Unit 10: Alcohols

Distinction by Victor Meyer's Method

Victor Meyer’s test distinguishes primary, secondary, and tertiary alcohols based on the colour produced when the alcohol is converted to an iodoalkane, treated with nitrous acid (generated in situ from NaNO₂/HCl), and then reacted with phenol.

Procedure

1. Convert the alcohol to its corresponding iodoalkane using red phosphorus and iodine (or HI).
   R‑OH + PI₃ → R‑I + H₃PO₃
2. Treat the iodoalkane with silver nitrite (AgNO₂) to form the nitroalkane.
   R‑I + AgNO₂ → R‑NO₂ + AgI↓
3. Add nitrous acid (generated from NaNO₂ + dilute HCl) to the nitroalkane, which undergoes the nitro‑alkane‑to‑nitrosoalkane rearrangement, finally reacting with phenol to give a coloured product.

Observations

• Primary alcohol: Gives a red colour.
• Secondary alcohol: Gives a blue colour.
• Tertiary alcohol: No colour change (remains colourless).

The colour difference arises from the differing stability of the nitroso intermediates formed during the reaction.

Preparation of Alcohols

From Haloalkanes (Nucleophilic Substitution)

Hydrolysis of alkyl halides with aqueous alkali yields alcohols via an SN1 or SN2 mechanism depending on the halide structure.

R‑X + NaOH (aq) → R‑OH + NaX   (X = Cl, Br, I)

Example: CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr (ethyl bromide → ethanol).

From Primary Amines (Diazotization Followed by Hydrolysis)
Primary aliphatic amines are converted to diazonium salts, which upon hydrolysis give alcohols.
R‑CH₂‑NH₂ + NaNO₂ + 2HCl → R‑CH₂‑N₂⁺Cl⁻ + 2H₂O   (diazotization)
R‑CH₂‑N₂⁺Cl⁻ + H₂O → R‑CH₂‑OH + N₂ + HCl   (hydrolysis)

Example: CH₃CH₂CH₂NH₂ (propylamine) → CH₃CH₂CH₂OH (propan‑1‑ol).

From Esters (Acidic or Alkaline Hydrolysis)
Esters undergo hydrolysis to give an alcohol and a carboxylic acid (or its salt).
R‑COO‑R′ + NaOH → R‑COONa + R′‑OH   (saponification)
R‑COO‑R′ + H₂O (H⁺) → R‑COOH + R′‑OH   (acidic hydrolysis)

Example: Ethyl acetate hydrolysis → ethanol + acetic acid.

Industrial Methods

Oxo Process (Hydroformylation): Alkenes react with syngas (CO + H₂) in the presence of a cobalt or rhodium catalyst to give aldehydes, which are subsequently hydrogenated to alcohols.
CH₂=CH₂ + CO + H₂ → CH₃CH₂CHO →[H₂/Ni] CH₃CH₂OH
Hydroboration‑Oxidation of Ethene: Ethene undergoes hydroboration with BH₃·THF followed by oxidation with H₂O₂/NaOH to yield ethanol.
CH₂=CH₂ + BH₃ → CH₃CH₂‑BH₂ →[H₂O₂/NaOH] CH₃CH₂OH
Fermentation of Sugars: Yeast catalyzes the conversion of glucose to ethanol and carbon dioxide.
C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂


Common Terms and Commercial Alcohols


Term
Definition


Absolute alcohol
Ethanol containing ≤0.5% water (≥99.5% pure). Used as a solvent and in chemical synthesis.
Power alcohol
Blend of ethanol (≈10‑20%) with petrol, used as a motor fuel to reduce emissions.
Denatured alcohol (methylated spirit)
Ethanol rendered unfit for drinking by addition of toxic substances (e.g., methanol, pyridine, dyes). Employed as a cleaning agent and fuel.
Rectified spirit
Ethanol‑water azeotrope containing about 95% ethanol and 5% water (by volume). Obtained by simple distillation of fermented mash.
Alcoholic beverages
Drinks containing ethanol produced by fermentation: beer (3‑8% v/v), wine (10‑12% v/v), spirits (40‑60% v/v).



Chemical Properties of Alcohols
Reaction with Hydrogen Halides (HX)
Alcohols undergo nucleophilic substitution with hydrogen halides to give alkyl halides. The reactivity follows the order: tertiary > secondary > primary.
R‑OH + HX → R‑X + H₂O   (X = Cl, Br, I)

Example: tert‑Butyl alcohol + HCl → tert‑Butyl chloride + H₂O (SN1).

Reaction with Phosphorus Halides (PX₃ and PCl₅)
These reagents replace the ‑OH group with a halogen, producing alkyl halides and phosphorus‑containing by‑products.

With PCl₃:
3 R‑OH + PCl₃ → 3 R‑Cl + H₃PO₃
With PCl₅:
R‑OH + PCl₅ → R‑Cl + POCl₃ + HCl

Example: Ethanol + PCl₃ → Chloroethane + H₃PO₃.

Reaction with Thionyl Chloride (SOCl₂)
Thionyl chloride converts alcohols to alkyl chlorides with the evolution of gaseous SO₂ and HCl, making purification easy.
R‑OH + SOCl₂ → R‑Cl + SO₂↑ + HCl↑

Example: Isopropanol + SOCl₂ → Isopropyl chloride + SO₂ + HCl.

Action with Reactive Metals
Alcohols react with active metals (e.g., Na, K) to produce alkoxides and hydrogen gas.
2 R‑OH + 2 Na → 2 R‑ONa + H₂↑

Example: 2 Ethanol + 2 Sodium → 2 Sodium ethoxide + Hydrogen.

Dehydration (Acid‑Catalysed Elimination)
Heating an alcohol with concentrated sulfuric acid leads to elimination of water, forming an alkene. The mechanism follows E1 for tertiary alcohols and E2 for primary/secondary.
R‑OH ⟶[conc. H₂SO₄, Δ] Alkene + H₂O

The ease of dehydration: 3° > 2° > 1°. According to Zaitsev’s rule, the more substituted alkene is the major product.
Example: Butan‑2‑ol + H₂SO₄ → But‑2‑ene (major) + But‑1‑ene (minor).

Oxidation
The oxidation outcome depends on the degree of the alcohol.

Primary alcohols: First oxidized to aldehydes, further to carboxylic acids.
RCH₂OH →[O] RCHO →[O] RCOOH
Secondary alcohols: Oxidized to ketones (no further oxidation under normal conditions).
R₂CHOH →[O] R₂CO
Tertiary alcohols: Resistant to oxidation; no reaction under mild oxidizing agents.

Examples:

Ethanol + [O] (PCC) → Acetaldehyde; Acetaldehyde + [O] (KMnO₄) → Acetic acid.
Propan‑2‑ol + [O] (Na₂Cr₂O₇/H₂SO₄) → Acetone.
tert‑Butyl alcohol + [O] → No reaction.


Catalytic Dehydrogenation
Passing alcohol vapours over a heated copper catalyst (573 K) removes hydrogen, yielding carbonyl compounds.
Primary: RCH₂OH ⟶[Cu, 573 K] RCHO + H₂
Secondary: R₂CHOH ⟶[Cu, 573 K] R₂CO + H₂

Example: Ethanol ⟶[Cu, 573 K] Acetaldehyde + H₂.

Esterification (Fischer Esterification)
Alcohols react with carboxylic acids in the presence of a strong acid catalyst (usually conc. H₂SO₄) to give esters and water.
R‑OH + R′‑COOH ⟶[H₂SO₄] R′‑COO‑R + H₂O

Example: Ethanol + Acetic acid ⟶[H₂SO₄] Ethyl acetate + Water.

Iodoform Test (Specific for Ethanol and Secondary Alcohols Containing CH₃CH(OH)‑)
Ethanol and secondary alcohols having the structure CH₃CH(OH)‑R give a yellow precipitate of iodoform (CHI₃) when treated with iodine and sodium hydroxide.
R‑CH(OH)‑CH₃ + 4 I₂ + 6 NaOH → R‑COONa + CHI₃↓ + 5 NaI + 5 H₂O

Observation: Formation of a yellow solid (iodoform) indicates a positive test.
Example: Ethanol + I₂/NaOH → CHI₃ (yellow) + Sodium formate.

Summary
This chapter has provided a thorough account of monohydric alcohols, covering their nomenclature, isomerism, and classification into primary, secondary, and tertiary types. The Victor Meyer test offers a reliable laboratory method to distinguish these classes based on colour development. Various preparation routes—from haloalkanes, amines, esters, and industrial processes such as the oxo process, hydroboration‑oxidation, and fermentation—were detailed with balanced chemical equations. Common terminology related to commercial alcohol products was clarified, and the characteristic chemical reactions (with HX, phosphorus halides, thionyl chloride, metals, dehydration, oxidation, catalytic dehydrogenation, esterification, and the iodoform test) were explained with mechanisms, examples, and relevant formulae. Mastery of these concepts equips students to predict the behaviour of alcohols in synthetic pathways and to identify them qualitatively in the laboratory.