AgNO3 times and KOH products for three bromoalkanes
Three bottles contain 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane. Equal volumes of each are added to separate tubes containing 10 mL of 0.10 M AgNO3 in ethanol at 25 °C. A precipitate appears in 12 min, 3 min and immediately, respectively. The same halides are then refluxed separately with 0.10 M KOH in aqueous ethanol. The products are separated: the first gives butan-1-ol as the major product, the second gives butan-2-ol, and the third gives 2-methylpropan-2-ol together with a colourless gas that decolourises bromine water. A student proposes that the AgNO3 order proves the same mechanism operates in KOH. The teacher asks the group to evaluate this claim, assign mechanisms, and explain the gas. The tubes are kept in the same water bath, and the first tube warms only slowly while the third warms almost at once.
1. Interpret the AgNO3 precipitation times and assign the likely mechanism in each case. Does the order prove that the KOH reactions follow the same mechanism?
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Ag+ assists the departure of Br− by precipitating AgBr, so the rate depends mainly on the stability of the carbocation that would form:
- 1-bromobutane: primary carbocation is very unstable; precipitation is slow (12 min).
- 2-bromobutane: secondary carbocation is more stable; precipitation is faster (3 min).
- 2-bromo-2-methylpropane: tertiary carbocation is most stable; precipitation is immediate.
The order tertiary > secondary > primary is the characteristic SN1 order. The rapid warming in the third tube is also consistent with a fast exothermic solvolysis.
However, the AgNO3/ethanol test does not prove that the KOH reaction uses the same mechanism. Ag+ is a special electrophilic catalyst that pulls off halide, favouring SN1. In KOH, OH− is a strong nucleophile and a strong base, so SN2 and E2 can compete, especially with primary and tertiary substrates. The mechanism must be decided from the product pattern and the substrate structure, not from the silver nitrate test alone.
2. For the KOH reactions, assign the major mechanism and product in each case. Explain why the tertiary bromide gives a colourless gas as well as an alcohol.
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With KOH in aqueous ethanol:
- 1-Bromobutane: primary and unhindered; SN2 is favoured. OH− attacks the carbon bearing Br from the back side, giving butan-1-ol as the major product. Some elimination to but-1-ene is possible but is minor under these conditions.
- 2-Bromobutane: secondary; SN1 and SN2 compete. Substitution gives butan-2-ol, and elimination gives but-2-ene (major alkene) and but-1-ene. The question states butan-2-ol is the major alcohol.
- 2-Bromo-2-methylpropane: tertiary and highly hindered; SN2 is very slow. The tertiary carbocation can either be attacked by water/OH− to give 2-methylpropan-2-ol, or lose a β-proton to give 2-methylpropene. The alkene is the colourless gas that decolourises bromine water.
The tertiary bromide gives more elimination because the carbocation is stable and the β-hydrogen is accessible, while back-side attack by OH− is blocked by the three methyl groups. The more substituted alkene is also more stable, which favours E1/E2.
3. The student repeats the KOH reaction with 1-bromobutane but uses pure water instead of aqueous ethanol. Predict the effect on the rate and product distribution, and state whether the AgNO3 test would still be useful for predicting the outcome.
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1-Bromobutane is only slightly soluble in water and is much more soluble in ethanol–water mixtures. In pure water, the reaction mixture may become two-phase, so the effective contact between the halide and OH− is poor and the observed rate may fall even though water is more polar than ethanol–water. If the halide is dispersed well, the SN2 reaction still gives butan-1-ol, but the rate depends strongly on stirring and on the solubility of KOH and the halide. A polar protic solvent such as water also solvates OH− strongly, reducing its nucleophilicity compared with a less solvating solvent.
The AgNO3/ethanol test would not be a reliable predictor for the pure-water KOH reaction. The AgNO3 test uses Ag+ assistance and ethanol as solvent; it probes carbocation formation, whereas the KOH reaction with a primary halide is mainly SN2. The test can rank halides under SN1-like conditions, but it cannot be used to predict the rate or product distribution in a different solvent and with a strong nucleophile.