Let's take the simplest unsymmetrical and unsaturated hydrocarbon and the simplest symmetrical and unsaturated hydrocarbon. They will be respectively propene and butene-2. These are alkenes, and they like to undergo addition reactions. 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 attach - they are all equivalent. And in the case of propene, two options are possible: 1-bromopropane and 2-bromopropane. However, it was experimentally proved that 2-bromopropane noticeably predominates 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 the rule, which is called by his name.
Markovnikov's rule
Applies to unsymmetrical alkenes and alkynes. When water or hydrogen halides are attached to such molecules, their hydrogen is sent to the most hydrogenated carbon atom in the double bond (that is, the one that contains the most carbon atoms to itself). This works for the last propene example: the central carbon atom carries only one hydrogen, and the onethat on the edge - as many as two, so hydrogen bromide clings to the extreme carbon atom with hydrogen, and bromine to the central one, and 2-bromopropane is obtained.
Of course, the rule is not woven out of thin 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 takes place in several stages. It starts with an organic molecule being attacked by a hydrogen cation (a proton, in general); it attacks one of the carbon atoms in the double bond, because the electron density there is increased. A positively charged proton is always looking for regions with an increased electron density, therefore it (and other particles that behave in the same way) is called an electrophile, and the reaction mechanism, respectively, is an electrophilic addition.
A proton attacks the molecule, penetrates into it, and a positively charged carbonium ion is formed. And here, just the same, there is an explanation for Markovnikov's rule: the most stable of all possible carbcations is formed, and the secondary cation is more stable than the primary, the tertiary is more stable than the secondary, and so on (there are many more ways to stabilize the carbcation). And then everything is easy - a negatively charged halogen, or an OH group is attached to a positive charge, and the final product is formed.
If at first some inconvenient carbocation was suddenly formed, it can rearrange so that it is convenient and stable (an interesting effect is associated with this, that sometimes during such reactions the added halogen or hydroxyl group ends up on another atom altogethercarbon that did not have a double bond, simply because the positive charge in the carbocation shifted to the most stable position).
What can affect the rule?
Because it is based on the distribution of electron density in the carbocation, various kinds of substituents in the organic molecule can influence. For example, a carboxyl group: it has oxygen hooked to carbon through a double bond, and it pulls the electron density from the double bond to itself. Therefore, in acrylic acid, a stable carbocation is at the end of the chain (away from the carboxyl group), that is, one that would be less beneficial under normal conditions. This is one example where the reaction goes against Markovnikov's rule, but the general mechanism of electrophilic addition is preserved.
Peroxide Harash effect
In 1933, Morris Harash carried out the same reaction of hydrobromination of unsymmetrical alkenes, but in the presence of peroxide. And again, the reaction products contradicted Markovnikov's rule! The Kharash effect, as it was later called, consisted in the fact that in the presence of peroxide, the entire reaction mechanism changes. Now it is not ionic, as before, but radical. This is due to the fact that the 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 the carbocation, is more stable - secondary, so bromine itself is at the end of the chain.
Hereapproximate description of the Kharash 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 connections has its own reasons.
In hydrogen chloride, the bond between hydrogen and chlorine is quite strong. And if in radical reactions initiated by temperature and light there is enough energy to break it, the radicals formed during the decomposition of peroxide are practically incapable of doing this, and the reaction with hydrogen chloride is very slow due to the peroxide effect.
In hydrogen iodine, the bond breaks much more easily. However, the iodine radical itself turns out to have an extremely low reactivity, and the Harash effect again almost does not work at all.
