4.2.2 (d) Trend in the rates of Hydrolysis of Primary Haloalkanes

Syllabus

(d) Explanation of the trend in the rates of hydrolysis of Primary Haloalkanes in terms of the bond enthalpies of Carbon–Halogen bonds (C–F, C–Cl, C–Br and C–I)

What does this mean?

Hydrolysis is a reaction in which water breaks - although in this case we are still talking about the reaction in which OH^- ions substitute Halogen atoms in Haloalkanes.

By adding Silver Nitrate solution to the OH^-/Haloalkane mixture we can time the reactions and compare the rate of this Hydrolysis.

There are two trends to consider when thinking of Carbon-Halogen bonds.

If the the polarity of the bond is the important factor then Chloroalkanes should precipitate before Bromoalkanes, which would be faster than Iodoalkanes because the more polar the bond the more the nucleophiles would be attracted.

If the strength of the bond is the most important then the weakest C-X bond should break fastest and Iodoalkanes should react faster than Bromoalkanes, with Chloroalkanes reacting most slowly.

And this is exactly what happens.

Carbon-Fluorine bonds are so strong that they would not easily break and so hydrolysis would be very unlikely to happen at all.

And since Fluoride ions don't precipitate with Silver Nitrate we wouldn't be able to time it this way.

Video

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