JEE Chemistry Alcohols Phenols and Ethers Notes Set 03

Structure of Alcohols and Reducing Agents

1. The carbon bearing the OH group is sp3 hybridised and so is the oxygen. Two of oxygen's sp3 orbitals form sigma bonds (to carbon and to hydrogen) and two hold lone pairs.
2. The lone pairs push the bonds together, so the C–O–H angle in methanol is about 108.5 degrees, a little less than the tetrahedral 109 degrees 28 minutes.

Which reducing agent reduces which functional group (yes means it reduces the group):

Summary: hydrogen with a catalyst reduces everything in the table, LiAlH4 reduces everything except C=C, and NaBH4 is the mildest, reducing only aldehydes, ketones and acid chlorides.

Ethers: Structure and Naming

1. Ethers have the general formula R–O–R' (alkoxyalkanes) and the formula CnH2n+2O. Example: CH3–O–CH2CH3 is methoxyethane (ethyl methyl ether).
2. An ether can be seen as an alkyl derivative of an alcohol (R–OH, replacing H by R) or as a dialkyl derivative of water.
3. Simple (symmetrical) ethers have identical groups, R–O–R; mixed (unsymmetrical) ethers have different groups, R–O–R'.
4. The oxygen is sp3 hybridised with two lone pairs, so the molecule is bent. The C–O–C angle is about 110 degrees, larger than the 105 degrees of water because of repulsion between the bulky alkyl groups. The molecule has a small dipole moment and is slightly polar.

Preparation of Ethers

Preparation of Phenol

Phenol (C6H5OH, carbolic acid, hydroxybenzene) has the OH group on an sp2 carbon of the benzene ring. It was first isolated from the middle oil fraction of coal tar.

The first four methods are laboratory methods; cumene, Raschig and Dow processes are industrial.

Nucleophilic Substitution in Chlorobenzene

1. Aryl halides are very unreactive towards nucleophilic substitution under ordinary conditions because the C–Cl bond has partial double bond character through resonance with the ring. Aqueous NaOH does not react with chlorobenzene at room temperature, whereas an alkyl chloride (R–Cl) gives ROH.
2. At 300 C and high pressure, chlorobenzene reacts with NaOH to give sodium phenoxide.
3. Electron-withdrawing groups at the ortho and para positions make substitution easier, so the order of ease is: chlorobenzene < 2-nitrochlorobenzene < 2,4-dinitrochlorobenzene < 2,4,6-trinitrochlorobenzene.
4. 2,4,6-Trinitrochlorobenzene reacts even with warm water to give 2,4,6-trinitrophenol (picric acid), because the nitro groups strongly withdraw electrons (–I and –M effects) and stabilise the intermediate.