If we replace the substitution of n-butyl bromide with a substitution of t-butyl bromide, we get the reaction shown in the margin. It turns out that, kinetically, this reaction is first order: its rate depends only on the concentration of tert-BuBr—it doesn’t matter how much hydroxide you add: the rate equation is simply , rate = k t 1 [-BuBr] .

The reason for this is that the reaction happens in two steps: first the bromide leaves, to generate a carbocation, and only then does the hydroxide ion move in to attack, forming the alcohol.

In the SN1 mechanism, the formation of the cation is the rate-determining step. This makes good sense: a carbocation is an unstable species and so it will be formed slowly from a stable neutral organic molecule. But once formed, being very reactive, all its reactions will be fast, regardless of the nucleophile. The rate of disappearance of t-BuBr is therefore simply the rate of the slow first step: the hydroxide nucleophile is not involved in this step and therefore does not appear in the rate equation and hence cannot affect the rate. If this is not clear to you, think of a crowd of people trying to leave a railway station or a football match through some turnstiles. It doesn’t matter how fast they walk, run, or are driven away in taxis afterwards; it is only the rate of struggling through the turnstiles that determines how fast the station or stadium empties.

Once again, this rate equation is useful because we can determine whether a reaction is SN1 or SN2. We can plot the same graphs as we plotted before. If the reaction is SN2, the graphs look like those we have just seen. But if it is SN1, the graphs in the margin show what happens when we vary [t-BuBr] at constant [NaOH] and then vary [NaOH] at constant [t-BuBr]. The slope of the fi rst graph is simply the first-order rate constant because rate = k1[t-BuBr]. But the slope of the second graph is zero. The rate-determining step does not involve NaOH so adding more of it does not speed up the reaction. The reaction shows first-order kinetics (the rate is proportional to one concentration only) and the mechanism is called SN1, that is, Substitution, Nucleophilic, 1st order. This observation is very significant. The fact that the nucleophile does not appear in the rate equation means that not only does its concentration not matter—its reactivity doesn’t matter either! We are wasting our time opening a tub of NaOH to add to this reaction—water will do just as well. All the oxygen nucleophiles in the table above react at the same rate with t-BuBr although they react at very different rates with MeI. Indeed, SN1 substitution reactions are generally best done with weaker, non-basic nucleophiles to avoid the competing elimination reactions discussed in Chapter 17.

● The rate of an SN1 reaction depends on: • the carbon skeleton • the leaving group along with the usual factors of temperature and solvent. But NOT the nucleophile.

مواضيع ذات صلة

A closer look at steric effects

Adjacent C=C or C=O π systems increase the rate of SN2 reactions

An adjacent C=C π system stabilizes a carbocation: allylic and benzylic carbocations

Alkyl substituents stabilize a carbocation

Shape and stability of carbocations

A closer look at the SN1 reaction

? How can we decide which mechanism (SN1 or SN2) will apply to a given organic compound

Significance of the SN2 rate equation

Mechanisms for nucleophilic substitution

Resolutions using diastereoisomeric salts

Separating enantiomers is called resolution

Chiral compounds with no stereogenic centres