Take the simplest asymmetric and unsaturated hydrocarbon and the simplest symmetric and unsaturated. They will be propene and butene-2, respectively. These are alkenes, and they like to enter into reactions of accession. Let, for example, it be the addition of hydrogen bromide. In the case of butene-2, only one product is possible - 2-bromobutane, to which of the carbon atoms bromine would not join - they are all the same. And in the case of propene, two options are possible: 1-bromopropane and 2-bromopropane. However, it was experimentally proved that 2-bromopropane significantly prevails in the products of the hydrohalogenation reaction. The same is true for the hydration reaction: propanol-2 will be the main product.
To explain this pattern, Markovnikov formulated a rule, which is called by his name.
Markovnikov Rule
Extends to asymmetric alkenes and alkynes. When water or hydrogen halides are attached to such molecules, their hydrogen is sent to the most hydrogenated carbon atom in a double bond (that is, to the one that contains most of the carbon atoms with it). This works in the last example with propene: the central carbon atom has only one hydrogen with it, and the one with the edge has two, so hydrogen bromide clings to the extreme carbon atom, and bromine to the central one, and 2-bromopropane is obtained.
Of course, the rule is not woven from the air, and there is a normal explanation for it. However, this will require a more detailed study of the reaction mechanism.
Addition Reaction Mechanism
The reaction proceeds in several stages. It begins with the fact that an organic molecule is attacked by a hydrogen cation (proton, in general); he attacks one of the carbon atoms in a double bond, because the electron density there is increased. A positively charged proton is always looking for regions with increased electron density, therefore it (and other particles behaving in the same way) is called an electrophile, and the reaction mechanism, respectively, is an electrophilic attachment.
A proton attacks a molecule, penetrates into it, and a positively charged carbonium ion forms. And here, just the same, there is an explanation of the Markovnikov rule: the carbcation is formed the most stable of all possible, and the secondary cation is more stable than the primary, tertiary is more stable than the secondary and so on (there are many more ways to stabilize the carbation). And then everything is easy - a negatively charged halogen, or the OH group joins a positive charge, and the final product is formed.
If at first some uncomfortable carbcation formed, it can rearrange in a way that is convenient and stable (this has such an interesting effect that sometimes during such reactions the attached halogen or hydroxyl group appears in general at another carbon atom that has nothing to do with double bond, simply because the positive charge in the carbocation has shifted to the most stable position).
What can affect the rule?
Since it is based on the distribution of electron density in the carbocation, various kinds of substituents located in the organic molecule can influence. For example, a carboxyl group: there is oxygen in it that is attached to carbon through a double bond, and it pulls the electron density from the double bond onto itself. Therefore, in acrylic acid, a stable carbcation is at the end of the chain (away from the carboxyl group), that is, such that under normal conditions it would be less advantageous. This is one example where the reaction goes against the Markovnikov rule, however, the general mechanism of electrophilic addition remains.
Harash Peroxide Effect
In 1933, Morris Harash conducted the same hydrobromination reaction of asymmetric alkenes, but in the presence of peroxide. And again, the reaction products contradicted Markovnikov’s rule! The Kharash effect, as it was later called, was that in the presence of peroxide the whole reaction mechanism changes. Now it is not ionic, as before, but radical. This is due to the fact that peroxide itself first breaks down into radicals, which give rise to a chain reaction. Then a bromine radical is formed, then an organic molecule with bromine. But the radical, like carbcation, more stable is secondary, therefore bromine itself appears at the end of the chain.
Here is a rough description of the Harash effect in chemical reactions.
Selectivity
It is worth mentioning that this effect only works when hydrogen bromide is added. With hydrogen chloride and hydrogen iodide, nothing of the kind is observed. Each of these compounds has its own reasons.
In hydrogen chloride, the bond between hydrogen and chlorine is strong enough. And if in radical reactions initiated by temperature and light, there is enough energy to break it off, the radicals formed during the decomposition of peroxide are practically incapable of this, and the reaction with hydrogen chloride due to the peroxide effect is very slow.
In iodine hydrogen, the bond breaks much easier. However, the iodine radical itself appears to have extremely low reactivity, and the Harash effect again almost completely does not work.