Methylamine
Methylamine is the simplest primary aliphatic amine, an organic compound with the molecular formula CH₃NH₂ and a molecular weight of 31.06 g/mol.[1][2] It appears as a colorless, flammable gas under standard conditions, with a boiling point of −6 °C, a melting point of −93 °C, and a pungent fishy odor similar to ammonia.[3][4] Industrially produced by the catalytic reaction of methanol with ammonia, typically over an alumina or silica-alumina catalyst, it serves as a key intermediate in the synthesis of pharmaceuticals (such as ephedrine and theophylline), pesticides, herbicides, fungicides, solvents, dyes, and fuel additives.[5][6][7] While essential for legitimate chemical manufacturing, methylamine has gained notoriety as a precursor in the illicit synthesis of methamphetamine, prompting strict regulatory controls on its distribution despite its broad industrial utility.[2]
Chemical Properties
Molecular Structure and Bonding
Methylamine has the chemical formula CH₃NH₂, representing the simplest primary aliphatic amine formed by substituting one hydrogen atom in ammonia (NH₃) with a methyl (CH₃) group.[1] The nitrogen atom is sp³ hybridized, utilizing four equivalent hybrid orbitals to form three σ-bonds—one to the carbon atom and two to hydrogen atoms—while accommodating a lone pair of electrons in the remaining orbital. This lone pair confers nucleophilicity and basicity, as it can donate electrons to form coordinate bonds or accept protons./Amines/Properties_of_Amines/Basic_Properties_of_Amines) The tetrahedral electron arrangement around nitrogen yields a trigonal pyramidal molecular geometry, distorted by lone pair-bond pair repulsions that compress bond angles below the ideal 109.5°. Microwave spectroscopy data indicate an H-N-H bond angle of approximately 106° and a C-N-H angle of about 112°, with the C-N bond length measured at 1.471 Å (r₀ structure).[8][9] These structural features arise from the pyramidal conformation, which inverts rapidly at room temperature due to a low barrier of about 8 kJ/mol. In comparison to ammonia, the methyl substituent exerts an inductive electron-donating effect, increasing electron density on nitrogen and enhancing basicity, as the lone pair becomes more available for protonation.[10] Both molecules support hydrogen bonding, with methylamine acting as a donor through its N-H bonds and as an acceptor via the nitrogen lone pair; however, the nonpolar methyl group reduces overall polarity relative to ammonia and introduces greater van der Waals interactions. Relative to higher primary alkylamines, methylamine exhibits similar bonding but diminished hydrogen bonding capacity per molecule as chain length increases, owing to fewer relative N-H sites amid bulkier alkyl groups, though primary amines retain two donor hydrogens unlike secondary or tertiary variants.[11]/Amines/Properties_of_Amines/Basic_Properties_of_Amines)Physical and Thermodynamic Properties
Methylamine exists as a colorless gas with a pungent, ammonia-like odor under standard conditions of 25 °C and 1 atm.[2] Its melting point is -93.1 °C, and its normal boiling point is -6.3 °C, rendering it gaseous at ambient temperatures.[12] The liquid phase has a density of approximately 0.66 g/cm³ near its boiling point, while the vapor density is 1.08 relative to air, indicating vapors heavier than air.[13] Methylamine is miscible with water and soluble in common organic solvents such as ethanol and ether.[2] Vapor pressure at 20 °C is 304 kPa (3.0 atm).[14] Critical constants include a critical temperature of 430.6 K (157.45 °C), critical pressure of 7.61 MPa, and critical density of approximately 222 kg/m³.[2][15]| Property | Value | Conditions | Source |
|---|---|---|---|
| Standard enthalpy of formation (ΔH_f°) | -23.0 kJ/mol | Gas phase, 298 K | [16] |
| Standard Gibbs free energy of formation (ΔG_f°) | 32.3 kJ/mol | Gas phase, 298 K | [16] |
| Standard entropy (S°) | 243 J/mol·K | Gas phase, 298 K | [16] |