JEE Mains Chemistry · Haloalkanes and Haloarenes

Classification, Structure and Physical Properties

An organic halide is named by the carbon that holds the halogen: sp³ for alkyl, allylic and benzylic halides, sp² for vinylic and aryl halides, whose C–X bond is shorter, stronger and less polar; boiling point and density rise with the size and number of halogen atoms.

All Haloalkanes and Haloarenes notes JEE Mains Chemistry strategy

Why this matters

Thirteen PYQs, all multiple choice, three from 2026. Five classify a halide by the carbon that holds the halogen or compare the C–Cl bond of an aryl halide with that of an alkyl halide; four compare boiling points, melting points, densities or polarity; four ask the formula or the use of a polyhalogen compound.

Concept 1 of 3: Types of organic halides and the aryl C–X bond

Look only at the carbon that carries the halogen. If it is sp³, the halide is alkyl, allylic (that carbon sits next to a C=C) or benzylic (it sits next to a benzene ring). If it is sp², the halide is vinylic (on a C=C carbon) or aryl (on a ring carbon). In vinylic and aryl halides a lone pair of the halogen spreads into the π system, so the C–X bond gains some double-bond character.

Definition

Alkyl halides are 1°, 2° or 3° by the number of carbons on the carbon that holds X.
Allylic: X on an sp³ carbon next to C=C, as in $\mathrm{CH_2{=}CH{-}CH_2{-}Cl}$ . Benzylic: X on an sp³ carbon attached to a ring, as in $\mathrm{C_6H_5CH_2Cl}$ .
Vinylic: X on an sp² carbon of C=C, as in $\mathrm{CH_2{=}CHCl}$ . Aryl: X on a ring carbon, as in $\mathrm{C_6H_5Cl}$ .
Geminal dihalides (alkylidene halides) carry both X on one carbon; vicinal dihalides (alkylene halides) carry them on adjacent carbons.
In an aryl or vinylic halide, resonance gives the C–X bond partial double-bond character, and the carbon is sp² (more s character). So the bond is shorter and stronger (C–Cl 169 pm in chlorobenzene against 177 pm in an alkyl chloride) and less polar, with less negative charge on the halogen.

Class	Carbon that holds X	Example	What follows
Alkyl, 1°	sp³ carbon bonded to one other carbon	$\mathrm{CH_3CH_2CH_2Cl}$	Reacts mainly by SN2
Alkyl, 3°	sp³ carbon bonded to three other carbons	$\mathrm{(CH_3)_3C{-}Cl}$	Reacts mainly by SN1 or elimination
Allylic	sp³ carbon next to a C=C	$\mathrm{CH_2{=}CH{-}CH_2{-}Cl}$	Ionises easily: the allyl cation is resonance-stabilised
Benzylic	sp³ carbon attached to a benzene ring	$\mathrm{C_6H_5CH_2Cl}$ , $\mathrm{C_6H_5CH(Cl)CH_3}$	Ionises easily: the benzyl cation is resonance-stabilised
Vinylic	sp² carbon of a C=C	$\mathrm{CH_2{=}CHCl}$	Partial C=Cl character; no SN1 or SN2 under normal conditions
Aryl	sp² carbon of a benzene ring	$\mathrm{C_6H_5Cl}$	Partial C=Cl character; substituted only under harsh conditions or with nitro groups ortho or para
Geminal dihalide	Both X on one carbon	Ethylidene chloride, $\mathrm{CH_3CHCl_2}$ (1,1-dichloroethane)	Common name ends in -idene
Vicinal dihalide	X on two adjacent carbons	Ethylene dichloride, $\mathrm{ClCH_2CH_2Cl}$ (1,2-dichloroethane)	Common name ends in -ene

Allylic and benzylic halides are sp³ at the C–X carbon; vinylic and aryl halides are sp².

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 22 Jan 2026 Shift 1 · Q44Moderate

Example 1 · Haloalkanes and Haloarenes · Classification, Structure and Physical Properties

As compared with chlorocyclohexane, which of the following statements correctly apply to chlorobenzene? (A) The magnitude of negative charge is more on chlorine atoms (B) The

C - Cl

bond has partial double bond character (C)

C - Cl

bond is less polar (D)

C - Cl

bond is longer due to repulsion between delocalised electrons of the aromatic ring and lone pairs of electrons of chlorine. (E) The

C - Cl

bond is formed using

{sp}^{2}

hybridised orbital of carbon. Choose the correct answer from the options given below:

Allylic is not vinylic

In an allyl halide such as

\mathrm{CH_2{=}CH{-}CH_2Cl}

the halogen is on the sp³ carbon next to the double bond. A statement that allylic halides carry X on an sp² carbon is false; that describes a vinylic halide.

Ethylidene and ethylene dichloride

The -idene name puts both halogens on one carbon: ethylidene chloride is 1,1-dichloroethane. Ethylene dichloride has them on adjacent carbons: 1,2-dichloroethane.

Resonance makes the aryl C–Cl bond shorter and less polar

Delocalisation of a chlorine lone pair into the ring gives the bond partial double-bond character. The bond gets shorter, not longer, and chlorine carries less negative charge than in an alkyl chloride.

Drill 4 more on types of organic halides and the aryl c–x bond

Concept 2 of 3: Boiling point, melting point, density and polarity of halides

Boiling point follows the attraction between molecules. Larger, more polarisable halogens and longer chains give stronger van der Waals forces, so the boiling point rises; branching makes a molecule more compact and lowers it. Melting point also depends on how well molecules pack in a crystal, which is why a symmetric isomer can melt highest without boiling highest. Density rises as heavy halogen atoms replace hydrogen.

Definition

Same alkyl group: $\mathrm{R{-}I > R{-}Br > R{-}Cl > R{-}F}$ in boiling point.
Same halogen: boiling point rises with chain length and falls with branching: 1-bromobutane (375 K) > 2-bromobutane (364 K) > 2-bromo-2-methylpropane (346 K).
Dichlorobenzenes: the para isomer melts highest (323 K) because it is the most symmetric and packs best; the ortho isomer boils highest (453 K) because it is the most polar.
Density rises with the number and mass of halogen atoms: $\mathrm{CH_2Cl_2 < CHCl_3 < CCl_4}$ , and $\mathrm{n{-}C_3H_7Cl < n{-}C_3H_7Br < n{-}C_3H_7I}$ . Bromo, iodo and polychloro compounds are denser than water.
In a 1,2-dihaloethene the two C–X dipoles add in the cis isomer and cancel in the trans isomer, so the cis isomer is polar and boils higher.
Haloalkanes dissolve only slightly in water: breaking water's hydrogen bonds costs more energy than the new attractions release.

Property	Trend	Reason
Boiling point, changing the halogen	$\mathrm{RI > RBr > RCl > RF}$	A larger, more polarisable halogen gives stronger van der Waals forces
Boiling point, longer chain	$\mathrm{CH_3Cl < C_2H_5Cl < n{-}C_3H_7Cl}$	A larger surface gives stronger London forces
Boiling point, branched isomers	Falls with branching	A branched molecule is more nearly spherical, with less contact area
Melting point of dichlorobenzenes	para (323 K) > ortho (256 K) > meta (249 K)	The symmetric para isomer packs best in the crystal
Boiling point of dichlorobenzenes	ortho (453 K) > para (448 K) > meta (446 K)	The ortho isomer has the largest dipole
Density	$\mathrm{CH_2Cl_2 < CHCl_3 < CCl_4}$ ; iodides densest	More and heavier halogen atoms in about the same volume
Dipole moment of $\mathrm{CH_3X}$	$\mathrm{CH_3Cl > CH_3F > CH_3Br > CH_3I}$	Charge × bond length is largest for C–Cl; the C–F bond is very short
cis against trans 1,2-dihaloethene	cis is polar and boils higher; trans has almost no dipole	In the trans isomer the two C–X dipoles point opposite ways and cancel

Symmetry raises the melting point; polarity and size raise the boiling point.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 8 Apr 2026 Shift 2 · Q41Moderate

Example 2 · Haloalkanes and Haloarenes · Classification, Structure and Physical Properties

Given below are two statements : Statement-I : Due to increase in van der Waals forces, the order of boiling points is

{CH}_{3}{CH}_{2}{CH}_{2}I > {CH}_{3}{CH}_{2}I > {CH}_{3}I

. Statement-II : As p-dichlorobenzene (1,4-dichlorobenzene) is more symmetric, its melting point is higher than that of o-dichlorobenzene (1,2-dichlorobenzene); however, its boiling point is lower than that of o-dichlorobenzene. In the light of the above statements, choose the correct answer from the options given below :

Symmetry raises the melting point, not the boiling point

1,4-Dichlorobenzene melts far above its isomers because it packs well, yet it boils below 1,2-dichlorobenzene, which has the larger dipole. Do not carry the melting-point order over to boiling points.

Methyl fluoride is not the most polar methyl halide

Fluorine is the most electronegative halogen, but the C–F bond is so short that the dipole moment of

\mathrm{CH_3F}

is slightly below that of

\mathrm{CH_3Cl}

Iodides are the densest, not the lightest

For the same alkyl group, density rises from chloride to bromide to iodide. An order that puts the iodo compound lowest has it backwards.

Drill 3 more on boiling point, melting point, density and polarity of halides

Concept 3 of 3: Polyhalogen compounds: formulas and uses

A handful of compounds with several halogen atoms are asked as facts: what each is used for, and how many halogen atoms it carries. Write the formula first; the count and the use then follow.

Definition

Dichloromethane $\mathrm{CH_2Cl_2}$ : solvent, paint remover, aerosol propellant; harms the central nervous system.
Chloroform $\mathrm{CHCl_3}$ : solvent, once an anaesthetic. Air and light oxidise it to the poisonous gas phosgene, $\mathrm{COCl_2}$ , so it is kept in dark bottles filled to the top.
Iodoform $\mathrm{CHI_3}$ : antiseptic, because of the iodine it releases.
Carbon tetrachloride $\mathrm{CCl_4}$ : solvent and raw material for freons; once used in fire extinguishers; it depletes ozone.
Freons are chlorofluorocarbons, such as freon-12, $\mathrm{CCl_2F_2}$ , made from $\mathrm{CCl_4}$ by the Swarts reaction: refrigerants and aerosol propellants.
DDT $\mathrm{(ClC_6H_4)_2CH{-}CCl_3}$ : insecticide; not biodegradable, so it builds up in fatty tissue.

Compound	Formula	Use or fact
Dichloromethane (methylene chloride)	$\mathrm{CH_2Cl_2}$	Paint remover, solvent and aerosol propellant
Trichloromethane (chloroform)	$\mathrm{CHCl_3}$	Solvent; stored in dark, full bottles because air and light turn it into phosgene
Triiodomethane (iodoform)	$\mathrm{CHI_3}$	Antiseptic, through the free iodine it releases
Tetrachloromethane (carbon tetrachloride)	$\mathrm{CCl_4}$	Fire extinguisher (earlier), solvent, feedstock for freons
Freon-12 (dichlorodifluoromethane)	$\mathrm{CCl_2F_2}$	Refrigerant and aerosol propellant; a CFC with 2 Cl
DDT (p,p′-dichlorodiphenyltrichloroethane)	$\mathrm{C_{14}H_9Cl_5}$	Non-biodegradable insecticide; 5 Cl
Gammaxene (lindane, BHC)	$\mathrm{C_6H_6Cl_6}$	Insecticide; 6 Cl
Chloropicrin (trichloronitromethane)	$\mathrm{CCl_3NO_2}$	Insecticide and war gas; 3 Cl
Chloral (trichloroethanal)	$\mathrm{CCl_3CHO}$	Raw material for DDT; 3 Cl

Learn each compound with its formula: most questions ask for a use or an atom count.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2023 · 31 January 2023 · Q137Moderate

Example 3 · Haloalkanes and Haloarenes · Classification, Structure and Physical Properties

In the following halogenated organic compounds the one with maximum number of chlorine atoms in its structure is:

A freon needs both chlorine and fluorine

Freons are chlorofluorocarbons. A compound with fluorine but no chlorine, such as

\mathrm{C_2F_4}

\mathrm{C_2HF_3}

, is not a freon.

Chloroform is kept full and dark

Chloroform is stored in dark bottles filled to the brim to keep out air and light, which oxidise it to phosgene. The storage rule is a common statement question.

Drill 3 more on polyhalogen compounds: formulas and uses

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (3)

Types of organic halides and the aryl C–X bond8 rows

Class	Carbon that holds X	Example	What follows
Alkyl, 1°	sp³ carbon bonded to one other carbon	$\mathrm{CH_3CH_2CH_2Cl}$	Reacts mainly by SN2
Alkyl, 3°	sp³ carbon bonded to three other carbons	$\mathrm{(CH_3)_3C{-}Cl}$	Reacts mainly by SN1 or elimination
Allylic	sp³ carbon next to a C=C	$\mathrm{CH_2{=}CH{-}CH_2{-}Cl}$	Ionises easily: the allyl cation is resonance-stabilised
Benzylic	sp³ carbon attached to a benzene ring	$\mathrm{C_6H_5CH_2Cl}$ , $\mathrm{C_6H_5CH(Cl)CH_3}$	Ionises easily: the benzyl cation is resonance-stabilised
Vinylic	sp² carbon of a C=C	$\mathrm{CH_2{=}CHCl}$	Partial C=Cl character; no SN1 or SN2 under normal conditions
Aryl	sp² carbon of a benzene ring	$\mathrm{C_6H_5Cl}$	Partial C=Cl character; substituted only under harsh conditions or with nitro groups ortho or para
Geminal dihalide	Both X on one carbon	Ethylidene chloride, $\mathrm{CH_3CHCl_2}$ (1,1-dichloroethane)	Common name ends in -idene
Vicinal dihalide	X on two adjacent carbons	Ethylene dichloride, $\mathrm{ClCH_2CH_2Cl}$ (1,2-dichloroethane)	Common name ends in -ene

Allylic and benzylic halides are sp³ at the C–X carbon; vinylic and aryl halides are sp².

Boiling point, melting point, density and polarity of halides8 rows

Property	Trend	Reason
Boiling point, changing the halogen	$\mathrm{RI > RBr > RCl > RF}$	A larger, more polarisable halogen gives stronger van der Waals forces
Boiling point, longer chain	$\mathrm{CH_3Cl < C_2H_5Cl < n{-}C_3H_7Cl}$	A larger surface gives stronger London forces
Boiling point, branched isomers	Falls with branching	A branched molecule is more nearly spherical, with less contact area
Melting point of dichlorobenzenes	para (323 K) > ortho (256 K) > meta (249 K)	The symmetric para isomer packs best in the crystal
Boiling point of dichlorobenzenes	ortho (453 K) > para (448 K) > meta (446 K)	The ortho isomer has the largest dipole
Density	$\mathrm{CH_2Cl_2 < CHCl_3 < CCl_4}$ ; iodides densest	More and heavier halogen atoms in about the same volume
Dipole moment of $\mathrm{CH_3X}$	$\mathrm{CH_3Cl > CH_3F > CH_3Br > CH_3I}$	Charge × bond length is largest for C–Cl; the C–F bond is very short
cis against trans 1,2-dihaloethene	cis is polar and boils higher; trans has almost no dipole	In the trans isomer the two C–X dipoles point opposite ways and cancel

Symmetry raises the melting point; polarity and size raise the boiling point.

Polyhalogen compounds: formulas and uses9 rows

Compound	Formula	Use or fact
Dichloromethane (methylene chloride)	$\mathrm{CH_2Cl_2}$	Paint remover, solvent and aerosol propellant
Trichloromethane (chloroform)	$\mathrm{CHCl_3}$	Solvent; stored in dark, full bottles because air and light turn it into phosgene
Triiodomethane (iodoform)	$\mathrm{CHI_3}$	Antiseptic, through the free iodine it releases
Tetrachloromethane (carbon tetrachloride)	$\mathrm{CCl_4}$	Fire extinguisher (earlier), solvent, feedstock for freons
Freon-12 (dichlorodifluoromethane)	$\mathrm{CCl_2F_2}$	Refrigerant and aerosol propellant; a CFC with 2 Cl
DDT (p,p′-dichlorodiphenyltrichloroethane)	$\mathrm{C_{14}H_9Cl_5}$	Non-biodegradable insecticide; 5 Cl
Gammaxene (lindane, BHC)	$\mathrm{C_6H_6Cl_6}$	Insecticide; 6 Cl
Chloropicrin (trichloronitromethane)	$\mathrm{CCl_3NO_2}$	Insecticide and war gas; 3 Cl
Chloral (trichloroethanal)	$\mathrm{CCl_3CHO}$	Raw material for DDT; 3 Cl

Learn each compound with its formula: most questions ask for a use or an atom count.

Watch out for (8)

Allylic is not vinylic
→ Types of organic halides and the aryl C–X bond
Ethylidene and ethylene dichloride
→ Types of organic halides and the aryl C–X bond
Resonance makes the aryl C–Cl bond shorter and less polar
→ Types of organic halides and the aryl C–X bond
Symmetry raises the melting point, not the boiling point
→ Boiling point, melting point, density and polarity of halides
Methyl fluoride is not the most polar methyl halide
→ Boiling point, melting point, density and polarity of halides
Iodides are the densest, not the lightest
→ Boiling point, melting point, density and polarity of halides
A freon needs both chlorine and fluorine
→ Polyhalogen compounds: formulas and uses
Chloroform is kept full and dark
→ Polyhalogen compounds: formulas and uses

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