STATE AUTONOMOUS EDUCATIONAL INSTITUTION

SECONDARY VOCATIONAL EDUCATION IN THE NOVOSIBIRSK REGION

"KUPINSKY MEDICAL COLLEGE"

TOOLKIT

« »

for independent work students

in Chemistry

Section: Organic chemistry

Theme: Item organic chemistry.

Theory of the structure of organic compounds

Specialty: 34.02.01 "Nursing" 1 course

Kupino

2015 academic year

Considered at the meeting

subject - cyclic methodological commission on

general education disciplines, general humanitarian and

socio-economic, mathematical

and natural science cycle

Protocol from 2015

Chairman ______________ /__________________/

Vede Irina Viktorovna

Explanatory note to the methodological guide

Toolkit intended for in-depth study Topics « Types of carbon atom hybridization ».

Practice shows that many students find it difficult to determine the types of hybridization of carbon atoms and species chemical bond in the study of organic compounds.

The purpose of the manual is to help students learn to identify the types of hybridization of carbon atoms and the types of chemical bonds in organic compounds. This manual is recommended for 1st year students of the specialty 34.02.01 Nursing. The manual contains theoretical material on the topic, tables for systematizing knowledge, exercises for independent work and detailed answers for each of the tasks.

The manual is aimed at developing the skills of independent work with educational material, search and use of information, formation and development creativity, increased interest in the discipline.

I'm always ready to learn

but I don't always like it

when they teach me

W. Churchill

Types of carbon atom hybridization

The electronic structure of the carbon atom in the ground state is 1s 2 2s 2 2p 2 , there are two unpaired electrons on the p-orbitals of the 2nd level. This allows the carbon atom to form only two covalent bonds by the exchange mechanism. However, in all organic compounds, carbon forms four covalent bonds, which becomes possible as a result of hybridization of atomic orbitals.

Hybridization is the interaction of atomic orbitals with close energy values, accompanied by the formation of new "hybrid" orbitals.

Hybridization is a process that requires energy costs, but these costs are more than offset by the energy released during the formation more covalent bonds. the resulting "hybrid" orbitals are shaped like an asymmetric dumbbell and differ sharply from the initial orbitals of the carbon atom.

Three types of hybridization are possible for a carbon atom: sp 3 -hybridization- interacting orbitals are shown by blue arrows:

sp 2 -hybridization:

sp hybridization:

Hybrid orbitals of the carbon atom are able to participate in the formation of only -bonds, p-orbitals unaffected by hybridization form only -bonds. It is this feature that determines spatial structure organic molecules.

Hybridization
atomic orbitals of carbon

A covalent chemical bond is formed using common bonding electron pairs of the type:

Form a chemical bond, i.e. only unpaired electrons can create a common electron pair with a “foreign” electron from another atom. When writing electronic formulas, unpaired electrons are located one by one in the orbital cell.
atomic orbital is a function that describes the density of the electron cloud at each point in space around the nucleus of an atom. An electron cloud is a region of space in which an electron can be found with a high probability.
For agreement electronic structure carbon atom and the valency of this element use the concept of excitation of the carbon atom. In the normal (unexcited) state, the carbon atom has two unpaired 2 R 2 electrons. In an excited state (when energy is absorbed) one of 2 s 2-electrons can pass to free R-orbital. Then four unpaired electrons appear in the carbon atom:

Recall that in electronic formula atom (for example, for carbon 6 C - 1 s 2 2s 2 2p 2) large numbers in front of the letters - 1, 2 - indicate the number of the energy level. Letters s and R indicate the shape of the electron cloud (orbitals), and the numbers to the right above the letters indicate the number of electrons in a given orbital. All s- spherical orbitals:

On the second energy level except 2 s-there are three orbitals 2 R-orbitals. These 2 R-orbitals have an ellipsoidal shape, similar to dumbbells, and are oriented in space at an angle of 90 ° to each other. 2 R-Orbitals denote 2 R X , 2R y and 2 R z according to the axes along which these orbitals are located.

Shape and Orientation
p-electron orbitals

When chemical bonds are formed, electron orbitals acquire the same shape. Yes, in saturated hydrocarbons mixed one s-orbital and three R-orbitals of a carbon atom to form four identical (hybrid) sp 3-orbitals:

It - sp 3 - hybridization.
Hybridization– alignment (mixing) of atomic orbitals ( s and R) with the formation of new atomic orbitals, called hybrid orbitals.

Four sp 3 hybrid orbitals
carbon atom

Hybrid orbitals have an asymmetric shape, elongated towards the attached atom. Electron clouds repel each other and are located in space as far as possible from each other. At the same time, the axes of four sp 3-hybrid orbitals turn out to be directed to the vertices of the tetrahedron (regular triangular pyramid).
Accordingly, the angles between these orbitals are tetrahedral, equal to 109°28".
The tops of electron orbitals can overlap with the orbitals of other atoms. If electron clouds overlap along a line connecting the centers of atoms, then such a covalent bond is called sigma( )-bond. For example, in a C 2 H 6 ethane molecule, a chemical bond is formed between two carbon atoms by overlapping two hybrid orbitals. This is a connection. In addition, each of the carbon atoms with its three sp 3-orbitals overlap with s-orbitals of three hydrogen atoms, forming three -bonds.

Scheme of overlapping electron clouds
in the ethane molecule

There are three possibilities for a carbon atom. valence states With different type hybridization. Except sp 3-hybridization exists sp 2 - and sp-hybridization.
sp 2 -Hybridization- mixing one s- and two R-orbitals. As a result, three hybrid sp 2 -orbitals. These sp 2 -orbitals are located in the same plane (with axes X, at) and are directed to the vertices of the triangle with an angle between the orbitals of 120°. unhybridized
R-orbital is perpendicular to the plane of the three hybrid sp 2 orbitals (oriented along the axis z). Upper half R-orbitals are above the plane, the lower half is below the plane.
Type of sp 2-hybridization of carbon occurs in compounds with a double bond: C=C, C=O, C=N. Moreover, only one of the bonds between two atoms (for example, C=C) can be a bond. (The other bonding orbitals of the atom are directed to opposite sides.) The second bond is formed as a result of the overlap of non-hybrid R-orbitals on both sides of the line connecting the nuclei of atoms.

Orbitals (three sp 2 and one p)
carbon atom in sp 2 hybridization

Covalent bond formed by lateral overlap R-orbitals of neighboring carbon atoms is called pi( )-bond.

Education
- communications

Due to less overlap of orbitals, the -bond is less strong than the -bond.
sp-Hybridization is a mixing (alignment in form and energy) of one s- and one
R-orbitals with the formation of two hybrid sp-orbitals. sp- Orbitals are located on the same line (at an angle of 180 °) and directed in opposite directions from the nucleus of the carbon atom. Two
R-orbitals remain unhybridized. They are placed perpendicular to each other.
directions - connections. On the image sp-orbitals are shown along the axis y, and the unhybridized two
R-orbitals - along the axes X and z.

Atomic orbitals (two sp and two p)
carbon in the state of sp-hybridization

The triple carbon-carbon bond CC consists of a -bond that occurs when overlapping
sp-hybrid orbitals, and two -bonds.
The relationship between such parameters of the carbon atom as the number of attached groups, the type of hybridization and the types of chemical bonds formed is shown in Table 4.

Covalent bonds of carbon

Number of groups,
related
with carbon

Type of
hybridization

Types
participating
chemical bonds

Examples of compound formulas

sp 3

Four - communications

sp 2

Three - communications and
one is connection

sp

Two - communications
and two connections

H-CC-H

Exercises.

1. What electrons of atoms (for example, carbon or nitrogen) are called unpaired?

2. What does the concept of "shared electron pairs" mean in compounds with a covalent bond (for example, CH 4 or H 2 S )?

3. What are the electronic states of atoms (for example, FROM or N ) are called basic, and which are excited?

4. What do the numbers and letters mean in the electronic formula of an atom (for example, FROM or N )?

5. What is an atomic orbital? How many orbitals are in the second energy level of an atom FROM and how do they differ?

6. What is the difference between hybrid orbitals and the original orbitals from which they were formed?

7. What types of hybridization are known for the carbon atom and what are they?

Answers to exercises

1. Electrons that are one per orbital are called unpaired electrons. For example, in the electron diffraction formula of an excited carbon atom, there are four unpaired electrons, and the nitrogen atom has three:

2. Two electrons involved in the formation of one chemical bond is called a common electron pair. Usually, before the formation of a chemical bond, one of the electrons of this pair belonged to one atom, and the other electron belonged to another atom:

3. The electronic state of an atom, in which the order of filling of electronic orbitals is observed: 1s 2, 2s 2, 2p 2, 3s 2, 3p 2, 4s 2, 3d 2, 4p 2, etc., is called the ground state. In an excited state, one of the valence electrons of an atom occupies a free orbital with a higher energy, such a transition is accompanied by the separation of paired electrons. Schematically it is written like this:

Whereas in the ground state there were only two valence unpaired electrons, in the excited state there are four such electrons.

5. An atomic orbital is a function that describes the density of an electron cloud at each point in space around the nucleus of a given atom. There are four orbitals on the second energy level of the carbon atom - 2s, 2p x, 2p y, 2p z. These orbitals are:
a) the shape of the electron cloud (s is a ball, p is a dumbbell);
b) p-orbitals have different orientations in space - along mutually perpendicular axes x, y and z, they are denoted p x, p y, p z.

6. Hybrid orbitals differ from the original (non-hybrid) orbitals in shape and energy. For example, the s-orbital is the shape of a sphere, p is a symmetrical figure-eight, sp-hybrid orbital is an asymmetric figure-eight.
Energy differences: E(s)< E(sр) < E(р). Таким образом, sp-орбиталь – усредненная по форме и энергии орбиталь, полученная смешиванием исходных s- и p-орбиталей.

7. Three types of hybridization are known for the carbon atom: sp 3 , sp 2 and sp (see the text of lesson 5).

9. -connection - covalent bond, formed by frontal overlapping of orbitals along a line connecting the centers of atoms.
-bond - a covalent bond formed by lateral overlap of p-orbitals on both sides of the line connecting the centers of atoms.
- Bonds are shown by the second and third lines between the connected atoms.

10.

The method of valence bonds makes it possible to visually explain the spatial characteristics of many molecules. However, the usual idea of ​​the shapes of the orbitals is not enough to answer the question why, if the central atom has different - s, p, d- valence orbitals, the bonds formed by it in molecules with the same substituents turn out to be equivalent in their energy and spatial characteristics. In the twenties XIX years century Linus Pauling proposed the concept of hybridization of electron orbitals. Hybridization is understood as an abstract model of alignment of atomic orbitals in shape and energy.

Examples of the shape of hybrid orbitals are presented in Table 5.

Table 5. Hybrid sp, sp 2 , sp 3 orbitals

The concept of hybridization is convenient to use when explaining geometric shape molecules and bond angles (examples of tasks 2–5).

Algorithm for determining the geometry of molecules by the VS method:

a. Determine the central atom and the number of σ-bonds with terminal atoms.

b. Compose electronic configurations of all atoms that make up the molecule and graphic images of external electronic levels.

in. According to the principles of the VS method, the formation of each bond requires a pair of electrons, in general case, one for each atom. If there are not enough unpaired electrons for the central atom, it should be assumed that the atom is excited with the transition of one of the pair of electrons to a higher energy level.

d. Suggest the need and type of hybridization, taking into account all bonds and, for elements of the first period, unpaired electrons.

e. Based on the above conclusions, depict the electronic orbitals (hybrid or not) of all atoms in the molecule and their overlap. Make a conclusion about the geometry of the molecule and the approximate value of bond angles.

e. Determine the degree of polarity of the bond based on the values ​​of the electronegativity of the atoms (Table 6) Determine the presence of a dipole moment based on the location of the centers of gravity of the positive and negative charges and/or the symmetry of the molecule.

Table 6. Electronegativity values ​​of some elements according to Pauling

Task examples

Exercise 1. Describe the chemical bond in the CO molecule using the BC method.

Solution (fig.25)

a. Compose the electronic configurations of all the atoms that make up the molecule.

b. To form a bond, it is necessary to create socialized electron pairs

Figure 25. Scheme of bond formation in a CO molecule (without hybridization of orbitals)

Conclusion: In the CO molecule there is a triple bond C≡O

For the CO molecule, we can assume the presence sp-hybridization of the orbitals of both atoms (Fig. 26). Paired electrons that do not participate in bond formation are on sp hybrid orbital.

Figure 26. Scheme of bond formation in a CO molecule (taking into account the hybridization of orbitals)

Task 2. Based on the VS method, suggest the spatial structure of the BeH 2 molecule and determine whether the molecule is a dipole.

The solution of the problem is presented in Table 7.

Table 7. Determination of the geometry of the BeH 2 molecule

	Electronic configuration		Notes
a.			The central atom is beryllium. It needs to form two ϭ-bonds with hydrogen atoms
b.	H: 1 s 1 Be: 2 s 2		The hydrogen atom has an unpaired electron, the beryllium atom has all the electrons paired, it must be transferred to an excited state
in.	H: 1 s 1 Be*: 2 s 1 2p 1		If one hydrogen atom bonded to beryllium at the expense of 2 s-electron of beryllium, and the other - due to 2 p-electron of beryllium, then the molecule would not have symmetry, which is energetically unjustified, and the Be–H bonds would not be equivalent.
G.	H: 1 s 1 Be*: 2( sp) 2		It should be assumed that there sp- hybridization
d.			Two sp-hybrid orbitals are located at an angle of 180 °, the BeH 2 molecule is linear
e.		Electronegativity χ H = 2.1, χ Be = 1.5, therefore the bond is covalent polar, the electron density is shifted to the hydrogen atom, a small negative charge δ– appears on it. On the beryllium atom δ+. Since the centers of gravity of the positive and negative charge coincide (it is symmetric), the molecule is not a dipole.

Similar reasoning will help to describe the geometry of molecules with sp 2 - and sp 3 hybrid orbitals (Table 8).

Table 8. Geometry of BF 3 and CH 4 molecules

Task 3. Based on the VS method, suggest the spatial structure of the H 2 O molecule and determine whether the molecule is a dipole. There are two possible solutions, they are presented in tables 9 and 10.

Table 9. Determination of the geometry of the H 2 O molecule (without orbital hybridization)

	Electronic configuration	Graphic image orbitals external level	Notes
a.
b.	H: 1 s 1O:2 s 2 2p 4
in.			There are enough unpaired electrons to form two ϭ-bonds with hydrogen atoms.
G.			hybridization can be neglected.
d.
e.

Thus, a water molecule must have a bond angle of about 90°. However, the angle between bonds is approximately 104°.

This can be explained

1) repulsion of closely spaced hydrogen atoms.

2) Hybridization of orbitals (Table 10).

Table 10. Determination of the geometry of the H 2 O molecule (taking into account the hybridization of orbitals)

	Electronic configuration	Graphical representation of outer level orbitals	Notes
a.			The central atom is oxygen. It needs to form two ϭ-bonds with hydrogen atoms.
b.	H: 1 s 1O:2 s 2 2p 4		The hydrogen atom has an unpaired electron, the oxygen atom has two unpaired electrons.
in.			The hydrogen atom has an unpaired electron, the oxygen atom has two unpaired electrons.
G.			An angle of 104° suggests the presence sp 3 - hybridization.
d.			Two sp 3-hybrid orbitals are located at an angle of approximately 109°, the H 2 O molecule is close to a tetrahedron in shape, the decrease in the bond angle is explained by the influence of an electron non-bonding pair.
e.		Electronegativity χ H = 2.1, χ O = 3.5, therefore the bond is covalent polar, the electron density is shifted to the oxygen atom, a small negative charge 2δ– appears on it. On the hydrogen atom δ+. Since the centers of gravity of the positive and negative charges do not coincide (it is not symmetrical), the molecule is a dipole.

Similar reasoning makes it possible to explain the bond angles in the ammonia molecule NH 3 . Hybridization involving unshared electron pairs is usually assumed only for the orbitals of atoms of period II elements. Bond angles in molecules H 2 S = 92°, H 2 Se = 91°, H 2 Te = 89°. The same is observed in the series NH 3 , РH 3 , AsH 3 . When describing the geometry of these molecules, one traditionally either does not resort to the concept of hybridization or explains the decrease in the tetrahedral angle by the increasing influence of the lone pair.

The most common hybridizations are sp, sp 2 , sp 3 and sp 3 d 2 . Each type of hybridization corresponds to a certain spatial structure of the molecules of the substance.

sp hybridization. This type of hybridization is observed when an atom forms two bonds due to electrons located on the s-orbital and on the same p-orbital (of the same energy level). In this case, two hybrid q-orbitals are formed, directed in opposite directions at an angle of 180 º (Fig. 22).

Rice. 22. Scheme of sp-hybridization

During sp-hybridization, linear triatomic molecules of the type AB 2 are formed, where A is the central atom, in which hybridization occurs, and B are the attached atoms, in which hybridization does not occur. Such molecules are formed by atoms of beryllium, magnesium, as well as carbon atoms in acetylene (C 2 H 2) and in carbon dioxide(CO 2).

Example 5 Explain the chemical bond in the BeH 2 and BeF 2 molecules and the structure of these molecules.

Solution. beryllium atoms in normal condition do not form chemical bonds, because do not have unpaired electrons (2s 2). In the excited state (2s 1 2p 1), electrons are in different orbitals; therefore, when bonds are formed, sp hybridization occurs according to the scheme shown in Fig. 22. Two hydrogen or fluorine atoms are attached to two hybrid orbitals, as shown in fig. 23.

1) 2)

Rice. 23. Scheme of the formation of molecules BeH 2 (1) and BeF 2 (2)

The resulting molecules are linear, the bond angle is 180º.

Example 6 According to experimental data, the CO 2 molecule is linear, and both bonds of carbon with oxygen are the same in length (0.116 nm) and energy (800 kJ / mol). How is this data explained?

Solution. These data on the carbon dioxide molecule are explained by the following model of its formation.

The carbon atom forms bonds in an excited state in which it has four unpaired electrons: 2s 1 2p 3 . When bonds are formed, sp hybridization of orbitals occurs. The hybrid orbitals are directed in a straight line in opposite directions from the atomic nucleus, and the remaining two pure (non-hybrid) p-orbitals are located perpendicular to each other and to the hybrid orbitals. All orbitals (hybrid and non-hybrid) contain one unpaired electron.

Each oxygen atom, which has two unpaired electrons in two mutually perpendicular p-orbitals, is attached to a carbon atom with an s-bond and a p-bond: an s-bond is formed with the participation of a hybrid carbon orbital, and a p-bond is formed by overlapping pure p-orbitals of carbon atoms and oxygen. The formation of bonds in a CO 2 molecule is shown in Fig. . 24.

Rice. 24. Scheme of the formation of the CO 2 molecule

The bond multiplicity equal to two explains the greater bond strength, and sp hybridization explains the linear structure of the molecule.

The mixing of one s and two p orbitals is called sp 2 hybridization. With this hybridization, three equivalent q-orbitals are obtained, located in the same plane at an angle of 120º (Fig. 25).

Rice. 25. Scheme of sp 2 hybridization

The AB 3 type molecules formed during this hybridization have the shape of a flat right triangle with A atoms in the center and B atoms at its vertices. Such hybridization occurs in the atoms of boron and other elements of the third group and in carbon atoms in the C 2 H 4 molecule and in the CO 3 2- ion.

Example 7 Explain the formation of chemical bonds in the ВН 3 molecule and its structure.

Solution. Experimental studies indicate that in the BH 3 molecule all three B–H bonds are located in the same plane, the angles between the bonds are 120º. This structure of the molecule is explained by the fact that valence orbitals occupied by unpaired electrons (2s 1 2p 2) are mixed in the boron atom in an excited state and it forms bonds with sp 2 hybrid orbitals. The diagram of the VN 3 molecule is shown in Fig. . 26.

Rice. 26. Scheme of formation of the ВН 3 molecule

If one s- and three p-orbitals participate in hybridization ( sp 3 hybridization), then as a result four hybrid orbitals are formed, directed towards the vertices of the tetrahedron, i.e. oriented at angles of 109º28¢ (~109.5º) to each other. The resulting molecules have a tetrahedral structure. Hybridization of this type explains the structure of saturated hydrocarbons, carbon compounds with halogens, many silicon compounds, the ammonium cation NH 4 +, etc. A classic example this hybridization is the methane molecule CH 4 (Fig. 27)

Rice. 27. Scheme of the formation of chemical bonds in the CH 4 molecule

If one s-, three p- and two d-orbitals participate in hybridization ( sp 3 d 2 - hybridization), then six hybrid orbitals appear, directed to the vertices of the octahedron, i.e. oriented at 90º angles to each other. The resulting molecules have an octahedral structure. Hybridization of this type explains the structure of compounds of sulfur, selenium and tellurium with halogens, for example, SF 6 and SeF 6, and many complex ions: 2–, 3–, etc. On fig. 28 shows the formation of a sulfur hexafluoride molecule.

Rice. 28. Scheme of the SF 6 molecule

Chemical bonds involving hybrid orbitals are very strong. If the s-bond energy formed by "pure" s-orbitals is taken as unity, then the bond energy during sp hybridization will be 1.43, with sp 2 hybridization 1.99, with sp 3 hybridization 2.00, and with sp 3 d 2 hybridization 2.92. The increase in bond strength is explained by the more complete overlap of hybrid orbitals with non-hybrid ones during the formation of a chemical bond.

In addition to the considered types of hybridization, in chemical compounds there are hybridizations sp 2 d, sp 3 d, sp 3 d 3, sp 3 d 3 and others. With sp 2 d hybridization, molecules and ions have square shape, with sp 3 d-hybridization - the shape of a trigonal bipyramid and with sp 3 d 3 -hybridization - a pentagonal bipyramid. Other types of hybridization are rare.

Example 8 The equations of two similar reactions are given:

1) CF 4 + 2HF = H 2 CF 6; 2) SiF 4 + 2HF = H 2 SiF 6

Which of them is impossible from the point of view of the formation of chemical bonds?

Solution. For the formation of H 2 CF 6, sp 3 d 2 hybridization is necessary, but in the carbon atom, valence electrons are at the second energy level, in which there are no d-orbitals. Therefore, the first reaction is in principle impossible. The second reaction is possible because sp 3 d 2 hybridization is possible in silicon.

carbon atom model

The valence electrons of a carbon atom are located in one 2s orbital and two 2p orbitals. 2p orbitals are located at an angle of 90° to each other, and the 2s orbital has spherical symmetry. Thus, the arrangement of carbon atomic orbitals in space does not explain the occurrence of bond angles 109.5°, 120°, and 180° in organic compounds.

To resolve this contradiction, the notion hybridization of atomic orbitals. To understand the nature of the three options for the arrangement of bonds of the carbon atom, ideas about three types of hybridization were needed.

We owe the emergence of the concept of hybridization to Linus Pauling, who did a lot to develop the theory of chemical bonding.

The concept of hybridization explains how a carbon atom changes its orbitals to form compounds. Below we will consider this process of orbital transformation step by step. At the same time, it should be borne in mind that the division of the hybridization process into stages or stages is, in fact, nothing more than a mental device that allows a more logical and accessible presentation of the concept. Nevertheless, the conclusions about the spatial orientation of the bonds of the carbon atom, which we eventually come to, are fully consistent with real situation affairs.

Electronic configuration of the carbon atom in the ground and excited state

The figure on the left shows electronic configuration carbon atom. We are only interested in the fate of the valence electrons. As a result of the first step, which is called excitement or promotion, one of the two 2s electrons moves to a free 2p orbital. At the second stage, the hybridization process itself takes place, which can be somewhat conventionally imagined as a mixture of one s- and three p-orbitals and the formation of four new identical orbitals from them, each of which retains the properties of the s-orbital by one quarter and the properties of p-orbitals. These new orbitals are called sp 3 - hybrid. Here, the superscript 3 denotes not the number of electrons occupying the orbitals, but the number of p-orbitals that took part in the hybridization. Hybrid orbitals are directed to the vertices of the tetrahedron, in the center of which there is a carbon atom. Each sp 3 hybrid orbital contains one electron. These electrons participate in the third stage in the formation of bonds with four hydrogen atoms, forming bond angles of 109.5°.

sp3 - hybridization. methane molecule.

The formation of planar molecules with 120° bond angles is shown in the figure below. Here, as in the case of sp 3 hybridization, the first step is excitation. At the second stage, one 2s and two 2p orbitals participate in hybridization, forming three sp 2 -hybrid orbitals located in the same plane at an angle of 120° to each other.

Formation of three sp2 hybrid orbitals

One p-rorbital remains unhybridized and is located perpendicular to the plane of sp 2 hybrid orbitals. Then (third step) two sp 2 hybrid orbitals of two carbon atoms combine electrons to form a covalent bond. Such a bond, formed as a result of the overlap of two atomic orbitals along the line connecting the nuclei of an atom, is called σ-bond.

The formation of sigma and pi bonds in the ethylene molecule

The fourth stage is the formation of a second bond between two carbon atoms. The bond is formed as a result of the overlapping of the edges of unhybridized 2p orbitals facing each other and is called π-bond. The new molecular orbital is a set of two regions occupied by electrons of the π-bond - above and below the σ-bond. Both bonds (σ and π) together make up double bond between carbon atoms. And finally, the last, fifth step is the formation of bonds between carbon and hydrogen atoms using the electrons of the four remaining sp 2 hybrid orbitals.

Double bond in the ethylene molecule

The third and last type of hybridization is shown on the example of the simplest molecule containing a triple bond, molecules acetylene. The first step is the excitation of the atom, the same as before. At the second stage, hybridization of one 2s and one 2p orbitals occurs with the formation of two sp-hybrid orbitals that are at an angle of 180°. And the two 2p orbitals necessary for the formation of two π bonds remain unchanged.

Formation of two sp-hybrid orbitals

The next step is the formation of a σ-bond between two sp-hybridized carbon atoms, then two π-bonds are formed. One σ bond and two π bonds between two carbons together make up triple bond. Finally, bonds are formed with two hydrogen atoms. The acetylene molecule has a linear structure, all four atoms lie on the same straight line.

We have shown how the three main types of molecular geometry in organic chemistry arise as a result of various transformations of the atomic orbitals of carbon.

Two methods can be proposed for determining the type of hybridization of various atoms in a molecule.

Method 1. Most general way suitable for any molecules. Based on the dependence of the bond angle on hybridization:

a) bond angles of 109.5°, 107° and 105° indicate sp 3 hybridization;

b) a valence angle of about 120 ° - sp 2 - hybridization;

c) valence angle 180°-sp-hybridization.

Method 2. Suitable for most organic molecules. Since the type of bond (single, double, triple) is associated with geometry, it is possible to determine the type of its hybridization by the nature of the bonds of a given atom:

a) all bonds are simple - sp 3 -hybridization;

b) one double bond - sp 2 -hybridization;

c) one triple bond - sp-hybridization.

Hybridization is a mental operation of transforming ordinary (energetically most favorable) atomic orbitals into new orbitals, the geometry of which corresponds to the experimentally determined geometry of molecules.

Continuation. For the beginning, see № 15, 16/2004

Lesson 5
atomic orbitals of carbon

A covalent chemical bond is formed using common bonding electron pairs of the type:

Form a chemical bond, i.e. only unpaired electrons can create a common electron pair with a “foreign” electron from another atom. When writing electronic formulas, unpaired electrons are located one by one in the orbital cell.
atomic orbital is a function that describes the density of the electron cloud at each point in space around the nucleus of an atom. An electron cloud is a region of space in which an electron can be found with a high probability.
To harmonize the electronic structure of the carbon atom and the valency of this element, the concepts of excitation of the carbon atom are used. In the normal (unexcited) state, the carbon atom has two unpaired 2 R 2 electrons. In an excited state (when energy is absorbed) one of 2 s 2-electrons can pass to free R-orbital. Then four unpaired electrons appear in the carbon atom:

Recall that in the electronic formula of an atom (for example, for carbon 6 C - 1 s 2 2s 2 2p 2) large numbers in front of the letters - 1, 2 - indicate the number of the energy level. Letters s and R indicate the shape of the electron cloud (orbitals), and the numbers to the right above the letters indicate the number of electrons in a given orbital. All s- spherical orbitals:

At the second energy level except 2 s-there are three orbitals 2 R-orbitals. These 2 R-orbitals have an ellipsoidal shape, similar to dumbbells, and are oriented in space at an angle of 90 ° to each other. 2 R-Orbitals denote 2 p x, 2r y and 2 pz according to the axes along which these orbitals are located.

When chemical bonds are formed, the electron orbitals acquire the same shape. So, in saturated hydrocarbons, one s-orbital and three R-orbitals of a carbon atom to form four identical (hybrid) sp 3-orbitals:

It - sp 3 - hybridization.
Hybridization– alignment (mixing) of atomic orbitals ( s and R) with the formation of new atomic orbitals, called hybrid orbitals.

Hybrid orbitals have an asymmetric shape, elongated towards the attached atom. Electron clouds repel each other and are located in space as far as possible from each other. At the same time, the axes of four sp 3-hybrid orbitals turn out to be directed to the vertices of the tetrahedron (regular triangular pyramid).
Accordingly, the angles between these orbitals are tetrahedral, equal to 109°28".
The tops of electron orbitals can overlap with the orbitals of other atoms. If electron clouds overlap along a line connecting the centers of atoms, then such a covalent bond is called sigma()-bond. For example, in a C 2 H 6 ethane molecule, a chemical bond is formed between two carbon atoms by overlapping two hybrid orbitals. This is a connection. In addition, each of the carbon atoms with its three sp 3-orbitals overlap with s-orbitals of three hydrogen atoms, forming three -bonds.

In total, three valence states with different types of hybridization are possible for a carbon atom. Except sp 3-hybridization exists sp 2 - and sp-hybridization.
sp 2 -Hybridization- mixing one s- and two R-orbitals. As a result, three hybrid sp 2 -orbitals. These sp 2 -orbitals are located in the same plane (with axes X, at) and are directed to the vertices of the triangle with an angle between the orbitals of 120°. unhybridized
R-orbital is perpendicular to the plane of the three hybrid sp 2 orbitals (oriented along the axis z). Upper half R-orbitals are above the plane, the lower half is below the plane.
Type of sp 2-hybridization of carbon occurs in compounds with a double bond: C=C, C=O, C=N. Moreover, only one of the bonds between two atoms (for example, C=C) can be a bond. (The other bonding orbitals of the atom are directed in opposite directions.) The second bond is formed as a result of the overlap of non-hybrid R-orbitals on both sides of the line connecting the nuclei of atoms.

Covalent bond formed by lateral overlap R-orbitals of neighboring carbon atoms is called pi()-bond.

Education - communications

Due to less overlap of orbitals, the -bond is less strong than the -bond.
sp-Hybridization is a mixing (alignment in form and energy) of one s- and one
R-orbitals with the formation of two hybrid sp-orbitals. sp- Orbitals are located on the same line (at an angle of 180 °) and directed in opposite directions from the nucleus of the carbon atom. Two
R-orbitals remain unhybridized. They are placed perpendicular to each other.
directions - connections. On the image sp-orbitals are shown along the axis y, and the unhybridized two
R-orbitals - along the axes X and z.

The triple carbon-carbon bond CC consists of a -bond that occurs when overlapping
sp-hybrid orbitals, and two -bonds.
The relationship between such parameters of the carbon atom as the number of attached groups, the type of hybridization and the types of chemical bonds formed is shown in Table 4.

Table 4

Covalent bonds of carbon

Number of groups related with carbon	Type of hybridization	Types participating chemical bonds	Examples of compound formulas
4	sp 3	Four - connections
3	sp 2	Three - connections and one is connection
2	sp	Two - connections and two connections	H-CC-H

Exercises.

1. What electrons of atoms (for example, carbon or nitrogen) are called unpaired?

2. What does the concept of "shared electron pairs" mean in compounds with a covalent bond (for example, CH 4 or H 2 S )?

3. What are the electronic states of atoms (for example, C or N ) are called basic, and which are excited?

4. What do the numbers and letters mean in the electronic formula of an atom (for example, C or N )?

5. What is an atomic orbital? How many orbitals are in the second energy level of a C atom and how do they differ?

6. What is the difference between hybrid orbitals and the original orbitals from which they were formed?

7. What types of hybridization are known for the carbon atom and what are they?

8. Draw a picture of the spatial arrangement of orbitals for one of the electronic states of the carbon atom.

9. What chemical bonds are called and what? Specify-and-connections in connections:

10. For the carbon atoms of the compounds below, indicate: a) the type of hybridization; b) types of its chemical bonds; c) bond angles.

Answers to exercises for topic 1

Lesson 5

1. Electrons that are one per orbital are called unpaired electrons. For example, in the electron diffraction formula of an excited carbon atom, there are four unpaired electrons, and the nitrogen atom has three:

2. Two electrons participating in the formation of one chemical bond are called common electron pair. Usually, before the formation of a chemical bond, one of the electrons of this pair belonged to one atom, and the other electron belonged to another atom:

3. The electronic state of the atom, in which the order of filling of electronic orbitals is observed: 1 s 2 , 2s 2 , 2p 2 , 3s 2 , 3p 2 , 4s 2 , 3d 2 , 4p 2 etc. are called main state. AT excited state one of the valence electrons of the atom occupies a free orbital with a higher energy, such a transition is accompanied by the separation of paired electrons. Schematically it is written like this:

Whereas in the ground state there were only two valence unpaired electrons, in the excited state there are four such electrons.

5. An atomic orbital is a function that describes the density of an electron cloud at each point in space around the nucleus of a given atom. There are four orbitals on the second energy level of the carbon atom - 2 s, 2p x, 2r y, 2pz. These orbitals are:
a) the shape of the electron cloud ( s- ball, R- dumbbell);
b) R-orbitals have different orientations in space - along mutually perpendicular axes x, y and z, they are denoted p x, r y, pz.

6. Hybrid orbitals differ from the original (non-hybrid) orbitals in shape and energy. For example, s-orbital - the shape of a sphere, R- symmetrical figure eight, sp-hybrid orbital - asymmetric figure eight.
Energy Differences: E(s) < E(sp) < E(R). In this way, sp-orbital - an orbital averaged in shape and energy, obtained by mixing the initial s- and p-orbitals.

7. Three types of hybridization are known for the carbon atom: sp 3 , sp 2 and sp (see the text of lesson 5).

9. -bond - a covalent bond formed by frontal overlapping of orbitals along a line connecting the centers of atoms.
-bond - a covalent bond formed by lateral overlap R-orbitals on either side of the line connecting the centers of atoms.
- Bonds are shown by the second and third lines between the connected atoms.