Structure and Classification of Alcohols

– In this subject we will talk about Structure and Classification of Alcohols.

What are Alcohols?

– Alcohols are organic compounds containing hydroxyl (-OH) groups.

– They are some of the most common and useful compounds in nature, in industry, and around the house.

– The word alcohol is one of the oldest chemical terms, derived from the early Arabic al-kuhl.

– Originally it meant (the powder) and later (the essence). Ethyl alcohol, distilled from wine, was considered to be (the essence) of wine.

– Ethyl alcohol (grain alcohol) is found in alcoholic beverages, cosmetics, and drug preparations. Methyl alcohol (wood alcohol) is used as a fuel and solvent.

– Isopropyl alcohol (rubbing alcohol) is used as a skin cleanser for injections and minor cuts.

– Alcohols are synthesized by a wide variety of methods, and the hydroxyl group may be converted to most other functional groups.

– For these reasons, alcohols are versatile synthetic intermediates.

Structure and Classification of Alcohols

Structure of alcohols

– The structure of an alcohol resembles the structure of water, with an alkyl group replacing one of the hydrogen atoms of water.

– The following Figure compares the structures of water and methanol.

Structure and Classification of Alcohols

Structure and Classification of Alcohols
Comparison of the structures of water and methyl alcohol

– Both have Sp3 – hybridized oxygen atoms, but the C-O-H bond angle in methanol (108.9°) is considerably larger than the H-O-H bond angle in water (104.5°) because the methyl group is much larger than a hydrogen atom.

– The bulky methyl group counteracts the bond angle compression caused by oxygen’s nonbonding pairs of electrons.

– The O-H bond lengths are about the same in water and methanol (0.96 Å), but the C-O bond is considerably longer (1.4 Å), reflecting the larger covalent radius of carbon compared to hydrogen.

Classification of alcohols

– One way of organizing the alcohol family is to classify each alcohol according to the type of carbinol carbon atom: the one bonded to the -OH group.

– If this carbon atom is primary (bonded to one other carbon atom), the compound is a primary alcohol.

A secondary alcohol has the -OH group attached to a secondary carbon atom, and a tertiary alcohol has it bonded to a tertiary carbon.

– When we studied alkyl halides, we saw that primary, secondary, and tertiary halides react differently. The same is true for alcohols.

– We need to learn how these classes of alcohols are similar and under what conditions they react differently.

– The following figure shows examples of primary, secondary, and tertiary alcohols.

– Compounds with a hydroxyl group bonded directly to an aromatic (benzene) ring are called phenols.

– Phenols have many properties similar to those of alcohols, while other properties derive from their aromatic character.

Structure and Classification of Alcohols
Classification of alcohols. Alcohols are classified according to the type of carbon atom (primary, secondary, or tertiary) bonded to the hydroxyl group. Phenols have a hydroxyl group bonded to a carbon atom in a benzene ring.

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