Chapter 10: Alkyl Halides – Chemistry 2nd Year Notes

This chapter covers Alkyl Halides from the 2nd Year (FSc Part-II) Chemistry syllabus of the Punjab Curriculum and Textbook Board (PTB/PCTB). Alkyl halides (haloalkanes) have the general formula R-X, where a halogen replaces a hydrogen atom of an alkane. The carbon to halogen bond is polar and reactive, which makes alkyl halides ideal starting materials for alcohols, ethers, amines and, above all, Grignard reagents.

Here you will learn how alkyl halides are classified and prepared, their nucleophilic substitution (SN1 and SN2) and elimination reactions, and the preparation and uses of the Grignard reagent.

Learning Objectives

• Define alkyl halides and classify them as primary, secondary and tertiary.
• Describe methods of preparing alkyl halides.
• Explain nucleophilic substitution and distinguish SN1 from SN2.
• Describe elimination (dehydrohalogenation) reactions.
• Describe the preparation and synthetic uses of the Grignard reagent.
• State the order of reactivity of alkyl halides.

Key Concepts

What Are Alkyl Halides

An alkyl halide has the general formula R-X. The carbon to halogen bond is polar because the halogen is more electronegative than carbon, giving the carbon a small positive charge that is attacked by nucleophiles. They are classified as primary, secondary or tertiary according to whether the carbon bearing the halogen is joined to one, two or three other carbons. The reactivity of the carbon to halogen bond is R-I greater than R-Br greater than R-Cl greater than R-F.

Preparation

Alkyl halides can be prepared from alkanes by free-radical halogenation, from alkenes by adding a hydrogen halide following Markownikoff rule, and, most usefully, from alcohols by replacing the hydroxyl group using reagents such as HX, PCl3, PCl5 or thionyl chloride.

Nucleophilic Substitution and Elimination

The most important reactions are nucleophilic substitutions, in which a nucleophile replaces the halogen. In the SN2 mechanism the reaction occurs in one step as the nucleophile attacks while the halide leaves, favoured by primary halides. In the SN1 mechanism the halide leaves first to form a carbocation, then the nucleophile attacks, favoured by tertiary halides. With aqueous potassium hydroxide an alkyl halide gives an alcohol, while with hot alcoholic potassium hydroxide it undergoes elimination of hydrogen halide to give an alkene.

The Grignard Reagent

When an alkyl halide reacts with magnesium in dry ether it forms a Grignard reagent, R-Mg-X. The carbon attached to magnesium is strongly nucleophilic, so Grignard reagents react with many compounds to build larger molecules: they give alkanes with water, primary alcohols with methanal, secondary alcohols with other aldehydes, tertiary alcohols with ketones, and carboxylic acids with carbon dioxide. They must be kept perfectly dry, because water destroys them.

Important Definitions

Formulas & Rules

Item	Fact
General formula	R-X; classify 1, 2, 3 (primary, secondary, tertiary)
Reactivity	R-I greater than R-Br greater than R-Cl greater than R-F
Mechanisms	SN2 favours primary; SN1 favours tertiary (via carbocation)
Aqueous vs alcoholic KOH	aqueous gives alcohol (substitution); alcoholic gives alkene (elimination)
Grignard	R-X + Mg in dry ether gives R-MgX; with CO2 gives a carboxylic acid

Diagrams & Illustrations

SN1 versus SN2: a comparison showing the one-step bimolecular SN2 (nucleophile attacks as the halide leaves, favoured by primary halides) and the two-step unimolecular SN1 (a carbocation forms first, favoured by tertiary halides).

Grignard reagent: a map showing the reactions of R-MgX with water (alkane), methanal (primary alcohol), aldehydes (secondary alcohol), ketones (tertiary alcohol) and carbon dioxide (carboxylic acid).

Solved Examples & Numericals

Example 1: Substitution product

What is formed when ethyl bromide is heated with aqueous potassium hydroxide? Nucleophilic substitution occurs: the hydroxide replaces bromide, giving ethanol and potassium bromide.

Example 2: Substitution versus elimination

How can ethyl bromide be converted to ethene? Heat it with alcoholic potassium hydroxide; this eliminates hydrogen bromide (dehydrohalogenation) and forms ethene.

Example 3: Grignard synthesis

How would you make a carboxylic acid using a Grignard reagent? React R-MgX with carbon dioxide, then hydrolyse with dilute acid; the product has one more carbon than R.

Short Questions & Answers

Long Questions & Answers

Q1: Describe the nucleophilic substitution reactions of alkyl halides (SN1 and SN2).

Because the carbon to halogen bond is polar, the carbon carries a partial positive charge and is attacked by nucleophiles, which replace the halogen. In the SN2 mechanism the reaction occurs in a single step: the nucleophile approaches from the side opposite the halogen and forms a bond at the same time as the carbon to halogen bond breaks, passing through a transition state and giving inversion of configuration; its rate depends on both the alkyl halide and the nucleophile, and it is favoured by primary halides with little steric hindrance. In the SN1 mechanism the reaction occurs in two steps: first the halide leaves to give a carbocation (the slow step), and then the nucleophile attacks it; its rate depends only on the alkyl halide, and it is favoured by tertiary halides, which form the most stable carbocations. Typical products are alcohols with aqueous potassium hydroxide and nitriles with alcoholic potassium cyanide.

Q2: Explain the preparation and synthetic importance of the Grignard reagent.

The Grignard reagent is an organomagnesium compound R-Mg-X, prepared by adding an alkyl halide to magnesium turnings in dry ether. In this reagent the carbon attached to magnesium is strongly nucleophilic, which makes it one of the most useful tools in organic synthesis. With water or dilute acid it gives an alkane; with methanal it gives a primary alcohol; with other aldehydes it gives secondary alcohols; with ketones it gives tertiary alcohols; and with carbon dioxide, followed by hydrolysis, it gives a carboxylic acid with one more carbon than the original halide. Because these reactions form new carbon to carbon bonds, the Grignard reagent lets chemists build larger molecules from simple alkyl halides. It must be prepared and used in completely dry conditions, since even a trace of water destroys it.

Q3: Distinguish between substitution and elimination reactions of alkyl halides.

Alkyl halides can react in two competing ways, and the conditions decide which dominates. In a substitution reaction a nucleophile replaces the halogen while the carbon skeleton stays the same, for example ethyl bromide with aqueous potassium hydroxide gives ethanol. In an elimination reaction a molecule of hydrogen halide is removed from two adjacent carbons to create a double bond, for example ethyl bromide with hot alcoholic potassium hydroxide gives ethene. Thus the same reagent gives different products depending on the solvent: aqueous conditions favour substitution while hot concentrated alcoholic conditions favour elimination. Tertiary halides and strong bulky bases also favour elimination, whereas primary halides favour substitution.

MCQs with Answers

Quick Revision Summary

• Alkyl halide R-X; classify primary, secondary, tertiary; the carbon to halogen bond is polar and reactive.
• Reactivity: R-I greater than R-Br greater than R-Cl greater than R-F.
• Nucleophilic substitution: SN2 (one step, primary) and SN1 (carbocation, tertiary).
• Aqueous KOH gives substitution (alcohol); alcoholic KOH gives elimination (alkene).
• Grignard R-MgX from R-X plus magnesium in dry ether; builds carbon to carbon bonds.
• Grignard plus carbon dioxide gives a carboxylic acid; plus aldehyde or ketone gives alcohols.

Exam Tips

• Contrast SN1 and SN2 in a small table: steps, molecularity and which halide is favoured.
• Remember aqueous KOH gives alcohol (substitution), alcoholic KOH gives alkene (elimination).
• Learn the full set of Grignard reactions; a very common long question.
• Always mention that Grignard reagents need dry conditions.
• Reactivity order R-I greater than R-Br greater than R-Cl greater than R-F is a frequent MCQ.