Alkanes

• Organic Chemistry (A-level) exam

  <|>Alkane (saturated hydrocarbon):

  • Combustion (complete and incomplete)
  • Free-radical substitution
  • Cracking (elimination): alkane → alkene + alkane (no oxygen, high temperature, zeolite catalyst)
• Alkene (unsaturated hydrocarbon):

  • Addition (electrophilic addition):
  • Hydrogen (H2 (g)): CH2=CH2 + H2 → CH3CH3 (140 ℃, Ni catalyst)
  • Steam (H2O (g)): CH2=CH2 + H2O → CH3CH2OH (330 ℃, 6MPa, H3PO4)
  • Hydrogen Halides (HX (aq)): CH2=CH2 + HBr → CH3CH2Br (conc. HX, r.t.p.)
  • Halogens (X2 (aq)): CH2CH2 + Br2 → CH2BrCH2Br (r.t.p.)
  • Test for the presence of C=C bond (decolourisation of Br2)
• Oxidation:

  • Cold Dilute Acidified Manganate(VII) Solution (KMnO4)
  • Hot Concentrated Acidified Manganate(VII) Solution (KMnO4)
  • Addition Polymerisation
• Halogenoalkane:

  • Substitution (nucleophilic substitution):
  • Aqueous Alkali (OH- (aq)): CH3CH2Br + NaOH → CH3CH2OH + NaBr / CH3CH2Br + H2O → CH3CH2OH + HBr (Heated under reflux)
  • KCN (CN- (in ethanol)): CH3CH2Br + CN- → CH3CH2CN + Br- (heated under reflux)
  • Ammonia (NH3 (in ethanol)): CH3CH2Br + NH3 → CH3CH2NH2 + HBr (heated)
  • Mechanism:
  • Primary Halogenoalkane (SN2): S stands for substitution, N stands for nucleophilic, 2 is the rate of reaction; depends on both conc. of halogenoalkane and hydroxide ions present.
• • Tertiary halogenoalkanes (SN1): two-step mechanism, where a carbocation is produced, due to the stability of the carbocation – due to the inductive effect of the alkyl groups attached to the C atom; depends on only the conc. of the halogenoalkane (slow-step)
  • Secondary halogenoalkane (SN1 and SN2)
  • Elimination: CH3CHBrCH3 + NaOH(ethanol) → CH2=CHCH3 + H2O + NaBr
• Alcohol:

  • Hydrogen bonding causes the higher boiling point than expected compared to other organic molecules with similar relative molecular masses.
  • Combustion: C2H5OH + 3O2 → 2CO2 + 3H2O
  • Substitution (forming halogenoalkane (nucleophilic substitution)):
  • CH3CH2OH + HCL → CH3CH2Cl + H2O
  • Substitution with sodium metal: C2H5OH + Na → C2H5O-Na + + H2(g)
  • Esterification: CH3CH2COOH + CH3CH2OH ↔ CH3CH2COOC2H5 + H2O
  • Hydrolysis of Esters:
  • With an acid (H2SO4) – catalyst: Reverses the preparation of an ester from an alcohol and a carboxylic acid.
• • With an alkali (NaOH(aq)): fully hydrolysed
  • Dehydration (Elimination): CH3CH2OH → CH2=CH2 + H2O
  • Oxidation (using potassium dichromate(VI) solution, K2Cr2O7, acidified with dilute H2SO4 – Orange Cr2O7 2- (aq) reduced to green Cr 3+ (aq), warming of reaction mixture required)
• Nitrile:

  • Hydrolysis: CH3CH2CN + HCl + 2H2O → CH3CH2COOH + NH4Cl
  • Reduced to an amine (NH2): – CN + 4[H] → – CH2NH2
• Carboxylic Acid:

  • Dissociation: CH3COOH(aq) ↔ CH3COO-(aq) + H+(aq)
  • Neutralisation (Alkali): CH3COOH + NaOH → CH3COONa + H2O
  • Reactive metals: 2CH3COOH + Mg → (CH3COO)2Mg + H2
  • Carbonates: 2CH3COOH + K2CO3 → 2CH3COOK + H2O + CO2
  • Reduction: CH3COOH + 4[H] → CH3CH2OH + H2O
• Aldehyde & Ketone:

  • Reduction (reducing agents: NaBH4 (sodium tetrahydridoborate) Or LiAlH4 (lithium tetrahydridoaluminate)):
• Nucleophilic addition with HCN:

  • Increases the length of the hydrocarbon chain
  • Mechanism of Nucleophilic addition
• Testing for the carbonyl group:

  • Tri-iodomethane (Alkaline iodine solution test)
  • Tollens reagent
  • Fehling’s solution
  • 2,4-DNPH (2,4-dinitrophenylhydrazine) – condensation reaction
• C=C → Electrophilic addition

  C=O → Nucleophilic addition
• Benzene:

  • Organic hydrocarbons containing one or more benzene rings are called arenes.
  • Benzene molecule is a planar, perfectly symmetrical molecule
  • Each carbon atom in the hexagonal ring is sp2 hybridised sharing:
  • All three are σ (sigma) bonds; leaves one electron to spare contributing to a π (pi) bond
• Substitution (Cl & Br) - Electrophilic substitution:

  • Br2 molecule forms a dative (co-ordinate) bond with Iron (III) bromide by donating a lone pair of electrons from one bromine atom into an empty 3d orbital in the iron
  • This creates the electrophile (Br +) which is attracted to the 'electron-rich' benzene ring
  • Halogen carriers used in the mechanism are FeCl3, AlCl3, and FeBr3
• Halogenation of alkylarenes:

  • Halogen atom substitutes into the benzene ring at positions 2 or 4
  • Excess chlorine gas can form 1-methyl-2,4,6-trichlorobenzene
  • C-X bond in halogenoarenes is stronger than in halogenoalkanes due to partial double bond character
• Free-radical substitution (Cl & Br) into the alkylbenzene side-chain:

  • In excess chlorine, all three hydrogen atoms will be replaced by chlorine atoms
• Nitration - Electrophilic substitution:

  • Conc. HNO3 & conc. H2SO4 create the electrophile nitronium ion (NO2 + ion)
  • Reflux with benzene at 55 ℃ produces nitrobenzene
  • Further nitration yields 1,3,5-trinitrobenzene
• Alkylation or Acylation (Friedel-Crafts reaction):

  • Introduces a side-chain into a benzene ring
  • Mechanism involves oxidation of the side-chain to form carboxylic acid
• Phenol:

  • Melts at 43 ℃ due to hydrogen bonding
  • Weakly acidic, with a conjugate base (phenoxide ion) that has its negative charge spread over the whole ion
  • Phenol reacts with electrophiles more readily than benzene
• Carboxylic Acids:

  • Neutralisation with alkali produces carboxylate salt
  • Weak acids with O-H bond weakened by the carbonyl group, C=O
  • Electron-withdrawing groups next to the -COOH group make the acid stronger
• Oxidation Of HCOOH (methanoic acid):

  • Can undergo further oxidation with strong oxidising agents like alkaline potassium manganate(VII) or potassium dichromate(VI)
• Oxidation Of (COOH)2 (ethanedioic acid):

  • Oxidised by strong oxidising agents and used to standardise potassium manganate(VII) solution
  • An autocatalysis reaction where Mn2 + acts as the catalyst
• Nucleophilic substitution, forming acyl chlorides:

  • Replacement of carboxylic acid's -OH with a Cl atom using reagents like Phosphorus(V) chloride, Phosphorus(III) chloride, or Sulfur dichloride oxide
• Acyl chlorides are more reactive than carboxylic acid, hence are more used in compound synthesis
• The carbonyl carbon in acyl chlorides has electrons drawn away from it by electronegative atoms (O and Cl), giving it a large partial positive charge, open to nucleophiles
• Reactions with acyl chlorides will cause the C-Cl bond to break and HCl(g) to be given off as white fumes
• Hydrolysis of acyl chlorides occurs at room temperature and involves a lone pair on the oxygen atom in water attacking the δ+ carbonyl carbon atom, resulting in the formation of carboxylic acid and HCl(g)
• The ease of hydrolysis follows the order: acyl chloride > chloroalkane > aryl chloride
• Esterification occurs when acyl chlorides react with alcohols or phenol to form esters and HCl
• Nucleophilic substitution with amines involves amines with a lone pair of electrons acting as nucleophiles and attacking the carbonyl carbon atom in acyl chlorides, resulting in the formation of a substituted amide
• Amines can be classified into three classes: primary, secondary, and tertiary
• Ammonia and amines act as bases due to the lone pair of electrons on the nitrogen atom
• Formation of ethylamine involves the reaction of excess ammonia with bromoethane to avoid the formation of secondary and tertiary amines
• The strength of ammonia and amines as bases depends on the availability of the lone pair of electrons on their nitrogen atom to bond with an H+ ion
• Formation of phenylamine includes the reduction of nitrobenzene by heating with tin and concentrated hydrochloric acid
• Diazotisation is used in the synthesis of dyes and involves the reaction of phenylamine with nitrous acid to form a diazonium salt, which then reacts with an alkaline solution of phenol in a coupling reaction to form an azo dye
• Amino acids have a general structure of RCH(NH2)COOH
• Interactions between the molecules within amino acids are possible due to its basic –NH2 group and its acidic –COOH group, forming zwitterions as they carry two charges
• Solutions of amino acids are amphoteric and can act as buffer solutions