A molecule with only single bonds is called a “saturated” compound.  But, if the compound contains multiple bonds between two atoms, then it is called an “unsaturated” compound. Multiple bond between two atoms is termed as unsaturation. Let’s learn what is a Degree of Unsaturation.

Here are a few examples,

  1. C2H6  –  Ethane  is  saturated,  since all bonds are single bonds & there aren’t any pi bonds
  2. C2H4  –  Ethene  is unsaturated, since there’s a double bond or 1 pi bond  between two carbon atoms
  3. C2H2  –  Ethyne  is unsaturated, since there’s one triple bond or two pi bonds between two carbon atoms

The above example shows that each pi bond reduces Hydrogen atoms from the molecule by 2. Thus, each pi bond introduces one “degree of unsaturation.”

Degree of Unsaturation

A triple bond ( as in ethyne ) counts for two degrees of unsaturation, because a triple bond contains two pi bonds. Consider Propene  –   C3H6  (C=C-C) & CycloPropane/C3H6 (3 C’s at the vertices of a triangle joined by single bonds between any 2 C’s), for example. Note that there are no pi bonds in cyclopropane, yet its chemical formula is identical to that of propene. Thus, a ring is said to introduce one degree of unsaturation, just like a pi bond. We can compute the total degree of unsaturation for a molecule by adding No. of double bonds, 2 x (No. of triple bonds) & no. of rings.

A large number of isomeric compounds can be drawn from the same molecular formula. It is a remarkable fact that regardless of which isomer you have drawn, it must have the same degree of unsaturation as any other isomer — molecular formula dictates the degree of unsaturation, according to the formula below:

Degree of Unsaturation = [2 + (2 x no. of Carbon atoms) + (no. of Nitrogen atoms) – (no. of Hydrogen atoms) – (no. of Halogen atoms)]/2

Any compound C3H6 must have one degree of unsaturation, because [2+(2×3)-6]/2=1. As such, we know that this formula must correspond to a compound either one double bond or one ring (as in propene and cyclopropene, shown previously). Likewise, any compound with the formula C5H8 must include one of the following four structural features:

  1. Two double bonds
  2. One double bond and one ring
  3. Two rings
  4. One triple bond

The degree of unsaturation (also known as the index of hydrogen deficiency (IHD) or rings plus double bonds) formula is used in organic chemistry to help draw chemical structures. The formula lets the user determine how many rings, double bonds, and triple bonds are present in the compound to be drawn. It does not give the exact number of rings or double or triple bonds, but rather the sum of the number of rings and double bonds plus twice the number of triple bonds. The final structure is verified with use of NMR, mass spectroscopy and IR spectroscopy, as well as qualitative inspection.

For more details, you may refer NCERT Chemistry part-2 for class XI . Here are the links to chapter 12 and chapter 13 of NCERT chemistry book where you will find this topic.

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