JEE Chemistry Alcohols Phenols and Ethers Notes Set 07

Dihydric Alcohols (Diols)

Diols are of two kinds. Geminal diols have both OH groups on the same carbon; they are hydrates of carbonyl compounds and are usually unstable (some, such as chloral hydrate, are stable because of intramolecular hydrogen bonding). Vicinal diols have the OH groups on adjacent carbons and are called glycols; ethylene glycol is the simplest. A vicinal diol with all four carbon substituents alkyl groups, such as 2,3-dimethylbutane-2,3-diol, is called pinacol.

Preparation of Glycols

1. Hydroxylation of alkenes: cold dilute alkaline KMnO4 (Baeyer's reagent) or OsO4 adds two OH groups on the same side (syn addition) and gives a vicinal diol.
2. Hydrolysis of halohydrins: an alkene with HOCl gives a halohydrin, and aqueous KOH converts it to the glycol (propene gives propane-1,2-diol).
3. Acid-catalysed hydrolysis of oxiranes: ethylene oxide with dilute acid gives ethylene glycol.
4. Hydrolysis of vicinal dihalides with aqueous KOH: 1,2-dibromoethane gives ethylene glycol.
5. Reductive coupling of carbonyl compounds (an important route to pinacols): acetone with magnesium amalgam (or Mg) followed by water gives pinacol, and acetaldehyde gives butane-2,3-diol.
6. Reduction of glyoxal.

Reactions of Glycols

1. Glycols show the usual reactions of alcohols; each OH can react (with Na, PCl5, HX and so on).
2. With organic and inorganic acids they form mono- and diesters.
3. Polymerisation: ethylene glycol with terephthalic acid gives the polyester terylene (dacron).
4. Acetal (ketal) formation: with an aldehyde or ketone and an acid catalyst, a glycol gives a cyclic acetal (a dioxolane). Aldehydes are more reactive than ketones. Acetals are stable in alkaline medium and are not reduced, but they are hydrolysed easily by acid, so the glycol is used to protect carbonyl groups.
5. Oxidation: depending on the oxidant, ethylene glycol can give glycolaldehyde, glyoxal, glycolic acid, glyoxylic acid or oxalic acid.
6. Oxidative cleavage: periodic acid (HIO4) or lead tetraacetate cleaves the C-C bond of a vicinal diol to two carbonyl compounds (propane-1,2-diol gives acetaldehyde and formaldehyde). A cis-diol (cyclic intermediate) is cleaved instantly, whereas a trans-diol cleaves much more slowly or not at all. HIO4 also cleaves a molecule where a CHOH group is next to a C=O group.
7. Dehydration: heating ethylene glycol with concentrated H2SO4 gives 1,4-dioxane (intermolecular), while dehydration with ZnCl2 gives acetaldehyde.

Pinacol-Pinacolone Rearrangement

Pinacol (2,3-dimethylbutane-2,3-diol) heated with dilute H2SO4 rearranges to pinacolone (3,3-dimethylbutan-2-one, methyl tert-butyl ketone).
Steps: (1) one OH is protonated, (2) water leaves to give a tertiary carbocation, (3) a group on the neighbouring carbon migrates to the cationic carbon, leaving a carbocation stabilised by the other OH, (4) loss of H+ gives the ketone.
Order of migratory aptitude: H, then phenyl, then ethyl, then methyl. Choose the OH that leaves to form the more stable carbocation. Ring expansion or ring contraction can also occur where possible.

\[ \ce{(CH3)2C(OH)-C(OH)(CH3)2 ->[H2SO4] (CH3)3C-CO-CH3 + H2O} \]

Trihydric Alcohol: Glycerol

Glycerol is propane-1,2,3-triol.
Preparation: (1) alkaline hydrolysis (saponification) of oils and fats gives soap and glycerol as a by-product; (2) synthetically from propene: chlorination at 500 °C gives allyl chloride, hydrolysis with aqueous KOH gives allyl alcohol, HOCl adds to give a chlorohydrin, and aqueous KOH gives glycerol.

\[ \ce{CH3CH=CH2 ->[Cl2, 773 K] CH2=CHCH2Cl ->[aq. KOH] CH2=CHCH2OH ->[HOCl] CH2OHCH(OH)CH2Cl ->[aq. KOH] CH2OHCH(OH)CH2OH} \]

Reactions of glycerol:
1. With concentrated HNO3 and concentrated H2SO4 it gives glyceryl trinitrate (nitroglycerine); glyceryl trinitrate and dinitrate absorbed on kieselguhr is dynamite.
2. With oxalic acid: at about 110 °C glycerol gives glycerol monoformate and then formic acid; at 260 °C allyl alcohol is formed.
3. With HI: a small amount of HI (excess glycerol) gives allyl iodide, which with more HI gives 2-iodopropane (isopropyl iodide) with the release of I2.
4. Dehydration: heating with KHSO4 (or dehydrating agents) gives acrolein (CH2=CH-CHO), which has an irritating smell and is used as a test for glycerol.
Uses: glycerol is miscible with water and sweet in taste; it is used to make the polyester glyptal, in medicines, cosmetics and textile processing, and in explosives.