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Halogen Derivatives: Haloalkanes & Haloarenes revision notes

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How we count: 7× asked means 7 past JEE Main questions in the PrepWiser bank needed that concept, and "last" is the latest year one appeared. ★ marks ideas asked 8 or more times.

Classification, the C–X bond and physical properties

Types of organic halides
4× asked, last 2026
sp3 C–X: alkyl (1∘,2∘,3∘), allylic CHX2=CH−CHX2X, benzylic CX6HX5CHX2Xsp2 C–X: vinylic CHX2=CHX, aryl CX6HX5X\begin{gathered}sp^3\text{ C–X: alkyl } (1^\circ,2^\circ,3^\circ),\ \text{allylic } \ce{CH2=CH-CH2X},\ \text{benzylic } \ce{C6H5CH2X}\\ sp^2\text{ C–X: vinylic } \ce{CH2=CHX},\ \text{aryl } \ce{C6H5X}\end{gathered}
Degree = number of C on the C bearing X. Allylic and benzylic X sit on the sp3sp^3 C next to a C=C or an aromatic ring.
Cl
alkyl (sp³)
Cl
allylic (sp³)
Cl
benzylic (sp³)
Cl
vinylic (sp²)
Cl
aryl (sp²)
  • Not 'X near a C=C ⇒ vinylic'. Vinylic means X on the C=C carbon itself. In CHX2=CH−CHX2Cl\ce{CH2=CH-CH2Cl} the Cl carbon is sp3sp^3: allylic, and a haloalkene (not a haloalkyne).
Nature of the C–X bond
1× asked, last 2020
CXδ+−XXδ−:CHX3F, CHX3Cl, CHX3Br, CHX3Ilength (pm) 139, 178, 193, 214;enthalpy (kJ mol−1) 452, 351, 293, 234\begin{gathered}\ce{C^{\delta+}-X^{\delta-}}:\quad \ce{CH3F},\ \ce{CH3Cl},\ \ce{CH3Br},\ \ce{CH3I}\\ \text{length (pm) } 139,\ 178,\ 193,\ 214;\qquad \text{enthalpy (kJ mol}^{-1})\ 452,\ 351,\ 293,\ 234\end{gathered}
Down group 17 C–X grows longer and weaker: R–I reacts fastest, R–F hardly at all. μ: CHX3Cl>CHX3F>CHX3Br>CHX3I\ce{CH3Cl}>\ce{CH3F}>\ce{CH3Br}>\ce{CH3I}.
139C–F178C–Cl193C–Br214C–IC–X length (pm)452C–F351C–Cl293C–Br234C–IC–X enthalpy (kJ mol⁻¹)sheet values
Down the group: longer and weaker, so R–I reacts fastest
1.86CHX3Cl\ce{CH3Cl}1.85CHX3F\ce{CH3F}1.83CHX3Br\ce{CH3Br}1.64CHX3I\ce{CH3I}μ (D)
Dipole moments (NCERT): F is most polar per bond but C–F is short, so CH₃Cl wins
C–X bond in haloarenes and vinyl halides
6× asked, last 2026
C–Cl: 169 pm in CX6HX5Cl vs 178 pm in CHX3Cl\text{C–Cl: } 169\ \text{pm in } \ce{C6H5Cl}\ \text{vs } 178\ \text{pm in } \ce{CH3Cl}
Cl's lone pair joins the ring π system (partial C=Cl); the C is sp2sp^2 and holds the bond pair tighter. Same in CHX2=CHCl\ce{CH2=CHCl}.
Cl
chlorobenzene
 
 
Cl + C -
C=Cl⁺, − ortho
 
 
Cl + C -
C=Cl⁺, − para
178C–Cl in CHX3Cl\ce{CH3Cl}169C–Cl in CX6HX5Cl\ce{C6H5Cl}pm
Partial double bond: 9 pm shorter (sheet)
  • So the aryl/vinyl C–X bond is shorter, stronger and less polar. These halides resist nucleophiles (sp2sp^2 C, unstable PhX+\ce{Ph+}, electron-rich ring repels Nu): no SN1S_N1, no AgX with AgNOX3\ce{AgNO3}.
  • Not 'lone-pair repulsion lengthens the aryl C–Cl'. Resonance makes it shorter.
Physical properties
1× asked, last 2026
bp: RI>RBr>RCl>RF;CHX3CHX2CHX2I>CHX3CHX2I>CHX3Idichlorobenzenes: mp p 323>o 256 K;bp o 453>p 448 K\begin{gathered}\text{bp: } \ce{RI} > \ce{RBr} > \ce{RCl} > \ce{RF};\qquad \ce{CH3CH2CH2I} > \ce{CH3CH2I} > \ce{CH3I}\\ \text{dichlorobenzenes: mp } p\ 323 > o\ 256\ \text{K};\quad \text{bp } o\ 453 > p\ 448\ \text{K}\end{gathered}
More mass and surface, stronger van der Waals forces; branching lowers bp. Para packs best (high mp); polar ortho boils higher.
195F249Cl276Br316ICHX3X\ce{CH3X} b.p.316CHX3I\ce{CH3I}345CX2HX5I\ce{C2H5I}375CX3HX7I\ce{C3H7I}iodides b.p.256o mp323p mp453o bp448p bpdichlorobenzene (sheet)K
b.p. rises with the halogen’s mass and the chain (standard values); para packs best (m.p.), polar ortho boils higher

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Practise Halogen Derivatives: Haloalkanes & Haloarenes

16 more concepts in 5 topics

Preparation; Nucleophilic substitution: SN2, SN1, stereochemistry; Elimination and reactions with metals; Haloarenes: reactions; and Polyhalogen compounds. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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Preparation

  • From alcohols2×
  • From hydrocarbons4×
  • Halogen exchange: Finkelstein and Swarts2×
  • Preparing haloarenes4×

Nucleophilic substitution: SN2, SN1, stereochemistry

  • Nucleophiles and ambident ions5×
  • SN2: one step, back-side attack5×
  • SN1: carbocation route and conditions5×
  • SN1 reactivity: cation stability12×
  • Leaving group and nucleophile strength5×
  • Chirality, inversion and racemisation6×

Elimination and reactions with metals

  • β-Elimination and Saytzeff rule8×
  • Substitution vs elimination3×
  • Grignard reagent and Wurtz coupling4×

Haloarenes: reactions

  • Nucleophilic substitution in haloarenes4×
  • Electrophilic substitution in haloarenes

Polyhalogen compounds

  • Polyhalogen compounds: uses and hazards2×

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