Preparation of Alcohols: Different Methods and Reactions

Walk into a hardware store and be greeted by the acrid smell of paint thinners, many of which are haloarenes. These chemicals find effective use in most of the industrial set-ups and go into many syntheses for the production of pharmaceutical products, pesticides, and polymers. Haloarenes represent very important substances that find wide application both in modern chemistry and industry. Haloarenes or aryl halides are aromatic compounds in which one or more hydrogens in the ring have been replaced by the halogen atoms chlorine, bromine, fluorine, or iodine. These compounds are of structural importance and have some very special chemical properties as a result of these halogen atoms. The electronic environments in the ring are disposed around the halogens in the haloarenes, and they display some reactivity patterns quite useful in synthetic organic chemistry.

Haloarenes

Haloarenes, otherwise known as aryl halides, are defined as aromatic compounds in which a certain number of hydrogen atoms in the aromatic ring have been replaced by any of the halogen atoms, such as fluorine, chlorine, bromine, or iodine. More explicitly, a bijou example would be chlorobenzene or bromobenzene. The halogen atoms of a haloarene are held directly to the carbon atoms of the aromatic ring, rendering them very different chemically—in properties—from their aliphatic relations.

It is the electronegativity of the halogen atoms that most strikingly influences the chemical reactivity of haloarenes. The electronegative atoms in the halo group remove electron density from the aromatic ring by the inductive effect, leading to polarization of the carbon-halogen bond. Logical inferences can thus be made about this reaction, such that the polarized bond in haloarenes would be able to participate in nucleophilic substitution reactions, though the aromatic ring itself is relatively stable against electrophilic attack. The important haloarene compounds thus form basic building blocks from which to work in organic synthesis: a wellspring of possible transformations that may eventually lead to the manipulation of higher functionalities.

Haloarene Reactions

Grignard Reagent — 1: Formation

Of the most important reactions of haloarenes forms Grignard reagent. Grignard reagents are organomagnesium compounds prepared by the reaction of haloarenes with magnesium in dry ether. For Example, in the reaction of Bromobenzene or bromomethane with magnesium, phenylmagnesium bromide is obtained. This phenylmagnesium bromide remarkable intermediate of organic synthesis is prepared in the following reaction:
C₆H₅{Grignard reagents are very nucleophilic and react to a variety of electrophiles to make carbon-carbon linkages necessary in putting together more complicated organic structures.

All three types of monohydric alcohols can be prepared by the use of Grignard reagents. Grignard reagents form additional compounds by nucleophile attack with aldehydes and ketones which on hydrolysis with dilute acid yields alcohol.

Mechanism

For example:

Grignard Reagent - 2: Reactions

Grignard reagents derived from haloarenes are thus quite versatile reagents in synthetic chemistry. They react with most carbonyl groups, such as aldehydes and ketones, in which process the alcohols are then produced. One example is the interaction of phenylmagnesium bromide with formaldehyde to form benzyl alcohol:

$\mathrm{C} 6 \mathrm{H}_ 5 \mathrm{MgBr}+\mathrm{H}_ 2 \mathrm{CO} \rightarrow \mathrm{C}_ 6 \mathrm{H}_ 5 \mathrm{CH}_ 2 \mathrm{OH}$

This is one of the most classical ways of synthesizing alcohols, namely the broad utility of Grignard reagents from haloarenes.

Alcohols are produced by the reaction of Grignard reagents with aldehydes and ketones. The first step of the reaction is the nucleophilic addition of Grignard reagent to the carbonyl group to form an adduct. Hydrolysis of the adduct yields alcohol.

Mechanism

Anhydrides are formed by heating of two $(-\mathrm{COOH})$ groups to remove $\left(\mathrm{H}_2 \mathrm{O}\right)$ molecule. Aldehydes, ketones, carboxylic acids, and derivatives on reduction yield alcohols. A number of reducing agents link $\mathrm{Zn} / \mathrm{HCl}, \mathrm{Na} / \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}, \mathrm{LiAlH}_4$ or NaBH₄ can be used for this purpose. These derivatives are reduced by nascent hydrogen into corresponding alcohols.

Some examples include,

$\left(\mathrm{CH}_3 \mathrm{CO}\right)_2 \mathrm{O}+4 \mathrm{H} \xrightarrow[\text { ether }]{\mathrm{LiAlH}_4} \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{OH}+\mathrm{CH}_3 \mathrm{COOH}$

$\mathrm{CH}_3 \mathrm{COOC}_2 \mathrm{H}_5+4 \mathrm{H} \xrightarrow[\text { or } \mathrm{LiAlH}_4]{\mathrm{Na} / \mathrm{C}_5 \mathrm{H}^2} 2 \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{OH}$

LiAlH₄ Reduction

Haloarenes can also be reduced to form simpler hydrocarbons. Reduction of haloarenes is done using reducing agents like Lithium aluminum hydride and sodium borohydride. Lithium aluminum hydride is a stronger reducing agent and is used to reduce haloarene to form the corresponding hydrocarbon. For example, chlorobenzene is reduced to benzene:C₆H₅Cl

By nature, NaBH₄ is milder than most other reducing agents and usually finds application only for the reduction of less reactive substrates. It does have the capability, though, to allow some very selective reductions in more complex molecules.

NaBH₄ can only reduce keto groups. But $\mathrm{LiAlH}_4$ can reduce even anhydrides and esters. $\mathrm{LiAlH}_4$ is a very good reducing agent because the (Al) atom present in ir is more covalent than the (B) atom in $\mathrm{NaBH}_4$ Therefore, Al has more tendency to gain the electrons, thus, it will try to keep the electrons to itself and hence H^- will go in a particular manner. Thus, LiAlH₄ is better reducing agent than $\mathrm{n} \mathrm{NaBH}_4$

Mechanism
The mechanism for LiAlH₄ occurs in the following steps:

1. Deprotonation
   
2. Nucleophilic attack by the hydride ion
   
3. Nucleophilic attack by the hydride ion
   
4. Leaving group removal
   
5. Alkoxide is protonated
   

Some examples include:

Relevance and Applications

Haloarenes find application in many industrial and academic uses in general as precursors in the synthesis of various pharmaceuticals, such as antiseptics and anesthetics. For instance, chlorobenzene is used for making painkillers such as aspirin and the antiseptic called chlorhexidine.

In the field of agrochemicals, haloarenes are important in the preparation of and about pesticides and herbicides. Also, they are utilized in the preparation of the infamous insecticide DDT. However, this is highly restricted in its application today since it poses a great risk to environmental conditions.

Haloarenes play quite an equally significant role when it comes to polymer chemistry. One of the most widely used plastics, polyvinyl chloride, is obtained from the polymerization of the haloarene vinyl chloride. PVC is very vital because of the strength attained and is thus quite durable for construction, packaging, and medical tools.

Models using haloarenes are often reacted to explore the mechanism of the reaction and sometimes to design new synthetic methods. Reactions of haloarenes, especially with Grignard reagents and reducing agents like LiAl₄ and NaBH₄ , have been very informative about organic synthesis and materials science.

Recommended video(Preparation of Alcohols)

Some Solved Examples

Example 1
Question:

Which of the following alcohols could not be synthesized by reduction of aldehyde?

1) (correct)

2)

CH₃CH₂OH

3)

4)

Solution:

As we learnt ,

Reduction of aldehydes yields primary alcohol while ketones are reduced to give secondary alcohols

This alcohol is secondary alcohol and can be prepared by reduction of the ketone.

Therefore, option (1) is correct.

Example 2
Question:
Which of the following compounds will most readily be dehydrated to give alkene under acidic conditions?

1) 1-Pentanol
2) 4-Hydroxypentan-2-one (correct)
3) 3-Hydroxypentan-2-one
4) 2-Hydroxycyclopentanone

Solution:
As we learned, the dehydration of alkenes involves a carbocationic intermediate. The ease of dehydration of the given compounds can be explained on the basis of the stability of the carbocation formed.

In the case of options (3) and (4), a secondary carbocation is formed, but the presence of an electron-withdrawing group adjacent to the positively charged carbon intensifies the charge and hence destabilizes the species. However, in option (2), a secondary carbocation is formed, but the electron-withdrawing group is present farther away; as a result, the effect of this group is diminished and hence the carbocation is relatively more stable. Also, the alkene produced in (3) is conjugated with the carbonyl group, which increases the product stability.

Therefore, option (2) is correct.

Example 3
Question:

The major product of the following reaction is:

1) (correct)

2)

3)

4)

Solution:

Markovnikov addition of Br⁺ and EtO^- takes place to form the ether

Therefore, option (1) is correct.

Summary

Haloarenes are important aromatic compounds that contain one or more halogen atoms attached to the ring, and their presence has a significant effect on the course of a variety of chemical reactions. Industrially, the conversion of haloarenes into Grignard reagents provides a route to the production of more complex organic molecules, and when combined with their reduction byLiAlH₄ and NaBH₄ ,respectively, it enhances their applicability. Thus, the backbone of pharmaceuticals, agrochemicals, and polymer chemistry includes the synthesis of vital drugs, pesticides, and plastics, which is formed by haloarenes. It is precisely their chemical properties that make haloarenes especially useful for industrial and academic research users. The technologies behind haloarenes can only diffuse into many fields and are sure to develop in importance with the progress of science and technology. It means that any haloarene-related knowledge related to its properties and insight into its application will directly lead to new, innovative findings and developments that truly give pride of place to haloarenes in modern chemistry.