Clues about functional groups from the molecular formula
The molecular formula of a compound gives important clues to the nature of the functional group(s) present in the molecule.
These clues are given by both the number and the types of atoms present.
it is possible to distinguish between saturated functional groups and unsaturated functional groups by calculating the total number of double bonds and/or rings present in the compound. This number is called the number of double bond equivalents or degrees of unsaturation.
CH3CH=CHCH2CH3
Note that the number of oxygen atoms is not included in the calculation because replacing a C=C with a C=O does not affect the total number of hydrogens in the compound. Note the similarity between the formula for pentan-2-one (C5H10O) and pentene (C5H10).
Thus, for a compound containing one oxygen, by calculating the number of double bond equivalents it is possible to establish that the compound may be an aldehyde or a ketone OR may be an ether or an alcohol. Other spectroscopic or chemical evidence is required to then distinguish between the two functional groups that are possible in each case.
These clues are given by both the number and the types of atoms present.
For example, the presence of a single oxygen atom indicates that the compound may be an alcohol (ROH), an ether (ROR′), an aldehyde (RCH=O) or a ketone (RR′C=O) AND that it cannot be a carboxylic acid (RCO2H) nor an ester (RCO2R′).
it is possible to distinguish between saturated functional groups and unsaturated functional groups by calculating the total number of double bonds and/or rings present in the compound. This number is called the number of double bond equivalents or degrees of unsaturation.
CH3CH=CHCH2CH3
A ring is a "double bond equivalent" because converting a carbon chain into a ring involves removing two hydrogens from non-adjacent carbons. Forming a double bond also involves removing two hydrogens, in this case from adjacent carbons.
Thus cyclopentane and pent-2-ene both have the molecular formula C5H10.
A general formula for calculation of the number of double bond equivalents (DBE) present in a molecule from its molecular formula is given below.
DBE= ½(2N4 + 2 + N3 – N1 )
DBE= ½(2N4 + 2 + N3 – N1 )
N4 = number of tetravalent atoms (C)
N1 = number of monovalent atoms (H or halogen)
N3 = number of trivalent atoms (N)
N1 = number of monovalent atoms (H or halogen)
N3 = number of trivalent atoms (N)
Understanding the formula:
2n+2 is the maximum number of hydrogens for the given number of carbons (N4)
IF no nitrogens are present (for every N introduced, you must also introduce an H).
N1 (the actual number of hydrogens or halogens).
If N1 is less than 2n+2, there will be a double bond equivalent.
Since 2H are removed in forming each π or ringbond, the difference is divided by 2.
2n+2 is the maximum number of hydrogens for the given number of carbons (N4)
IF no nitrogens are present (for every N introduced, you must also introduce an H).
N1 (the actual number of hydrogens or halogens).
If N1 is less than 2n+2, there will be a double bond equivalent.
Since 2H are removed in forming each π or ringbond, the difference is divided by 2.
Note that the number of oxygen atoms is not included in the calculation because replacing a C=C with a C=O does not affect the total number of hydrogens in the compound. Note the similarity between the formula for pentan-2-one (C5H10O) and pentene (C5H10).Thus, for a compound containing one oxygen, by calculating the number of double bond equivalents it is possible to establish that the compound may be an aldehyde or a ketone OR may be an ether or an alcohol. Other spectroscopic or chemical evidence is required to then distinguish between the two functional groups that are possible in each case.