1. CH 2-2 Hydrogen Deficiency & Constitutional Isomers of Hydrocarbons
Hydrogen Deficiency (HD) is a tool for identifying unknown organic compounds. Consider an unknown: C4H8
• HD is a measure of unsaturation, or deficiency in the number of Hydrogens (due to pi bonds or rings):
HD = (Max # H’s – Actual # H’s)
• Max # H’s ?????
= 2n+2 (n = #carbons)
= 10
HD = (10 – 8) = 2 (deficient 2 H’s)
So C4H8 with an HD of 2 could have either one pi bond (alkene), or one ring as each would take the place of 2H’s.

2. Hydrocarbon Constitutional Isomers
C4H8 Alkene or Alkane with ring (Cycloalkane)
cycloalkane
alkene
Any other isomers?

3. Isomers for C4H8
cis
trans

4. Alkyne Constitutional Isomers: C4H6
• What is the HD = _____ Lets start by drawing all Constitutional Isomers with a triple bond. 4
terminal alkyne
internal alkyne

5. Additional Constitutional Isomers: C4H6
• Remember HD = 4. How many additional Constitutional Isomers can you draw? Remember, with HD = 4 you could have: 1 triple bond (two pi bonds), OR 1 ring and 1 double bond; OR 2 double bonds; OR 2 rings.