Organic Chemistry Part 5 : Phenols

Animated Clipping of Phenols

3D Pictures of Different Types of Phenols

Phenols

Phenol — also known as carbolic acid — is an aromatic organic compound with the formula C6H5OH. It is a white crystalline solid that is volatile. The molecule consists of a phenyl group (-C6H5) bonded to a hydroxyl group (-OH). It is mildly acidic, but requires careful handling due to its propensity to cause burns.

Through phenols you understand combinations which contain one or more hydroxyl groups directly connected to the benzoic nucleus. Phenols can be similar to tertiary alcohols through the fact that they also contain the hydroxyl group connected to a carbon atom (which is not bound to a hydrogen atom).

After the number of hydroxyl groups in the molecule, you can distinguish:

- phenols with a single hydroxyl group, meaning mono hydroxyl phenols

- phenols with more than one hydroxyl groups in the molecule, meaning poly hydroxyl phenols

Mono hydroxyl phenols have not isomers because they are produced by de mono substitution of the benzoic nucleus. Containing more than one hydroxyl groups bound in different positions to the aromatic nucleus, the poly hydroxyl phenols exist under the form of isomers. The denomination of phenols is based on the fact that they are considered the results of the substitution of the aromatic hydrocarbons. Properly said, C₆H₅-OH is called hydroxybenzene.

Phenols 3D Structures

Structure and nomenclature

Hydroxybenzene, C₆H₅-OH, is normally called phenol and is the simplest mono hydroxyl phenol. His homologue is hydroxytoluene, (CH₃)C₆H₄OH, at which you can distinguish 3 isomers, after the relative position of the substitutes at the aromatic nucleus: o-cresol, m-cresol and p-cresol.

Phenols are, normally, solid substances (at the usual temperature), with a pervading and unpleasant smell. Phenols are a little soluble in water,

Physical properties

Phenols are, normally, solid substances (at the usual temperature), with a pervading and unpleasant smell. Phenols are a little soluble in water, but completely soluble in organic solvents.

Phenols

Obtaining methods

1. Alkaline melting method

Ar-OH + H₂SO₄ ----> Ar-SO₃H (sulfonic acid)-----> Ar-SO₃Na + H₂O ------> Na₂SO₃ + Ar-OH (phenol)

2. The cumin procedure

Benzene + CH2=CH-CH3---->  Cumin + O_{2  ------------>} Cumin hydro peroxide -------> Ar-OH + acetone

Preparation of Phenols

Chemical reactions

1. The reaction with metals and bases

Ar-OH + Na ----->Ar-ONa(sodium phenoxide) + H₂O

Ar-OH + NaOH-------> Ar-ONa + H₂O

Ar-ONa + R-X ----> Ar-OR (mix ether) + NaX

2. Substitutions reactions (at the nucleus)(-OH is a first grade substitute)

 Substitutions reactions

3. Hydrogenation

Ar-OH + 3H₂ ------>Ar-OFeCl₂ + HCl

4. The identification of phenols (FeCl3)

Ar-OH + FeCl₃------>Ar-OFeCl₂ + HCl

- Phenol + FeCl3 violet

- Cresol + FeCl3 blue

- Hydro quinine + FeCl3 blue

- Alfa-naphthol +FeCl3 green

- Beta-naphthol + FeCl3 green

Reaction of  Phenols

5. Phenols oxidation - at the oxidation, the phenols react different than the alcohols.

Phenols oxidation

Resonance structures of phenol

Notice that three of the four contributing structures possess a positive charge on the oxygen atom of the molecule. Thus, the true hybrid structure must possess a partial positive charge. Because oxygen is an electronegative element, the electrons in the oxygen-hydrogen bond orbital are attracted to the oxygen atom, resulting in a partially positive hydrogen.

Loss of a hydrogen ion to a base creates a phenoxide ion that is resonance stabilized.Notice that upon removal of the hydroxy hydrogen by a base, the phenoxide anion results. This anion is resonance stabilized by delocalization of an electron pair throughout the molecule, as shown by the contributing structures

Use

» Phenol - plastic masses, medicine industry, coloring industry and perfumes

» Cresols - plastic masses, explosive industry

» Naphthols - coloring industry

» Hydro quinine - photographic technique, coloring industry

Uses of Phenols