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Hydrogen bonds are a kind of very intense intermolecular interaction that holds together polar molecules that have hydrogen bonded to oxygen, nitrogen, sulfur, or some halogen, and also to any other molecule that has these same atoms with free or not pairs of electrons. shared. The hydrogen bond can be described as a three-centered covalent bond in which the three centers are two high-electronegativity atoms and a hydrogen atom acting as a bridge between the two, which is why this type of bond used to be called hydrogen bond interaction.
Of all the intermolecular forces, including dipole-dipole attractive forces and London dispersion forces, hydrogen bonds are the strongest and are responsible for the high boiling point of low molecular weight compounds. such as water or ethanol. They are also responsible for the solubility of most of the most water-soluble substances known, including some alcohols and polyols such as glycerin.
How are hydrogen bonds formed?
Hydrogen bonds are formed between two functional groups that may or may not be the same, but serve two different roles in hydrogen bond formation.
hydrogen bond donor groups
On the one hand, in order for a hydrogen bond to form, a molecule needs to have a hydrogen-donating group. This usually consists of a group containing at least one hydrogen atom covalently bonded to an electronegative atom such as an oxygen, nitrogen, halogen, or sulfur atom. These groups are those that contribute the hydrogen atom that is part of the hydrogen bond, which is why they are called donor groups.
hydrogen bond accepting groups
The acceptor groups are functional groups that contain at least one electronegative atom among those mentioned above, which has at least one pair of free or unshared electrons. This pair of electrons are the ones that this atom uses to bond to the polarized hydrogen of the hydrogen donor group.
The receiving group of one molecule can be the same receiving group of another. For example, a molecule that has a hydroxyl group (–OH) can use that group as a donor in one hydrogen bond, as well as acceptor of two hydrogen bonds, acting as an acceptor group, as shown in the following image.
On the other hand, there are also molecules that have polar groups with highly electronegative atoms that can act as acceptors of hydrogen bonds but not as donors, which is why these compounds cannot form intermolecular hydrogen bonds with other similar molecules, although they can. form hydrogen bonds with other molecules that have donor groups.
The following image shows an example of a molecule that has various groups capable of forming hydrogen bonds, some as donors, others as acceptors and another as both:
Examples of Hydrogen Bonded Molecules
Water
Water is a small molecule that can form many hydrogen bonds. It has two O–H bonds, so each water molecule can form two hydrogen bonds as a donor. Additionally, the oxygen atom has two unshared pairs of electrons, so it can also form two hydrogen bonds as an acceptor, so each water molecule can form a total of four hydrogen bonds.
hydrogen fluoride
Hydrogen fluoride or HF has a highly polarized F–H bond (in fact, it is the most polarized hydrogen bond in existence). In addition to this, the fluorine atom possesses three additional lone pairs of electrons, so it can form three hydrogen bonds as an acceptor for the hydrogen atom. For this reason, HF can form four hydrogen bonds in total. However, since each HF molecule can only form one bond as a donor, a sample of HF molecules will only be able to form on average two hydrogen bonds each.
ethanol
Ethanol or ethyl alcohol is an organic compound related to water. It is the second simplest alcohol that exists and has a hydroxyl group in its structure that can donate one hydrogen and receive two to form a total of three simultaneous hydrogen bonds. This ability makes ethanol miscible (soluble in all proportions) with water, since each ethanol molecule can form multiple hydrogen bonds with this solvent.
methylamine
Methylamine is the simplest primary amine. It is an organic compound with the formula CH 3 NH 2 that has an amino group.
This group has two N–H bonds and nitrogen also has a pair of unpaired electrons, so this compound can form three simultaneous hydrogen bonds, two as a donor of the hydrogen atom and one as an acceptor.
Ammonia
Ammonia is to amines what water is to alcohols. It is an inorganic compound with the formula NH 3 that has three N–H bonds while nitrogen has only one lone pair of electrons.
Consequently, and as in the case of HF, ammonia can form a total of four simultaneous hydrogen bonds, but between ammonia molecules, only two hydrogen bonds can form on average, one as a donor and one as an acceptor. , since there will not be enough acceptor groups for all the donor groups.
methanol with water
For the same reasons as ethanol, methanol can form hydrogen bonds with other methanol molecules, but it can also form up to three hydrogen bonds with water molecules.
This makes methanol also miscible with water, and methanol-water solutions can be prepared in any ratio.
ethanol with acetone
Acetone is an organic compound with the formula C 3 H 6 O, which has two methyl groups linked to a carbonyl group (C=O). Having no O–H, N–H, S–H, or X–H bonds (X represents a halogen), the acetone molecule cannot act as a hydrogen bond donor. For this reason, acetone cannot form intermolecular hydrogen bonds with itself.
However, the oxygen atom of the carbonyl group has two unshared pairs of electrons, so acetone can receive two hydrogen bonds. This means that acetone can form hydrogen bonds with molecules that have donor groups, such as a water molecule or with an ethanol molecule. For this reason, acetone is soluble in ethanol and vice versa.
pyridine with ammonia
Pyridine is an example of a heterocyclic aromatic compound with a nitrogen forming part of the ring that possesses an unshared pair of electrons and is also not compromised on the aromaticity of the compound. This is a case similar to the previous one, since it does not have groups with hydrogens linked to O, N, S or X, it cannot act as a donor molecule in the hydrogen bond, but nitrogen can act as an acceptor. For this reason, pyridine can form hydrogen bonds with other donor molecules, such as ammonia.
Purines and pyrimidines
Life develops and thrives in water, largely thanks to the formation of millions of hydrogen bonds. Much of the secondary, tertiary, and quaternary structure of proteins is due to hydrogen bonding, and the same is true of the structure of our genetic material. Both DNA and RNA can form chains of complementary sequences thanks to the hydrogen bonds that are formed between the purines and pyrimidines that make up the nitrogenous bases of these nucleic acids.
For example, adenine, which forms the nitrogenous base of the nucleoside adenosine, forms two hydrogen bonds with thymine in thymidine, which is a purine.
On the other hand, guanosine, which is a nucleoside that contains guanine, another purine, forms three hydrogen bonds with cytosine, which is part of cytidine.
References
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