What is a Hydrogen Bond
Hydrogen bonding is an intermolecular bond that occurs when there is a hydrogen atom in a molecule bonded to an electronegative and smaller element such as F, O and N.
The hydrogen bond is the strongest of the so-called secondary bonds and is responsible for the boiling point of molecules where it occurs. When hydrogen is bonded to an atom such as oxygen, the higher difference in electronegativity between the two elements causes the bonding electrons to be attracted to the more electronegative element.
This implies that the type of bond is polar, i.e. the center of gravity of the positive charges does not coincide with the center of gravity of the negative charges. Therefore hydrogen has a partial positive charge δ+ and the most electronegative element with a partial negative charge δ -.
When these polar molecules come near each other, the hydrogen will be attracted to the more electronegative element in the other molecule. A so-called hydrogen bridge bond is formed. This type of bond is a special type of dipole-dipole interaction and can occur in molecules such as HF, H₂O and NH₃.
The formation of this bond is evidenced by the boiling temperatures of hydrogen fluoride, water and ammonia.
One of the factors affecting the boiling point of a compound is its molecular mass.
In fact, if we consider the boiling points of compounds formed by an element of group 14 and hydrogen, it happens that as the molar mass of the compound increases, the boiling point increases and therefore CH₄ has the lowest boiling point and SnH₄. high boiling temperature.
However, this tendency does not occur for HF when compared to compounds containing hydrogen and other halogens. In fact HF, despite being the compound with the lowest molar mass, has the highest boiling temperature.
Similar verification trend for compounds containing group 15 and group 16 elements with hydrogen.
Hydrogen Bounding Result
So for hydrogen fluoride, water and ammonia it is clear that there is an attractive intermolecular force. Therefore a substance with a high amount of thermal energy needs to be conquered before it boils.
Hydrogen bonding is also found in some organic compounds or alcohols and in primary and secondary amines. Alcohols are actually characterized by the functional group -OH in which hydrogen assumes a partial positive charge δ+ and oxygen a partial negative charge δ-. Instead the primary and secondary amines have -NH₂ and -NH- groups, respectively, and so hydrogen takes on a partial positive charge δ+ and nitrogen a partial negative charge δ-.
The boiling point of alcohol is actually higher than that of ether. The boiling point of ethanol CH₃CH₂OH is 78.37 °C while the dimethyl ether CH₃OCH₃ isomer of ethanol which has no hydrogen bonds has a boiling point of – 24.8 °C.
In some complex molecules the hydrogen bond may also be of intermolecular type. In fact, it is responsible for the interactions inherent in the secondary structure of the protein (the α helix and the β sheet). This level of organization is the result of hydrogen bonds between amino acids of the same chain or between amino acids of different chains. A hydrogen bond is established between the hydrogen atom bonded to the nitrogen of each peptide bond and the oxygen of the -C=O group of the overlapping peptide bond.
Even in DNA, which is composed of a double helix, each nucleotide has a lateral skeleton, which allows it to covalently bond with adjacent nucleotides, and a nitrogenous base, which is present on the opposite side. Establishes hydrogen bonds with nitrogenous bases. Edge. which stabilizes the structure.