Reaction Parameters – Haloalkanes

00:00 Let’s have a look at the reaction parameters.

00:04 Nucleophiles - atoms, ions or molecules that have an electron pair that may be donated in forming a covalent bond to an electrophile (or a Lewis acid). They have a negative charge, partial negative charge, an electron pair or pi electrons available.

00:23 Let’s have a look at some examples of nucleophiles used in SN reactions. Negative ones include our friend hydroxide, methoxide, it’s the CH3O-, cyanide (CN-), NH2- and dicarbide.

00:39 Neutral nucleophiles include water, methanol and ammonia, but are not restricted; there are a great many others. It’s possible to classify a species in order of its nucleophilicity. That is to say how willing it is to donate it’s electrons in the formation of a new bond. The most reactive nucleophiles are therefore said to be more nucleophilic than those less reactive ones. In general, for a given element, negatively charged species are more nucleophilic - hydroxide, cyanide, even sometimes, other halides.

01:18 Nucleophilicity decreases on moving from left to right in a period of the periodic table and nucleophilicity increases from top to bottom i.e. with increasing size, along a group of the periodic table (basicity varies, however, in the opposite manner).

01:36 Thus, if you look at the bottom of the board, you’ll see the order of the nucleophilicity - nitrile or cyanide being the highest, followed by iodide, methoxide, hydroxide, bromide, chloride, ammonia, fluoride, acetate, alcohol and water; the alcohol, of course, being able to beat methanol in this... in this case. Let’s also have a look at the solvent effects.

02:05 Polar protic solvents solvate the anion that is produced, thus decreasing its nucleophilicity.

02:13 Polar aprotic solvents such as dimethylsulphoxide or dimethylformamide solvate the counter-ion, thus leaving the anion more naked and therefore, more nucleophilic. So, in the case of an SN2 reaction, polar aprotic solvents are the best to achieve the highest degree of nucleophilicity for your given nucleophile. In different solvents, the order of... the order of nucleophilicity can indeed change slightly... slightly as a direct consequence of this.

02:45 Let’s have a look at the alkyl group. The degree of substitution, as we saw in the case of the alkanes, in the previous lecture, can have a marked influence on the reactivity of a haloalkane and the degree of substitution on the carbon has a great effect on whether it’s SN2 or as we will see, SN1. The order of reactivity is thus in terms of an SN2 reaction and let me talk you through it. In the case of an SN2 reaction, what you’re dependent upon is a nucleophile directly attacking something which has a leaving group, in this case, a halide that wishes to leave. When you have something like a methyl or primary alkyl group as part of your haloalkane, these are sterically less congested. With that we’re talking about just the idea that hydrogen is a small atom and it doesn’t get in the way of the reaction. If, on the other hand, we have three alkyl groups such as in the case of our tertiary, there is insufficient space for a nucleophile to be able to attack effectively and therefore, carry out its bimolecular SN2 exchange of electrons.

03:55 Now, let’s quickly have a look in the context of an SN2 reaction of the leaving group. Bearing in mind that the leaving group, in this case our halide, is also technically a nucleophile.

04:07 It has a negative charge and it may actually want to react again. Conjugate bases of a strong acid are usually good leaving groups, so things like iodide, chloride and bromide, although fluoride, less so.