Chapter 8, Problem 28P

Interpretation Introduction

Interpretation:

The structures of the major organic products formed in the reaction of $\text{1-pentene}$ with each of the given reagents are to be written.

Concept introduction:

Alkenes are unsaturated hydrocarbons that react with substances that add to the double bond and they show addition reactions.

When an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogens, and the halogen adds to the carbon that has fewer hydrogens. This rule is called Markovnikov’s rule.

During hydroboration oxidation, hydrogen forms a bond with the carbon atom that has fewer hydrogens attached to it and the hydroxyl atom forms a bond with the carbon atom that has a greater number of hydrogens attached to it. This is a rule opposite to the Markovnikov’s addition.

Answer to Problem 28P

Solution:

Explanation of Solution

(a) Reaction of $\text{1-pentene}$ with hydrogen chloride.

The given alkene, $\text{1-pentene}$, is an unsymmetrical alkene. When $\text{1-pentene}$ reacts with hydrogen chloride, the major organic product formed is $\text{2-chloropentane}$.

Hydrogen chloride gets added to the double bond of $1-\text{pentene}$ in accordance with the Markovnikov’s rule. The hydrogen atom in hydrogen chloride adds to the carbon $\left(\text{C1}\right)$ that has a greater number of hydrogen atoms attached to it, while the chlorine adds to the carbon $\left(\text{C2}\right)$ that has a fewer number of hydrogen atoms attached to it.

(b) Reaction of $\text{1-pentene}$ with dilute sulfuric acid.

This reaction is an acid catalyzed electrophilic addition reaction of alkenes in which water molecule adds to the double bond in $\text{1-pentene}$. Acid-catalyzed hydration converts alkenes to alcohols and the addition takes place according to Markovnikov’s rule.

A molecule of water adds to the double bond of $\text{1-pentene}$ in accordance with the Markovnikov’s rule. The hydrogen atom in water molecule adds to the carbon $\left(\text{C1}\right)$ that has a greater number of hydrogen atoms, while the $\text{-OH}$ group adds to the carbon $\left(\text{C2}\right)$ that has a fewer number of hydrogen atoms.

The addition mechanism for this reaction follows the Markovnikov’s rule. Therefore, the major organic product for the above acid-catalyzed electrophilic addition reaction is $\text{2-pentanol}$.

(c) Reaction of $\text{1-pentene}$ with diborane in diglyme, followed by basic hydrogen peroxide

Hydroboration-oxidation leads to the overall hydration of an alkene. In hydroboration-oxidation, $\text{H}$ and $\text{OH}$ addition occurs in a syn fashion with a regioselectivity opposite to that of the Markovnikov’s rule.

The hydrogen atom in the water molecule adds to the carbon $\left(\text{C2}\right)$ that has a greater number of hydrogen atoms, while the $\text{-OH}$ group adds to the carbon $\left(\text{C1}\right)$ that has a fewer number of hydrogen atoms.

In case of $\text{1-pentene}$, this leads to the formation of $\text{1-pentanol}$ as the major product.

(d) Reaction of $\text{1-pentene}$ with bromine in carbon tetrachloride.

Bromine reacts rapidly with alkenes by electrophilic addition. The products are called vicinal dibromides, meaning that the bromine atoms get attached to adjacent double bonded carbon atoms. It is carried out in suitable solvents like ${\text{CCl}}_4{\text{ or CHCl}}_3$.

A molecule of bromine adds across the double bond in $\text{1-pentene}$ to form a $\text{1,2-dibromopentane}$ as a major organic product. It is a vicinal dibromide.

(e) Reaction of $\text{1-pentene}$ with bromine in water.

Chlorine and bromine react with alkenes in aqueous solution to give the corresponding vicinal halohydrins – compounds that add a halogen and hydroxyl group on adjacent carbon atoms in the alkene. The halogen atom forms a bond with that carbon atom in alkene, which has a greater number of hydrogen atoms, while the hydroxyl group bonds to that carbon atom in alkene, which has a fewer number of hydrogen atoms.

In the reaction of $\text{1-pentene}$ with bromine water, $\text{1-bromo-2-pentanol}$(vicinal bromohydrin) forms as a major organic product. The bromine atom gets attached to the C1 atom while the hydroxyl group gets attached to the $\text{C2}$ atom in $\text{1-pentene}$.

(f) Reaction of $\text{1-pentene}$ with peroxyacetic acid.

Peroxyacid transfers oxygen to the double bond of alkene to yield epoxides, which is a three-membered oxygen-containing ring.

When $\text{1-pentene}$ reacts with peroxyacetic acid, $\text{1,2-epoxypentane}$ is the major organic product, where the oxygen atom has formed a three-membered ring with both the carbon atoms in alkene.

(g) Reaction of $\text{1-pentene}$ with ozone.

Ozone is a powerful electrophile and reacts with alkenes to cleave the double bond between two oxygen atoms in the molecule, forming an ozonide.

When $\text{1-pentene}$ is treated with ozone, it forms the corresponding ozonide as shown in the above reaction. The three oxygen atoms in ozone form a five-membered ring with the two carbon atoms that contain the double bond in $1-\text{pentene}$.

(h) Product of part (g) treated with zinc in water

Ozonides are formed as a result of the reaction of ozone with an alkene. Ozonides undergo hydrolysis in water giving carbonyl compounds. Depending upon the structure of the starting alkene, various carbonyl compounds such as formaldehyde, aldehydes, or ketones are formed.

When corresponding ozonide of $\text{1-pentene}$ reacts further with zinc in the presence of water, two products are formed as major products — formaldehyde and butanaldehyde.

(i) Product of part (g) is treated with dimethyl sulfide.

Ozonides are formed as a result of the reaction of ozone with an alkene. Ozonides undergo hydrolysis in water, giving carbonyl compounds. Depending upon the structure of the starting alkene, various carbonyl compounds such as formaldehyde, aldehydes, or ketones are formed.

When corresponding ozonide of $\text{1-pentene}$ reacts further with dimethyl sulfide, two products are formed as major products — formaldehyde and butanaldehyde.