
- Solubility:
- Amines more soluble in water compared to hydrocarbons of similar molecular weight due to ability to form with water (ethylamine vs ethane)
- Solubility of amines in water decreases as size of alkyl group increases, as nature of alkyl group becomes more dominant (octylamine vs ethylamine)
- Amines less soluble in water than alcohols of similar molecular weight because amines have one less hydrogen bond donor site () compared to alcohols () (ethylamine vs ethanol)
- Boiling points:
- Amines have higher boiling points than hydrocarbons of similar molecular weight due to presence of intermolecular hydrogen bonding between amine molecules (propylamine vs propane)
- Boiling points of amines increase with increasing molecular weight and surface area, as strength of intermolecular forces increases (ethylamine vs butylamine)
- have higher boiling points than , which in turn have higher boiling points than (ethylamine vs diethylamine vs triethylamine)
- Trend due to decreasing number of N-H hydrogen bond donor sites from primary to tertiary amines
- Amines have lower boiling points than alcohols of similar molecular weight because N-H···N hydrogen bond is weaker than O-H···O hydrogen bond in alcohols (propylamine vs propanol)
Chemical properties of amines
- Basicity: Amines are generally basic due to the lone pair of electrons on the nitrogen atom, which can accept a proton
- : The lone pair also makes amines good nucleophiles, allowing them to participate in various reactions
- : A reaction where undergo elimination to form alkenes
- : A method used to distinguish between primary, secondary, and tertiary amines based on their reactivity with benzenesulfonyl chloride
Key Terms to Review (28)
Alkylamines: Alkylamines are a class of organic compounds that contain a nitrogen atom bonded to one or more alkyl groups. They are an important group of amines that exhibit unique structural and chemical properties, making them crucial in the context of understanding amines and their basicity.
Ammonium Salts: Ammonium salts are a class of chemical compounds formed by the reaction between ammonia (NH3) and an acid. They are characterized by the presence of the ammonium cation (NH4+) and a corresponding anion from the acid. These salts are widely used in various applications, including agriculture, industrial processes, and household products.
Basicity: Basicity is a measure of the strength or ability of a chemical species to accept a proton (H+) and form a conjugate acid. It is a fundamental concept in organic chemistry that plays a crucial role in understanding the reactivity and properties of various organic compounds, including those involved in SN2 reactions, aromatic heterocycles, amines, and their reactions.
Basicity constant, Kb: The basicity constant, \(K_b\), measures the strength of a base in solution, specifically how well an amine can attract and hold a proton (H+). It quantitatively expresses the equilibrium between the amine in its basic form and its corresponding protonated form in solution.
Boiling Points: Boiling point is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid, and bubbles of vapor form inside the liquid. This property is crucial in understanding the behavior and characteristics of various organic compounds, including alkanes, alcohols, carboxylic acids, and amines.
Bond Angles: Bond angles refer to the geometric arrangement of atoms around a central atom in a molecule, determined by the number and type of bonds formed. This concept is crucial in understanding the structures and properties of various organic compounds.
Chirality: Chirality is a fundamental concept in organic chemistry that describes the three-dimensional arrangement of atoms in a molecule. It refers to the property of a molecule that is non-superimposable on its mirror image, resulting in the existence of two distinct forms known as enantiomers. Chirality is a crucial factor in understanding the behavior and properties of various organic compounds, including their interactions with living systems.
Chirality centers: A chirality center in organic chemistry is an atom, typically carbon, that has four different groups attached to it, leading to non-superimposable mirror image forms of the molecule. These centers are crucial for determining the 3D spatial orientation of molecules, affecting their chemical behavior and interactions.
Enantiomers: Enantiomers are a pair of stereoisomers that are non-superimposable mirror images of each other. They have the same molecular formula and connectivity, but differ in the spatial arrangement of their atoms, resulting in a unique handedness or chirality.
Hinsberg Test: The Hinsberg test is a method used to distinguish between primary, secondary, and tertiary amines. It involves the reaction of an amine with a benzenesulfonyl chloride reagent, which allows for the identification of the amine type based on the solubility of the resulting product.
Hofmann Elimination: The Hofmann elimination is a type of elimination reaction that involves the removal of a hydrogen atom and a leaving group from a quaternary ammonium salt, resulting in the formation of an alkene. This reaction is particularly important in the context of preparing alkenes, understanding the structure and properties of amines, and the reactions of amines.
Hofmann elimination reaction: The Hofmann elimination reaction is a chemical process where an amine is converted into an alkene through the treatment with excess methyl iodide, followed by silver oxide (Ag2O), and then heating with water. This reaction involves the removal of a hydrogen atom and a leaving group from adjacent carbon atoms in a substrate, leading to the formation of a double bond.
Hydrogen Bonds: Hydrogen bonds are a type of noncovalent interaction that occurs when a hydrogen atom covalently bonded to a highly electronegative element, such as nitrogen, oxygen, or fluorine, is attracted to another nearby highly electronegative element. This attractive force is significantly stronger than a typical dipole-dipole interaction and plays a crucial role in the structure and properties of various molecules and biological systems.
Hydrophobic: Hydrophobic refers to the aversion or repulsion of water molecules by certain molecules or surfaces. This property arises from the inability of water to form hydrogen bonds with these substances, leading to a minimization of contact between the water and the hydrophobic entity.
Lone Pair: A lone pair, also known as an unshared pair, refers to a pair of valence electrons in an atom that are not involved in covalent bonding with other atoms. These electron pairs are not shared with neighboring atoms and remain solely within the atom's electron cloud.
N-C Bond: The N-C bond is a covalent chemical bond formed between a nitrogen (N) atom and a carbon (C) atom. This bond is an important structural feature in various organic compounds, particularly in the context of amines, which are a class of organic compounds containing a nitrogen atom attached to one or more alkyl or aryl groups.
N-H: The N-H term refers to the nitrogen-hydrogen bond, which is a key structural feature in organic compounds known as amines. This bond plays a crucial role in determining the properties and reactivity of amine compounds.
Nucleophilicity: Nucleophilicity refers to the ability of a species to donate electrons and form a covalent bond with an electrophilic center. It is a key concept in organic chemistry that governs the reactivity and selectivity of many important reactions, including substitution, addition, and elimination reactions.
O-H: The O-H functional group, also known as the hydroxyl group, consists of an oxygen atom bonded to a hydrogen atom. This group is a key structural feature in many organic compounds and plays a crucial role in their physical and chemical properties, particularly in the context of infrared spectra and the structure and properties of amines.
Primary Amines: Primary amines are organic compounds containing a nitrogen atom bonded to two hydrogen atoms and one alkyl or aryl group. They are a class of amines that play a crucial role in various organic chemistry topics, including the chemistry of amides, the structure and properties of amines, the synthesis of amines, the reactions of amines, and the spectroscopy of amines.
Pyramidal inversion: Pyramidal inversion is a spontaneous process in which the spatial arrangement of atoms at a trivalent (three-coordinate) nitrogen atom in an amine changes, resembling an umbrella flipping inside out. This process allows molecules like amines to invert their three-dimensional shape without breaking any bonds.
Pyramidal Inversion: Pyramidal inversion is a process that occurs in organic chemistry, particularly in the context of the structure and properties of amines. It describes the rearrangement of the substituents around a nitrogen atom, resulting in the inversion of the nitrogen's pyramidal geometry.
Quaternary ammonium salts: Quaternary ammonium salts are compounds where an ammonium ion (NH4+) has four organic groups attached to it, typically through a nitrogen atom, and is paired with a negatively charged ion (anion). These substances are known for their surfactant and antiseptic properties.
Secondary Amines: Secondary amines are a class of organic compounds that contain a nitrogen atom bonded to two alkyl or aryl groups. They are characterized by the presence of two carbon-nitrogen bonds, distinguishing them from primary amines which have one carbon-nitrogen bond and tertiary amines which have three carbon-nitrogen bonds.
Sp³ Hybridized: sp³ hybridization is a type of orbital hybridization that occurs in certain organic molecules, where the central atom has four sp³ hybrid orbitals. This hybridization results in a tetrahedral geometry around the central atom, with bond angles of approximately 109.5 degrees.
Tert-butylamine: tert-Butylamine is a primary amine with the chemical formula (CH3)3CNH2. It is a colorless, flammable liquid with a strong, fishy odor. tert-Butylamine is an important organic compound that is commonly used in the synthesis of various chemicals and pharmaceuticals.
Tertiary Amines: Tertiary amines are a class of organic compounds where a nitrogen atom is bonded to three alkyl or aryl groups. They are an important subgroup of amines, which are organic compounds containing a nitrogen atom with a lone pair of electrons.
Tetrahedral Geometry: Tetrahedral geometry refers to the three-dimensional spatial arrangement of atoms or groups of atoms in a molecule, where the central atom is bonded to four other atoms or groups in a symmetrical tetrahedral configuration. This geometric structure is a fundamental concept in understanding the structure and properties of various organic and inorganic compounds.