Drawing Organic Structures

1. Drawing Organic Structures • Organic compounds are held together by covalent bonds • a covalent bond may be represented by a pair of dots, or by a single line

2. Molecular formula • Use methane as an example • CH4 • molecular formula • shows relative numbers of atoms • conveys no sense of structure or bonding • conveys no sense of chemistry

3. Writing Molecular Formulas • The number of carbon atoms is specified first • The number of hydrogen atoms is specified next • The remaining atoms are then specified in (usually alphabetical) order • cyclohexane: C6H12 • benzoic acid: C7H6O2 • ethyl para-aminobenzoate: C9H11NO2 • picryl chloride: C6H2ClN3O6

4. Full structural formula • every atom specified • every bond explicitly shown • methane is simple • two structures convey the same information about as efficiently

5. Now lets try it with cholesterol as the example… • Molecular formula • C27H46O

6. Full structural formula

7. How do we simplify this? • recognize that every organic compound contains carbon • delete the symbol for carbon atoms except for those explicitly involved in a functional group (CHO, COOH, CN)

8. Lets do it…

9. Can we make it simpler…? • We have removed all the carbon symbols (every organic compound contains carbon) • Practically every carbon compound contains hydrogen • delete symbol for hydrogen except where it is part of a functional group (e.g. CHO, OH)

10. Lets try that… • Dramatic simplification • structural features are visible • three rings • length and shape of side chains

11. Lets try that… • Dramatic simplification • structural features are visible • functional groups are visible • OH group of alcohol • C=C double bond

12. Lets try that… • Dramatic simplification • structural features are visible • functional groups are visible • information is more available • how long does it take to find the important features?

13. The molecules of this experiment • cyclohexane • C6H12 • toluene • C7H8 • benzoic acid • C7H6O2 • ethyl para-aminobenzoate • C9H11NO2

14. Properties • all electrically neutral • all soluble in non-polar solvents (e.g. ether) • none soluble in water to an appreciable degree • cyclohexane & toluene are liquids • benzoic acid and ethyl para-aminobenzoate are solids

15. Benzoic acid • a carboxylic acid • has one acidic hydrogen • weak acid • only slightly ionized • pKa ≈ 4 • reacts with base to give benzoate anion and water

16. benzoic acid molecules are converted by strong base into benzoate anions • benzoate anions are converted by strong acids into benzoic acid molecules

17. benzoic acid neutral, covalent molecules soluble in non-polar solvents (e.g. ether) almost insoluble in water sodium benzoate sodium cations and benzoate anions almost insoluble in non-polar organic solvents (e.g. ether) soluble in water A comparison

18. Ethyl para-aminobenzoate • has both ester (neutral) and amine (base) functional groups • has one basic nitrogen • weak base: gives practically no OH— in water • reacts with acid to give alkylammonium cation

19. ethyl p-aminobenzoate molecules are converted by strong acid into alkylammonium cations • the alkylammonium cations are converted by strong bases into ethyl p-aminobenzoate molecules

20. ethyl p-aminobenzoate neutral, covalent molecules soluble in non-polar solvents (e.g. ether) almost insoluble in water the alkylammonium salt alkylammonium cations and sulfate or chloride anions almost insoluble in non-polar organic solvents (e.g. ether) soluble in water A comparison