Constitutional Isomers
Constitutional isomers are molecules with the same molecular formula but different structural formula.
Stereoisomerism
Stereoisomers are molecules with the same molecular formula and structural formula but different arrangement in space.
Cis-trans Isomerism
For a molecule to exhibit cis-trans isomerism, it must have:1) a covalent bond with restricted rotation - e.g. presence of pi bond or ring structure,AND2) non-identical groups attached to each end of this bond with restricted rotation
Enantiomerism
For a molecule to exhibit enantiomerism (i.e. to be chiral), its mirror image must be non-superimposable onto itself. These two non-superimposable mirror images are called enantiomers.Molecules which exhibit enantiomerism:1) often contain one or more chiral centres2) do not possess any internal plane (or centre) of symmetry
Cl2 (g) or Br2 (l); UV light or heat
[Alkanes] Free-radical substitutionObservations: Greenish-yellow Cl2 (g) decolourised or Reddish-brown Br2 (l) decolourised
Gaseous HCl, HBr or HI, room temperature
[Alkenes] Electrophilic addition of hydrogen halids (H—X adds across the double bond)
Cl2 (g) / Br2 (l) or Cl2 / Br2 dissolved in CCl4 Room temperature
[Alkenes] Electrophilic Addition of Halogens (X2 adds across the double bond)Observations: Greenish yellow Cl2/reddish brown Br2 decolourised
Br2 (aq), room temperature
[Alkenes] Electrophilic addition of bromine water. Yellow-orange Br2 (aq) decolourised. *distinguishing test
Steam; H3PO4 catalyst at high temperatures and high pressuresOR 1. Conc. H2SO4, cold2. H2O
[Alkenes] Electrophilic addition of water/steam (OH and H adds across the double bond)
H2 gas; Ni catalyst. Room temperature and pressure
[Alkenes] Reduction. Platinum/Palladium catalyst at rtp can also be used.
KMnO4, dilute NaOH, cold
[Alkenes] Mild Oxidation. Observations: Purple KMnO4 is decolourised and a brown precipitate of MnO2 is formed. *distinguishing test
KMnO4, dilute H2SO4, heat
[Alkenes] Oxidative cleavage. Observations: Purple KMnO4 is decolourised. Note that effervescence of carbon dioxide is possible for selected alkenes. *Cannot be carried out with potassium dichromate (VI)(*distinguishing test)
Cl2 (g), FeCl3 (s) as Lewis Acid catalyst, warm OR Cl2 (g), Fe(s), warm
[Arenes] Electrophilic substitution with chlorine (Chlorination) Observations: Decolourisation of greenish-yellow Cl2 (g) and white fumes of HCl (g)
Br2(l), FeBr3(s) as Lewis acid catalyst, warm ORBr2(l), Fe(s), warm
[Arenes] Electrophilic substitution with bromine (bromination) Observations: decolourisation of reddish-brown Br2(l) and white fumes of HBr(g)
concentrated HNO3concentrated H2SO4 as Bronsted-Lowry acid catalyst, maintained at 55 degrees Celsius
[Arenes] Electrophilic substitution with concentrated nitric acid (nitration) Observation: Pale Yellow oily liquid formed
chloroalkane, AlCl3 (s) as Lewis acid catalyst, warm
[Arenes] Electrophilic substitution with halogenoalkanes (Friedel-Crafts alkylation) Observation: white fumes of HCl (g)
Cl2(g), FeCl3(s) as Lewis acid catalyst, rt and absence of UV
[Arenes] Electrophilic sub with chlorine of methyl benzene Observations:decolourisation of greenish-yellow Cl2(g) and white fumes of HCl (g)
Br2(l), FeBr3(s) as Lewis acid catalyst, rt and absence of UV
[Arenes] Electrophilic sub with bromine of methyl benzene. Observations:decolourisation of reddish-brown Br2(l) and white fumes of HBr (g)
Concentrated HNO3, Concentrated H2SO4 as Brønsted-Lowry acid catalyst, maintained at 30 degrees Celsius
[Arenes] Electrophilic sub/nitration of methyl benzene. Yellow oily liquid formed.
Chloroalkane, AlCl3 (s) as Lewis acid catalyst, rt
[Arenes] Electrophilic sub with halogenoalkanes of methyl benzene Observations: white fumes of HCl (g)
Cl2 (g), UV light or heat. Greenish-yellow Cl2 decolourises slowly.
[Arenes] Side-chain free radical sub of methyl benzene Observation: Greenish-yellow Cl2(g) decolourises slowly
Br2(l), UV light or heat
[Arenes] Side-chain free radical sub of methyl benzene. Observations: Reddish brown Br2(l) decolourises slowly.
KMnO4(aq), dilute H2SO4, heat. Purple KMnO4(aq) decolourises and white precipitate of benzoic acid is formed
[Arenes] Side-chain oxidation of methyl benzene.
KMnO4(aq), dilute NaOH, heat. purple KMnO4(aq) decolourises and brown precipitate of manganese dioxide, MnO2(s) is formed
[Arenes] Side-chain oxidation of methyl benzene
Distinguishing test: Add bromine in tetrachloromethane dropwise with shaking to 1 cm3 of each compound in separate test tubes.
Benzene: Br2 in CCl4 remains reddish-brown1-methylcyclohexene: Reddish-brown Br2 in CCl4 decolourises
Isomers
Isomers are molecules with the same molecular formula but different arrangements of the atoms.
Chiral Centre
A chiral centre is any atom with a tetrahedral geometry around it that is bonded to four different groups.
Dilute NaOH, heat
[Halogenoalkanes] Nucleophilic Sub with OH- to form alcohols
Ethanolic KCN
[Halogenoalkanes] Nucleophilic Sub with CN- to form nitriles
Dilute H2SO4, heat
Acidic hydrolysis of nitriles: R-CN --> RCO2H + NH4+
Dilute NaOH, heat
Alkaline hydrolysis of nitriles: R-CN --> R-CO2- + NH3
LiAlH4 in dry ether or H2 (g), Ni catalyst, high pressure
Reduction of Nitriles: R-CN --> RCH2NH2
Ethanolic concentrated NH3, heat in sealed tube
[Halogenoalkanes] Nu Sub with NH3 to form amines R-X --> R-NH2 + HX
Ethanolic KOH, heat
[Halogenoalkanes] Elimination, major pdt is the one with the largest number of alkyl groups attached to the double bonded carbon atoms
Distinguishing test for halogenoalkanes
1. Add NaOH (aq) and heat. (R-OH is formed) Then cool. 2. Add excess dilute HNO3. (removes unreacted OH- so brown Ag2O ppt will not be formed in next step. 3. Add AgNO3 (aq) and observe the colour of precipitate formed. (Ag+ + X- --> AgX)4. Add conc. and dil NH3. chloroalkane: white ppt AgCl, dissolves in dilute aq NH3. AgX + 2NH3 --> [Ag(NH3)2]+ + X-bromoalkane: cream ppt AgBr, dissolves in conc NH3iodoalkane: yellow ppt AgI insoluble even in conc NH3.
Distinguishing test for halogenoalkanes
Add ethanolic AgNO3 and heat. Rate of ppt formation RI>RBr>RCl
Na (s), room temperature
[hydroxy] acid-metal reaction with sodium. aliphatic alcohols: slow effervescence of hydrogen gas Phenol: Rapid effervescence of hydrogen gas
NaOH (aq), room temperature
[Phenol] Acid base rxn with sodium hydroxidecloudy mixture dissolves to form colourless homogenous solution
Conc. HCl, room temperature
[hydroxy] Nu Sub with hydrogen halidesTertiary alcohol: Solution turns cloudy
Conc HCl, ZnCl2, heat
[hydroxy] Nu Sub with hydrogen halidesprimary and secondary alcohol: Solution turns cloudy
Lucas Test
[hydroxy] tertiary: immediate cloudiness appears in solution secondary: cloudiness appears within 5 min at rt primary: no cloudiness appears until warmed
Conc HBr, heat or NaBr, conc. H2SO4 to generate HBr in situ, heat
[hydroxy] Nu Sub with HBr bromoalkane formed
PCl3, rt or P, Br2, heat or P, I2, heat
[hydroxy] Nu sub with phosphorus halides to form corresponding halogenoalkanes and H3PO3
PCl5, room temperature
[hydroxy] Nu sub with phosphorus halides to form chloroalkane. good distinguishing test if carboxylic acid is absent. Dense white fumes of HCl produced
SOCl2, warm
[hydroxy] Nu Sub with thionyl chlorideSO2 gas and white fumes of HCl produced. Good distinguishing test if carboxylic acid absent.
Excess conc H2SO4, heat or conc H3PO4, heat or Al2O3, heat
[hydroxy] Dehydration/Elimination. Only alcohols with at least one H atom on the carbon atom adjacent to the C atom bearing the hydroxy group can undergo dehydration.
Alcohol, Carboyxlic acid, few drops of conc H2SO4, heat
[hydroxy] Condensation with carboxylic acids. reaction slow and reversible
Acyl chloride, alcohol, rt
[aliphatic alcohol] condensation with acyl chlorides. white fumes of HCl produced.
acyl chloride, phenol, rt
[phenol] condensation with acyl chloride. white fumes of HCl produced.