Chapter 24, Problem 24.35P

(a)

Interpretation Introduction

Interpretation:

The function of the sodium hydride has to be determined in the 1^st step along with the ${\text{pk}}_{\text{a}}$ value of the cyclopentadiene with the reason for its remarkable acidity.

Concept introduction:

$p{K}_a$:

$p{K}_a$ is the negative log of acid dissociation constant that determines the strength of the acid. Lower the $p{K}_a$ value stronger is the acid.

${\text{pK}}_{\text{a1}}$ is the 1^st dissociation constant for the acid and ${\text{pK}}_{\text{a2}}$ is the 2^nd dissociation constant of the acid.

Aromaticity:

Aromaticity is a property of cyclic, planar structures with a ring of resonance bonds that gives increased stability compared to the other geometric or connective arrangements with the same set of atoms. Aromatic molecules are very stable and do not easily break up to take part in reactions.

There are some rules for a compound to be aromatic i.e. they have to be planar, cyclic and must contain $\left(\text{4n}\text{+}\text{2}\right)\text{π}$ electrons where n is integer.  These rules are called Huckel’s rule of aromaticity.

Explanation of Solution

In the given question the 1^st step is,

The mechanism as follows,

Here $\text{NaH}$ acts as base and abstracts the acidic hydrogen from cyclopentadiene.  The driving force for this step is the aromaticity.  On loosing on hydrogen cyclopentadienyl anion is formed which is cyclic, planar and having $\text{6π}$ electrons i.e. $\left(\text{4n}\text{+}\text{2}\right)\text{π}$ electrons and thus cyclopentadienyl becomes aromatic and attains extra stability.  This is the reason for the acidity of cyclopentadiene molecule.

The $p{K}_a$ value of the cyclopentadiene is $\text{16}$ which less than other hydrocarbons.  This is due to the aromaticity of the cyclopentadienyl anion.

(b)

Interpretation Introduction

Interpretation:

The reaction pathway by which B is converted to C that has to be identified.

Concept introduction:

Diels Alder reaction:

The Diels-Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene which is called dienophile to form a substituted cyclohexene derivative.  It is one type of pericyclic reaction with a concerted mechanism and more specifically it is thermally allowed $[4+2]$ cycloaddition.  Dienes attached to some electron donating group are good HOMO and dienophile with electron withdrawing group are good LUMO.  The mechanism is as follows,

Explanation of Solution

According to the question,

Here B is the diene and the dienophile consists of electron withdrawing groups.

Thus via Diels Alder reaction the product is obtained.

(c)

Interpretation Introduction

Interpretation:

The role of the carbon dioxide added to the reaction mixture in step 2 of the conversion of (E) to (F) has to be determined.

Concept introduction:

Lactone formation:

Lactones are cyclic carboxylic esters  that are formed by intramolecular esterification of the corresponding hydroxycarboxylic acids which takes place spontaneously when th ering that is formed is five or six membered.  The reaction pathway is as follows,

Explanation of Solution

The reaction given is,

In this reaction in the presence of base lactone is hydrolised to give back acid and alcohol.  Now if the amount of base is more in medium then again the backward reaction will be favoured and lactone will be formed again.  So the reactant will be formed again.  Hence to maintain the $\text{pH}$ of the solution carbon dioxide is added.

Carbon dioxide reacts with water to give mild amount of carbonic acid that dissociates in water to a little extent to give less amount of proton which is sufficient to balance the $\text{pH}$.  If amount of proton is more then all the base will neutalise and hyrolises of lactone will not be possible.  Hence mild carbon dioxide is used.

  $\begin{array}{l}{\text{CO}}_{\text{2}}\text{+}{\text{H}}_{\text{2}}\text{O}\stackrel{}{\rightleftarrows }{\text{H}}_{\text{2}}{\text{CO}}_{\text{3}}\\ \\ {\text{H}}_{\text{2}}{\text{CO}}_{\text{3}}\stackrel{}{\rightleftarrows }{\text{H}}^{\text{+}}\text{+}{\text{HCO}}_{\text{3}}^{\text{-}}\end{array}$

(d)

Interpretation Introduction

Interpretation:

A reaction pathway has to suggest for the conversion of (H) to (I) via radical mechanism.

Concept introduction:

Radical reaction:

A free radical reaction is a chemical reaction involving free radicals.  Radical reaction contain three steps that are respectively chain initiation, chain propagation and chain termination.

Explanation of Solution

The reaction given,

According to the question, the reaction proceeds via radical pathway and the 1^st step involves a radical initiator to form ${\text{Bu}}_{\text{3}}\text{Sn•}$ radical.  The mechanism is as follows,

As a radical initiator AIBN is used.  The reaction undergoes via two steps which are initiation and propagation respectively.  Initiation step involves a radical initiator to form ${\text{Bu}}_{\text{3}}\text{Sn•}$ radical and these radical further breaks the carbon iodine bond forming new radical that again abstracts hydrogen from ${\text{Bu}}_{\text{3}}\text{SnH}$ to form new ${\text{Bu}}_{\text{3}}\text{Sn•}$ radical.

(e)

Interpretation Introduction

Interpretation:

The steps in which the chiral centers of Corey lactone can be determined has to be shown with the mechanism.

Concept introduction:

Chiral centre:

Chiral centre is defined as an atom bonded to four different chemical species.  It is a stereo centre that holds the atom in such way that the structure may not be superimposable to its mirror image.  They give optical isomerism.

Explanation of Solution

In the question it is given that the Corey lactone has four chiral centers which are pointed below,

Now the steps where the chiral centers can be distinguished are given below,

In this step due to hydrolisation of lactone, ring opening occurs along with the formation of acid and alcohol.

In this step normal ${\text{S}}_{\text{N}}{}^{\text{2}}$ occurs followed by neighboring group participation via the formation of cyclo-iodonium ring.

In this step normal acid base reaction occurs followed by normal ${\text{S}}_{\text{N}}{}^{\text{2}}$ and the alcohol group is protected.

Via radical mechanism this step occurs to break the carbon-halogen bond.

Here the ether is converted to alcohol.

Here alcohol is oxidized to aldehyde with chromic oxide.

In all these steps the chiral centres can be distinguished.

(f)

Interpretation Introduction

Interpretation:

The definition of resolution has to be given along with the rationale for using a chiral, enantiomerically pure amine for the resolution.

Concept introduction:

Resolution:

Chiral resolution in stereochemistry is a process for the seperation of racemic compounds into their enantiomers.  Now racemic mixture is the mixture that has equal amounts of left and right handed enantiomers of the chiral molecule.

Explanation of Solution

According to the question the compound F formed is resolved by (+)-ephedrine.  The structures of compound (F) and (-)-ephedrine are given below,

The process of racemisation follows acid base reaction between compound F and (+)-ephedrine.  The two enantiomers can be converted to two diastereomeric salts those are [(+)(+)] and [(-)(+)].  Now being diastereomers they can be easily separated as diastereomers have different physical properties.  Thus racemisation process works to separate two enantiomers.

One of the salts is shown below,

(g)

Interpretation Introduction

Interpretation:

The mechanism for the reaction pathway from D to E that undergoes via Baeyer-Villiger oxidation has to be shown.

Concept introduction:

Baeyer-Villiger oxidation:

The Baeyer-Villiger oxidation is an organic rearrangement reaction that forms ester from a ketone or a lactone (cyclic ketone) using peroxides or peroxyacids as the oxidant.  This is carbon to oxygen migration rearrangement.  The mechanism is as follows,

Explanation of Solution

The reaction given in the question is,

The mechanism is as follows,

This is the pathway for D to E via Baeyer-Villiger oxidation.  The process starts with acid base reaction followed by ${\text{S}}_{\text{N}}{}^{\text{2}}$ reaction.  The major step is the carbon to oxygen migration and thus the ester is formed from the ketone.