Organic- HalogenoAlkanes Flashcards
What are halogenoalkanes, and how are they produced?
Halogenoalkanes are alkanes with one or more halogen atoms attached.
Production Methods:
1. Free-radical substitution of alkanes with Cl₂ or Br₂ in UV light.
- Electrophilic addition of alkenes with hydrogen halides (HX) or halogens (X₂) at room temperature.
- Substitution of alcohols using:
HX (or KBr with H₂SO₄ or H₃PO₄ to form HX).
PCl₃ and heat.
PCl₅ at room temperature.
SOCl₂.
Describe the steps of free-radical substitution.
Initiation: UV light breaks the halogen bond (homolytic fission), forming two radicals.
Propagation: Radicals react with alkanes to produce a chain reaction.
Termination: Two radicals combine to form a stable molecule, stopping the chain reaction.
How are halogenoalkanes produced via electrophilic addition?
Reactants: Alkenes react with HX (hydrogen halides) or X₂ (halogens) at room temperature.
Mechanism:
HX: Hydrogen acts as an electrophile, bonding to one carbon. The halide bonds to the more substituted carbon (Markovnikov’s Rule).
X₂: One halogen acts as an electrophile, and the other as a nucleophile.
How are halogenoalkanes classified?
Primary: Halogen is attached to a carbon bonded to one alkyl group.
Secondary: Halogen is attached to a carbon bonded to two alkyl groups.
Tertiary: Halogen is attached to a carbon bonded to three alkyl groups.
reagant and conditions
What conditions are required for free-radical substitution of alkanes?
Reagent: Cl₂ or Br₂.
Condition: Ultraviolet (UV) light.
What conditions are required for electrophilic addition of alkenes?
Reagents: HX (e.g., HCl, HBr) or X₂ (e.g., Cl₂, Br₂).
Condition: Room temperature.
Write the free-radical substitution reaction of methane with chlorine.
Initiation-
Cl2—uv light—> Cl⋅ + Cl⋅
Propagation-
CH4 + Cl⋅ —> CH3 ⋅ + HCl
CH3 ⋅ + Cl2 —> CH3Cl + Cl⋅
Termination-
Cl⋅ + Cl⋅ —> Cl2
CH3 ⋅ + Cl⋅ —> CH3Cl
CH3 ⋅ + CH3 ⋅ —> C2H6
What is Markovnikov’s rule, and how does it apply to the electrophilic addition of hydrogen halides?
Markovnikov’s rule states that in the addition of HX to an alkene, the hydrogen atom bonds to the carbon with the greater number of hydrogen atoms (the less substituted carbon).
Describe the mechanism for the electrophilic addition of bromine to ethene.
- Electrophile Formation:
Bromine approaches the electron-rich double bond, inducing a dipole: Br𝛿+ - Br𝛿- - Bond Formation:
The π-electrons attack Br𝛿+, forming a carbocation and a bromide ion (Br− ). - Nucleophilic Attack:
Br− attacks the carbocation, forming 1,2-dibromoethane
Write the reaction of ethanol with SOCl₂ to produce chloroethane.
CH3CH2OH +SOCl2—>CH3CH2Cl + HCl + SO2
How do you classify halogenoalkanes as primary, secondary, or tertiary?
Primary: Halogen is attached to a carbon bonded to one alkyl group
Secondary: Halogen is attached to a carbon bonded to two alkyl groups.
Tertiary: Halogen is attached to a carbon bonded to three alkyl groups.
List the reagents and conditions for substituting an alcohol to form a halogenoalkane.
Reaction with HX (hydrogen halide):
ROH + HX → R-X + H₂O
Reaction with KCl (and concentrated H₂SO₄ or H₃PO₄):
ROH + KCl + H₂SO₄ → R-Cl + H₂O + KHSO₄
Reaction with PCl₃ (requires heat):
3ROH + PCl₃ → 3R-Cl + H₃PO₃
Reaction with PCl₅ (room temperature):
ROH + PCl₅ → R-Cl + HCl + POCl₃
Reaction with SOCl₂:
ROH + SOCl₂ → R-Cl + HCl + SO₂
What is homolytic fission, and where does it occur in free-radical substitution?
- Homolytic fission is the breaking of a covalent bond where each atom takes one electron from the bond, forming two free radicals.
- Occurs during the initiation step
Eg- Cl₂ → Cl· + Cl·
reagant, conditions, reaction type, product
Reaction of Halogenoalkanes with NaOH(aq)
Reagent: Sodium hydroxide (NaOH) in aqueous solution
Conditions: Heat under reflux
Reaction type: Nucleophilic substitution
Product: Alcohol
Example:
CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻
reagant, conditions, reaction type, product
Reaction of Halogenoalkanes with KCN in Ethanol
Reagent: Potassium cyanide (KCN) in ethanol
Conditions: Heat under reflux
Reaction type: Nucleophilic substitution
Product: Nitrile (extends the carbon chain by one atom)
Example:
CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻
reagant, conditons, reaction type, product
Reaction of Halogenoalkanes with NH₃ in Ethanol
Reagent: Ammonia (NH₃) in ethanol
Conditions: Heat under pressure, excess NH₃
Reaction type: Nucleophilic substitution
Product: Amine
Example:
CH₃CH₂Br + NH₃ → CH₃CH₂NH₂ + HBr
reagant, reaction type, purpose
Reaction of Halogenoalkanes with Aqueous Silver Nitrate in Ethanol
Reagent: Aqueous silver nitrate (AgNO₃) in ethanol
Reaction type: Nucleophilic substitution
Purpose: Identifying halogen type via precipitate formation
Example:
CH₃CH₂Br + H₂O → CH₃CH₂OH + Br⁻
Ag⁺ + Br⁻ → AgBr (cream precipitate)
Observations of Reaction of Halogenoalkanes with Aqueous Silver Nitrate in Ethanol
Observations:
Chlorides → White precipitate (AgCl)
Bromides → Cream precipitate (AgBr)
Iodides → Yellow precipitate (AgI)
Example:
CH₃CH₂Br + H₂O → CH₃CH₂OH + Br⁻
Ag⁺ + Br⁻ → AgBr (cream precipitate)
reagent, conditions, reaction type, product
Reaction with NaOH in Ethanol
Reagent: Sodium hydroxide (NaOH) in ethanol
Conditions: Heat under reflux
Reaction type: Elimination
Product: Alkene
Example:
CH₃CH₂Br + NaOH → CH₂=CH₂ + H₂O + NaBr
SN2 Mechanism of Nucleophilic Substitution
Occurs in primary halogenoalkanes
One-step mechanism: Nucleophile attacks carbon while the leaving group departs simultaneously
Rate depends on the concentration of both halogenoalkane and nucleophile
Example:
CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻
SN1 Mechanism of Nucleophilic Substitution
Occurs in tertiary halogenoalkanes
Two-step mechanism:
C-X bond breaks heterolytically to form a carbocation (rate-determining step).
Nucleophile attacks the carbocation.
Rate depends only on the concentration of the halogenoalkane.
Example:
(CH₃)₃CBr → (CH₃)₃C⁺ + Br⁻
(CH₃)₃C⁺ + OH⁻ → (CH₃)₃COH
Reactivity of Halogenoalkanes and C-X Bond Strengths
Reactivity depends on bond strength:
C–F > C–Cl > C–Br > C–I (strongest to weakest)
Iodoalkanes are the most reactive; fluoroalkanes are the least reactive.
Reactivity can be tested with aqueous silver nitrate:
Faster precipitate formation indicates higher reactivity.
Stability of Carbocations in SN1 Mechanism
Carbocation stability: Tertiary > Secondary > Primary
Tertiary carbocations are stabilized by the positive inductive effect of alkyl groups.
Primary carbocations are less stable, making SN1 mechanisms unfavorable for them.
What determines the rate of SN2 reactions?
Both the concentration of halogenoalkane and nucleophile.
