Question

3. Give the order of reactivity of the alkyl halides you reacted with aqueous silver nitrate. Does this ordering match what you would expect based on the alkyl group? Explain. Does this ordering match what you would expect based on leaving group? Explain. (To decide, you must compare only the three primary halides. Comparing a primary bromide with a tertiary chloride, for example, will not give a valid comparison of leaving groups as the substitution is also different.) 4. Give the order of reactivity of the alkyl halides you reacted with iodide in acetone. Does this ordering match what you would expect based on the alkyl group? Explain.

Answer 1

Pomparing halogenoalkane perd lies In this case, Various halogenoalkanes are treated cu a solution of silver nitrate in a mixture of ethanol and water . Nothing else is added. After varying lengths of time precipitates appear as halide ions. react with the silver ions prsent. As long as you are doing everything under controlled conditions, the & time taken gives a good guide to the reactivity of the halogenoalkanes - Lbe quither the precipitale appears, Id the more reactive the halogenoalkane. - You have to keep the type of halogenoalkan a constant, but vary the halogen. CH₂ - CH2 - CH2 - CHE CI CH₂ - CH CH CH B (Hz-CH-CH₂ - CH₂ - 1 The time taken for a precipitate of silvas halide to no appear will defend on how much of everything you use. . and the temperature at which the reaction is carried out •A Primary iodo compound produces a Precipitate quite. quickly • A Primary bromo compound takes longer to give a precipitate of Primary chloro compound probably won't give any Precipitate until well after you have lost interest. in the whole thing The order of reactivity reflects the strengths of the carbon-halogen bonds. The caston- iodine bond is the weakest and the carbon-chlorine the strongest. of the three bonds. In order for a halide ion to be produced, the carton-halogen bond has to be broken. The weaker the bond, the easier that is

If you have looked at the mechanisms for these. reactions, you will know that a lone pair on a water molecule attacks the slightly positive carbon atom. attached to the halogen. It is slightly positive. because most of the halogens are more electronegative i than caston, and so Pull electoons away from the Carbon- l It is tempting to think that will the reaction d be faster if the electroneantivity difference is greater larger The slight positive charge on the carton will be if it is attached to a chlorine atom than to an rodine That means that there will be more attraction- between a lone Pair on the water and a carbon atom attached to a chlorine atom than if it was attached to an lodine atom. The electronegativity difference between carton and iodine is negligible However, the fastest reaction is with an odoalkane. In these reactions, bond strength is the main factor deciding the relative rates of reaction. The finkelstein reaction entails the convertion of an alkyl chloride of an alkyl boomide to an. alkyl iodide by treatment with a solution of sodium iodide in acetone. sodium iodide is soluable in acetone while su di um chloride and sodium bromide are not. The reaction is driven toward Products by mass action due to the precipitation of the Poorly soluble Nall or NaBr-.

Alkyl halides differ greatly in the ease with. which they undergo the finkelstein reaction. The reaction works well for primary lecicept for neopentuj] halides, and exceptionally well for allyl, benzyl and a-carbonyl whalides. Secondary halides are for Less reactive - vinyl, aryl and testiary alkyl halides are unreactive, as a result, the reaction of Nai in acetone can be used as a quantitative test to determine which of the aforementioned classes an. unknown alkyl halide belongs to, with the exception of alkyl iodides, as they held the same product upon. substitution