Q.52. - Organic Chemistry

Question

Question: a. Draw the anti and gauche conformations for ethylene glycol $\left(OHC{H}_{2}C{H}_{2}OH\right)$ .

b. Ethylene glycol is unusual in that the gauche conformation is more stable than the anti conformation. Offer an explanation. 

Step-by-Step Solution

Answer

Answer

a. 

b. The gauche conformation of ethylene glycol is more stable due to hydrogen bonding.

Step 1: Gauche conformation

The gauche conformation is obtained when two bulky molecules have a ${60}^0$ angle. The gauche conformation is obtained by rotating the single bond.

 

• The gauche conformation is less stable when the bulky groups are sterically crowded, such as methyl ethyl, etc.
• The gauche conformation is more stable when the bulky groups have extra bonding, such as hydrogen bonding, ionic bonding, etc. 

Step 2: Anti conformation

The anti-confirmation is obtained when two bulky molecules have  ${180}^0$ angle.

Step 3: Explanation

a. The given ethylene glycol  $\left(OHC{H}_{2}C{H}_{2}OH\right)$ has anti and gauche conformations. These conformations are shown as:

Anti and gauche conformations of ethylene glycol

 

 b. Usually, the anti-conformation is the more stable conformation. However, in the case of the molecule that can form hydrogen bonding, they stabilize the gauche conformation.

 

In the ethylene molecule, the gauche conformation is the most stable due to the formation of a hydrogen bond between OH-OH. In anti-conformation, hydrogen bonding is not possible.