Organic Chemistry R&C, Type of Reactions and Mechanisms

From Alkenes to Alkanes

R&C: H₂ (g) , Ni catalyst, heat

Rxn: Reduction

Describe FRS mechanism

Free-radical Substitution

R&C: Limited X₂ and UV light

3 Steps: IPT

1) Initiation step

• Breaking of X₂ to form X radicals

2) Propagation step

• Reacting alkane with X radical

• Reacting alkane radical with X₂

3) Termination step

• Reacting radicals together (3 possible steps)

From Alcohols to Alkenes

R&C: Excess conc H₂SO_4, heat

(OR Al₂O₃ high temperature)

Rxn: Elimination

From Halogenoalkanes (R-X) to Alkenes

R&C: KOH, ethanol, heat

(OR ethanolic KOH, heat)

Rxn: Elimination

From Alkenes to Halogenoalkanes (R-X)

R&C: dry gaseous HX

Rxn: Electrophilic Addition

Describe Electrophilic Addition Mechanism

Electrophilic Addition

2 Steps:

• Step 1: slow step

  • Identify δ+ and δ- , Full arrow from π electrons from C=C attacking δ+

  • Draw carbocation + and anion

• Step 2: fast step

  • Electron rich anion attacks carbocation

  • Forming new bond

From Alkene to Dihalogenoalkane

R&C: X₂ in inert organic solvent (e.g CCl4)

Rxn: Electrophilic Addition

Observation: pale-yellow Cl₂ / orange-red Br₂ decolorises

From Alkene to Halohydrin

R&C: Aqueous X₂

Rxn: Electrophilic Addition

Observation: pale-yellow Cl₂ / orange Br₂ decolorises

From Alkene to Alcohol

R&C:

Industrial: H₂O(g) , H₃PO₄ catalyst (phosphoric acid) ,high temperature , high pressure

Laboratory: conc H₂SO₄ , room temperature, followed by heating with liquid water

Rxn: Electrophilic Addition

From Alkene to Diol

R&C: cold, alkaline KMNO₄

Rxn: Mild oxidation

Observation: Purple KMNO₄ solution turns green (MnO₄^2- )

followed by formation of brown ppt of MnO₂

Strong Oxidation of Alkene

R&C: Hot KMNO₄

Rxn: Strong Oxidation

Observation: Purple KMNO₄ decolourises.

If CO₂ gas formed, gas evolves forms white ppt with limewater.

Don’t change anything, the structure.

Splitting the C=C, and H just add O,

be aware of whether there is CO₂ formed

ID Test for C=C bond

1) Add a few drops of Br₂ (aq) | orange Br₂ decolourises

2) Add a few drops of acidified KMnO₄ | Purple KMnO₄ decolourises

3) Add a few drops of cold alkaline KMnO₄ | Purple KMnO₄ solution turns green (MnO₄^2- ) and brown ppt is formed (MnO₂)

ID Test for Terminal Alkene ( =CH₂ group)

Test: Add few drops of acidified KMnO₄ , pass any gas evolved through lime water

Observations:

• Purple KMnO₄ decolourises

• For Terminal Alkene ( =CH₂ group), gas evolved forms white ppt with limewater

Benzene to Cyclohexane

R&C: H₂ , Ni catalyst, heat

Rxn: Reduction

Benzene to Halogenoarenes

R&C: X₂ , anhydrous AlX₃ / FeX₃ catalyst, heat

Rxn: Electrophilic Substitution

Describe Electrophilic Substitution

Electrophilic Substitution

3 Steps:

• Step 1: Generation of Electrophile

• Step 2: Electrophilic Substitution (Slow step)

  • Benzene ring attacks E⁺ forming arenium ion

• Step 3: Restoration of aromaticity and regeneration of catalyst (Fast step)

  • C-H bond full arrow to “+” arenium ion

  • + anion

Benzene to Nitrobenzene

R&C: conc HNO₃ , conc H₂SO₄ catalyst, 50 °C

Rxn: Electrophilic Substitution

Benzene to Alkylbenzene

Friedal-Crafts Alkylation

R&C: RX, anhydrous AlX₃ catalyst, heat

Rxn: Electrophilic Substitution

Benzene to Aromatic carbonyl compounds

Friedal-Crafts Acylation

R&C: RCOCl, anhydrous AlX₃ catalyst, heat

Alcohol to Halogenoalkanes (R-X)

1. With PCl₅

   ROH + PCl₅ = RCl + POCl₃ + HCl

   R&C: PCl₅ (s), room temperature

   Rxn: Nucleophilic Substitution

2. With PX₃

   3ROH + PX₃ = 3 RX + H₃PO₃

   R&C: PX₃, heat

   Rxn: Nucleophilic Substitution

3. With SOCl₂

   ROH + SOCl₂ = RCl + SO₂ (g) + HCL (g)

   R&C: SOCl₂, heat

   Rxn: Nucleophilic Substitution

Describe S_N1 Mechanism

Nucleophilic Substitution

Usually for Tertiary 3° H-X (Have to draw 3D structure)

2 Steps

• Step 1: (Slow step)

  • Identify δ+ and δ- from C-X polar bond.

  • Full arrow from C-X bond attacking δ-

  • Forms from tetrahedral to carbocation (trigonal planar), halide ion

• Step 2: (Fast step)

  • Nucleophile attack C+ equal probability

  • Just have to draw one product

Describe S_N2 Mechanism

Nucleophilic Substitution

Usually for Primary 1° H-X (Have to draw 3D structure)

1 Step

• Nucleophile OH^- “backside attack” the halogenoalkane

  (H-X)

• Pentavalent transition state forms

  • new C-O bond forms, C-X begins to break (Dotted lines)

• Structure is now inverted + Halide ion

From Halogenoalkane (R-X) to Alcohol

R&C: NaOH (aq) (or KOH (aq) ) , heat

Rxn: Nucleophilic Substitution

From Halogenoalkane (R-X) to Nitriles (CN group)

R&C: KCN (or NaCN), ethanol, heat

Rxn: Nucleophilic Substitution

From R-CN to R-CO₂H

R&C: H₂SO₄ (aq) , heat

Rxn: Acid Hydrolysis

From R-CN to R-CO₂^-

R&C: NaOH (aq) , heat

Rxn: Alkaline Hydrolysis

From R-CN to R-CH₂NH₂

R&C: LiAlH₄ , dry ether

Rxn: Reduction

From R-X to R-NH₂ (1° Amine)

R&C: excess NH₃ , ethanol, heat in sealed tube

Rxn: Nucleophilic Substitution

ID Test for Halogenoalkanes

4 Step Package:

1) Heat R-X with NaOH

2) Cool the mixture

3) Acidify with dilute HNO₃

4) Add AgNO₃ (aq)

I, Br, Cl (fastest to slowest)

I: yellow ppt

Br: pale cream ppt

Cl: white ppt

From Aldehydes to 1° Alcohol

R&C:

1) LiAlH₄ , dry ether

(OR NaBH₄ , methanol)

OR

2) H₂ , Ni catalyst, heat

Rxn: Reduction

From Ketones to 2° Alcohol

R&C:

1) LiAlH₄ , dry ether

(OR NaBH₄ , methanol)

OR

2) H₂ , Ni catalyst, heat

Rxn: Reduction

From Carboxylic Acid to Primary Alcohol

R&C: LiAlH₄ , dry ether

Rxn: Reduction

From 1° Alcohol to Aldehydes

R&C: Acidified K₂Cr₂O₇ (aq), immediate distillation

Rxn: Oxidation

Observation: Orange K₂Cr₂O₇ (aq) turns green

From 1° Alcohol to Carboxylic Acid

R&C:

• Acidified K₂Cr₂O₇ (aq), heat under reflux

  OR

• Acidified KMnO₄ (aq), heat under reflux

Rxn: Oxidation

Observation:

• Orange K₂Cr₂O₇ (aq) turns green

  OR

• Purple KMnO₄ (aq) decolourises

From 2° Alcohol to Ketones

R&C:

• Acidified K₂Cr₂O₇ (aq), heat under reflux

  OR

• Acidified KMnO₄ (aq), heat under reflux

Rxn: Oxidation

Observation:

• Orange K₂Cr₂O₇ (aq) turns green

  OR

• Purple KMnO₄ (aq) decolourises

Alcohols to Esters

1) R-OH + RCOOH

R&C: RCOOH, conc H₂SO₄, heat under reflux

Rxn: Condensation

OR

2) R-OH + RCOCl

R&C: RCOCl, room temperature

Rxn: Condensation

Phenols to Esters

Ring-OH + COCl

R&C:

1. Phenol in NaOH(aq)

2. RCOCl, room temperature

Rxn: Condensation

ID Test for Alipathic Alcohols

Test:
1) Add PCl₅

Observations: For 1°/2°/3°, white fumes of HCl (g) will be evolved

2) Oxidation: Add Acidified K₂Cr₂O₇ (aq) / KMnO₄ (aq)

Observations: Orange K₂Cr₂O₇ (aq) turns green / Purple KMnO₄ (aq)

For 3°,

Orange K₂Cr₂O₇ (aq) remains orange / Purple KMnO₄ (aq) remains purple

3) Iodoform Test: Add iodine, NaOH (aq) and heat using a hot water bath

Observations:

With -CH(CH₃)OH or -COCH₃ group, pale yellow ppt of CHI₃ will be formed.

ID Test for Phenols

1) Add neutral FeCl₃ (aq)

Observation: Violet colouration will be observed

2) Add Br₂ (aq)

Observation: Orange Br₂ (aq) decolourises,

White ppt of 2,4,6-tribromophenol formed

Carbonyl compounds (Aldehydes/ Ketones) to cyanohydrins

R&C:HCN, trace amount of NaCN

Rxn: Nucleophilic Addition

Describe Nucleophilic Addition

Nucleophilic Addition

• Generation of Nucleophile

• 2 Steps:

• Step 1: (Slow step)

  • Identify δ+ and δ- , Full arrow from π electrons from C=C attacking δ- , O

  • Lone pair from Nucleophile attacks δ+, C

• Step 2: (Fast step)

  • Lone pair from O^- attacks δ+ H

Cyanohydrins to COOH

R&C: H₂SO₄ (aq), heat

Rxn: Acid Hydrolysis

Cyanohydrins to CH₂NH₂

R&C: LiAlH₄ , dry ether

OR H₂ (g), Ni catalyst, heat

Rxn: Reduction

ID Test for Hydroxy compounds

Remember table form

1) 2,4 - DNPH , condensation, orange ppt

2) Tollens’ reagent, oxidation, silver mirror

3) Fehling’s solution, oxidation, red-brown ppt

4) Alkaline I₂ (aq), heat, iodoform test, pale yellow ppt

From Alkylbenzene to Benzoic acid

R&C: Acidified KMnO₄ (aq), heat

Observations: Purple KMnO₄ (aq) decolourises

RCN to RCOOH

1) Acid Hydrolysis

R&C: H₂SO₄ (aq), heat

2) Alkaline Hydrolysis followed by acid-base reaction

R&C:

• 1) NaOH (aq), heat

• 2) acidify with H₂SO₄ (aq)

RCOOH to RCOO^-

1) Redox

R&C: Na/ K/ Mg

Observation: Effervescence H₂ (g), gives ‘pop' sound with lighted splint

2) Acid-Base reaction

R&C: NaOH (aq) / KOH (aq) / NH₃ (aq)

3) Acid-carbonate reaction

R&C: NaCO₃ (aq) / NaHCO₃ (aq)

Observation: Effervescence of CO₂ (g), forms white ppt with limewater

RCOOH to Ester

R&C: Alcohol, conc H₂SO₄ (l) catalyst, heat

Rxn: Condensation

RCOOH to Acyl Chloride (RCOCl)

Rxn: Nucleophilic Acyl Substitution

1) PCl₅

2) PCl₃ , heat

3) SOCl₂ , heat

RCOOH to CO₂ (g) and H₂O (l)

R&C: Acidified KMnO₄ (aq), heat under reflux

Observations: Purple KMnO₄ (aq) decolourises, Eff CO₂ (g), forms white ppt with limewater

RCOOH to CO (g) and H₂O (l)

R&C: excess conc H₂SO₄ (l), heat

Rxn: Elimination

ID Test for RCOOH

1) Add Na₂CO₃ (aq) / NaHCO₃ (aq)

Observation: Eff CO₂ (g), forms white ppt with limewater

2) Add a few crystals of PCl₅ at room temperature

Observation: white fumes of HCl is liberated

RCOCl to RCOOH

R&C: water, room temp

Rxn: Hydrolysis

RCOCl to Esters

1) RCOCl + ROH

R&C: Alcohol , room temperature

Rxn: Condensation

OR

2) RCOCl + Phenol

R&C: Phenol in NaOH(aq) , room temperature

Rxn: Condensation

RCOCl to Amides

Rxn: Condensation

1) Primary Amides 1°

R&C: NH₃ , room temperature

2) Secondary Amides 2°

R&C: R’NH₂ (primary amine), room temperature

3) Tertiary Amides 3°

R&C: (R’)₂NH (secondary amine) , room temperature

ID Test for RCOCl

Test: Add AgNO₃ (aq)

Observations: white ppt of AgCl formed immediately

ID Test for Esters

Test:

1) Hydrolysis of esters (acidic/ alkaline)

2) Analyse alcohol and ID test for alcohols

From RCN to Amines

R&C:

• LiAlH₄ , dry ether

  OR

• H₂ (g), Ni catalyst, heat

Rxn: Reduction

*LiAlH₄ , dry ether recommended if got benzene ring; don’t want to reduce benzene ring

From Amides to Amines

R&C: LiAlH₄ , dry ether

Rxn: Reduction

From Nitrobenzene to Phenylamine

R&C:

1) Sn, conc HCl, hear

2) followed by NaOH (aq)

Rxn: Reduction

From RNH₂ to RNH₃⁺

R&C: HCl (aq) or H₂SO₄ (aq), room temperature

Rxn: acid-base reaction

From Phenylamine to 2,4,6 triphenylamine

R&C: Br₂ (aq)

Rxn: Electrophilic Substitution

Observation: orange Br₂ (aq) decolourises, white ppt formed

ID Test for Primary Amides

Test: Insert damp red litmus paper

Observations: NH₃ (g) turns damp red litmus paper blue