मराठी
Karnataka Board PUCPUC Science 2nd PUC Class 12
पाठ्यपुस्तक उत्तरे१५१३४
MCQ ऑनलाइन सराव परीक्षा२२
संकल्पना स्पष्टीकरण आणि व्हिडिओ६५९
अभ्यासक्रम

Reactions of Haloalkanes - Nucleophilic Substitution Reactions

Advertisements

Topics

Estimated time: 6 minutes
CBSE: Class 12

Key Points: Reactions of Haloalkanes — Nucleophilic Substitution Reaction

The C–X bond in alkyl halides is polarised (Cδ+–Xδ–), making alkyl halides reactive towards nucleophiles.

Two Types of SN Reactions

SN1 (Unimolecular Nucleophilic Substitution):

  • First-order kinetics: Rate = k[RX] (depends only on substrate concentration)
  • Two-step mechanism: Step 1 (slow) — ionisation to form carbocation; Step 2 (fast) — attack by nucleophile.
  • Intermediate: Trigonal planar carbocation.
  • More substituted alkyl halides react faster (more stable carbocation).
  • Reactivity order: R₃CX > R₂CHX > RCH₂X (3° > 2° > 1°)
  • Gives a racemic mixture (optically inactive product) because the nucleophile can attack from both faces.
  • For aryl/vinyl halides: Ar₂CX > Ar₂CHX > ArCH₂X = CH₂=CHX > CH₂=CHCH₂X

SN2 (Bimolecular Nucleophilic Substitution):

  • Second-order kinetics: Rate = k[RX][Nu] (depends on both substrate and nucleophile concentration)
  • One-step mechanism (concerted): Nucleophile attacks from the back side as leaving group departs simultaneously → Transition State is formed.
  • Results in Walden Inversion (inversion of configuration at the carbon — stereochemistry inverted).
  • Reactivity order: Methyl halide > Primary > Secondary > Tertiary (CH₃X > 1° > 2° > 3°)
  • The SN2 reaction rate depends on the concentration of both alkyl halide and nucleophile.

Shaalaa.com | mechanism of substitution reactions

Shaalaa.com


Next video


Shaalaa.com


mechanism of substitution reactions [00:02:49]
S
Series: series 1
0%


mechanism of substitution reactions
00:02:49  undefined
Carbocation Stability
00:09:37  undefined
Haloalkanes and Haloarenes part 17 (Nucleophilic Substitution reaction)
00:08:40  undefined
Haloalkanes and Haloarenes part 18 (Substitution SN1 reaction)
00:14:19  undefined
Haloalkanes and Haloarenes part 19 (Substitution SN2 reaction)
00:12:13  undefined
Haloalkanes and Haloarenes part 20 (Numerical SN1, SN2 reaction)
00:08:08  undefined
Haloalkanes and Haloarenes part 21 (Stereochemistry, optical isomers)
00:26:56  undefined

Related QuestionsVIEW ALL [124]

Which alkyl halide from the following pair would you expect to react more rapidly by an SN2 mechanism? Explain your answer.

\[\begin{array}{cc}\ce{CH3CH2CHCH3}\\\phantom{...}|\\\phantom{....}\ce{Br}\end{array}\] or \[\begin{array}{cc}\phantom{.....}\ce{CH3}\\\phantom{..}|\\\ce{H3C - C - Br}\\\phantom{..}|\\\phantom{....}\ce{CH3}\end{array}\]

Which of the following is a primary halide?

Which of the following is an example of SN2 reaction?

The treatment of alkyl chlorides with aqueous KOH leads to the formation of alcohols but in the presence of alcoholic KOH, alkenes are major products. Explain.

Which would undergo SN2 reaction faster in the following pair and why ?

CH3 – CH2 ​– Br and CH3 ​– CH2 ​– I

Assertion (A) : Nucleophilic substitution of iodoethane is easier than chloroethane.

Reason (R) : Bond enthalpy of C-I bond is less than that of C-Cl bond.

Give reason for the following:

The product formed during SN1 reaction is a racemic mixture.

Cyanide ion acts as an ambident nucleophile. From which end it acts as a stronger nucleophile in aqueous medium? Give reason for your answer.

Advertisements
Share
Notifications

Englishहिंदीमराठी
Login
Create free account


      Forgot password?
Course
Use app×