Halogen Derivatives Notes for Class 12, IIT JEE & NEET

“Halogen derivatives” is the common name for the NCERT chapter Haloalkanes and Haloarenes, which is Chapter 6 of the current NCERT Class 12 Chemistry book (it was Chapter 10 in older books). In the CBSE 2026-27 scheme, it carries 6 marks. The chapter is also the base for the Alcohols, Phenols and Ethers and the Amines chapters, because they use the same ideas of nucleophilic substitution. These Halogen Derivatives Notes explain why each reaction happens, since this chapter rewards reasoning more than memory.

Halogen Derivatives Notes: Chapter Overview and Syllabus

Check the latest syllabi of CBSE and NTA (JEE Main) and the NMC syllabus for NEET for the exact lists.

Tip from the eSaral Chemistry team: For every substitution question, ask three things in order: is the carbon attached to the halogen primary, secondary or tertiary, how strong is the nucleophile, and is the solvent protic or aprotic? These three answers decide SN1 or SN2 almost every time.

Halogen Derivatives Notes 

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

Classification and Nomenclature in Halogen Derivatives Notes

Haloalkanes (alkyl halides) have a halogen on an $sp^3$ carbon, and haloarenes (aryl halides) have a halogen on an $sp^2$ carbon of an aromatic ring. Compounds are also classed by the number of halogens (mono-, di-, tri- and polyhalogen) and by the type of carbon.

IUPAC names: for haloalkanes, add the prefix chloro-, bromo-, fluoro- or iodo- to the alkane name, and number the chain from the end that gives the lowest locants (for example 2-chloropropane). For haloarenes, use chlorobenzene, and the prefixes o-, m- and p- (or numbers) for the positions in a disubstituted ring. Isomerism: haloalkanes show chain and position isomerism, and a compound like 2-bromobutane is chiral and shows optical isomerism.

Nature of the C–X Bond in Halogen Derivatives Notes

The C–X bond is polar, because the halogen is more electronegative than carbon. The carbon has a partial positive charge and the halogen has a partial negative charge, so the carbon is attacked by nucleophiles. The bond length increases and the bond enthalpy decreases from F to I:

The weaker the bond, the better the halogen leaves in a substitution reaction, so the reactivity order is $\mathrm{R{-}I} > \mathrm{R{-}Br} > \mathrm{R{-}Cl} > \mathrm{R{-}F}$.

Methods of Preparation of Haloalkanes in Halogen Derivatives Notes

Methods of Preparation of Haloarenes in Halogen Derivatives Notes

• Electrophilic substitution: $\mathrm{C_6H_6 + Cl_2 \xrightarrow{Fe\ \text{or}\ FeCl_3} C_6H_5Cl + HCl}$. Iodination needs an oxidising agent, and direct fluorination is not practical.
• Sandmeyer reaction: $\mathrm{ArN_2^{+}Cl^{-} \xrightarrow{CuCl/HCl} ArCl + N_2}$ (and $\mathrm{CuBr/HBr}$ for the bromide). With copper powder and HX instead, it is the Gattermann reaction.
• Iodides: $\mathrm{ArN_2^{+}X^{-} + KI \rightarrow ArI + N_2 + KX}$.
• Fluorides (Balz–Schiemann reaction): the diazonium fluoroborate is heated: $\mathrm{ArN_2^{+}BF_4^{-} \xrightarrow{\Delta} ArF + BF_3 + N_2}$.

Physical Properties in Halogen Derivatives Notes

• Boiling points rise with the size of the halogen: $\mathrm{R{-}F} < \mathrm{R{-}Cl} < \mathrm{R{-}Br} < \mathrm{R{-}I}$. For isomeric alkyl halides, the boiling point falls with branching, so $1^\circ > 2^\circ > 3^\circ$.
• Haloalkanes are only sparingly soluble in water, because they cannot form hydrogen bonds with water to replace the strong hydrogen bonds between water molecules.
• Among disubstituted haloarenes, the para isomer has the highest melting point, because it packs best in the crystal.

Nucleophilic Substitution in Haloalkanes: SN1 and SN2 in Halogen Derivatives Notes

The general reaction is $\mathrm{Nu^{-} + R{-}X \rightarrow R{-}Nu + X^{-}}$.

Cyanide and nitrite are ambident nucleophiles: KCN is ionic and attacks through carbon, while AgCN is covalent and attacks through nitrogen.

SN2 and SN1: Comparison

Allylic and benzylic halides react quickly by SN1, because their carbocations are stabilised by resonance.

Stereochemistry in Substitution

In an SN2 reaction, the nucleophile attacks from the side opposite to the leaving group, so the configuration is inverted. In an SN1 reaction, the planar carbocation is attacked from both sides, so an optically active reactant gives a (nearly) racemic product.

Elimination and Reactions with Metals in Halogen Derivatives Notes

Dehydrohalogenation: a haloalkane with a β-hydrogen, heated with alcoholic KOH, loses HX and gives an alkene. Saytzeff’s rule: the more substituted alkene is the major product.

$$\mathrm{CH_3CH_2CHBrCH_3 \xrightarrow{\text{alc. KOH}, \Delta} CH_3CH{=}CHCH_3\ (\text{major}) + CH_3CH_2CH{=}CH_2\ (\text{minor})}$$

Aqueous KOH gives substitution (alcohol), and alcoholic KOH gives elimination (alkene).

Grignard reagent: $\mathrm{R{-}X + Mg \xrightarrow{\text{dry ether}} R{-}MgX}$.

Wurtz reaction: $\mathrm{2R{-}X + 2Na \xrightarrow{\text{dry ether}} R{-}R + 2NaX}$. It works well only with two identical alkyl halides. Two different halides give a mixture of three alkanes.

Reactions of Haloarenes in Halogen Derivatives Notes

Why Haloarenes Are Less Reactive in Nucleophilic Substitution

1. Resonance: the lone pair of the halogen is delocalised into the ring, so the C–Cl bond has a partial double bond character and is shorter and stronger.
2. Hybridisation: the carbon is $sp^2$ (more s-character), so it holds the halogen more tightly than an $sp^3$ carbon.
3. Instability of the phenyl cation: a phenyl cation is not stabilised by resonance, so SN1 does not occur.
4. Repulsion: the electron-rich ring repels the incoming nucleophile.

Dow process: under drastic conditions, chlorobenzene gives phenol: $\mathrm{C_6H_5Cl + NaOH \xrightarrow{623\ K,\ 300\ atm} C_6H_5ONa \xrightarrow{H^+} C_6H_5OH}$.

Effect of nitro groups: electron-withdrawing groups at the ortho and para positions make substitution easier. 4-Nitrochlorobenzene reacts under milder conditions than chlorobenzene, 2,4-dinitrochlorobenzene reacts more easily still, and 2,4,6-trinitrochlorobenzene reacts even at room temperature. A nitro group at the meta position has little effect.

Electrophilic Substitution

The halogen deactivates the ring, but it is ortho- and para-directing, so nitration, sulphonation and Friedel–Crafts reactions give mainly the ortho and para products, with the para product usually as the major one.

Fittig and Wurtz–Fittig Reactions

$$\mathrm{2C_6H_5Br + 2Na \xrightarrow{\text{dry ether}} C_6H_5{-}C_6H_5 + 2NaBr} \quad \text{(Fittig)}$$ $$\mathrm{C_6H_5Br + CH_3Br + 2Na \xrightarrow{\text{dry ether}} C_6H_5CH_3 + 2NaBr} \quad \text{(Wurtz–Fittig)}$$

Polyhalogen Compounds in Halogen Derivatives Notes

Chloroform oxidises slowly in light and air: $\mathrm{2CHCl_3 + O_2 \xrightarrow{\text{light}} 2COCl_2 + 2HCl}$. It is therefore stored in dark, tightly closed bottles filled to the brim, with about 1% ethanol, which converts phosgene into harmless diethyl carbonate.

Key Facts in Halogen Derivatives Notes

Solved Examples from Halogen Derivatives Notes

Example 1 (rate laws): The concentrations of both the substrate and the nucleophile are doubled. How does the rate change for (a) $\mathrm{CH_3Br + OH^-}$ (SN2) and (b) $\mathrm{(CH_3)_3CBr + H_2O}$ (SN1)?

(a) For SN2, rate $= k[\mathrm{R{-}X}][\mathrm{Nu^-}]$, so the rate becomes $2 \times 2 = 4$ times. (b) For SN1, rate $= k[\mathrm{R{-}X}]$, so the rate becomes 2 times.

Example 2 (SN1 and SN2 order): Which reacts faster, $\mathrm{CH_3CH_2Br}$ or $\mathrm{(CH_3)_3CBr}$, by (a) SN2 and (b) SN1?

(a) SN2: $\mathrm{CH_3CH_2Br}$, because it is less hindered. (b) SN1: $\mathrm{(CH_3)_3CBr}$, because it gives a more stable tertiary carbocation.

Example 3 (elimination): Give the major product of 2-bromobutane with alcoholic KOH.

But-2-ene is the major product (Saytzeff’s rule), and but-1-ene is the minor product.

Example 4 (classification and names): Name and classify $\mathrm{CH_3CHClCH_3}$, $\mathrm{CH_2{=}CHCl}$ and $\mathrm{C_6H_5CH_2Cl}$.

$\mathrm{CH_3CHClCH_3}$ is 2-chloropropane (a secondary alkyl halide). $\mathrm{CH_2{=}CHCl}$ is chloroethene (a vinylic halide). $\mathrm{C_6H_5CH_2Cl}$ is (chloromethyl)benzene, or benzyl chloride (a benzylic halide).

Example 5 (ambident nucleophile): What is formed when ethyl bromide reacts with (a) KCN and (b) AgCN?

(a) $\mathrm{CH_3CH_2CN}$ (propanenitrile, an alkyl cyanide). (b) $\mathrm{CH_3CH_2NC}$ (ethyl isocyanide).

Example 6 (haloarene reactivity): Arrange chlorobenzene, 4-nitrochlorobenzene and 2,4-dinitrochlorobenzene in the increasing order of reactivity towards nucleophilic substitution.

Chlorobenzene < 4-nitrochlorobenzene < 2,4-dinitrochlorobenzene, because each nitro group at the ortho or para position makes the ring more electron-poor.

Example 7 (Fittig and Wurtz–Fittig): Give the products when (a) bromobenzene is treated with sodium in dry ether, and (b) bromobenzene and methyl bromide are treated together with sodium in dry ether.

(a) Biphenyl, $\mathrm{C_6H_5{-}C_6H_5}$. (b) Toluene, $\mathrm{C_6H_5CH_3}$, with some biphenyl and ethane as by-products.

Example 8 (boiling points): Arrange n-butyl chloride, isobutyl chloride and tert-butyl chloride in the decreasing order of their boiling points. Also arrange $\mathrm{CH_3CH_2CH_2Cl}$, $\mathrm{CH_3CH_2CH_2Br}$ and $\mathrm{CH_3CH_2CH_2I}$ in the increasing order.

For the isomers, the boiling point falls with branching: n-butyl chloride > isobutyl chloride > tert-butyl chloride. For the halides, the boiling point rises with the size of the halogen: $\mathrm{CH_3CH_2CH_2Cl} < \mathrm{CH_3CH_2CH_2Br} < \mathrm{CH_3CH_2CH_2I}$.

Common Mistakes in Halogen Derivatives Notes Problems

How to Study Halogen Derivatives Notes

1. Read the NCERT chapter first, and download the textbook from eSaral’s NCERT books for Class 12 page.
2. Make a two-column SN1 and SN2 table from memory, and add the leaving group, the solvent and the stereochemistry.
3. Write each named reaction (Finkelstein, Swarts, Sandmeyer, Gattermann, Wurtz, Fittig, Wurtz–Fittig and Dow) with the reagent and the product.
4. Revise with eSaral’s Class 12 Chemistry notes and JEE Main Chemistry revision notes, and test yourself with eSaral’s NEET chapter-wise PYQs and JEE Main chapter-wise PYQs.