6.2.1 Amines + 6.2.2 Amino acids, amides and chirality

An organic derivative of ammonia in which one or more hydrogen atoms are replaced by alkyl or aryl groups.

Primary: $RNH_2$; secondary: $R_2NH$; tertiary: $R_3N$.

By the number of alkyl or aryl groups directly bonded to nitrogen.

Aliphatic amines have alkyl groups; aromatic amines have an amino group directly attached to an aromatic ring.

$CH_3NH_2$, $C_2H_5NH_2$ and $C_6H_5NH_2$.

$(CH_3)_2NH$ and $(CH_3)_3N$.

Alkyl group + amine, e.g. ethylamine; or using amino- with a numbered carbon chain.

The nitrogen lone pair accepts a proton, forming a dative covalent bond.

A dative covalent bond from nitrogen to $H^+$.

They accept a proton to form substituted ammonium salts.

$RNH_2+HCl\rightarrow RNH_3^+Cl^-$

$C_2H_5NH_2+HCl\rightarrow C_2H_5NH_3^+Cl^-$

Methylammonium chloride, $CH_3NH_3^+Cl^-$.

Diethylammonium chloride, $(C_2H_5)_2NH_2^+Cl^-$.

Add aqueous sodium hydroxide to deprotonate the ammonium ion.

$CH_3NH_3Cl+NaOH\rightarrow CH_3NH_2+NaCl+H_2O$

They accept protons from water, producing hydroxide ions.

$RNH_2+H_2O\rightleftharpoons RNH_3^++OH^-$

Excess ethanolic ammonia, heated in a sealed tube under pressure.

Nucleophilic substitution.

Its nitrogen lone pair attacks the electron-deficient carbon bonded to the halogen.

To reduce competing hydrolysis of the haloalkane.

To favour primary amine formation and reduce further substitution.

$RX+2NH_3\rightarrow RNH_2+NH_4X$

$CH_3CH_2Br+2NH_3\rightarrow CH_3CH_2NH_2+NH_4Br$

Nucleophilic attack forming an alkylammonium ion, followed by deprotonation.

A second ammonia molecule acting as a base.

The amine products are also nucleophiles and can undergo further substitution.

React a haloalkane with excess primary amine in ethanol.

React a haloalkane with excess secondary amine in ethanol.

A primary amine, $RNH_2$.

A secondary amine, $R_2NH$.

$C_2H_5Br+2C_2H_5NH_2\rightarrow(C_2H_5)_2NH+C_2H_5NH_3Br$

A quaternary ammonium salt, $R_4N^+X^-$.

Fractional distillation, using differences in boiling points.

Nitrobenzene.

Tin and concentrated hydrochloric acid, heated under reflux, followed by aqueous sodium hydroxide.

To deprotonate the phenylammonium salt and release phenylamine.

$C_6H_5NO_2+6[H]\rightarrow C_6H_5NH_2+2H_2O$

$RCH(NH_2)COOH$

An amino group, $-NH_2$, and a carboxylic acid group, $-COOH$.

The amino group is attached to the carbon adjacent to the carboxylic acid carbon.

A variable side chain, which may be hydrogen or contain other functional groups.

2-Aminoethanoic acid and 2-aminopropanoic acid.

They contain an acidic carboxyl group and a basic amino group.

It accepts a proton, forming $-NH_3^+$.

It donates a proton, forming $-COO^-$ and water.

Its amino group is protonated, giving a positively charged ammonium species.

Its carboxylic acid group is deprotonated, giving a negatively charged carboxylate species.

$RCH(NH_2)COOH+HCl\rightarrow[RCH(NH_3^+)COOH]Cl^-$

$RCH(NH_2)COOH+NaOH\rightarrow RCH(NH_2)COO^-Na^++H_2O$

Both amino groups are protonated to $-NH_3^+$.

Their carboxylic acid group reacts with an alcohol using a strong acid catalyst, forming an ester and water.

Heat with an alcohol and an acid catalyst, such as concentrated sulfuric acid.

$RCH(NH_2)COOH+R'OH\rightleftharpoons RCH(NH_2)COOR'+H_2O$

The amino group is also protonated to $-NH_3^+$.

A species containing both positive and negative charges but with no overall charge.

A proton transfers from $-COOH$ to $-NH_2$, forming $-COO^-$ and $-NH_3^+$.

$RCH(NH_3^+)COO^-$

Their zwitterionic forms experience strong electrostatic attractions in the solid state.

An organic compound containing a carbonyl group directly bonded to nitrogen.

$RCONH_2$

$RCONHR'$

Primary amides have two N–H bonds; secondary amides have one N–H bond.

$RCONR'R''$

$HCONH_2$ and $CH_3CONH_2$.

A substituent attached directly to the nitrogen atom.

$CH_3CH_2CONHCH_3$

React an acyl chloride with a primary amine by nucleophilic addition–elimination.

Nucleophilic addition–elimination.

The amide linkage $-CONH-$ joining amino acid residues.

Compounds with the same structural formula but different arrangements of atoms in space.

E/Z isomerism and optical isomerism.

Stereoisomerism involving non-superimposable mirror-image molecules called enantiomers.

A tetrahedral carbon atom bonded to four different atoms or groups.

With an asterisk (*).

A pair of non-superimposable mirror-image stereoisomers.

The molecules cannot be placed exactly on top of one another with all atoms aligned.

Tetrahedral, with bond angles of approximately $109.5^\circ$.

Solid wedges project towards the viewer; dashed wedges project away.

Find a tetrahedral carbon bonded to four different groups, comparing complete side chains.

It is not tetrahedral and cannot have four different single-bonded groups.

Glycine, because its α-carbon is bonded to two hydrogen atoms.

Their α-carbon has four different groups, allowing optical isomerism.

They rotate it by equal angles in opposite directions under identical conditions.

They generally have identical properties in achiral environments but may behave differently in chiral environments.

An equal mixture of two enantiomers with no overall optical rotation.

Two enantiomers.

A reaction in which a covalent bond is broken by reaction with water.

A reaction in which two molecules join by forming a new covalent bond, eliminating a small molecule, usually water.