| Halogenoalkane | Precipitate Color | Rate of Precipitation | Bond Enthalpy (C-X) | |---|---|---|---| | Fluoroalkane | No precipitate (AgF soluble) | Very slow (usually not seen) | Very high (484 kJ/mol) | | Chloroalkane | White (AgCl) | Slow (minutes, needs warming) | 338 kJ/mol | | Bromoalkane | Cream (AgBr) | Faster (seconds to minutes) | 276 kJ/mol | | Iodoalkane | Yellow (AgI) | Instant (room temperature) | 238 kJ/mol |
: Favored by hot, ethanolic conditions (e.g., KOH in ethanol). Common Worksheet Reactions Reagent Conditions Type of Reaction Organic Product Warm, aqueous Nucleophilic Substitution Alcohol (e.g., Ethanol) Hot, ethanolic Nucleophilic Substitution Nitrile (e.g., Propanenitrile) NH₃ (excess) Hot, concentrated Nucleophilic Substitution Amine (e.g., Ethylamine) Hot, ethanolic Elimination Alkene (e.g., Ethene) Mechanism Tips
OH⁻ │ ▼ H─O─H │ H H H H │ │ │ │ R─C ── C─R ───────► R─C ═ C─R + H₂O + X⁻ │ │ H X (δ-) Isomerism in Elimination Products
| | Model Answer/Key Points | | :--- | :--- | | Define a nucleophile. | A nucleophile is a species with a lone pair of electrons, ready to donate to an electron-deficient carbon atom. Nucleophiles act as electron pair donors in reaction mechanisms. | | Why are halogenoalkanes attacked by nucleophiles? | The carbon-halogen bond is polar because the halogen is more electronegative, creating a δ⁺ carbon (electrophilic center) that attracts nucleophiles. | | In the reaction of a primary halogenoalkane with aqueous KOH, outline the mechanism. | Draw the SN2 mechanism showing a one-step process where the OH⁻ attacks from the opposite side of the leaving group (X⁻). Include partial bonds (---) in the transition state. | | What are the products when a secondary halogenoalkane is heated with ethanolic KOH? | The major product is the most substituted alkene (Saytzeff's rule). For example, 2-bromobutane produces but-2-ene (major) and but-1-ene (minor). | | Why are iodoalkanes the most reactive in nucleophilic substitution? | The C-I bond is the weakest despite being the least polar. Its low bond enthalpy makes it the easiest to break, leading to faster reactions. | reactions of halogenoalkanes 1 chemsheets answers exclusive
For example, bromoethane reacts with ethanolic KOH to form ethene.
R−X+2NH3→R−NH2+NH4+X−cap R minus cap X plus 2 cap N cap H sub 3 right arrow cap R minus cap N cap H sub 2 plus cap N cap H sub 4 raised to the positive power cap X raised to the negative power 3. Elimination Reactions (Formation of Alkenes) Under different conditions, hydroxide ions ( OH−cap O cap H raised to the negative power
is a halogen (F, Cl, Br, or I). The reactivity stems from two main factors: | Halogenoalkane | Precipitate Color | Rate of
double bond, stereoisomers ( E/Z or cis/trans isomers) can form. If you are working through your chemistry assignments and
E2 (bimolecular, concerted)
Under altered reaction conditions, hydroxide ions can behave as (proton acceptors) rather than nucleophiles. This shifts the reaction from substitution to elimination. Reagent: Potassium hydroxide ( ) or Sodium hydroxide ( Nucleophiles act as electron pair donors in reaction
Nucleophiles are species possessing a lone pair of electrons available for donation to form a new covalent bond. Chemsheets materials emphasize three primary nucleophilic substitution reactions. Reaction A: Hydrolysis (Formation of Alcohols) Aqueous sodium hydroxide ( NaOH(aq)NaOH sub open paren a q close paren end-sub ) or potassium hydroxide ( KOH(aq)KOH sub open paren a q close paren end-sub Conditions: Warm / Reflux. Nucleophile: Hydroxide ion ( :OH−:OH raised to the negative power General Equation:
When a halogenoalkane reacts with a hydroxide ion under different conditions, a competing reaction takes place: . Instead of acting as a nucleophile, the hydroxide ion acts as a base (a proton acceptor), removing a hydrogen atom to form an alkene. Reagent: Potassium Hydroxide ( ) or Sodium Hydroxide (