Physical and Chemical Properties — Revision Notes

⚡ 30-Second Revision

Basicity: — Lone pair on N. Aliphatic > NH$_3$ > Aromatic. Aqueous basicity: $2^\circ > 1^\circ > 3^\circ$ (methyl), $2^\circ > 3^\circ > 1^\circ$ (ethyl). Aromatic amines are weaker due to resonance.\n- Boiling Point: $1^\circ > 2^\circ > 3^\circ$ (isomeric) due to H-bonding. Alcohols > Amines.\n- Solubility: Lower amines water soluble due to H-bonding with water.\n- Carbylamine Test: Only $1^\circ$ amines (R-NH$_2$) $\xrightarrow{CHCl_3, KOH}$ R-NC (foul smell).\n- **Nitrous Acid (HNO$_2$, 0-5 $^\circ$C):**\n * $1^\circ$ Aliphatic: ROH + N$_2$ gas.\n * $1^\circ$ Aromatic: Ar-N$_2^+$Cl$^-$ (stable diazonium salt).\n * $2^\circ$: R$_2$N-N=O (N-nitrosoamine, yellow oily).\n * $3^\circ$ Aliphatic: R$_3$NH$^+$NO$_2^-$ (salt).\n- Acylation: $1^\circ, 2^\circ$ amines $\xrightarrow{R'COCl / (R'CO)_2O}$ Amides. $3^\circ$ amines do not react.\n- Alkylation: $RNH_2 \xrightarrow{R'X}$ $RNHR' \xrightarrow{R'X}$ $R_2NR' \xrightarrow{R'X}$ $R_3N^+R'X^-$. (Mixture of products).\n- Electrophilic Substitution (Aromatic Amines): $-NH_2$ is strong activating, ortho-para director. Polysubstitution (e.g., tribromination). Protection (acetylation) needed for mono-substitution.

2-Minute Revision

Amines, derived from ammonia, exhibit distinct physical and chemical properties. Physically, their boiling points are higher than non-polar compounds but lower than alcohols, primarily due to intermolecular hydrogen bonding in primary ($1^\circ$) and secondary ($2^\circ$) amines.

Tertiary ($3^\circ$) amines lack N-H bonds, thus having lower boiling points. Lower molecular weight amines are water-soluble. Chemically, amines are basic and nucleophilic due to the lone pair on nitrogen.

Basicity is enhanced by electron-donating alkyl groups but diminished by resonance in aromatic amines. In aqueous solution, solvation effects significantly influence basicity, leading to $2^\circ > 1^\circ > 3^\circ$ for methylamines.

Key reactions include the carbylamine test, specific to $1^\circ$ amines, producing foul-smelling isocyanides. Reaction with nitrous acid differentiates $1^\circ, 2^\circ, 3^\circ$ amines: $1^\circ$ aliphatic give N$_2$ gas and alcohol; $1^\circ$ aromatic give stable diazonium salts; $2^\circ$ give oily N-nitrosoamines.

$1^\circ$ and $2^\circ$ amines undergo acylation to form amides, while $3^\circ$ amines do not. Aromatic amines are highly activated towards electrophilic substitution, often requiring protection of the amino group to control selectivity.

5-Minute Revision

Amines are organic bases with a nitrogen atom possessing a lone pair of electrons. This lone pair dictates their fundamental properties. \n\nPhysical Properties:\n* Boiling Points: $1^\circ$ and $2^\circ$ amines form intermolecular hydrogen bonds (N-H...

N), leading to higher boiling points than $3^\circ$ amines (which lack N-H bonds). However, amine H-bonds are weaker than alcohol H-bonds, so alcohols have higher boiling points than amines of comparable mass.

For isomeric amines, the order is generally $1^\circ > 2^\circ > 3^\circ$.\n* Solubility: Lower amines are water-soluble due to their ability to form hydrogen bonds with water molecules. Solubility decreases as the hydrocarbon part increases.

\n* Odour: Lower amines have characteristic 'fishy' or ammonia-like odours.\n\nChemical Properties:\n1. Basicity: Amines are basic due to the lone pair on nitrogen. \n * Aliphatic amines are generally stronger bases than ammonia due to the +I effect of alkyl groups.

\n * Aromatic amines are weaker bases than aliphatic amines (and often weaker than ammonia) because the lone pair is delocalized into the benzene ring by resonance, making it less available for protonation.

\n * Aqueous Basicity Order: This is a balance of inductive effect, steric hindrance, and solvation of the conjugate acid. For methylamines: (CH$_3$)$_2$NH > CH$_3$NH$_2$ > (CH$_3$)$_3$N > NH$_3$.

For ethylamines: (C$_2$H$_5$)$_2$NH > (C$_2$H$_5$)$_3$N > C$_2$H$_5$NH$_2$ > NH$_3$.\n2. Alkylation (Hofmann Ammonolysis): Amines react with alkyl halides to form higher amines and eventually quaternary ammonium salts.

This often yields a mixture of products.\n $RNH_2 \xrightarrow{R'X} R_2NH \xrightarrow{R'X} R_3N \xrightarrow{R'X} R_4N^+X^-$\n3. Acylation: $1^\circ$ and $2^\circ$ amines react with acid chlorides or anhydrides to form amides.

$3^\circ$ amines do not react as they lack an N-H bond.\n $RNH_2 + R'COCl \rightarrow RNHCOR' + HCl$\n4. Carbylamine Reaction (Isocyanide Test): A specific test for $1^\circ$ amines (aliphatic and aromatic).

Heating with CHCl$_3$ and alcoholic KOH produces foul-smelling isocyanides (R-NC).\n $RNH_2 + CHCl_3 + 3KOH \xrightarrow{\text{heat}} R-NC + 3KCl + 3H_2O$\n5. **Reaction with Nitrous Acid (NaNO$_2$/HCl, 0-5 $^\circ$C):** Used to distinguish amines.

\n * $1^\circ$ Aliphatic: Forms unstable diazonium salt, decomposes to alcohol and N$_2$ gas (effervescence).\n * $1^\circ$ Aromatic: Forms stable diazonium salt (Ar-N$_2^+$Cl$^-$), important for synthesis.

\n * $2^\circ$ (Aliphatic/Aromatic): Forms N-nitrosoamine (yellow oily layer).\n * $3^\circ$ Aliphatic: Forms trialkylammonium nitrite salt (soluble).\n6. Electrophilic Substitution (Aromatic Amines): The $-NH_2$ group is a strong activating and ortho-para directing group.

Direct bromination leads to 2,4,6-tribromoaniline. To achieve mono-substitution or control nitration, the amino group is protected by acetylation (forming acetanilide), which reduces its activating power.

Prelims Revision Notes

1
Basicity: — Amines are basic due to the lone pair on nitrogen. \n * Factors: Inductive effect (+I groups increase basicity), resonance effect (delocalization decreases basicity), solvation effect (stabilization of conjugate acid by H-bonding with water). \n * Order (Gas Phase): $3^\circ > 2^\circ > 1^\circ > NH_3$ (purely inductive effect). \n * Order (Aqueous Solution): \n * Methylamines: $(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3$ \n * Ethylamines: $(C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 > NH_3$ \n * Aromatic Amines: Weaker bases than aliphatic amines due to resonance delocalization of the lone pair into the ring. Aniline is weaker than ammonia.\n2. Boiling Points: \n * $1^\circ$ and $2^\circ$ amines form intermolecular H-bonds (N-H...N). \n * $3^\circ$ amines cannot form intermolecular H-bonds among themselves (no N-H bond). \n * Order (isomeric amines): $1^\circ > 2^\circ > 3^\circ$. \n * Alcohols have higher boiling points than amines of comparable mass due to stronger O-H...O H-bonds.\n3. Solubility: Lower amines are soluble in water due to H-bonding with water. Solubility decreases with increasing size of hydrophobic alkyl/aryl group.\n4. Carbylamine Reaction (Isocyanide Test): \n * Specific for: Primary amines (aliphatic and aromatic). \n * Reagents: $CHCl_3 + alcoholic KOH$, heat. \n * Product: Foul-smelling isocyanide (R-NC).\n5. **Reaction with Nitrous Acid (NaNO$_2$/HCl, 0-5 $^\circ$C):** \n * **Primary Aliphatic Amines (R-NH$_2$):** Form unstable diazonium salts, which decompose to alcohols (ROH) and evolve nitrogen gas ($N_2$). \n * **Primary Aromatic Amines (Ar-NH$_2$):** Form relatively stable aromatic diazonium salts (Ar-N$_2^+$Cl$^-$), important synthetic intermediates. \n * **Secondary Amines (R$_2$NH):** Form N-nitrosoamines (R$_2$N-N=O), yellow oily compounds. \n * **Tertiary Aliphatic Amines (R$_3$N):** Form trialkylammonium nitrite salts (R$_3$NH$^+$NO$_2^-$).\n6. Acylation: \n * Reactants: $1^\circ$ and $2^\circ$ amines (due to N-H bonds) with acid chlorides ($R'COCl$) or acid anhydrides ($(R'CO)_2O$). \n * Product: Amides ($RNHCOR'$ or $R_2NCOR'$). \n * **$3^\circ$ amines: Do not undergo acylation.\n7. Alkylation (Hofmann Ammonolysis):** Amines react with alkyl halides ($R'X$) to form higher substituted amines and eventually quaternary ammonium salts ($R_4N^+X^-$). Often yields a mixture.\n8. Electrophilic Substitution (Aromatic Amines, e.g., Aniline): \n * The $-NH_2$ group is a strong activating group and an ortho-para director due to +R effect. \n * Bromination: With bromine water, gives 2,4,6-tribromoaniline (white precipitate). \n * Nitration: Direct nitration gives a mixture of o-, m-, p-nitroanilines (significant meta product due to anilinium ion formation in acidic medium). To get p-nitroaniline, protect $-NH_2$ by acetylation first (form acetanilide), then nitrate, then hydrolyze. \n * Sulphonation: Forms sulphanilic acid (zwitterionic).

Vyyuha Quick Recall

To remember the basicity order of methylamines in water: Secondary Primary Tertiary Ammonia (SPT A) - (CH$_3$)$_2$NH > CH$_3$NH$_2$ > (CH$_3$)$_3$N > NH$_3$. \n\nFor Nitrous Acid reactions: 1Alcohol, 1Aromatic Diazonium, 2Nitroso, 3Salt.

\n* 1Alcohol: $1^\circ$ Aliphatic $\rightarrow$ Alcohol + N$_2$ gas. \n* 1Aromatic Diazonium: $1^\circ$ Aromatic $\rightarrow$ Diazonium salt. \n* 2Nitroso: $2^\circ$ Amine $\rightarrow$ N-nitrosoamine (yellow oil).

\n* 3Salt: $3^\circ$ Aliphatic $\rightarrow$ Ammonium Salt.