what information does the index of hydrogen deficiency provide from a mass spectrum
The correct answer and explanation is:
Answer:
The Index of Hydrogen Deficiency (IHD), also known as the degree of unsaturation, provides information about the number of rings and/or multiple bonds (double bonds or triple bonds) present in a compound based on its molecular formula, which can be derived from the mass spectrum.
Explanation (300 words):
The Index of Hydrogen Deficiency (IHD) is a valuable tool in organic chemistry used to estimate the number of rings and unsaturation (double or triple bonds) in a molecule. It is calculated from the molecular formula, which is often determined from a mass spectrum by identifying the molecular ion peak (M⁺) and its exact mass.
From the mass spectrum, once the molecular formula is established, the IHD is calculated by comparing the number of hydrogens in the molecule to the number in a fully saturated hydrocarbon (alkane) with the same number of carbons. Each ring or double bond reduces the number of hydrogens by two compared to the saturated analog. A triple bond reduces hydrogens by four.
The formula for calculating IHD is: IHD=2C+2+N−H−X2\text{IHD} = \frac{2C + 2 + N – H – X}{2}
where:
- CC = number of carbons
- HH = number of hydrogens
- NN = number of nitrogens
- XX = number of halogens (F, Cl, Br, I)
Oxygen and sulfur do not affect IHD.
What IHD tells you:
- IHD = 0 means the molecule is fully saturated with no rings or double/triple bonds.
- IHD = 1 indicates either one double bond or one ring.
- IHD = 2 could indicate two double bonds, two rings, one ring plus one double bond, or one triple bond.
Why is this useful with mass spectra?
Mass spectrometry often gives the molecular ion mass, allowing deduction of the molecular formula through isotopic patterns and fragmentation analysis. With the formula, calculating IHD helps narrow down the possible structures by indicating the presence of unsaturation or ring systems, which significantly helps chemists identify or confirm molecular structure.
In summary, the IHD derived from a mass spectrum’s molecular formula is essential for determining the structural features like rings and multiple bonds in an unknown organic compound.