Hydrogen bond (hydrogen bond and its application)
One, hydrogen bond
1. Concept: a special intermolecular force.
2. Formation conditions: ① H is connected to atoms (F, O, N) with large electronegativity and small radius; ② There are atoms with high electronegativity and small radius nearby (F, O, N).
3. Expression: X—H…y-.
Hydrogen bond is an electrostatic interaction, which is another force besides Van der Waals force an intermolecular force. The size of hydrogen bonds is between chemical bonds and van der Waals forces. It is not a chemical bond, but it has bond length and bond energy. Hydrogen bonds are saturated and directional.
Second, the existence of hydrogen bonds
1. Intermolecular hydrogen bonds. Examples include C2H5OH, CH3COOH, H2O, HF, and NH3.
2. Intramolecular hydrogen bonding. For example, when there are -CHO, -COOH, -OH, and -NO2 in the ortho position of phenol, the special structure of the ring consists of hydrogen bonds.
Hydrogen bonds also exist in macromolecules.
Third, the strength of hydrogen bonds
(1), x—h…y ——: The greater the electronegativity of x and y, the greater the ability to attract electrons The stronger the hydrogen bond is, the stronger it is. For example, F has greater electronegativity and has the strongest ability to obtain electrons, so F-H…F is the strongest hydrogen bond.
(2) The order of hydrogen bond strength: F-H…F > O-H…O > O-H…N > N-H…N (Note: C atoms have weak electron-absorbing ability and generally do not form hydrogen bonds).
Comparison of hydrogen bond strength
Four. The effect on the melting point of substances
Substances that can form hydrogen bonds between molecules generally have higher melting and boiling points. This is because in addition to breaking the Van der Waals force, when a solid melts or a liquid vaporizes, it must also break the hydrogen bonds between molecules, which requires more energy consumption. Among the same compounds, substances that can form intermolecular hydrogen bonds have higher melting points than substances that cannot form intermolecular hydrogen bonds. For example, the hydrides of VIA group elements, from H2Te, H2Se to H2S, as the relative molecular weight decreases, the intermolecular force decreases, and the melting point decreases in turn. However, O-H … O hydrogen bond *** is formed between H2O molecules, the intermolecular force is strengthened, and the melting point of H2O suddenly rises.
The formation of intramolecular hydrogen bonds lowers the melting point of the substance. For example, o-nitrophenol, m-nitrophenol, and p-nitrophenol have melting points of 45°C, 96°C, and 114°C, respectively. This is because there are intermolecular hydrogen bonds in m-nitrophenol and p-nitrophenol, and part of them must be broken when melting, so the melting point is high. However, o-nitrophenol has a lower melting point because it forms intramolecular hydrogen bonds but not intermolecular hydrogen bonds.
The effect of verbs (abbreviation of verb) on the solubility of substances
If hydrogen bonds are formed between solute molecules and solvent molecules, the solubility of the solute will increase dramatically.
Ammonia, for example, is more soluble in water than other gases. At 20°C, one volume of water absorbs 700 volumes of ammonia. The solubility of ammonia in water is particularly high because water molecules and ammonia molecules combine with each other through hydrogen bonds to form ammonia hydrate; ethanol, ethylene glycol, glycerin, etc. It can be miscible with water in any proportion, all from this.
If the solute molecules form intramolecular hydrogen bonds, the solubility in polar solvents decreases and the solubility in nonpolar solvents increases.
Six. Effects on acidity of organic compounds
Take carboxylic acids as an example. There are many factors that affect the acidity of carboxylic acid. Anything that makes a carboxylic acid anion more stable than a carboxylic acid will make the carboxylic acid more acidic. Conversely, carboxylic acids are less acidic. Make the hydrogen-bonded carboxylate anion more stable and make the carboxylic acid more acidic. In protic solvents, if the carboxylate anion can be stabilized by hydrogen bonding, an increase in its acidity will also be observed. For example, the pKa of acetic acid in different solvents is shown in Table 1.
The formation of intramolecular hydrogen bonds also affects the acidity of carboxylic acids. The most typical example is the acidity of o-hydroxybenzoic acid, because its carboxylate anion can form a hydrogen bond with the o-hydroxyl group, which greatly improves the stability of the anion, so the acidity (PKa=2.98) is higher than that of hydroxybenzoic acid (PKa=4.57) Much stronger.
Seven. Effects on Viscosity and Surface Tension of Substances
When intermolecular hydrogen bonds are formed, intermolecular forces increase, fluidity decreases, and viscosity increases. In general, substances that can form intermolecular hydrogen bonds are more viscous than substances that cannot form intermolecular hydrogen bonds. Alcohols and carboxylic acids can form intermolecular hydrogen bonds, but alkanes, ketones, and esters cannot, so the viscosity of alcohols and carboxylic acids is higher than that of alkanes, ketones, and esters of the same molecular weight. Polyols such as glycerol, phosphoric acid, and distillate are usually viscous liquids due to the large number of hydrogen bonds between molecules.
Intramolecular hydrogen bonds have a different effect on the viscosity of compounds than intermolecular hydrogen bonds. Compounds with intramolecular hydrogen bonds have smaller intermolecular forces, higher molecular mobility, and lower viscosity than compounds with intermolecular hydrogen bonds. For example, the viscosity ratio of o-hydroxybenzaldehyde is smaller than that of the enantiomers; among the isomers of nitrophenol, the ortho isomer is less viscous.
The high surface tension of water is rooted in the hydrogen bonds between water molecules. The surface energy of a substance is related to the intermolecular force, because the surface molecules are attracted by the molecules inside the liquid and squeezed into the liquid, so the energy is high, and the surface has a tendency to shrink automatically, as shown in Table 2.
Of the liquid substances listed in the table, the surface energy of water is higher because there areStrong hydrogen bonds. If a surfactant is added to destroy the hydrogen bond system of the surface layer, the surface energy can be reduced, which is of great significance in industrial production.
Eight, affecting the density of substances
The greater the intermolecular force, the tighter the molecular arrangement, and the greater the density. As the number of carbon atoms increases, the intermolecular forces and density of linear alkane molecules increase.
Intermolecular hydrogen bonding also affects the density of a compound. For example, alcohols can form intermolecular hydrogen bonds, and the density of low-carbon alcohols is higher than that of alkanes with similar molecular weights; with the increase of molecular weight, the proportion of hydrocarbon groups increases, hindering the formation of intermolecular hydrogen bonds, and the density of higher alcohols with similar molecular weights and alkanes The density difference gradually decreases. Ethylene glycol molecules contain two hydroxyl groups, which are more capable of forming hydrogen bonds. The density of ethylene glycol is 1.113 gcm-3, which is higher than ethanol (0.789 gcm-3) with the same carbon number and propanol (0.804 gcm-3) with similar molecular weight. Carboxylic acids can form strong hydrogen bonds, and the density of carboxylic acids is higher than that of corresponding alkanes and ethers, and also higher than that of corresponding alcohols.
If hydrogen bonds are formed between molecules, association may occur, and the result of molecular association will affect the density of the substance. For example, nH2O(H2O)n, in addition to simple H2O molecules, also has association molecules such as (H2O)2, (H2O)3, …, (H2O)n of liquid water at room temperature.
Reducing the temperature is conducive to the combination of water molecules.
Nine. The role of hydrogen bonds in living matter
Life is composed of proteins, nucleic acids, sugars, lipids and other organic substances, as well as water and inorganic salts. These substances have the characteristic of living together, in which hydrogen bonding plays a key role. A protein is a polypeptide chain molecule formed by the condensation of a certain sequence of amino acids, rich in the ability to form hydrogen bonds. In the polypeptide main chain, N-H acts as a proton donor, and C=O acts as a proton acceptor, forming C=O…H-N hydrogen bonds with each other, which determines the secondary structure of the protein. In a deoxyribonucleic acid (DNA) molecule, two polynucleotide strands are joined by hydrogen bonds between the bases (C = O…H-n and C = N…H-n), namely adenine (A) and thymine (T ) pair to form two hydrogen bonds, and guanine (G) pairs with cytosine (C) to form three hydrogen bonds, coiled between the coils of the double helix. Once the hydrogen bonds are broken, the structure between empty molecules will change, and the biological and physiological functions will be lost.