Is Naphthol Polar or Nonpolar? Detailed Explanation with Examples
Have you ever stepped into an elderly person's closet and detected a peculiar and pungent odor right away? It's a distinct smell that younger generations aren't familiar with, as the substance that emits it has been phased out in recent years. You may wonder what's causing the odor. It's known as mothballs. Mothballs were used in clothes as a fumigant to prevent moths and other insects from destroying the textiles.
Naphthalene, a polyaromatic hydrocarbon, is the primary ingredient in conventional mothballs. Polyaromatic denotes the presence of more than one benzene ring, while hydrocarbon denotes the presence of only carbon and hydrogen atoms. Today, we'll hear about naphthol, a naphthalene derivative. Naphthol is very similar to its parent compound, naphthalene, with the exception of the hydroxyl (-OH) group. Let's take a look at some of this molecule's main features together!
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Amino Naphthol
Naphthol is one of two colorless, crystalline organic compounds derived from naphthalene and belonging to the phenol family; the molecular formula for both is C10H7OH. Both compounds have long been associated with the production of dyes and dye intermediates, but they also have significant applications in other industries.
The compound 1-naphthol, or alpha naphthol, is used directly in some dyes and is converted to compounds that are eventually integrated into other dyes. Heating 1-naphthalenesulfonic acid with caustic alkali or heating 1-naphthalene amine with water under pressure produces it.
The compound 2-naphthol, also known as b naphthol, is the most common naphthalene-based chemical intermediate. It's made by combining 2-naphthalenesulfonic acid and caustic soda, and it's used to make a variety of dyes and dye intermediates, as well as tanning agents, antioxidants, and antiseptics. It has been shown to cause cancer.
Structure
A hydroxyl group is bonded to a naphthalene ring to form naphthol. 1-naphthol and 2-naphthol are two isomers of naphthol (compounds with the same chemical formula but different atom connectivity). The hydroxyl group is bound to a different carbon in the naphthalene ring in the two isomers.
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Aromaticity is another significant structural characteristic of naphthol. It has alternating double and single bonds in its rings, which is typical of aromatics.
Polarity
Since it comprises a hydroxyl group, naphthol is a strongly polar molecule, with the oxygen atom attracting electron density to itself through the bonds. A difference in electronegativity (an atom's ability to attract electrons) between one or more atoms is needed for a molecule to be polar, and oxygen is more electronegative than both carbon and hydrogen in the case of naphthol. As a result, it can 'hog' more electron density in the form of bonds, causing it to become polar.
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Solubility
Naphthol is very flexible in terms of the solvents it is soluble in (things in which it can form solutions). It can make a hydrogen bond with other alcohol-based (polar) solvents including ethanol, methanol, and isopropanol due to the presence of the hydroxyl group. Because of its ability to form hydrogen bonds, it is easily soluble in these types of solvents.
Alcoholic Alpha Naphthol
Molisch's test is a biochemical test for detecting the presence of carbohydrates in solution, named after the Austrian chemist H. Molisch (1856–1937), who invented it. A small amount of alcoholic alpha-naphthol is applied to the test solution, followed by a gradual pour of concentrated sulphuric acid down the test tube's rim. The creation of a violet ring at the junction of the two liquids indicates a positive reaction.
Pyridylazo Naphthol
The orange dye pyridylazo-2-naphthol (PAN) is widely used as an acid-base indicator. Since it can form chelates with metal ions, it's a good indicator for complexometric titrations. PAN may also be used as a spectrophotometric reagent to remove metal chelates from an organic solvent.
Furfural and 1 Naphthol
The rapid furfural test is similar to Molisch's test, except that concentrated hydrochloric acid is used instead of concentrated sulfuric acid, and the solution is boiled. To ethanolic 1-Naphthol and concentrated hydrochloric acid, a dilute sugar solution is added.
FAQs on Naphthol: Definition, Structure & Properties
1. What is naphthol and what is its chemical formula?
Naphthol is an aromatic organic compound derived from naphthalene. Structurally, it is a naphthalene molecule where one hydrogen atom has been replaced by a hydroxyl (-OH) group. Its general chemical formula is C₁₀H₇OH. Naphthols are typically crystalline solids and are considered derivatives of phenol.
2. What is the main difference between 1-naphthol and 2-naphthol?
The main difference between 1-naphthol (alpha-naphthol) and 2-naphthol (beta-naphthol) lies in the position of the hydroxyl (-OH) group on the naphthalene ring structure. They are positional isomers.
In 1-naphthol, the -OH group is attached to the carbon atom at position 1 (C1).
In 2-naphthol, the -OH group is attached to the carbon atom at position 2 (C2).
3. What are the primary industrial uses of naphthol?
Naphthols are valuable intermediates in the chemical industry with several important applications. Their primary uses include:
Dye Synthesis: They are extensively used as coupling agents to produce a wide variety of vibrant azo dyes and pigments.
Pharmaceuticals: They serve as a starting material for synthesising various drugs and medicinal compounds.
Agrochemicals: Used in the production of certain insecticides and fungicides.
Perfumes: Derivatives of naphthol are used in the fragrance industry.
Antioxidants: They are also used as antioxidants, especially in the manufacturing of rubber.
4. Why is naphthol classified as a phenol and not an alcohol?
Naphthol is classified as a type of phenol because its hydroxyl (-OH) group is directly attached to a carbon atom within an aromatic ring (the naphthalene ring system). In contrast, an alcohol has its -OH group bonded to a saturated, non-aromatic carbon atom. This direct attachment to an aromatic ring gives the hydroxyl group in naphthol acidic properties, which is a characteristic feature of phenols, rather than the neutral properties of alcohols.
5. How does the reactivity of the hydroxyl group in naphthol compare to that in phenol?
The hydroxyl group in naphthols makes the ring system highly activated towards electrophilic substitution, even more so than in phenol. The naphthalene ring is inherently more reactive than the benzene ring. Consequently, reactions like azo coupling, a key step in dye formation, occur more readily and under milder conditions with naphthols compared to phenol. This enhanced reactivity makes naphthols exceptionally useful in synthesis.
6. What is the role of naphthol in the synthesis of azo dyes?
In the synthesis of azo dyes, naphthol acts as the 'coupling component'. The process involves an electrophilic substitution reaction where a diazonium salt (the 'diazo component') attacks the electron-rich naphthalene ring of the naphthol. This reaction, known as azo coupling, forms a stable compound containing an azo group (-N=N-), which links the two aromatic rings. This resulting azo compound is highly conjugated and, therefore, intensely coloured, forming the basis of a dye.
7. What are the key physical properties of naphthols that a student should know?
For the CBSE/NCERT syllabus, students should be familiar with these key properties:
Appearance: Naphthols are typically white to yellowish crystalline solids with a mild phenolic odour.
Solubility: They are sparingly soluble in cold water but dissolve in hot water and aqueous alkaline solutions (like NaOH). They are also soluble in organic solvents such as ethanol and ether.
Melting Point: The two isomers have distinct melting points. 1-naphthol melts around 96°C, while 2-naphthol has a higher melting point of about 122°C.
8. How can 1-naphthol and 2-naphthol be distinguished using a chemical test?
A simple chemical test to distinguish between the two isomers is the reaction with a neutral ferric chloride (FeCl₃) solution. Although both react, they produce different colours.
1-Naphthol gives a bluish-violet or purple colouration.
2-Naphthol gives a greenish or milky colouration.
