Advices For Studying Organic Chemistry

2.14C HYDROGEN BONDS

1. Hydrogen bond: the strong dipole-dipole attractions between hydrogen atoms
   bonded to small, strongly electronegative atoms (O, N, or F) and nonbonding
   electron pairs on other electronegative atoms.

1. Bond dissociation energy of about 4-38 KJ mol–1 (0.96-9.08 Kcal mol–1).


2. H-bond is weaker than an ordinary covalent bond; much stronger than the
   dipole-dipole interactions.

δZ− Hδ+ δZ− Hδ+

A hydrogen bond (shown by red dots)

Z is a strongly electronegative element, usually oxygen, nitrogen, or fluorine.

O H

H 3 CH 2 C δ− δ+

O

H

CH 2 CH 3

δ− δ+

The dotted bond is a hydrogen bond.

Strong hydrogen bond is limited to

molecules having a hydrogen atom

attached to an O, N, or F atom

2. Hydrogen bonding accounts for the much higher boiling point (78.5 °C) of
   ethanol than that of dimethyl ether (–24.9 °C).


3. A factor (in addition to polarity and hydrogen bonding) that affects the melting
   point of many organic compounds is the compactness and rigidity of their
   individual molecules.

H 3 CC

CH 3

CH 3

OH

CH^3 CH^2 CH^2 CH^2 OH CH^3 CHCH^2

CH 3

CH^3 CH^2 CH

CH 3

OH OH

tert-Butyl alcohol Butyl alcohol Isobutyl alcohol sec-Butyl alcohol
(mp 25 °C) (mp –90 °C) (mp –108 °C) (mp –114 °C)