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The hydrogen bond is a special force generated by the H in the molecule and other atoms in or between molecules
.
1.
Formation of hydrogen bonds
The boiling point of H 2 O is much higher than that of H 2 S, H 2 Se, and H 2 Te.
The boiling point of HF is much higher than that of HCI, HBr, and Hl.
This phenomenon is difficult to explain by van der Waals forces
.
The O and F in the H 2 O and HF molecules are elements of the second period, with a small radius, that is, the covalent bond between HF and HO is strong; at the same time, the electronegativity of O and F is large, and H is the bond between HF and HO.
The electron pair is far away from H, the H in the molecule is obviously positive (electron deficiency), while O and F are obviously negative (electron rich)
.
The electron-deficient sun produces electrostatic interaction with electron-rich atoms in nearby molecules (such as O and F).
Figure 6-38 Schematic diagram of intermolecular hydrogen bonds formed by H 2 O molecules
Hydrogen bonds are represented by ".
.
From the examples of H 2 O and HF, it can be seen that the formation of hydrogen bonds must meet two conditions:
(1) There is hydrogen that forms bonds with atoms with a small radius and a large electronegativity (making the H atoms partially positively charged)
.
(2) There are atoms with large electronegativity, small radius and lone electron pairs (partially negatively charged electron pair donors)
.
The atoms that meet the requirements are mainly F, O, and N, that is, H that forms bonds with F, O, and N can form hydrogen bonds with F, O, and N atoms
.
The hydrogen bond formed by Cl-H and the hydrogen provided by CH is very weak
Hydrogen bonds are divided into intramolecular hydrogen bonds and intermolecular hydrogen bonds
.
Intramolecular hydrogen bonding means that the atoms that form covalent bonds with hydrogen and the atoms that form hydrogen bonds with hydrogen belong to the same molecule.
Intermolecular hydrogen bonds mean that the atoms that form covalent bonds with hydrogen and the atoms that form hydrogen bonds with hydrogen do not belong to the same molecule, such as hydrogen bonds formed between H 2 O molecules, hydrogen bonds formed between NH 3 molecules, NH 3 and H 2 hydrogen bonds formed between the O molecules
.
Figure 6-39 Intramolecular hydrogen bonds formed by nitric acid and o-nitrophenol
The hydrogen bond has saturation and directionality
.
Due to the small size of H atoms, there are generally only 2 atoms around 1 H, that is, 1 H forms 1 hydrogen bond, so the hydrogen bond is saturated
The strength of the hydrogen bond is related to the electronegativity of the atoms on both sides of the H.
The greater the electronegativity of the atom, the stronger the hydrogen bond and the greater the bond energy (Table 6-8)
Table 6-8 Bond energy of several hydrogen bonds
2.
The effect of hydrogen bonds on the properties of compounds
The hydrogen bond affects the physical properties of the compound
.
When there are hydrogen bonds between molecules, the force between molecules increases, which increases the melting and boiling points of the substance
.
For example, the boiling point of CH 3 CH 2 OH (78°C) is much higher than the boiling point of H 2 COCH 3 (-25°C), because CH 3 CH 2 OH forms a stronger hydrogen bond between molecules; There is a hydrogen bond, the boiling point of HF is the highest in the hydrogen halide sequence, that is, HF>HI>HBr>HCI; similarly, a strong hydrogen bond is formed between H 2 O molecules, making it the highest boiling point in the same series of hydrides, namely H 2 O> H 2 Te> H 2 Se> H 2 S
.
Due to the strong hydrogen bonds between molecules, H 2 O and HF molecules associate, and they often exist in the form of (H 2 O) 2 , (H 2 O) 3 , (HF) 2 , (HF) 3 and so on at room temperature
.
The arrangement of (H 2 O) 2 in water is the tightest, and the proportion of dimerized water is the largest at 4°C, so the density of water is the largest at 4°C
.
When a molecule can form intramolecular hydrogen bonds, it is bound to weaken the formation of intermolecular hydrogen bonds
.
Therefore, the boiling point and melting point of compounds capable of forming intramolecular hydrogen bonds are low
.
For example, p-nitrophenol can only form intermolecular hydrogen bonds, and its melting point is relatively high (113-114°C), while o-nitrophenol , which is capable of intra-molecular hydrogen bonds , has a relatively low melting point (44-45°C)
.
3.
Non-classical hydrogen bonding
The classic hydrogen bond is a strong interaction between the hydrogen in AH (a small radius and large electronegativity) and an atom with a small radius and large electronegativity, while AH (no special effect on the radius and electronegativity of A) The weak interaction between hydrogen and other atoms in (requirement) is called non-classical hydrogen bond
.
For example, the double hydrogen bond between H 3 N→BH 3 molecules, the hydrogen bond between chloroform and benzene, etc.
(Figure 6-40)
.
The experimental results show that the solubility of chloroform in benzene is obviously greater than that of 17/4 trichloroethane , indicating that the hydrogen atom of CHCl 3 forms a hydrogen bond with the conjugated electron of the benzene ring
.
Figure 6-40 Schematic diagram of hydrogen bonds between H 3 N→BH 3 molecules and between chloroform and benzene