What is the result of the reaction methylcyclohexanol + HCl?

Answer

Hi Kristel,

A universal step-by-step plan for the many thousands of organic reactions is not possible, the properties of organic substances vary too much for that. A good understanding of organic chemistry presupposes a good knowledge of the properties of molecules, and how those properties are influenced by electronic effects.

Let’s take a look at this reaction. We look at the properties of the different reaction partners. Hydrogen chloride, HCl, is a strong acid. So it will tend to donate a hydrogen ion. At the reaction partner we are now looking for an atom (group) that has the ability to absorb this hydrogen ion. The only group that has this ability is the -OH group. A hydrogen ion can be taken up via a free electron pair of oxygen. This is shown in figure 1. I also note that an alcohol is an extremely weak base, the pKa of the conjugate acid of an alcohol is about -2 to -3. Nevertheless, this is sufficient to absorb the hydrogen ion from HCl, because the pKa of HCl is about -7. (I assume you know Brönsted’s acid-base theory, know what a pKa value of an acid is, and where is the location of an acid-base equilibrium).

The result is now that the alcohol group has been converted into an oxonium ion. The pKa of its conjugate acid, H3O+ is -1.74 which is much less than the pKa of its conjugate acid of the -OH group, H2O with a pKa of 15.74. This makes H2O a very good flight group, unlike OH- which is a bad flight group. Thus, in the second step of the reaction, H2O is split off and a carbocation (a positively charged carbon atom) is formed. This here is a tertiary carbocation, which means that the positively charged carbon atom is bonded to three other carbon atoms. Carbon atoms have a slight electron donating effect, and this reduces the positive charge on the carbocation. This stabilizes it.

In the next step, a base present, and in this case H2O, will take up a hydrogen ion from a carbon atom next to the carbocation. This is shown in step 3, figure 3. The choice of carbon atom is governed by Zaitsev’s rule: “The hydrogen ion is taken from the carbon atom with the fewest hydrogen atoms” (“the poor get poorer”). The double bond is formed simultaneously with the bonding electrons of hydrogen.

Thus, of the two possible alternatives presented in Figure 4, only the left product, 1-methylcyclohexene, is formed. The right product is not formed.

As you can see, this reaction can be explained perfectly logically.