氢键是指在氢原子与电负性很大的原子X 以共价键相结合时，同另外一个具有孤 对电子对的原子Y 形成的一个弱的键， 是一种由氢原子参加成键的特殊的化学 键。，如下式中虚线所示：A－H…B 其中A，B 是氧、氮或氟等电负性大且原 子半径比较小的原子。

氢键的键长
原子A 到B 的距离。比范德华半径之和要小，但比共 价半径之和大很多。 例如甲酸分子之间的氢键，两个氧原子之间的距离为 O.267nm，而范德华半径之和约为0.35nm．共价半径 之和为0．162nm。单体甲酸分子中的O－H 键长为 0.098nm，其二聚体的O－H 键长为0.104nm，O－ H…O 键长为0.267nm，因此形成氢键以后O－H 键长 发生了变化，H…O 键的键长为O.163nm，要长于O 一 H 键的键长。
Cl
Cl Cl ( Ⅰ) O
Example:
Cl O O N
Cl
OH
+
Cl
O ( Ⅰ)
Cl Cl Cl O
O
+
( Ⅱ)
Cl
Cl
N N
( Ⅱ)
OH
Cl
O ( Ⅲ)
Cl
O
Cl Cl O
O Br
O
O N N N
( Ⅲ)
+
Br2
Cl
O ( Ⅳ) Cl
( Ⅳ)
+
N N N
Cl O
O
O
N N N
丙环唑、噁醚唑异构体均在缩合反应过程中产生， 主要是由于1，2，4－三唑在碱性条件下容易转化 为1，3，4－三唑造成的。
H H

C O


H H C O
H
H
C O




Exceptions to the octet rule.
1. Molecules with second period elements (eg. C, N and O) without octet electronic configurations will be extremely reactive.


Part 1: THE COVALENT BOND
I. Lewis Structures
II.Resonance
III.Hybrid orbitals
IV.Aromaticity
V. Functional Groups
•Lewis structure: the electron-pair bond
F F F P F F
O S
H 3C
CH3
H3C
O S
CH3
d-orbitals
no d-orbitals
either of these structures would be considered correct
I. Lewis Structures
Resonance structures
O CH3 C NH2 CH3 C O NH2 O = CH3 C + NH2
SF6,
H2SO4
The heat of hydrogenation of benzene is less exothermic by about 21 kcal mol-1 (88kj mol-1) than one would have expected from Lewis structure 1 on the basis of known properties of single and double bonds.
Na + Cl
Na
+ Cl
covalent bonds: sharing of electrons
H + Cl
H Cl
electro negativity, : relative attraction for electrons in a bond - increases going up and to the right in the periodic table
O O O
O

O O




Rules for Resonance Structures:
6. The more stable a structure is (when taken by itself), the greater its contribution to the hybrid.

Tautomer and resonance: more subtle cases
Crystalline omeprazole exists as solid solutions of two tautomers in a continuous composition range, and this raises questions pertaining to the definition of the term polymorph.
H C H
methylene
Exceptions to the octet rule.
2. Molecules with third period elements (eg. Si, P and S) can exceed octet electron configurations because they have d-orbitals available for bonding.
more C O stable H
H H H
C O


O
H

O
H
C
C
more stable
Rules for evaluating the relative energies of resonance structures:
d. Resonance contributors with negative charge on electronegative atoms are more stable.
H
H C O H
C O H

H
H
C O




Rules for evaluating the relative energies of resonance structures:
a. The more covalent bonds a structure has the more stable it is.
1.5 bond order
Rules for Resonance Structures:
2. In writing resonance structures we are only allowed to move electrons.
O3
O O O
O O O
Rules for Resonance Structures:
more C O stable H
H H H
C O


O
H

O
H
C
C
more stable
Rules for evaluating the relative energies of resonance structures:
b. Structures in which all of the atoms have a complete valence shell of electrons are more stable.
delocalized electrons
-
HCO2
O H C O H C
O O = H C
O½ O ½-
•neither of these accurately describes the formate ion •actual species is an average of the two (resonance hybrid)
+ 3H2
1
Resonance structures The superposition of two or more Lewis structures with different electron distributions but identical nuclear positions into a composite picture is called resonance.
more stable major contributor
less stable minor contributor
Draw resonance structures for the following species. If the structures are not equivalent, indicate which would be the major contributor. CH3NO2 CH2CHO–
question:?

在 以三唑类杀菌剂为主要生产产品的化工企业，1、 2、4-三氮唑的质量关乎几乎所有三唑类杀菌剂的质量， 关乎企业生产效益和市场竞争力。1、2、4-三氮唑合 成中异构体的分离技术显得尤为重要。 ××有限公司预计投资70万用于1、2、4-三氮唑 中异构分离技术的改进，提高1、2、4-三氮唑含量， 主要是控制其中异构体1、3、4-三氮唑的含量。在保 证1、2、4-三氮唑生产成本不影响最终三唑类的杀菌 剂成本的前提下，对1、2、4-三氮唑合成技术进行优 化，对分离提纯技术进行改进,设计一条可以工业化的 合理的小试路线。
3. All of the structures must be proper Lewis structures.