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  Hydrogen Bonding Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Hydrogen Bonding

Hydrogen bond is the most powerful intermolecular force influencing neutral (uncharged) molecules. After comparing the boiling points of methane (CH4) -161ºC, the ammonia (NH3) -33ºC, water (H2O) 100ºC and hydrogen fluoride (HF) 19ºC, we see a great difference for these same sized molecules than expected from the data presented above for polar compounds. This is graphically shown in the chart. Simple hydrides of elements of the group IV, V, VI & VII exhibits expected rise in boiling point with molecular mass, but hydrides of the most electronegative elements (nitrogen, oxygen and fluorine) have abnormally high boiling points for their mass.

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The exceptionally strong dipole-dipole attractions that cause this behavior are called the hydrogen bond. The Hydrogen forms polar covalent bonds to more electronegative atoms like oxygen, and because a hydrogen atom is little bit small, the positive end of the bond dipole (the hydrogen) can come near to the neighboring nucleophilic or basic sites more closely than can other polar bonds. The Coulombic forces are in reverse proportional to the sixth power of the distance between dipoles making these interactions relatively strong, though they are still weak i.e. (ca. 4 to 5 kcal per mole) compared with most covalent bonds. The properties of water that are unique are largely due to the strong hydrogen bonding that occurs between its molecules. In the picture the hydrogen bonds are shows as magenta dashed lines.

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A donor is a molecule providing polar hydrogen for a hydrogen bond. An acceptor is the molecule that provides electron rich site to which the hydrogen is attracted. The Water and alcohols can serve as both acceptors and donors, and ethers, aldehydes, ketones and esters can function only as acceptors. Similarly, primary and secondary amines are both donors and acceptors but tertiary amines function only as acceptors. Once you are capable to identify the compounds that can exhibit intermolecular hydrogen bonding, relatively high boiling points they exhibit become understandable. The data in the table serve to illustrate this point.

Compound

Formula

Mol. Wt.

Boiling Point

Melting Point

dimethyl ether

CH3OCH3

46

-24ºC

-138ºC

ethanol

CH3CH2OH

46

78ºC

-130ºC

propanol

CH3(CH2)2OH

60

98ºC

-127ºC

diethyl ether

(CH3CH2)2O

74

34ºC

-116ºC

propyl amine

CH3(CH2)2NH2

59

48ºC

-83ºC

methylaminoethane

CH3CH2NHCH3

59

37ºC

 

trimethylamine

(CH3)3N

59

3ºC

-117ºC

ethylene glycol

HOCH2CH2OH

62

197ºC

-13ºC

acetic acid

CH3CO2H

60

118ºC

17ºC

ethylene diamine

H2NCH2CH2NH2

60

118ºC

8.5ºC

Alcohols boil noticeably higher than comparably sized ethers (first two entries) and isomeric 1º, 2º & 3º-amines, respectively, shows decreasing boiling points, with two hydrogen bonding isomers being considerably higher boiling than the 3º-amine. Also, hydrogen bonds i.e. O-H---O are clearly stronger than the hydrogen bonds N-H---N, as we see by comparing the propanol with the amines.

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As per the expectations, presence of two hydrogen bonding functions in a compound move up the boiling point even more. Acetic acid (the ninth entry) is an interesting case. A dimeric species that is shown in the figure on the right, held together by the two hydrogen bonds is an important component of the liquid state. If it is an exact illustration of the composition of this compound then we would expect its boiling point to be the same to that of a C4H8O4 compound (formula weight = 120). An appropriate approximation of such type of a compound is found in tetramethoxymethane, (CH3O)4C actually that is a bit larger and has a boiling point of 114ºC. So, the dimeric hydrogen bonded structure appears to be a good demonstration of acetic acid in the condensed state.

A related principle is worth noting at this state. Though the hydrogen bond is relatively weak (ca. 4 to 5 kcal per mole), when several such type of bonds exist the resulting structure can be quite robust. Hydrogen bonds between cellulose fibers confer immense strength to wood and related materials.

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