Unit 3: Alkyl Halides & Alcohols

February 20, 2026

Semester 2
BP202T

Introduction to Alkyl Halides & Alcohols

Unit 3 covers two very important classes of compounds with vast pharmaceutical applications. For Alkyl Halides, the SN1 and SN2 mechanisms are critical – they form the basis of reaction prediction. For Alcohols, you learn both the chemistry and the direct medicinal applications of compounds like Ethyl Alcohol, Glycerol, and Chloroform.

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Syllabus & Topics

  • 1Alkyl Halides: SN1 Reaction – Kinetics, order of reactivity, stereochemistry, rearrangement of carbocations.
  • 2Alkyl Halides: SN2 Reaction – Kinetics, order of reactivity, stereochemistry (Walden Inversion).
  • 3SN1 vs SN2: Factors affecting (substrate structure, solvent, nucleophile strength, leaving group).
  • 4Structure and uses of Ethylchloride, Chloroform, Trichloroethylene, Tetrachloroethylene.
  • 5Structure and uses of Dichloromethane, Tetrachloromethane, and Iodoform.
  • 6Alcohols: Qualitative tests (Lucas test, Iodoform test).
  • 7Structure and uses of Ethyl alcohol.
  • 8Structure and uses of Methyl alcohol (Methanol).
  • 9Structure and uses of Chlorobutanol and Cetosteryl alcohol.
  • 10Structure and uses of Benzyl alcohol, Glycerol, and Propylene glycol.

Learning Objectives

Draw the Mechanism: Write complete arrow-pushing mechanisms for SN1 and SN2 reactions.
Predict Reactivity Order: Rank primary, secondary, tertiary alkyl halides for SN1 and SN2.
Explain Walden Inversion: Describe the stereochemical outcome of the SN2 reaction.
Apply Qualitative Tests: Explain how Lucas Test distinguishes primary, secondary, and tertiary alcohols.
Know Pharmaceutical Uses: State the pharmaceutical use of Glycerol, Benzyl Alcohol, and Chloroform.

Frequently Asked Questions (FAQs)

Q1. What is the difference between SN1 and SN2?

  • SN1 reaction: A two-step unimolecular reaction that proceeds through a carbocation intermediate. It is favored by tertiary substrates and polar protic solvents.

  • SN2 reaction: A one-step bimolecular reaction involving simultaneous bond breaking and bond forming. It is favored by primary substrates and strong nucleophiles.

Q2. What is Walden Inversion?

In an SN2 reaction, the nucleophile attacks from the backside, leading to an inversion of configuration at the chiral center. This is called Walden inversion, often compared to an umbrella turning inside out.

Q3. What is the Lucas Test?

The Lucas Test uses a mixture of ZnCl₂ and concentrated HCl to distinguish between primary, secondary, and tertiary alcohols.

  • Tertiary alcohols → Immediate turbidity

  • Secondary alcohols → Turbidity after some time

  • Primary alcohols → No reaction at room temperature

Q4. What is the pharmaceutical use of Glycerol?

Glycerol (Glycerine) is used as:

  • A humectant in topical preparations

  • A sweetening agent and solvent in oral formulations

  • A base for suppositories

Q5. What is the use of Iodoform?

Iodoform (CHI₃) is used as an antiseptic agent. The Iodoform test (using I₂/NaOH) is also employed to detect methyl ketones and certain alcohols.