FAD 1018 BASIC CHEMISTRY 2 AMINE LECTURE 1 Notes

Amines: An Overview

Learning Outcomes

Categorize amines as primary ( $1°$ ), secondary ( $2°$ ), or tertiary ( $3°$ ).
Draw and name amines according to IUPAC nomenclature.
Explain the physical properties of amines: boiling points, solubility.
Compare the basicity of ammonia, aliphatic amines, and aromatic amines.
Explain the preparation of aromatic amines and primary aliphatic amines using nitriles.
Explain the preparation of primary, secondary, and tertiary aliphatic amines using amides and Hoffmann's degradation.
Explain the chemical properties with references to the reactions with acyl chloride, acid anhydrides, nitrous acid, and bromine water.

Structure and Classification

Amines are compounds and functional groups that contain a basic nitrogen atom.
Primary ( $1°$ ) amine: Nitrogen atom bonded to one carbon atom and two hydrogen atoms.
Secondary ( $2°$ ) amine: Nitrogen atom bonded to two carbon atoms and one hydrogen atom.
Tertiary ( $3°$ ) amine: Nitrogen atom bonded to three carbon atoms.
Ammonium salt: A salt formed when an amine is protonated.
Quaternary ( $4°$ ) ammonium salt: Nitrogen atom bonded to four carbon atoms, carrying a positive charge.

Nomenclature

Name is based on the longest carbon chain.
As a parent:
- The "-e" of alkane is replaced with "-amine."
- Substituents on the nitrogen atom have an "N-" prefix.
As a substituent:
- On a molecule with a higher priority functional group, the amine is named as a substituent.
- $-NH_2$ is called an amino group.
When an amino group is bonded to a benzene ring, the parent compound is called aniline.
- Examples:
  - Aniline
  - N,N-diethylaniline
  - 4-methylaniline (or p-toluidine)

Physical Properties

Low molecular mass amines have a characteristic fishy smell.
Amines are polar molecules and form intermolecular hydrogen bonds due to the high electronegativity of the nitrogen atom compared to the hydrogen atom.

Boiling Points

Amines have higher boiling points than alkanes, haloalkanes, and carbonyl compounds of comparable molar mass because they form hydrogen bonds.
Amines have lower boiling points than alcohols of comparable mass because the hydrogen bonds formed between amines are weaker than those in alcohols.
- Nitrogen (N) is less electronegative than Oxygen (O), making the hydrogen bonds in amines weaker.
Boiling Point Trend: carboxylic acid > alcohol > amine > carbonyl compound / haloalkane > alkane
Primary ( $1°$ ) amines have higher boiling points than secondary ( $2°$ ) and tertiary ( $3°$ ) amines.
- Primary amines can form more intermolecular hydrogen bonds.
- Secondary amines form fewer hydrogen bonds.
- Tertiary amines cannot form intermolecular hydrogen bonds at all.

Example

Matching compounds to their boiling points (1 = highest BP, 5 = lowest BP):

$C6H5NH_2$ (Aniline)
$CH3NHCH2CH_3$
$CH3NH2$
$(CH3)3N$
$(CH3)2NH$

Solubility

Small amines (less than six carbons) are soluble in water because they can form strong hydrogen bonds with water molecules.
Branching increases solubility because more branching leads to less surface area for hydrophobic interactions.
Larger molecules have more hydrophobic regions, reducing their ability to dissolve in water.
Most amines smell like rotting fish.
Aliphatic amines are generally more soluble than aromatic amines.

Basicity of Amines

Amines are basic because they have a lone pair of electrons on the nitrogen atom, allowing them to accept protons ( $H^+$ ).
Amines are also nucleophilic because the lone pair on nitrogen can attack electron-deficient species (electrophiles).
Amines react with acids to form salts.

Aromatic Amines

Aromatic amines (aniline) are less basic than ammonia and aliphatic amines.
Resonance effects: The lone pair of electrons on the nitrogen atom is delocalized with the $\pi$ electrons of the aromatic ring. This reduces electron availability for protonation.
Hybridization: Nonbonding electrons occupying an $sp^2$ orbital (aromatic amines) have greater s character (33%) and are more tightly held than those in the $sp^3$ orbital (aliphatic amines).

Basicity Comparison

Basicity increases in the order of: aniline < ammonia < methanamine < cyclohexanamine < ethanamine
$Kb$ (Base Dissociation Constant): A higher $Kb$ means the amine ionizes more in water, making it a stronger base.
$pKb = -log(Kb)$ : A lower $pK_b$ means a stronger base.
| Compound | $Kb$ | $pKb$ |
| --------------- | ------------- | -------- |
| ethanamine | $6.46 × 10^{-4}$ | 3.19 |
| cyclohexanamine | $4.57 × 10^{-4}$ | 3.34 |
| methanamine | $4.37 × 10^{-4}$ | 3.36 |
| ammonia | $1.82 × 10^{-5}$ | 4.74 |
| aniline | $4.27 × 10^{-10}$ | 9.37 |

Substituent Effects

Electron-donating substituents increase the basicity of amines, while electron-withdrawing substituents decrease the basicity.

	Substituent	Compound	$K<em>b$ \| $pK</em>b$	Effect
-OH (EDG)	4-aminophenol	$3.0 × 10^{-9}$	8.52	Increase basicity
-H	aniline	$4.2 × 10^{-10}$	9.38
-NO2 (EWG)	4-nitroaniline	$9.5 × 10^{-14}$	13.02	Decrease basicity

Question

If you wanted to predict the relative basicity of two amines, one with a cyano (-CN) substituent and another with a methyl (-CH3) substituent, which amine would you expect to be more basic?

Answer: B. The one with the methyl (-CH3) substituent because the methyl group is an electron-donating group (EDG), increasing electron density on nitrogen and making the amine more basic. In contrast, the cyano (-CN) group is a strong electron-withdrawing group (EWG), pulling electron density away and making the amine less basic.