Same molecular formula, different atom to atom bonding sequence.
The number of constitutional isomers increases sharply with size.
Drawing Constitutional Isomers Systematically. The molecular formula can tell us some important informationFor CnH2n+2 the molecule is said to be fully saturated – meaning it only contains single (σ) bonds and no rings.
For CnH2n the molecule must contain either one double bond or one ring.
The number of rings or π-bonds (called double bond equivalents or DBEs) can be calculated from the molecular formula.
Double bond equivalents (DBEs) =½(2n4 + n3 –n1 + 2)Example: C3H6On is number of atoms with subscripted valence n4 = C, n3 = N, n1 = H, halogen n4 = 3 n3 = 0 n1 = 6 DBEs = ½(6 + 0 6 + 2) = 1
Remember each DBE must be accounted for by a ring or a π-bond for any structure drawn for that molecular formula.
Using the molecular formula and the calculated double bond equivalents (DBEs) the potential functional groups can be determined.