Chapter 22, Problem 22.74AP

Interpretation Introduction

(a)

Interpretation:

The curved arrow mechanism, for base catalyzed exchange of hydrogen ${\text{H}}^{\text{a}}\text{,}{\text{H}}^{\text{c}}\text{,}{\text{H}}^{\text{d}}$ is to be drawn.

Concept introduction:

A carbonyl group has $-\text{I}$ effect. The $\alpha -$ hydrogen of carbonyl compound is acidic in nature. This is because of presence of $-\text{C}=\text{O}$ group in the vicinity, it forms resonating enolate ion. When a base is present in the reaction medium, this $\alpha -$ hydrogen can easily be replaced by other atoms, acidic in nature.

Answer to Problem 22.74AP

The curved arrow mechanism for base catalyzed exchange of hydrogen ${\text{H}}^{\text{a}}\text{,}{\text{H}}^{\text{c}}\text{,}{\text{H}}^{\text{d}}$ is given below.

Explanation of Solution

The structure of compound is given below.

Figure 1

When a base is present in the medium, it easily removes the $\alpha -$ hydrogen from the carbonyl compound because of $-\text{I}$ effect of carbonyl group. Here ${\text{H}}^{\text{a}}\text{,}{\text{H}}^{\text{c}}\text{,}{\text{H}}^{\text{d}}$ are at $\alpha -\text{C}$ of a double bond. When this $\alpha -$ hydrogen is removed by a base, molecule gives resonating structures of enolate ion. Enolate ion has one double bond of ene with a carbonyl double bond. This resonating structure stabilizes the ion. Now any acidic group or atom can replace the $\alpha -$ hydrogen. The hydrogen ${\text{H}}^{\text{a}}\text{,}{\text{H}}^{\text{c}}\text{,}\text{or}{\text{H}}^{\text{d}}$ given is exchanged for the deuterium by given mechanism. This is shown, with curved arrow mechanism, below.

Figure 2

Conclusion

The base catalyzed exchange of hydrogen ${\text{H}}^{\text{a}}\text{,}{\text{H}}^{\text{c}}\text{,}{\text{H}}^{\text{d}}$ with deuterium is easy, as these hydrogen atoms are acidic. The reaction mechanism for the same is shown in Figure 2.

Interpretation Introduction

(b)

Interpretation:

The less acidic character of $\alpha -\text{H}$, ${\text{H}}^{\text{b}}$, in comparison to other $\alpha -$ hydrogen’s ${\text{H}}^{\text{a}}\text{,}{\text{H}}^{\text{c}}\text{,}{\text{H}}^{\text{d}}$, is to be explained.

Concept introduction:

The $\alpha -$ hydrogen of carbonyl compound is acidic in nature. This is because of the presence of adjacent $-\text{C}=\text{O}$ group. If double bonds or electron donating groups are present in the chain then it affects acidity of $\text{α}-$ hydrogen. This is because; it increases the electron density of the said carbon.

Answer to Problem 22.74AP

The proton, ${\text{H}}^{\text{b}}$, is on the $\alpha -$ carbon bonded with one more double bond. So, this hydrogen is less acidic in nature.

Explanation of Solution

The $\alpha -\text{H}$, ${\text{H}}^{\text{b}}$, in the molecule is less acidic than hydrogen ${\text{H}}^{\text{a}}\text{,}{\text{H}}^{\text{c}}\text{,}{\text{H}}^{\text{d}}$. This hydrogen is on the $\alpha -$ carbon bonded with a double bond. The electron density on this carbon is more. The hydrogen atom is bonded to this high electron density carbon, cannot be taken up by base, leaving the electron pair on carbon. Therefore, formation of enolate ion is not feasible. So this hydrogen is less acidic in nature. This is shown in figure given below.

Figure 3

Conclusion

The $\alpha -$ hydrogen is acidic in nature. When this is bonded to carbon having high electron density, is not easily removable, even in the presence of a base as shown in Figure 3.

Interpretation Introduction

(c)

Interpretation:

The mechanism for exchange of ${\text{H}}^{\text{b}}$, in presence of a base in given equilibrium is to be explained.

Concept introduction:

In presence of the acid or base, the enone compounds remain in equilibrium with their isomers. Due to this, acidity of $\text{α}-$ hydrogen varies. If $\text{α}-$ hydrogen is on the carbon with single bonds, it is more acidic and can be exchanged. When the carbon is having double bond or electron releasing groups, the acidity of $\text{α}-$ hydrogen, decreases accordingly.

Answer to Problem 22.74AP

The isomer $3-$ cyclohexenone, has $\alpha -$ hydrogen on the carbon not having a double bond. So in presence of a base it is removable and can be exchanged with deuterium.

Explanation of Solution

The equilibrium of isomers, $2-$ cyclohexenone and $3-$ cyclohexenone, is given in the figure below.

Figure 4

The $\text{α}-$ carbon in $2-$ cyclohexenone is bonded with a double bond, so it has high electron density. Therefore, hydrogen on this carbon is less acidic. On the other hand, in $3-$ cyclohexenone, the $\alpha -$ carbon is bonded with all single bonds. Here the removal of oxygen is easy. So, formation of enolate ion is possible in $3-$ cyclohexenone, but not in $2-$ cyclohexenone.

The reaction mechanism is shown in figure below.

Figure 5

Conclusion

The $\alpha -\text{H}$, ${\text{H}}^{\text{b}}$, can be removed in presence of a base, as it shows acidic character in $3-$ cyclohexenone. In $2-$ cyclohexenone this does not happen, as enolate ion cannot be formed.

Interpretation Introduction

(d)

Interpretation:

The hydrogen that is replaced by deuterium in sex hormone, testosterone is to be identified.

Concept introduction:

The $\alpha -$ hydrogen of carbonyl compound is acidic in nature. This is because of presence of $-\text{C}=\text{O}$ group in the neighbor. A carbonyl group has $-\text{I}$ effect. It forms resonating enolate ion, in presence of a base in the reaction medium. This $\alpha -$ hydrogen can easily be replaced by other atoms or groups, acidic in nature.

Answer to Problem 22.74AP

The hydrogen at position “a” in vicinity to carbonyl carbon can be exchanged for deuterium in ${\text{CH}}_3{\text{O}}^{-}/{\text{CH}}_3\text{OD}$ as shown below.

Explanation of Solution

The structure of sex hormone testosterone is given below.

Figure 6

The hydrogen present at position “a” can be exchanged with deuterium, in sex hormone testosterone. This hydrogen, being at $\text{α}-$ position is acidic in nature. This can easily form enolate ion with carbonyl group, being in the vicinity. Now this hydrogen can be exchanged as in ${\text{CH}}_3{\text{O}}^{-}/{\text{CH}}_3\text{OD}$.

The mechanism for the same can be shown as given below.

Figure 7

Conclusion

The $\alpha -$ hydrogen to carbonyl carbon can be replaced with deuterium. As it is acidic in nature and presence of a base facilitates the reaction.