bond. As you move down Group 7, the bond length increases and strength decreases:
Chemsheets often asks why iodoalkanes react faster than fluoroalkanes. It comes down to . C-F: Very strong bond; requires massive energy to break. C-I: Very weak bond; breaks easily.
| Question type | Expected answer | |---------------|----------------| | Mechanism, 1° + NaOH(aq) | SN2, inversion, OH⁻ attacks C–Br | | Mechanism, 3° + H₂O | SN1, carbocation intermediate | | Elimination product from 2-bromopentane | Pent-2-ene (major – more substituted) + pent-1-ene (minor) | | Reagent for substitution to alcohol | Aqueous NaOH, warm | | Reagent for elimination | Ethanolic KOH, heat under reflux | | Why does iodoethane react faster than bromoethane in SN2? | C–I bond weaker, I⁻ better leaving group | | Test for chloroalkane vs iodoalkane | AgNO₃/ethanol – chloroalkane slow white ppt, iodoalkane fast yellow ppt | reactions of halogenoalkanes 1 chemsheets answers exclusive
For elimination questions involving secondary or tertiary halogenoalkanes, check all adjacent carbons for hydrogen atoms. List every possible alkene configuration, including
2-bromobutane is heated with ethanolic KOH. Name the major organic product and explain the reaction mechanism. A2. The major organic product is but-2-ene . The reaction proceeds via an E2 elimination mechanism. The ethoxide ion in ethanol abstracts a β-hydrogen from carbon 3. Simultaneously, the bromine on carbon 2 leaves, forming a double bond between C2 and C3. The reaction is stereospecific, typically yielding the more stable trans (E)-but-2-ene as the major product. C-F: Very strong bond; requires massive energy to break
, it attracts . Nucleophiles are species that possess a lone pair of electrons that they can donate to form a new covalent bond (e.g., The Trend in Reactivity: C-X Bond Enthalpy vs. Polarity
Below are key answers to the types of questions found in the Chemsheets AS 1139 halogenoalkanes resource. Reaction/Question Type Key Answer/Explanation bond is weaker (longer bond length) and breaks more easily. Reaction with KCNcap K cap C cap N Nucleophilic substitution (halogen replaced by CNcap C cap N OHcap O cap H Reaction with Elimination (forms an alkene, removes Formation of 2-bromobutane Elimination with gives a mixture of but-1-ene and but-2-ene. Ozone Depletion Mechanism (Radical initiation). acts as a catalyst. 5. Conclusion | C–I bond weaker, I⁻ better leaving group
R-X+OH−→R-OH+X−R-X plus OH raised to the negative power right arrow R-OH plus X raised to the negative power Bromoethane reacts with aqueous to yield ethanol and a bromide ion. B. Reaction with Potassium Cyanide (
| Reaction | Reagent(s) | Conditions | Product Type | Mechanism | |---|---|---|---|---| | Hydrolysis to alcohol | NaOH(aq) or KOH(aq) | Warm, aqueous | Alcohol (ROH) | SN1 or SN2 | | Water hydrolysis (slow) | H₂O + AgNO₃ (test) | Warm ethanol/water | Alcohol + AgX | SN1 | | Cyanide addition | KCN in ethanol | Warm | Nitrile (RCN) | SN2 | | Amine formation | Excess NH₃ in ethanol | Pressure, heat | Primary amine (RNH₂) | SN2 | | Elimination | NaOH in ethanol | Heat under reflux | Alkene | E1 or E2 | | Identification | AgNO₃ in ethanol | Warm | Silver halide precipitate | Hydrolysis then precipitation |