6 - Organic Chemistry & Analysis

• Kékulé Model of Benzene

  The Kékulé model was disproven as the structure could not explain all its chemical and physical properties.

  
  

  Lack of Reactivity:

  - Does not undergo electrophilic addition reactions.

  - Does not decolourise bromine

  - Therefore, no C=C bonds are present.

  
  

  Thermodynamic Stability:

  - Less energy is produced by the enthalpy change of hydrogenation than expected.

  - The actual structure is therefore more stable than this theoretical structure.

  
  

  Bond Length:

  - All the bond lengths in the actual structure are the same length.

  - Theoretical structure would have different bond lengths (C-C and C=C).

  
• Delocalised Model of Benzene

  Each carbon uses three (of four) electrons to bond with two C atoms and one H atom.

  
  

  Each C has an electron in a p-orbital.

  Adjacent p-orbitals overlap sideways, above and below the carbon ring, creating an area of high electron density - a system of pi-bonds.

  
• Nitration of Benzene


  Electrophilic substitution reaction.

  
  

  Reagents: HNO₃, H₂SO₄ , 50C

  
  

  Formation of electrophile:

  HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O

  
  

  NO₂⁺ accepts a pair of electrons from the benzene ring, forming an unstable intermediate.

  
  

  The intermediate breaks down to form nitrobenzene and H⁺

  
  

  H⁺ reacts with HSO₄⁻ to restore the catalyst.

  
• Halogenation of Benzene


  Do not react unless there is a halogen carrier present, e.g., AlCl₃, FeBr₃, as benzene is too stable to react with non-polar halogen molecules.

  
  

  Reagents: RTP, halogen carrier

  
  

  
  

  Formation of Electrophile:

  Br₂ + FeBr₃ → FeBr₄⁻ + Br⁺

  
  

  Br⁺ accepts a pair of electrons from the benzene ring, forming an unstable intermediate.

  
  

  Intermediate breaks down to form bromobenzene and H⁺

  
  

  H⁺ reacts with FeBr₄⁻ to reform the catalyst FeBr₃

  
• Alkylation of Benzene


  Electrophilic substitution reaction.

  
  

  Reagents: haloalkane, AlCl₃ catalyst (acts as halogen carrier).

  
  

  AlCl₃ generates the electrophile.

  
  

  Increases the number of carbon atoms in a compound.

  
• Acylation of Benzene


  Electrophilic substitution reaction.

  
  

  Reagents: acyl chloride, AlCl₃ catalyst

  
  

  Forms an aromatic ketone.

  
• Reactivity of Alkenes v Arenes

  Benzene, unlike alkenes, does not react with halogens unless a halogen carrier is present.

  
  

  This is due to the delocalised pi-bond system in benzene creating a lesser electron density than a C=C bond in an alkene.

  
  

  Alkenes have a higher electron density due to localised electrons above/below the C=C bond, which induces dipoles in non-polar halogen molecules.

  This creates δ⁻ and δ⁺, enabling the halogen to act like an electrophile.
• Phenols


  Hydroxyl functional group is directly bonded to an aromatic ring (benzene).
• Phenol Solubility


  Less soluble in water than alcohols due to non-polar benzene ring.
• Phenol Acidity


  Partially dissociates in water, meaning it is a weak acid.

  
  

  More acidic than alcohols but less acidic than carboxylic acids.

  
  

  Ethanol does not react with NaOH.

  Phenols and carboxylic acids react with NaOH.

  
  

  Only carboxylic acids are strong enough to react with Na₂CO₃ (weak base).

  
  

  Can be used to distinguish - carboxylic acids react with Na₂CO₃ and produce CO₂
• Phenol + Sodium Hydroxide

  Forms the salt sodium phenoxide and water

  
• Bromination of Phenol


  Electrophilic substitution reaction.

  
  

  Phenol reacts with Br₂ (aq) to form a white precipitate of 2,4,6-tribromophenol.

  
  

  Does NOT require a halogen carrier.

  
• Nitration of Phenol


  Electrophilic substitution reaction.

  
  

  Phenol reacts with dilute HNO₃ at room temperature.

  
  

  Forms a mixture of 2-nitrophenol and 4-nitrophenol.

  
• Reactivity of Benzene v Phenol

  Electrophilic substitution occurs more readily with phenol than benzene.

  
  

  This is caused by a lone pair of electrons from the -OH group being donated into the pi-system of phenol.

  
  

  This increases the electron density of phenol, attracting electrophiles more strongly than benzene, and the aromatic ring in phenol is therefore more susceptible to attack.
• Directing Groups


  OH and NH₂ are 2- and 4- directing groups.

  
  

  NO₂ is a 3- directing group.
• Oxidation of Aldehydes


  React with K₂Cr₂O₇/H₂SO₄ to form carboxylic acids.
• Reactivity of Ketones and Aldehydes

  C=O bond is polar, meaning it is susceptible to attack by nucleophiles.

  
  

  The nucleophile attacks the δ+ carbon atom, resulting in nucleophilic addition.
• Nucleophilic Addition of NaBH₄

  Aldehyde/ketone is warmed with NaBH₄ in aqueous solution.

  
  

  Aldehydes are reduced to primary alcohols.

  Ketones are reduced to secondary alcohols.

  
  

  NaBH₄ contains the nucleophile, hydride (H⁻), which is attracted to the δ+ carbon atom.

  
  

  Pi-bond in C=O breaks, forming a negatively charged intermediate.

  
  

  Oxygen of intermediate donates lone pair of electrons to H atom in a water molecule (aqueous solution).

  
  

  Forms alcohol and OH⁻

  
• Nucleophilic Addition of NaCN/H⁺

  CN⁻ attracted to δ+ carbon atom and donates electrons.

  
  

  Pi-bond in C=O breaks, forming a negatively charged intermediate.

  
  

  Oxygen of intermediate donates lone pair of electrons to H⁺ ion.

  
  

  Forms a hydroxynitrile.

  
• Using 2,4-DNPH


  Detects the presence of C=O groups, forming an orange precipitate if an aldehyde or ketone is present.

  
  

  Orange precipitate is purified by recrystallisation and its melting point determined. Compared to database to identify compound.
• Using Tollens' Reagent


  Distinguishes between aldehydes and ketones.

  
  

  Produces a silver mirror in the presence of an aldehyde.

  
  

  Silver ions are reduced to silver as the aldehyde is oxidised further (to a carboxylic acid).
• Solubility of Carboxylic Acids

  C=O and O-H bonds are polar, allowing carboxylic acids to form hydrogen bonds with water molecules.

  
  

  As the number of carbon atoms increases, solubility decreases.
• Reactions of Carboxylic Acids with Metals and Bases

  Metals:

  Forms carboxylate salt and H₂

  
  

  Metal Oxides:

  Forms carboxylate salt and H₂O

  
  

  Alkalis:

  Forms carboxylate salt and H₂O

  
  

  Carbonates:

  Forms carboxylate salt, H₂O and CO₂

  
• Acid Anhydrides


  Formed by the removal of water from two carboxylic acid molecules.

  
  

  Less reactive than acyl chlorides.

  
• Acyl Chloride


  React parent carboxylic acid with SOCl₂.

  
  

  Forms acyl chloride, SO₂ and HCl.

  
• Ester Nomenclature


  Named after the parent carboxylic acid, removing the -oic suffix and replacing with -oate.

  
  

  Alkyl chain attached to the O atom of the -COO group (from the alcohol) is the first word in the name.
• Esterification of Acyl Chlorides

  React with alcohol to form esters.

  
  

  Can also react with phenols to form esters.
• Esterification of Carboxylic Acids

  Reagents: conc. H₂SO₄ catalyst, alcohol

  
  

  Alcohol is warmed with the carboxylic acid.
• Esterification of Acid Anhydrides

  React with alcohol to form esters.
• Acid Hydrolysis of Esters

  Heated under reflux with dilute aqueous acid.

  
  

  Produces carboxylic acid and alcohol.

  
  

  Reversible.
• Alkali Hydrolysis of Esters


  Heated under reflux with dilute OH⁻ ions (aq)

  
  

  Produces carboxylate salt and alcohol.

  
  

  Irreversible.
• Acyl Chloride → Carboxylic Acid


  React with water to form carboxylic acid and HCl

  
• Acyl Chloride → Primary Amide


  Reagent: conc. NH₃

  
• Acyl Chloride → Secondary Amide


  Reagent: primary amine

  
• Basicity of Amines


  Act as bases due to the lone pair of electrons on nitrogen, which can accept a proton.

  
  

  React with dilute acids to form ammonium salts.
• Preparation of Aliphatic Amines


  Nucleophilic substitution of haloalkanes.

  
  

  Reagent: excess ethanolic ammonia.

  
  

  Ammonium salt is produced from haloalkane + NH₃

  
  

  Aqueous alkali is then added to create the amine.

  
• Preparation of Aromatic Amines

  Phenylamine is formed by the reduction of nitrobenzene.

  
  

  Reagents: reflux, tin, conc. HCl

  
  

  Forms the ammonium salt (⏣NH₃⁺Cl⁻).

  
  

  Aqueous alkali is then added to create the phenylamine.

  
• Reactions of Amines with Alkalis

  -COOH group reacts with alkalis to form salts.
• Reactions of Amines with Acids

  NH₂ group reacts with acids to form ammonium salts.

  
  

  NH₂ group becomes protonated, producing NH₃⁺
• Esterification of Amines


  Reagents: heat, conc. H₂SO₄, alcohol.

  
  

  The -COOH group is esterified, producing an ester.

  
  

  The NH₂ group is protonated, producing NH₃⁺