Example 2.2.
Write the structural formula for the compound 2,4,5 trimethyl-3-ethylhexane. Write the gross formula of this compound.
1. The main (the longest carbon chain) is recorded, i.e. the carbon skeleton of the alkane at the end of the proposed name is written. AT this example it is hexane and all carbon atoms are numbered:
C - C - C - C - C - C
2. In accordance with the numbers indicated in the formula, all substituents are placed.
C - C - C - C - C - C
CH 3 C 2 H 5 CH 3 CH 3
3. Observing the conditions of tetravalence of carbon atoms, fill the remaining free valencies of carbon atoms in the carbon skeleton with hydrogen atoms:
CH 3 - CH - CH - CH - CH - CH 3
CH 3 C 2 H 5 CH 3 CH 3
4. The number of carbon atoms in a given compound 11. The empirical formula of this compound is C 11 H 24
Isomerism of alkanes. Derivation of structural formulas of isomers.
Molecules that have the same composition but differ in different structures are called isomers. Isomers differ from each other in chemical and physical properties.
AT organic chemistry There are several types of isomerism. Limiting aliphatic hydrocarbons - alkanes have one character, the simplest type of isomerism. This type of isomerism is called structural or carbon skeleton isomerism.
In the molecules of methane, ethane and propane, there can be only one single order of connection of carbon atoms:
N N N N N
│ │ │ │ │ │
N - C - N N - C - C - N N - C - C - C - N
│ │ │ │ │ │
N N N N N
Methane ethane propane
If a hydrocarbon molecule contains more than three atoms, then the order of their connection to each other can be different. For example, C 4 H 8 butane may contain two isomers: linear and branched.
Example 2.3. Compose and name all possible isomers of C 5 H 12 pentane.
When deriving the structural formulas of individual isomers, one can proceed as follows.
1. According to the total number of carbon atoms in a molecule (5), I first write down a straight carbon chain - a carbon skeleton:
2. Then, “cleaving off” one extreme carbon atom, arrange them at the remaining carbons in the chain so as to get the maximum possible number completely new structures. With the elimination of one carbon atom from pentane, only one more isomer can be obtained:
3. It is impossible to get another isomer by rearranging the carbon "taken out" from the chain, since when rearranging it to the third carbon atom of the main chain, according to the rules for composing names, the numbering of the main chain will need to be done from right to left. By removing two carbon atoms from pentane, another isomer can be obtained:
4. Observing the conditions of tetravalence of carbon atoms, fill the remaining free valencies of carbon atoms in the carbon skeleton with hydrogen atoms
(See example 2.2.)
Note: it must be understood that by "bending" a molecule arbitrarily, a new isomer cannot be obtained. The formation of isomers is observed only when the original structure of the compound is disturbed. For example, the following connections
C - C - C - C - C and C - C - C
are not isomers, they are the carbon skeleton of the same pentane compound.
3. CHEMICAL PROPERTIES OF LIMITED HYDROCARBONS
(tasks №№ 51 - 75)
Literature:
N.L. Glinka. General chemistry. - L .: Chemistry, 1988, ch.XV, p. 164, p. 452-455.
Example 3.1. Using pentane as an example, characterize the chemical properties of alkanes. Specify the reaction conditions and name the reaction products.
Solution:
1. The main reactions of alkanes are hydrogen substitution reactions proceeding according to the free radical mechanism.
1.1. Halogenation h n
CH 3 - CH 2 - CH 2 - CH 2 - C H 3 + Cl 2 ¾¾® CH 3 - CH 2 - CH 2 - CH 2 - CH 2 Cl + HCl
pentane 1-chloropentane
CH 3 - C H 2 - CH 2 - CH 2 - CH 3 + Cl 2 ¾¾® CH 3 - CH - CH 2 - CH 2 - CH 3 + HCl
2-chloropentane
CH 3 - CH 2 - C H 2 - CH 2 - CH 3 + Cl 2 ¾¾® CH 3 - CH 2 - CH - CH 2 - CH 3 + HCl
3-chloropentane
At the first stage of the reaction in the pentane molecule, the substitution of the hydrogen atom will occur both at the primary and at the secondary carbon atom, resulting in the formation of a mixture of isomeric monochlorine derivatives.
However, the binding energy of a hydrogen atom with a primary carbon atom is greater than with a secondary carbon atom and greater than with a tertiary carbon atom, so the replacement of the hydrogen atom associated with the tertiary carbon atom is easier. This phenomenon is called selectivity. It is more pronounced in less active halogens (bromine, iodine). As the temperature rises, the selectivity decreases.
1.2. Nitration (reaction of M.M. Konovalov)
HNO 3 \u003d OHNO 2 Catalyst H 2 SO 4 conc.
As a result of the reaction, a mixture of nitro derivatives is formed.
t \u003d 120-150 0 С
CH 3 - CH 2 - CH 2 - CH 2 - C H 3 + OHNO 2 ¾¾® CH 3 - CH 2 - CH 2 - CH 2 - CH 2 NO 2 + H 2 O
pentane 1-nitropentane
t \u003d 120-150 0 С
CH 3 - C H 2 - CH 2 - CH 2 - CH 3 + OHNO 2 ¾¾® CH 3 - CH - CH 2 - CH 2 - CH 3 + H 2 O
NO 2 2-nitropentane
t \u003d 120-150 0 С
CH 3 - CH 2 - C H 2 - CH 2 - CH 3 + OHNO 2 ¾¾® CH 3 - CH 2 - CH - CH 2 - CH 3 + H 2 O
NO 2 3-nitropentane
1.3. Sulfonation reaction Concentrated H 2 SO 4 \u003d OHSO 3 H
CH 3 - CH 2 - CH 2 - CH 2 - C H 3 + OHSO 3 H ® CH 3 - CH 2 - CH 2 - CH 2 - CH 2 SO 3 H + H 2 O
pentane 1-sulfopentane
2. The reaction of complete oxidation - combustion.
C 5 H 12 + 8 (O 2 + 3.76 N 2) ® 5CO 2 + 6H 2 O + 8 × 3.76N 2
3. Thermal decomposition
C 5 H 12 ® 5C + 6H 2
4. Cracking - a cleavage reaction with the formation of an alkane and an alkene
CH 3 - CH 2 - CH 2 - CH 2 - CH 3 ¾¾® CH 3 - CH 3 + CH 2 \u003d CH - CH 3
pentane ethane propene
5. Isomerization reaction
CH 3 - CH 2 - CH 2 - CH 2 - CH 3 ¾¾® CH 3 ¾ C ¾ CH 3
CH 3 2,2-dimethylpropane
Example 3.2. Describe the methods of obtaining alkanes. Write the reaction equations that can be used to produce propane.
Solution:
1. Cracking of alkanes
CH 3 - CH 2 - CH 2 - CH 2 - CH 2 - CH 3 ® CH 3 - CH 2 - CH 3 + CH 2 \u003d CH - CH 3
hexane propane propene
2. Wurtz reaction
CH 3 - Cl + 2Na + Cl - CH 2 - CH 3 ® CH 3 - CH 2 - CH 3 + 2NaCl
chloromethane chloroethane propane
3. Recovery of halogenated alkanes
3.1. Hydrogen reduction
CH 3 - CH 2 - CH 2 - I + H - H® CH 3 - CH 2 - CH 3 + HI
1-iodopropane hydrogen propane
3.2. Recovery with hydrogen halide
CH 3 - CH 2 - CH 2 - I + H - I® CH 3 - CH 2 - CH 3 + I 2
1-iodopropane iodo-propane iodine
fusion
CH 3 - CH 2 - CH 2 - C \u003d O + NaOH ¾¾¾® CH 3 - CH 2 - CH 3 + Na 2 CO 3
sodium salt \ hydroxide propane carbonate
butanoic acid ONa sodium sodium (soda)
5. Hydrogenation of unsaturated hydrocarbons
5.1. Hydrogenation of alkenes
CH 2 \u003d CH - CH 3 + H 2 ® CH 3 - CH 2 - CH 3
propane propane
5.2. Alkyne hydrogenation
CH º C - CH 3 + 2H 2 ® CH 3 - CH 2 - CH 3
In substances, atoms are connected to each other in a certain sequence, and there are certain angles between pairs of atoms (between chemical bonds). All this is necessary for the characterization of substances, since their physical and chemical properties depend on this. Information about the geometry of bonds in substances is partially (sometimes completely) reflected in the structural formulas.
In structural formulas, the bond between atoms is represented by a bar. For example:
The chemical formula of water is H2O, and the structural formula is H-O-H,
The chemical formula of sodium peroxide is Na2O2, and the structural formula is Na-O-O-Na,
The chemical formula of nitrous acid is HNO2, and the structural formula is H-O-N=O.
When depicting structural formulas, dashes usually show the stoichiometric valency of the elements. Structural formulas based on stoichiometric valences are sometimes called graphic Such structural formulas carry information about the composition and arrangement of atoms, but do not contain correct information about the chemical bonds between atoms.
Structural formula - this is graphic image chemical structure molecules of a substance, which shows the order of bonding of atoms, their geometric arrangement. In addition, it clearly shows the valency of the atoms included in its composition.
For the correct writing of the structural formula of one or another chemical you must know and understand well what the ability of atoms to form a certain number of electron pairs with other atoms is. After all, it is valency that will help you draw chemical bonds. For example, given the molecular formula of ammonia NH3. You must write a structural formula. Keep in mind that hydrogen is always monovalent, so its atoms cannot be bonded to each other, therefore, they will be bonded to nitrogen.
To correctly write the structural formulas of organic compounds, repeat the main provisions of the theory of A.M. Butlerov, according to which there are isomers - substances with the same elemental composition, but with different chemical properties. For example, isobutane and butane. Their molecular formula is the same: C4H10, but the structural ones are different.
In a linear formula, each atom is written separately, so such an image takes up a lot of space. However, when drawing up a structural formula, you can specify the total number of hydrogen atoms for each carbon atom. And between neighboring carbons, draw chemical bonds in the form of lines.
Start writing isomers with a hydrocarbon of a normal structure, that is, with an unbranched chain of carbon atoms. Then reduce by one carbon atom, which is attached to another, internal carbon. Having exhausted all the options for writing isomers with a given chain length, reduce it by one more carbon atom. And again attach it to the inner carbon atom of the chain. For example, the structural formulas of n-pentane, isopentane, tetramethylmethane. Thus, a hydrocarbon with the molecular formula C5H12 has three isomers. Learn more about the phenomena of isomerism and homology in the following articles!
One of the most important tasks in chemistry is the correct compilation chemical formulas. A chemical formula is a written representation of the composition of a chemical substance using the Latin designation of the element and indices. For the correct compilation of the formula, we will definitely need the periodic table and knowledge simple rules. They are quite simple and even children can remember them.
How to write chemical formulas
The main concept in the preparation of chemical formulas is "valence". Valency is the property of one element to hold a certain number of atoms in a compound. The valence of a chemical element can be viewed in the periodic table, and you also need to remember and be able to apply simple general rules.
- The valence of a metal is always equal to the group number, provided that it is in the main subgroup. For example, potassium has a valence of 1, and calcium has a valence of 2.
- Non-metals are a little more difficult. A non-metal can have a higher and a lower valency. The highest valency is equal to the group number. The lowest valency can be determined by subtracting the element's group number from eight. When combined with metals, non-metals always have a lower valence. Oxygen always has a valence of 2.
- In the combination of two non-metals, the chemical element that is located to the right and above in the periodic table has the lowest valence. However, fluorine always has a valence of 1.
- One more thing important rule when placing coefficients! Total number The valencies of one element must always be equal to the total number of valences of another element!
Let's consolidate the knowledge gained on the example of a compound of lithium and nitrogen. The lithium metal has a valence of 1. The non-metal nitrogen is located in the 5th group and has a higher valence of 5 and a lower one of 3. As we already know, in compounds with metals, non-metals always have a lower valency, so nitrogen in this case will have a valency of three. We arrange the coefficients and get the desired formula: Li 3 N.
So, quite simply, we learned how to make chemical formulas! And for a better memorization of the algorithm for compiling formulas, we have prepared its graphical representation.
Instruction
To determine the valence of atoms when drawing up structural formulas, use periodic system. A three-dimensional structural formula will help to show exactly the distance of atoms in a molecule.
Sources:
- structural formula of substances
- Drawing up formulas of complex compounds
Some still shudder recall school chemistry lessons, in which it was necessary to draw up structural formulas hydrocarbons and their isomers. Meanwhile, there is nothing super complicated in this. It is enough to be guided in the preparation of formulas by a certain algorithm.
Instruction
Familiarize yourself with the molecular formula of a hydrocarbon. Based on it, first make a formula for an unbranched carbon skeleton (carbon chain).
Reduce the carbon chain by one atom. Arrange it like side branch carbon chain. Do not forget that the atoms that are located at the extreme atoms of the chain are side branches.
Determine which edge is closer to the side branch. Renumber the carbon chain starting from this edge. Arrange the hydrogen atoms according to carbon.
Determine if you can side branch at other carbons in the chain. In case of positive conclusions, make formulas. If this is not possible, reduce the main carbon chain by another atom and place it as another side branch. Please note: no more than 2 side branches can be placed near one carbon.
Arrange the serial numbers above from the edge to which the side branch is closest. Place hydrogen atoms near each atom, taking into account the valence of carbon.
Check again to see if there is any possibility of side branches on the other carbons in the main chain. If this is possible, then make formulas possible isomers, if not - reduce the carbon chain by another atom and arrange it as a side branch. Now number the entire chain of atoms and try again to compose formulas isomers. In the event that two side branches are already at the same distance from the edges of the chain, start numbering from the edge with more side branches.
Continue with these steps until you have exhausted all possibilities for the location of the side branches.
For ease of recording chemical composition and the structure of a chemical substance, certain rules were created for compiling chemical formulas using special symbols, symbols, numbers and auxiliary signs.
Instruction
Chemical formulas in writing chemical equations, schematic representation chemical processes, connections. For them, the so-called language is used, which is a set symbols, such as symbols of chemical elements, the number of atoms of each element in the described substance, etc.
Symbols of chemical elements - one or more letters of the Latin alphabet, of which the first capital. This is a schematic record of the full name of the element, for example, Ca is calcium or lat. Calcium.
The number of atoms is expressed in mathematical numbers, for example, H_2 is two hydrogen atoms.
There are several ways to write a chemical formulas: simplest, empirical, rational and . The simplest record reflects the ratio of chemical elements with an indication of the atomic mass, which is indicated after the sign of the chemical element as a subscript. For example, H_2O is the simplest formula of the water molecule, i.e. two hydrogen atoms and one oxygen atom.
empirical is different from the simplest topics, which reflects the composition of the substance, but not the structure of the molecules. The formula shows the number of atoms in one molecule, which is also shown as a subscript.
The difference between the simplest and empirical formulas is shown by the notation formulas benzene: CH and C_6H_6, respectively. Those. the simplest formula shows the direct ratio of carbon and hydrogen atoms, while the empirical formula says that the substance molecule contains 6 carbon atoms and 6 hydrogen atoms.
A rational formula clearly shows the presence of element atoms in a compound. Such groups are in parentheses, and their number is indicated by a subscript after the brackets. The formula also uses square brackets, which include complex compounds of atoms (compounds with a neutral molecule, ion).
The structural formula is depicted graphically in two- or three-dimensional space. Chemical bonds between atoms are depicted as lines, with the atoms indicated as many times as they are involved in the connection. The formula of a substance is most clearly expressed by a three-dimensional image, which shows the relative position of atoms and the distances between them.
Related videos
Hydrocarbon is organic matter, which consists of only two elements: carbon and hydrogen. It can be saturated, unsaturated with a double or triple bond, cyclic and aromatic.
A task.
Complex organic formulas are quite laborious in drawing them with the usual WORD methods. To solve this problem, special chemical editors have been created. They differ in specialization and their capabilities, in the degree of complexity of the interface and work in them, etc. In this lesson, we should get acquainted with the work of one of these editors by preparing a document file with the necessary formulas.General characteristics of the ChemSketh editor
Chemical Editor ChemSketch from the ACD / Labs software package of the Canadian company "Advanced Chemistry Development" according to functionality is not inferior to the ChemDraw editor and even surpasses it in some ways. Unlike ChemDraw (memory size 60 megabytes), ChemSketch takes up only about 20 megabytes of disk space. It is also important that the documents created using ChemSketch take up a small amount - only a few kilobytes. This chemical editor is more focused on working with organic formulas of an average level of complexity (there is a large library of ready-made formulas), but it is also convenient to create chemical formulas of inorganic substances in it. It can be used to optimize molecules in 3D space, calculate distances and bond angles between atoms in a molecular structure, and much more.
