Primary and Secondary Alcohols

Methods of Forming Primary and Secondary Alcohols

From Halogenoalkanes:

Primary Alcohols and Secondary Alcohols:

• Reaction: Halogenoalkane + Aqueous NaOH → Primary Alcohol + Halide ion.

• Reagents: NaOH _(aq)

• Conditions: heated under reflux

• Example: CHCH(Cl)CH + NaOH _(aq) → CHCH(OH)CH + NaCl Secondary Alcohol

  Mechanism

  1. The hydroxide ion (OH⁻) acts as a nucleophile, attacking the carbon atom bonded to the halogen in the halogenoalkane

  2. .The C—Br bond breaks heterolytically, with both electrons going to the bromide ion (Br⁻).

  3. This leads to the formation of a secondary alcohol and a halide ion.

From Carbonyl Compounds (Reduction):

• Primary Alcohols and Secondary Alcohols

  • Reaction: Aldehyde/Ketone + Reducing Agent → Primary Alcohol/Secondary Alcohol.

  • Reagents: NaBH₄ or LiAlH

  • Conditions: NaBH₄ is typically used in aqueous or alcoholic solution.

    LiAlH₄ is a stronger reducing agent and is used in anhydrous conditions (in dry ether).

  • Example: (CH₃)₂CO + + 2 [H] → C₃H₇OH

    Mechanism:

    1. The hydride ion (H⁻) from the reducing agent attacks the carbonyl carbon of the aldehyde, forming an alkoxide intermediate.

    2. The intermediate is then protonated by water or another proton donor to form the primary alcohol.

Reactions of Primary and Secondary Alcohols

With Hydrogen Halides:

Chloroalkane:

• Reaction: Alcohol + HCl → Chloroalkane + Water.

• Reagents/Conditions: HCl_(g) with anhydrous ZnCl₂ or SOCl₂ / PCl₅ / PCl₃

• Example: ROH + HCl → RCl + H₂O

Bromoalkane:

• Reaction: Alcohol + KBr → Bromoalkane + Water.

• Reagents: NaBr _(aq) / KBr and conc. H₂SO₄

• Conditions: Heat

• Example: ROH + KBr → RBr + H₂O + KHSO₄

Mechanism (in-situ reaction):

Subsitution Reaction

1. The hydroxyl group (OH) in the alcohol is protonated by the acid catalyst, making it a better leaving group.

2. The protonated alcohol loses a water molecule to form a carbocation intermediate.

3. The halide ion (Cl⁻, Br⁻, or I⁻) then attacks the carbocation, resulting in the formation of a halogenoalkane.

With Ethanoyl Chloride:

• Reaction: Alcohol + Ethanoyl Chloride → Ester + HCl

• Reagents: Acid Chloride, RCOCl

• Conditions: Room Temp.

• Observations: Steamy fumes of toxic HCl_(g), need fume cupboard

• Type of Reaction: Elimination

With Carboxylic Acids (Esterification):

• Reaction: Alcohol + Carboxylic Acid → Ester + Water (Catalyst: Concentrated H₂SO₄).

• Reagents: conc. H₂SO_{4 (aq)} and a carboxylic acid

• Conditions: Reflux

• Type of Reaction: Condensation