Organic Chemistry: Reagents & Conditions

Combustion of Alkanes

R&C: excess oxygen

• requires a spark: high activation energy

• only burn in the gaseous state

Free radical substitution

R&C: Cl₂ (g) or Br₂ (l), UV light at room temperature

Observations: Greenish-yellow / Reddish-brown decolourised

1. Initiation: Cl-Cl bond homolytically broken to give chlorine free radicals, with energy coming from light absorbed / heat supplied

2. Propagation: Highly reactive Cl• collides with methane molecule, abstracts H atom to produce HCl and •CH₃, CH₃ reacts with another molecule of Cl₂ to yield product & regenerate Cl•

3. Termination: Radicals needed for propagation are consumed, reaction cycle broken & chain reaction teminated

Electrophilic addition with halogens

R&C: Cl₂ / Br₂ dissolved in CCl₄, room temperature

Observation: Greenish-yellow / Reddish-brown decolourised

1. Non-polar Br₂ molecule approaches π electron cloud, high density of π electron cloud polarises electron cloud of Br₂

2. Br-Br undergoes heterolytic fission, forming bromide anion + bromide atom bonded to alkene to form (+)-charged carbocation intermediate

3. Bromide anion acts as nucleophile to attack carbocation intermediate to form product

Electrophilic addition with bromine water

R&C: Br₂, room temperature

Observation: Yellow-orange decolourised

Electrophilic addition of water/steam

R&C:

• industrial: steam, H₃PO₄ catalyst, high temperature & pressure

• lab: concentrated H₂O, H₂SO₄, cold

Reduction (Hydrogenation)

R&C: H₂ gas, Ni/Pt/Pd catalyst

Mild oxidation

R&C: KMnO₄, dilute NaOH, cold

Observation: Purple KMnO₄ decolourised, brown precipitate of MnO₂ formed

R&C: KMnO₄, dilute H₂SO₄, heat

Observation: Purple KMnO₄ decolourised, possible effervescence of CO₂

Different substrates give rise to different products:

Electrophilic substitution with concentrated nitric acid

R&C: concentrated HNO₃, concentrated H₂SO₄, heat at 55℃

Observation: pale yellow oily liquid formed

Friedel-Crafts Alkylation

R&C: RCl, anhydrous AlCl₃ catalyst, warm/heat

Observation: white fumes of HCl

Electrophilic substitution with concentrated nitric cid

R&C: concentrated HNO₃, concentrated H₂SO₄, maintained at 30℃

Observation: yellow oily liquid formed

Electrophilic substitution with chlorine/bromine

R&C:Cl₂ / Br₂, AlCl/Br₃ as Lewis acid catalyst, room temperature and absence of UV

Observation: decolourisation of greenish-yellow/reddish-brown, and white fumes of HCl/HBr

Free radical substitution (side-chain halogenation)

R&C: Cl₂ (g) or Br₂ (l), UV light at room temperature

Observation: greenish-yellow / reddish-brown decolourises slowly

• limited Cl₂ / excess alkylbenzene usually used to get monohalogenation

Side-chain oxidation

R&C: KMnO₄ (aq), dilute H₂SO₄, heat

Observation: purple KMnO₄ decolourises, white ppt. of benozic acid formed

Side-chain oxidation

1 – 

R&C: KMnO₄ (aq), dilute NaOH, heat

O: purple KMnO₄ decolourises, brown ppt. of MnO₂ formed

2 –

R&C: dilute H₂SO₄

O: white ppt. of benzoic acid formed

Distinguishing test for arenes and alkenes (Br₂ in CCl₄)

Add bromine in tetrachloromethane dropwise with shaking to 1cm³ of each compound in separate test tubes

Observations —

Benzene: Remains reddish brown

Alkene: Reddish-brown decolourises

Distinguishing test for alkenes and arenes (bromine water)

1. Add aqueous bromine dropwise with shaking to 1cm³ of each compound in separate test tubes

Observations:

Benzene: remains yellow-orange

Alkene: yellow-orange decolourises

Distinguishing test between alkenes and alkylbenzenes

R&C: KMnO₄, dilute H₂SO₄

Observations —

Alkene: effervescence of CO₂, purple KMnO₄, decolourises, no white precipitate (oxidative cleavage)

Alkylbenzene: purple KMnO₄, decolourises, white ppt. of benzoic acid

Distinguishing test for alkylbenzenes and tert-butylbenzene (3 alkyl groups attached to 1 alkyl group attached to benzene ring)

1. Add 5 drops of KMnO₄ to 1cm³ of dilute H₂SO₄ in test tube

2. To 1cm³ of each compound in separate test tubes, add 2 - 3 drops of the mixture with shaking. Warm this test tube in water bath for a few minutes

Observations —

Alkylbenzene: purple KMnO₄, decolourises, white ppt. of benzoic acid

tert-butylbenzene: KMnO₄ remains purple, no white ppt.

• lacks a H atom to the benzcylic carbon

Alkaline hydrolysis

R&C: dilute NaOH, heat

R&C: KCN in ethanol, heat

R&C: dilute H₂SO₄, heat

R&C: dilute NaOH, heat

R&C: LiAlH₄ in dry ether / H₂ (g),Ni catalyst

R&C: NH₃ in ethanol, heated under high pressure

R&C: NaOH in ethanol, heat

1. OH⁻ behaves like base > nucleophile in ethanol, removing Hᐩ from C atom adjacent to C-X bond

2. Electron pair from C-H bond moves to form C=C bond

3. C-X breaks heterolytically to give X⁻, HX formed

Distinguishing test for different types of halogen substitutents

1. Add NaOH & heat

2. Add excess dilute HNO₃ (remove unreacted OH⁻ so brown Ag₂O ppt will not be formed in next step)

3. Add AgNO₃, observe colour of precipitate formed (Agᐩ(aq) + X⁻(aq) → AgX(s))

4. Decant and wash precipitate with deionised water, add NH₃(aq) to check for solubility

Cl: white ppt.

Br: cream ppt.

I: yellow ppt.

rate of formation: Cl > Br > I (fastest)

R&C: Na(s), room temperature

Observation: slow effervescence of hydrogen gas

R&C: Na(s), room temperature

Observation: rapid effervescence of hydrogen gas

R&C: NaOH(aq), room temperature

Observation: cloudy mixture dissolves to form colourless homogeneous solution

R&C: anhydrous PCl₅

Observation: dense white fumes of HCl produced

R&C: PCl₃, room temperature / P, Br/I₂, heat

R&C: concentrated HCl

Observation: solution turns cloudy

R&C: concentrated HCl, ZnCl₂, heat

Observation: solution turns cloudy

R&C: concentrated HBr / NaBr, concentrated H₂SO₄, heat

For hydrogen iodide, use concentrated HI

R&C: SOCl₂, warm

Observation: SO₂ gas, white fumes of HCl produced

• distinguishing test for alcohol if no Carboxylic acid present

R&C: concentrated H₃PO₄ / H₂SO₄ catalyst, heat

• must have at least 1 H atom on the C adjacent to the C bearing hydroxy group

R&C: alcohol, Carboxylic acid, few drops of concentrated H₂SO₄, heat

• slow and reversible

R&C: acyl chloride, alcohol, room temperature

Observation: white fumes of HCl observed

R&C: acyl chloride, phenol, room temperature

Observation: white fumes of HCl produced

R&C: K₂Cr₂O₇, H₂SO₄, heat with immediate distillation

Observation: orange solution turns green

• only K₂Cr₂O₇ selective enough to oxidize primary alcohols to aldehydes (KMnO₄ will oxidize straight to Carboxylic acid)

R&C: K₂Cr₂O₇ / KMnO₄, H₂SO₄, heat under reflux

Observation: orange turns green / purple turns colourless

R&C: K₂Cr₂O₇ / KMnO₄, H₂SO₄, heat under reflux

Observation: orange turns green / purple turns colourless

R&C: I₂, NaOH, warm

Observation: yellow ppt. formed

R&C: dilute HNO₃ (2,4,6-trinitrophenol if conc.)

Observation: pale yellow liquid form

R&C: Br₂ (aq), room temp.

Observation: yellow-orange sol. decolourised, white ppt.

• Mono-bromination: Br₂(l) / Br₂ in CCl₄, low temp.

R&C: Br₂ (l) / Br₂ dissolved in CCl₄

Observation: reddish-brown liquid decolourised, white fumes of HBr evolved

R&C: neutral FeCl₃, room temp.

Observation: violet colouration

• Distinguishign test for phenols

R&C: HCN with trace KCN catalyst, cold

• KCN is used as a catalyst, CN- is regenerated at the end of the reaction

R&C 2,4-dinitrophenylhydrazine (2,4-DNPH), room temperature

Observation: orange ppt. formed

R&C: LiAlH₄ in dry ether, room temperature / H₂, Ni catalyst

• alkoxide ion is first formed, then water is added subsequently to protonate the alkoxide to form the alcohol

R&C: K₂Cr₂O₇ / KMnO₄, dilute H₂SO₄, heat under reflux

Observation: orange turns green / purple turns colourless

R&C: Tollen’s reagent, warm

Observation: silver mirror formed

[Ag(NH₃)₂]+ is unstable, must be freshly prepared by:

1. one drop of NaOH is added to 3cm³ of AgNO₃ to produce dark brown precipitate to Ag₂O

2. Dilute NH₃ is then added dropwise until brown precipitate first formed just dissolves

R&C: Fehling’s reagent, warm

Observation: reddish-brown precipitate formed

• ketone, benzaldehyde do not reduce Fehling’s reagent

R&C: I₂, with NaOH, warm

Observations: yellow crystals of CHI₃ formed

R & C: PCl₅

Observation: white fumes of HCl produced

R & C: LiAlH₄ in dry ether

R&C: KMnO₄, dilute H₂SO₄, heat under reflux

Observation: purple solution decolourises, effervescence of CO₂ observed

R&C: water

Observation: rapid reaction forming white fumes of HCl

R&C: primary amines (in excess) + acyl chlorides

• acyl chloride react readily with primary amines to form amides via condensation reaction

R & C: dilute NaOH, heat

R&C: dilute NaOH, heat, followed by acidification with dilute H₂SO₄