Example 2.2.
Write the structural formula for the compound 2,4,5 trimethyl-3-ethylhexane. Write the gross formula for this compound.
1. The main one (the longest carbon chain) is written down, i.e. The carbon skeleton of the alkane at the end of the proposed name is written down. IN in this example this is hexane and all carbon atoms are numbered:
S – S – S – S – S – S
2. In accordance with the numbers indicated in the formula, all substituents are placed.
S - S - S - S - S - S
CH 3 C 2 H 5 CH 3 CH 3
3. Observing the conditions for the tetravalence of carbon atoms, fill the remaining free valences 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 this compound is 11. The gross formula of this compound is C 11 H 24
Isomerism of alkanes. Conclusion structural formulas 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.
IN organic chemistry There are several types of isomerism. Saturated aliphatic hydrocarbons - alkanes - have the same 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
│ │ │ │ │ │
N – S – N N - S - S - N N - S - S - S - N
│ │ │ │ │ │
N N N N N N
Methane ethane propane
If a hydrocarbon molecule contains more than three atoms, then the order in which they are connected to each other may be different. For example, butane C 4 H 8 may contain two isomers: linear and branched.
Example 2.3. Compose and name all possible isomers of pentane C 5 H 12.
When deriving the structural formulas of individual isomers, you can proceed as follows.
1. According to the total number of carbon atoms in the molecule (5), I first write down the straight carbon chain - the carbon skeleton:
2. Then, “splitting off” one outermost carbon atom at a time, they are placed at the carbons remaining in the chain so as to obtain the maximum possible number of completely new structures. When one carbon atom is removed from pentane, only one more isomer can be obtained:
3. It is impossible to obtain another isomer by rearranging the carbon “removed” from the chain, since when rearranging it to the third carbon atom of the main chain, according to the naming rules, the numbering of the main chain will need to be done from right to left. By eliminating two carbon atoms from pentane, another isomer can be obtained:
4. Observing the conditions for the tetravalence of carbon atoms, fill the remaining free valences of carbon atoms in the carbon skeleton with hydrogen atoms
(See example 2.2.)
Note: it is necessary to understand that by “bending” a molecule arbitrarily, it is impossible to obtain a new isomer. The formation of isomers is observed only when the original structure of the compound is disrupted. For example, the connections below
S – S – S - S – S and S – S – S
are not isomers, they are carbon skeletons of the same pentane compound.
3. CHEMICAL PROPERTIES OF SATURATE HYDROCARBONS
(tasks no. 51 – 75)
Literature:
N.L. Glinka. General chemistry. – L.: Chemistry, 1988, chapter XV, paragraph 164, p. 452 – 455.
Example 3.1. Using pentane as an example, characterize the chemical properties of alkanes. Indicate the reaction conditions and name the reaction products.
Solution:
1. The main reactions of alkanes are hydrogen substitution reactions that occur via a free radical mechanism.
1.1. Halogenation h n
CH 3 – CH 2 – CH 2 – CH 2 – C N 3 + Cl 2 ¾¾® CH 3 – CH 2 – CH 2 – CH 2 – CH 2 Сl + HСl
pentane 1-chloropentane
CH 3 – C N 2 – CH 2 – CH 2 – CH 3 + Cl 2 ¾¾® CH 3 – CH – CH 2 – CH 2 – CH 3 + HСl
2-chloropentane
CH 3 – CH 2 – C N 2 – CH 2 – CH 3 + Cl 2 ¾¾® CH 3 – CH 2 – CH – CH 2 – CH 3 + HСl
3-chloropentane
At the first stage of the reaction in the pentane molecule, the replacement of the hydrogen atom will occur at both the primary and secondary carbon atoms, resulting in the formation of a mixture of isomeric monochloro 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 a hydrogen atom bonded to a tertiary carbon atom is easier. This phenomenon is called selectivity. It is more pronounced in less active halogens (bromine, iodine). As the temperature increases, selectivity weakens.
1.2. Nitration (reaction of M.M. Konovalov)
HNO 3 = OHNO 2 Catalyst H 2 SO 4 conc.
As a result of the reaction, a mixture of nitro derivatives is formed.
t = 120-150 0 C
CH 3 – CH 2 – CH 2 – CH 2 – C N 3 + OHNO 2 ¾¾® CH 3 – CH 2 – CH 2 – CH 2 – CH 2 NO 2 + H 2 O
pentane 1-nitropentane
t = 120-150 0 C
CH 3 – C N 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 = 120-150 0 C
CH 3 – CH 2 – C N 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 = OHSO 3 H
CH 3 – CH 2 – CH 2 – CH 2 – C N 3 + OHSO 3 H ® CH 3 – CH 2 – CH 2 – CH 2 – CH 2 SO 3 H + H 2 O
pentane 1-sulfopentane
2. Complete oxidation reaction - combustion.
C 5 H 12 + 8 (O 2 + 3.76 N 2) ® 5 CO 2 + 6 H 2 O + 8 × 3.76 N 2
3. Thermal decomposition
C 5 H 12 ® 5 C + 6 H 2
4. Cracking is a splitting reaction to form an alkane and an alkene
CH 3 – CH 2 – CH 2 – CH 2 – CH 3 ¾¾® CH 3 – CH 3 + CH 2 = 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 for 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 = 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. Reduction of halogenated alkanes
3.1. Reduction with hydrogen
CH 3 – CH 2 – CH 2 – I + H – H ® CH 3 – CH 2 – CH 3 + HI
1-iodopropane hydrogen propane
3.2. Hydrogen halide reduction
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 = 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 = CH – CH 3 + H 2 ® CH 3 – CH 2 – CH 3
propene propane
5.2. Hydrogenation of alkynes
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 between pairs of atoms (between chemical bonds) there are certain angles. All this is necessary to characterize substances, since their physical and chemical properties depend on this. Information about the geometry of bonds in substances is partially (sometimes completely) reflected in structural formulas.
In structural formulas, the connection between atoms is represented by a line. 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 valence of 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 graphic image the chemical structure of a molecule of a substance, which shows the order of connections between atoms and their geometric arrangement. In addition, it clearly shows the valency of the atoms included in its composition.
To correctly write the structural formula of one or another chemical substance you must know and imagine well what is the ability of atoms to form a certain amount electron pairs with other atoms. After all, it is valence that will help you draw chemical bonds. For example, given the molecular formula of ammonia NH3. You must write the 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. They have the same molecular formula: C4H10, but the structural ones are different.
IN linear formula Each atom is recorded separately, so this image takes up a lot of space. However, when writing a structural formula, you can indicate the total number of hydrogen atoms at each carbon atom. And between neighboring carbons, draw chemical bonds in the form of lines.
Start writing isomers with a hydrocarbon normal structure, that is, with an unbranched chain of carbon atoms. Then shorten it by one carbon atom, which you attach to another, internal carbon. Once you have exhausted all the spellings for isomers with a given chain length, shorten 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!
Based on these ideas, A. M. Butlerov developed principles for constructing graphic formulas of chemical substances. To do this, you need to know the valency of each element, which is depicted in the figure as the corresponding number of lines. Using this rule, it is easy to establish whether the existence of a substance with a certain formula is possible or impossible. So, there is a connection called methane and having the formula CH 4. A compound with the formula CH 5 is impossible, since carbon no longer has a free valency for the fifth hydrogen.
Let us first consider the principles of the structure of the most simply structured organic compounds. They are called hydrocarbons, since they contain only carbon and hydrogen atoms (Fig. 138). The simplest of these is the aforementioned methane, which has only one carbon atom. Let's add another similar atom to it and see what the molecule of a substance called ethane Each carbon atom has one valence occupied by its fellow carbon atom. Now we need to fill the remaining valencies with hydrogen. Each atom has three free valence bonds left, to which we will add one hydrogen atom. The resulting substance has the formula C 2 H 6 . Let's add another carbon atom to it.
Rice. 138. Complete and abbreviated structural formulas of organic compounds
Now we see that the average atom has only two free valences left. We will add a hydrogen atom to them. And to the outer carbon atoms we will add, as before, three hydrogen atoms. We get propane– a compound with the formula C 3 H 8. This chain can be continued, obtaining more and more new hydrocarbons.
But carbon atoms do not necessarily have to be arranged in a linear order in a molecule. Let's say we want to add another carbon atom to propane. It turns out that this can be done in two ways: attach it to either the outermost or middle carbon atom of propane. In the first case we get butane with the formula C 4 H 10. In the second case, the general, so-called empirical, formula will be the same, but the image in the picture, called structural formula, will look different. And the name of the substance will be slightly different: not butane, but isobutane
Substances that have the same empirical but different structural formulas are called isomers, and the ability of a substance to exist in the form of various isomers is isomerism. For example, we eat various substances having the same formula C 6 H 12 O 6, but they have different structural formulas and are different names: glucose, fructose or galactose.
The hydrocarbons that we have considered are called saturated hydrocarbons. In them, all carbon atoms are connected to each other by a single bond. But since the carbon atom is tetravalent and has four valence electrons, theoretically it can form double, triple and even quadruple bonds. Quadruple bonds between carbon atoms do not exist in nature, triple bonds are rare, but double bonds are present in many organic substances, including hydrocarbons. Compounds in which there are double or triple bonds between carbon atoms are called unlimited or unsaturated hydrocarbons. Let us again take a hydrocarbon molecule containing two carbon atoms, but connect them using a double bond (see Fig. 138). We see that now each carbon atom has two free bonds left, to each of which it can attach one hydrogen atom. The resulting compound has the formula C 2 H 4 and is called ethylene. Ethylene, unlike ethane, has fewer hydrogen atoms for the same number of carbon atoms. Therefore, hydrocarbons that have a double bond are called unsaturated in the sense that they are not saturated with hydrogen.
Instructions
To determine the valence of atoms when drawing up structural formulas, use the periodic system. A three-dimensional structural formula will help show the exact distance of atoms in a molecule.
Sources:
Some still remember with a shudder their school chemistry lessons, in which they had to make structural formulas hydrocarbons and their isomers. Meanwhile, there is nothing super complicated about this. It is enough to be guided by a certain algorithm when compiling formulas.
Instructions
Familiarize yourself with the molecular formula of a hydrocarbon. Based on it, first compose the formula for an unbranched carbon skeleton (carbon chain).
Reduce the carbon chain by one atom. Position it as a side branch of the carbon chain. Do not forget that the atoms that are located at the outermost atoms of the chain are side branches.
Determine which edge the side branch is closest to. Re-number the carbon chain starting at this end. Arrange the hydrogen atoms according to the carbons.
Determine whether it is possible to place a side branch at other carbon atoms in the chain. In case of positive findings, draw up 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.
Place the serial numbers above 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 other carbons in the main chain have possible side branches. 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 make formulas isomers. If there are already two side branches located at the same distance from the edges of the chain, start numbering from the edge that has more side branches.
Continue these steps until you have exhausted all the options for placing side branches.
For ease of recording chemical composition and the structure of a chemical substance, certain rules for compiling chemical formulas were created using special symbols, numbers and auxiliary signs.
Instructions
Chemical formulas in writing chemical equations, schematic representations of 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 substance being described, etc.
Symbols of chemical elements are one or more letters of the Latin alphabet, the first of which is capital. This is a schematic notation of the full name of an element, for example, Ca is calcium or lat. Calcium.
The number of atoms is expressed mathematical numbers, for example, H_2 is two hydrogen atoms.
There are several ways to write chemical formulas: simplest, empirical, rational and. The simplest record reflects the ratio of chemical elements indicating atomic mass, which is indicated after the sign of the chemical element as a subscript. For example, H_2O is the simplest formula of a 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 depicted 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 one says that a molecule of a substance contains 6 carbon atoms and 6 hydrogen atoms.
A rational formula clearly shows the presence of atoms of elements in a compound. Such groups are surrounded by parentheses, and their number is indicated by a subscript after the parentheses. The formula also uses square brackets, which enclose 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 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 relative position atoms and the distance between them.
Video on the topic
A hydrocarbon is an organic substance that contains only two elements: carbon and hydrogen. It can be saturated, unsaturated with a double or triple bond, cyclic and aromatic.
One of the most important tasks in chemistry is the correct composition of chemical formulas. A chemical formula is a written representation of the composition of a chemical substance using the Latin element designation and indices. For correct drafting For formulas, we will definitely need the periodic table and knowledge of simple rules. They are quite simple and even children can remember them.
The main concept when drawing up chemical formulas is “valency”. Valency is the property of one element to hold a certain number of atoms in a compound. The valency 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.
Let's consolidate the knowledge gained using the example of a compound of lithium and nitrogen. The metal lithium has a valency equal to 1. The nonmetal nitrogen is located in group 5 and has a higher valency of 5 and a lower valency of 3. As we already know, in compounds with metals, nonmetals always have a lower valency, therefore nitrogen is in in this case will have a valence of three. We arrange the coefficients and get the required formula: Li 3 N.
So, quite simply, we learned how to compose chemical formulas! And for better memorization algorithm for formulating formulas, we have prepared a graphical representation of it.
