1, equivalent hydrogen method

The determination of the substitution number of hydrocarbons is basically based on the number of hydrogen atoms at different positions. It can be judged by "equal hydrogen method". Three principles for judging "equivalent hydrogen" are:

(1) Hydrogen atoms on the same carbon atom are equivalent; Such as four hydrogen atoms in methane.

② The methyl groups attached to the same carbon atom are equivalent; For example, 12 hydrogen atoms on four methyl groups in neopentane.

③ Hydrogen atoms in symmetrical positions are equivalent; For example, six hydrogen atoms in ethane are equivalent, while 2,2,3,3-tetramethylbutane.

The 24 hydrogen atoms on the benzene ring are the same, and the 6 hydrogen atoms on the benzene ring are the same.

Before determining the isomers, we must first find out the symmetry plane and judge the equivalent hydrogen, so as to determine the number of isomers.

Example 2: Among alkanes with 10 or less, one alkane with no isomer in alkyl halide * * * has ().

A.2 kinds of B. 3 kinds of C. 4 kinds of D. 5 kinds.

2. Axis moving method

For polycyclic aromatic hydrocarbons with multiple benzene rings connected together, to judge whether they are isomers, an axis can be drawn, and then whether they are isomers can be judged by translation or turnover.

Example 3: The structural formula of naphthalene molecules can be expressed that they are equivalent. Benzo [a] pyrene is a strong carcinogen (existing in chimney ash, coal tar, smoke from burning tobacco and exhaust gas of internal combustion engine). Its molecule consists of five benzene rings, and its structural formula can be expressed as formula (I) or formula (II), which are also equivalent. Existing structural formulas (a) to (d), wherein

The equivalent structural formula of (1) and (i) and (II) is _ _ _ _ _ _ _ _ _ _;

(2) The isomers of formulas (i) and (II) are _ _ _ _ _ _ _ _.

3. Fix one trick and two methods

For judging the isomers of binary substituents, we can fix the position of one substituent and then move the position of another substituent to determine the number of isomers.

Example 4: If two hydrogen atoms in naphthalene molecule are replaced by bromine atoms respectively, the number of compounds formed is ().

A.5 B. 7 C. 8 D. 10

Fourthly, permutation and combination method.

Example 5: If two hydrogen atoms in naphthalene molecule are replaced by bromine atoms respectively, the number of compounds formed is (D).

A.5 B. 7 C. 8 D. 10

Example 6: There are three different groups, namely -X, -Y and-Z. If three hydrogen atoms on the benzene ring are substituted at the same time, the number of isomers that can be generated is

A. 10 B. 8 C. 6 D. 4