Glycol

Glycol

Glycol or Dihydric alcohols are compounds containing two hydroxyl groups. They are classified as α, β, γ… glycols, according to the relative positions of the two hydroxyl groups: α is the 1,2 glycol; β, 1,3; my. 1,4,…. Although it is unusual to find a compound with two hydroxyl groups attached to the same carbon atom, ether derivatives of these 1, 1 glycols are stable, e.g., acetals. The commonest glycols are the α-glycols.

Nemenclatrue

The common names of the α-glycols are derived from the corresponding alkene from which they may be prepared by direct hydroxylation, e.g.

                HOCH₂CH₂OH                      eethylene glycol

               (CH₃)₂COHCH₂OH               isobutene glycol

β-, γ-… Glycols are named as the corresponding polymethylene glycols, e.g., 

              HOCH₂CH₂OH                                  trimethylene glycol

              HOCH₂CH₂CH₂CH₂CH₂OH             pentamethylene glycol

According to the I.U.P.A.C. system of nomenclature, the class suffix is –diol, and numbers are used to indicate the positions of side-chains and the two hydroxyl groups, e.g.,

Preparation of Glycol

Ethylene glycol, glycon (ethane-1,2-diol), is  the simplest glycol, and may be prepared as follows:

1.  By passing ethylene into cold dilute alkaline permanganate solution.

2.  By passing ethylene into hypochlorous acid and then hydrolyzing the ethylene chlorohydrins by boiling with aqueous sodium hydrogen carbonate:

CH=CH₂ + HOCl    ⟶    CICH₂CH₂OH    (aq. NaHCO₃) ⟶   HOCH₂CH₂OH  +   NaCl  +  CO₂

3.  By boiling ethylene dibromide with aqueous sodium carbonate:

BrCH₂CH₂Br   +   NaCO₃  +   H₂O  ⟶  HOCH₂CH₂OH   +   2NaBr   +   CO₂ (50%)

The low yield in this reaction is due to conversion of some ethylene dibromide into vinyl bromide:

BrCH₂CH₂Br   +   Na₂CO₃    ⟶   CH₂=CHBr   +   NaBr   +   NaHCO₃

If aqueous sodium hyrooxide is used instead of sodium carbonate, vinyl bromide is again obtained as a by-product. The best yield of glycol is obtained by heating ethylene dibromide with potassium acetate in glacial acetic acid, and subsequently hyroolysing the glycol diacetate with hydrogen chloride in methanolic solution

BrCH₂CH₂Br   +   2CH₃CO₂K   ⟶   CH₂(OCOCH₃)CH₂(PCPCH₃)   +   2KBr    (HCl) ⟶    HOCH₂CH₂OH

                                                            (90%)                                                    (83-84%)

4. By treating ethylene oxide with dilute hydrochloric acid :

Glycol is prepared industrially by method (4), and by the catalytic reduction of methyl glycollate which is produced synthetically.

Glycol is a colorless viscous liquid, b.p.197^oC, and has a sweet taste (the prefix glyc- indicates that the compound has a sweet taste) it is miscible in all proportion with water and ethanol, but is insoluble in ether. It is widely used as a solvent and as an antifreeze agent.

The high boiling points and high solubility in water of polyhydric alcohols are due to hydrogen bonding involving all hydroxyl groups.

The chemical reactions of glycol are those which might have been expected of a monohydric primary alcohol. One hydroxyl group, however, is almost always completely attacked before the other reacts.