4.4 Bonding in Alcohols and Alkyl Halides

1. 4.4Bonding in Alcoholsand Alkyl Halides

2. H H H H O C Cl C Dipole Moments • alcohols and alkyl halides are polar d+ H d+ d+ d– d– H H m = 1.9 D m = 1.7 D

3. Dipole Moments • alcohols and alkyl halides are polar m = 1.9 D m = 1.7 D

4. Dipole-Dipole Attractive Forces d + d – d + d – d + d – d + d – d – d +

5. Dipole-Dipole Attractive Forces d + d – d + d – d + d – d + d – d – d +

6. 4.5Physical Properties of Alcohols and Alkyl Halides:Intermolecular Forces • Boiling point • Solubility in water • Density

7. Effect of Structure on Boiling Point CH3CH2CH3 CH3CH2OH • 44 4846 • -42 -32+78 • 0 1.91.7 CH3CH2F Molecularweight Boilingpoint, °C Dipolemoment, D

8. Effect of Structure on Boiling Point CH3CH2CH3 • 44 • -42 • 0 Molecularweight Boilingpoint, °C Dipolemoment, D Intermolecular forcesare weak. Only intermolecular forces are induced dipole-induced dipole attractions.

9. Effect of Structure on Boiling Point CH3CH2F • 48 • -32 • 1.9 A polar molecule;therefore dipole-dipoleand dipole-induceddipole forces contributeto intermolecular attractions. Molecularweight Boilingpoint, °C Dipolemoment, D

10. Effect of Structure on Boiling Point • 46 • +78 • 1.7 CH3CH2OH Highest boiling point;strongest intermolecularattractive forces. Hydrogen bonding isstronger than other dipole-dipole attractions. Molecularweight Boilingpoint, °C Dipolemoment, D

11. Figure 4.4 Hydrogen bonding in ethanol d– d+ d+ d–

12. Figure 4.4 Hydrogen bonding in ethanol

13. Boiling point increases with increasingnumber of halogens Compound Boiling Point • CH3Cl -24°C • CH2Cl2 40°C • CHCl3 61°C • CCl4 77°C Even though CCl4 is the only compound in this list without a dipole moment, it has the highest boiling point. Induced dipole-induced dipole forces are greatest in CCl4because it has the greatest number of Cl atoms. Cl is more polarizable than H.

14. But trend is not followed when halogenis fluorine • CH3CH2F -32°C • CH3CHF2 -25°C • CH3CF3 -47°C • CF3CF3 -78°C Compound Boiling Point

15. But trend is not followed when halogenis fluorine • CH3CH2F -32°C • CH3CHF2 -25°C • CH3CF3 -47°C • CF3CF3 -78°C Compound Boiling Point Fluorine is not very polarizable and induced dipole-induced dipole forces decrease with increasing fluorine substitution.

16. Solubility in water • Alkyl halides are insoluble in water. • Methanol, ethanol, isopropyl alcohol arecompletely miscible with water. • The solubility of an alcohol in waterdecreases with increasing number of carbons (compound becomesmore hydrocarbon-like).

17. d+ d– d+ d– d+ d– Figure 4.5 Hydrogen Bonding Between Ethanol and Water

18. Density • Alkyl fluorides and alkyl chlorides areless dense than water. • Alkyl bromides and alkyl iodides are more dense than water. • All liquid alcohols have densities of about 0.8 g/mL.