home Ladder Conjugate acids and bases. Acids and bases in organic chemistry. Conjugate acid and conjugate base. Acid-base balances, examples. Effect of substituents in a molecule on acidity and basicity. Modern ideas about acids and bases

Conjugate acids and bases. Acids and bases in organic chemistry. Conjugate acid and conjugate base. Acid-base balances, examples. Effect of substituents in a molecule on acidity and basicity. Modern ideas about acids and bases

Purpose of the lesson:

To form students' understanding of the influence of the acid-base properties of organic compounds on many physicochemical and biological processes occurring in the body.

To teach students to determine the acid-base properties of alcohols, phenols, thiols, and amines, depending on their structure.

The student must know: Bronsted acids and bases.

The student must be able to: determine the acidic and basic properties of organic compounds.

Modern ideas about acids and bases. Bronsted and Lowry

Important aspects of the reactivity of organic compounds are their acidic and basic properties. To describe the acidic and basic properties of chemical compounds, there are several theories - the Bronsted and Lowry theory, the Lewis theory, and a number of others. The most common is the theory of Bronsted and Lowry, or the protolithic theory.

According to the Bronsted-Lowry theoryacids are neutral molecules or ions that can donate a proton (proton donors), while bases are neutral molecules or ions that can accept a proton (proton acceptors).

According to Lewis theory acids are neutral molecules or ions capable of accepting an electron pair (electron pair acceptors), while bases are neutral molecules or ions capable of donating an electron pair (electron pair donors).

From this it follows that, theoretically, any compound that includes a hydrogen atom can donate it in the form of a proton and exhibit the properties of an acid. The ability to donate a proton can be shown not only by neutral molecules, but by charged particles - cations (NH 4 +) and anions of acids, for example HCl, ROH, HSO 4 -, etc.

As a foundation anions can act - particles that carry a negative charge, for example C1 -, OH -, HSO 4, NH 3. The bases can also be neutral molecules that include a heteroatom, for example, nitrogen, sulfur, oxygen, containing an unshared pair of electrons, for example alcoholROH.

Neutral molecules or charged ions that, depending on the nature of the second component, can exhibit the properties of acids or bases are called amphoteric.

Bronsted-Lowry theory. Conjugate acids and bases.

Acids and bases exhibit their properties only in the presence of each other. Not a single substance will donate a proton if there is no proton acceptor - a base in the system, and vice versa. they form conjugated acid-base pair in which the stronger the acid, the weaker its conjugate base, and the stronger the base, the weaker its conjugate acid.

An acid donates a proton to become a conjugate base, and a base accepts a proton to become a conjugate acid. An acid is usually denoted as AN and a base as B.

For example: HC1 ↔ H + + C1 -, HC1 is a strong acid; C1 - ion - conjugated weak base;

CH 3 COOH ↔ CH 3 COO - + H +, CH 3 COOH is a weak acid, and CH 3 COO - is an ion conjugated strong base.

The general view can be represented as follows: H + ‌ │: A + B ↔ H:B + + A: -

to-ta bases resist. resist.

to-ta basics

We have already said that the acidic properties of compounds are found only in the presence of a base, and the basic properties - in the presence of an acid, i.e. in compounds there is a certain acid-base balance, for the study of which H 2 O is used as a solvent. Relative to N 2 About how to acid or how to base determine the acid-base properties of compounds.

For weak electrolytes, acidity is quantified TO equal a reaction that consists in the transfer of H + from an acid to H 2 O as a base.

CH 3 COOH + H 2 O ↔ CH 3 COO - + H 3 O +

to-that basic base acid

CH 3 COO - - acetate ion, conjugate base;

H 3 O + - hydronium ion, conjugate acid.

Using the value of the equilibrium constant of this reaction and taking into account that the concentration of H 2 O is practically constant, we can determine the product K · called the acidity constant TO acidity (K A ).

;

The more K a, the stronger the acid. For CH 3 COOH K a \u003d 1.75 10 -5. such small values are inconvenient in practical work, therefore K a is expressed through RK A (pK = -ℓgTO A ). For CH 3 COOH pKa = 4.75. The smaller the pKa value, the stronger the acid.

The strength of the bases is determined by the value of pK ВН +.

An acid donates a proton to become a conjugate base, and a base accepts a proton to become a conjugate acid. Acid is usually denoted as AN and base as B.

For example: HC1- H + + C1 -, HC1 is a strong acid; C1 - ion - conjugated weak base;

CH 3 COOH - CH 3 COO - + H +, CH 3 COOH is a weak acid, and CH 3 COO - is an ion conjugated strong base.

The general view can be represented as follows:

H+¦: A + B H:B+ + A:-

to-ta bases resist. resist.

to-ta basics

We have already said that the acidic properties of compounds are found only in the presence of a base, and the basic properties - in the presence of an acid, i.e. in compounds there is a certain acid-base balance, for the study of which H 2 O is used as a solvent. With respect to H 2 O as an acid or as a base, the acid-base properties of the compounds are determined.

For weak electrolytes, acidity is quantified TO equal a reaction that consists in the transfer of H + from an acid to H 2 O as a base.

CH 3 COOH + H 2 O - CH 3 COO - + H 3 O +

to-that basic base acid

CH 3 COO - - acetate ion, conjugate base;

H 3 O + - hydronium ion, conjugate acid.

Using the value of the equilibrium constant of this reaction and taking into account that the concentration of H 2 O is practically constant, it is possible to determine the product K? called the acidity constant TO acidity (K A).

The more K a, the stronger the acid. For CH 3 COOH K a \u003d 1.75 10 -5. such small values are inconvenient in practical work, therefore K a is expressed through RK A (рК = -?g К A). For CH 3 COOH pKa = 4.75. The smaller the pKa value, the stronger the acid.

The strength of the bases is determined by the value of pK ВН +.

Acid properties of organic compounds with hydrogen-containing functional groups (alcohols, phenols, thiols, carboxylic acids, amines).

organic acids

In organic compounds, depending on the nature of the element with which H + is associated, the following acids are distinguished:

HE- acids (carboxylic acids, phenols, alcohols)

CH - acids (hydrocarbons and their derivatives)

NH- acids (amines, amides, imides)

SH- acids (thiols).

An acid center is an element and its associated hydrogen atom.

The strength of the acid will depend on anion stability, those. from the conjugate base, which is formed when H + is detached from the molecule. The more stable the anion, the higher the acidity of the compound.

The stability of the anion depends on a number of factors that contribute to charge delocalization. The higher the charge delocalization, the more stable the anion, the stronger the acidic properties.

Factors affecting the degree of delocalization:

1. Nature of the heteroatom in the acid center
2. Electronic effects of atoms of hydrocarbon radicals and their substituents
3. The ability of anions to solvate.
1. Dependence of acidity on heteroatom.

The nature of a heteroatom is understood as its electronegativity (E.O.) and polarizability. The more (E.O.) the easier the heterolytic gap in the molecule is carried out. In periods from left to right, with an increase in the charge of the nucleus, (E.O) increases, i.e. the ability of elements to hold a negative charge. As a result of the displacement of the electron density, the bond between the atoms is polarized. The more electrons and the larger the radius of the atom, the further the electrons of the outer energy level are located from the nucleus, the higher the polarizability and the higher the acidity.

Example: CH- NH- OH- SH-

increase in E.O. and acidity

C, N, O - elements of the same period. E.O. increases over time, acidity increases. In this case, the polarizability will not affect the acidity.

The polarizability of atoms in the period varies slightly, therefore, the main factor determining acidity is E.O.

Now consider OH-SH-

increased acidity

O, S - are in the same group, the radius in the group increases from top to bottom, therefore, the polarizability of the atom also increases, which leads to an increase in acidity. S has a larger atomic radius than O, so thiols exhibit stronger acidic properties than alcohols.

Compare three compounds: ethanol, ethanethiol and aminoethanol:

H 3 C - CH 2 - HE, H 3 C - CH 2 - SH and H 3 C - CH 2 - NH 2

1. Compare by radical - they are the same;
2. By the nature of the heteroatom in the functional group: S and O are in the same group, but S has a larger atomic radius, higher polarizability, therefore ethanethiol has stronger acidic properties
3. Now let's compare O and N. O has a higher EO, hence the acidity of alcohols will be higher.
2. Influence of the hydrocarbon radical and its substituents

It is necessary to draw students' attention to the fact that the compared compounds must have the same acid center and the same solvent.

Electron-withdrawing (EA) substituents contribute to the delocalization of the electron density, which leads to the stability of the anion and, accordingly, an increase in acidity.

Electron donating (ED) substituents on the contrary, they contribute to the concentration of electron density in the acid center, which leads to a decrease in acidity and an increase in basicity.

For example: monohydric alcohols exhibit weaker acidic properties compared to phenols.

Example: H 3 C > CH 2 > OH

1. The acid center is the same
2. The solvent is the same

In monohydric alcohols, the electron density shifts from the hydrocarbon radical to the OH group, i.e. the radical exhibits + I effect, then a large amount of electron density is concentrated on the OH group, as a result of which H + is more firmly bonded to O and the breaking of the O-H bond is difficult, therefore, monohydric alcohols exhibit weak acidic properties.

In phenol, on the contrary, the benzene ring is E.A., and the OH group is E.D.

Due to the fact that the hydroxyl group is included in the common p-p conjugation with the benzene ring, electron density delocalization occurs in the phenol molecule and acidity increases, tk. conjugation is always accompanied by an increase in acidic properties.

An increase in the hydrocarbon radical in monocarboxylic acids also affects the change in acid properties, and when substituents are introduced into the hydrocarbon, the acid properties change.

Example: in carboxylic acids, during dissociation, carboxylate ions are formed - the most stable organic anions.

In the carboxylate ion, the negative charge due to p, p-conjugation is distributed equally between two oxygen atoms, i.e. it is delocalized and, accordingly, less concentrated; therefore, the acid center in carboxylic acids is stronger than in alcohols and phenols.

With an increase in the hydrocarbon radical, which plays the role of E.D. the acidity of monocarboxylic acids decreases due to a decrease in q + on the carbon atom of the carboxyl group. Therefore, in the homologous series of acids, formic acid is the strongest.

With the introduction of E.A. substituent in a hydrocarbon radical, such as chlorine - the acidity of the compound increases, because due to the -I effect, the electron density is delocalized and q + on the C atom of the carboxyl group increases, therefore, in this example, trichloroacetic acid will be the strongest.

3. Influence of the solvent.

The interaction of molecules or ions of a solute with a solvent is called a process solvation. The stability of an anion essentially depends on its solvation in solution: the more the ion is solvated, the more stable it is, and the greater the solvation, the smaller the size of the ion and the less delocalization of the negative charge in it.

According to Lewis, the acidic and basic properties of organic compounds are measured by the ability to accept or donate an electron pair, followed by the formation of a bond. An atom that accepts an electron pair is an electron acceptor, and a compound containing such an atom should be classified as an acid. An atom that provides an electron pair is an electron donor, and a compound containing such an atom is a base.

Specifically, Lewis acids can be an atom, molecule or cation: proton, halides of elements of the second and third groups of the Periodic system, transition metal halides - BF3, ZnCl2, AlCl3, FeCl3, FeBr3, TiCl4, SnCl4, SbCl5, metal cations, sulfuric anhydride - SO3, carbocation. Lewis bases include amines (RNH2, R2NH, R3N), alcohols ROH, ethers ROR

According to Bronsted-Lowry, acids are substances capable of donating a proton, and bases are substances that accept a proton.

Conjugate acid and base:

HCN (acid) and CN- (base)

NH3 (base) and NH4+ (acid)

Acid-base (or protolytic) equilibrium is an equilibrium in which a proton (H +) participates.

HCOOH + H 2 O D H 3 O + + HCOO -

acid 2 base 1

H 2 O + NH 3 D NH 4 + + OH -.

acid 1 base 2 conjugate conjugate

acid 2 base 1

7. Types of isomerism in organic chemistry. Structural, spatial and optical isomerism. Chirality. Compatibility and configuration. R,S, Z,E - nomenclature.

There are two types of isomerism: structural and spatial (stereoisomerism). Structural isomers differ from each other in the order of bonds of atoms in a molecule, stereo-isomers - in the arrangement of atoms in space with the same order of bonds between them.

Structural isomerism: carbon skeleton isomerism, position isomerism, isomerism of various classes of organic compounds (interclass isomerism).

Structural isomerism

Isomerism of the carbon skeleton

Position isomerism is due to the different position of the multiple bond, substituent, functional group with the same carbon skeleton of the molecule:

Spatial isomerism

Spatial isomerism is divided into two types: geometric and optical.

Geometric isomerism is characteristic of compounds containing double bonds and cyclic compounds. Since free rotation of atoms around a double bond or in a cycle is impossible, substituents can be located either on one side of the plane of the double bond or cycle (cis position), or on opposite sides (trans position).

Optical isomerism occurs when a molecule is incompatible with its image in a mirror. This is possible when the carbon atom in the molecule has four different substituents. This atom is called asymmetric.

CHIRALITY, property of an object to be incompatible with its reflection in an ideal flat mirror.

Various spatial structures that arise due to rotation around simple bonds without violating the integrity of the molecule (without breaking chemical bonds) are called CONFORMATIONS.

The structure of alkanes. Sp3 is the state of carbon. Characterization of C-C and C-H bonds. The principle of free rotation. conformation. Methods of representation and nomenclature. Physical properties of alkanes.

According to Bronsted-Lowry, acids are substances capable of donating a proton, and bases are substances that accept a proton.

Conjugate acid and base:

HCN (acid) and CN- (base)

NH3 (base) and NH4+ (acid)

Acid-base (or protolytic) equilibrium is an equilibrium in which a proton (H +) participates.

HCOOH + H 2 O D H 3 O + + HCOO -

acid 2 base 1

H 2 O + NH 3 D NH 4 + + OH -.

acid 1 base 2 conjugate conjugate

acid 2 base 1

7. Types of isomerism in organic chemistry. Structural, spatial and optical isomerism. Chirality. Compatibility and configuration. R,S, Z,E - nomenclature.

Structural isomerism: carbon skeleton isomerism, position isomerism, isomerism of various classes of organic compounds (interclass isomerism).

Structural isomerism

Isomerism of the carbon skeleton

Position isomerism is due to the different position of the multiple bond, substituent, functional group with the same carbon skeleton of the molecule:

Spatial isomerism

Spatial isomerism is divided into two types: geometric and optical.

CHIRALITY, property of an object to be incompatible with its reflection in an ideal flat mirror.

Various spatial structures that arise due to rotation around simple bonds without violating the integrity of the molecule (without breaking chemical bonds) are called CONFORMATIONS.

All carbon atoms in alkane molecules are in the state sp 3 hybridization, the angle between the C-C bonds is 109 ° 28 ", therefore, the molecules of normal alkanes with a large number of carbon atoms have a zigzag structure (zigzag). The length of the C-C bond in saturated hydrocarbons is 0.154 nm

The C-C bond is covalent non-polar. The C-H bond is covalent and weakly polar, as C and H are close in electronegativity.

Physical properties

Under normal conditions, the first four members of the homologous series of alkanes are gases, C 5 -C 17 are liquids, and starting from C 18 are solids. The melting and boiling points of alkanes and their densities increase with increasing molecular weight. All alkanes are lighter than water, insoluble in it, but soluble in non-polar solvents (for example, in benzene) and are themselves good solvents.

· Melting and boiling points decrease from less branched to more branched.

Gaseous alkanes burn with a colorless or pale blue flame, releasing large amounts of heat.

The rotation of atoms around the s-bond will not break it. As a result of intramolecular rotation along C–C s-bonds, alkane molecules, starting from C 2 H 6 ethane, can take different geometric shapes.
Various spatial forms of a molecule, passing into each other by rotation around C–C s-bonds, are called conformations or rotational isomers(conformers).
The rotational isomers of a molecule are its energetically unequal states. Their interconversion occurs quickly and constantly as a result of thermal motion. Therefore, rotational isomers cannot be isolated individually, but their existence has been proven by physical methods.

alkanes .
methane, ethane, propane, butane –an

9. Hydrocarbons. Classification. Limit hydrocarbons of the methane series. homologous series. Nomenclature. Isomerism. Radicals. natural sources. Fischer-Tropsch synthesis. Preparation methods (from alkenes, carboxylic acids, halogen derivatives, by the Wurtz reaction)

General (generic) name of saturated hydrocarbons - alkanes .
The names of the first four members of the homologous series of methane are trivial: methane, ethane, propane, butane . Starting from the fifth name, they are formed from Greek numerals with the addition of a suffix –an

Radicals (hydrocarbon radicals) also have their own nomenclature. Monovalent radicals are called alkyls and are denoted by the letter R or Alk.
Their general formula is C n H 2n+ 1 .
The names of the radicals are formed from the names of the corresponding hydrocarbons by replacing the suffix -en to suffix -silt(methane - methyl, ethane - ethyl, propane - propyl, etc.).
Divalent radicals are named by changing the suffix -en on -ylidene(an exception is the methylene radical == CH 2).
Trivalent radicals have the suffix -ylidine

Isomerism. Alkanes are characterized by structural isomerism. If an alkane molecule contains more than three carbon atoms, then the order of their connection may be different. One of the isomers of butane ( n-butane) contains an unbranched carbon chain, and the other - isobutane - branched (isostructure).

The most important source of alkanes in nature is natural gas, mineral hydrocarbon raw materials - oil and associated petroleum gases.

The production of alkanes can be carried out by the Wurtz reaction, which consists in the action of metallic sodium on monohalogen derivatives of hydrocarbons.
2CH 3 -CH 2 Br (ethyl bromide) + 2Na -–> CH 3 -CH 2 -CH 2 -CH 3 (butane) + 2NaBr

from alkenes

C n H 2n + H 2 → C n H 2n+2

Fischer-Tropsch synthesis

nCO + (2n+1)H 2 → C n H 2n+2 + nH 2 O

The table shows that these hydrocarbons differ from each other in the number of groups - CH2-. Such a series of similar in structure, having similar chemical properties and differing from each other in the number of these groups is called a homologous series. And the substances that make it up are called homologues.

Formula	Name
CH 4	methane
C 2 H 6	ethane
C 3 H 8	propane
C 4 H 10	butane
C 4 H 10	isobutane
C 5 H 12	pentane
C 5 H 12	isopentane
C 5 H 12	neopentane
C 6 H 14	hexane
C 7 H 16	heptane
C 10 H 22	dean

10. Limit hydrocarbons (alkanes). Chemical and physical properties: radical substitution reactions. Halogenation, nitriding, sulfochlorination, sulfoxidation. The concept of chain reactions.

Physical properties

According to the Lowry-Bronsted theory, acids are substances capable of donating a proton, bases are substances that accept a proton:

If B is a strong base, then it is a weak acid. With help, you can characterize the degree of dissociation of an acid or conjugate acid. Along with the acidity constant, there is also the concept of the basicity constant and the corresponding

According to the Lewis theory, acids are compounds that can accept, bases can donate a pair of electrons.

In a broad sense, acids are compounds that supply a cation, in a particular case, a proton, or accept a pair of electrons with an atom or a group of atoms, etc.).

Bases accept a cation, in a particular case, a proton, or provide a pair of electrons with an atom or a group of atoms

The acidity or basicity of a substance is manifested in the process of interaction with another substance, in particular with a solvent, and therefore is relative.

Many substances have amphoteric properties. For example, water, alcohols and acids are capable of donating a proton when interacting with bases, and accepting it with acids. In the absence of acids and bases, the dual nature of such compounds is manifested in autoprotolysis:

The dissociation of an acid in a solvent means the transfer of a proton to the solvent:

In this regard, the strength of the acid is expressed by the dissociation constant, which is characteristic only for a given solvent. Proton transfer occurs only in highly ionizing and solvating solvents, such as water.

The degree of acid dissociation in the transition from an aqueous medium to an organic one decreases by 4-6 orders of magnitude.

Strongly solvating and ionizing solvents neutralize the strength of acids, while non-polar and low-iolar solvents, interacting with them at the level of hydrogen bonds, have a differentiating effect. In the latter case, the differences in the strength of the acids become more significant.

In inert, non-polar solvents, the probability of proton detachment is very small, although due to internal electronic effects, the bond can be highly polarized. Under such conditions, acidic properties manifest themselves in self-association of HA molecules or in association with proton acceptors, bases. In the latter case, the measure of acidity is the association constant with some base chosen as a standard. For example, the association constant of benzoic acid and diphenylguanidine in benzene is

The protonizing power of an acid is also expressed in terms of the acidity function, which characterizes the state of equilibrium during the complexation of acids and bases in organic solvents. The most commonly used bases are indicators that change color depending on the strength of the acid, which makes it possible to study the system by spectroscopic methods. In this case, it is important that bands of the associated free base are identified in the spectrum.

So, in an introductory medium, acids and bases form solvated ions, in an organic medium, ion pairs and their associates.

Close in meaning to the concept of association is the concept of complex formation: due to donor-acceptor and dative interactions, electron-donor-acceptor complexes, also called charge-transfer complexes, can form from ions and molecules.

Types of electron donors: I) compounds with heteroatoms. containing lone pairs of electrons, ethers, amines, sulfides, iodides, etc. For example: diethyl ether otlampn. ldmethylsulfpd triphenyl-phosphine-propyl iodide

2) compounds containing -bonds ethylene, acetylenes, benzene and its derivatives, other aromatic systems;

3) compounds capable of transferring electrons - bonds alkanes, cycloalkanes:

Types of electron acceptors: 1) metal compounds containing a vacant orbital (K-orbital): halides, etc., metal ions

2) compounds capable of accepting a pair of electrons per vacant antibonding halogens, mixed halogens

3) compounds with -bonds with strongly electronegative substituents participating in complex formation due to loosening tetracyanoethylene trinitrobenzene

Thus, either the donor can interact with the vacant acceptor, forming a new MO with a decrease in the energy of the system:

In organic chemistry, -complexes are of the greatest importance, and -complexes are characterized by instability constants, which are, in fact, their dissociation constants.

The dissociation and association constants of acids and bases still do not fully describe their properties. An important role in understanding many chemical processes, and in particular the phenomenon of catalysis, was played by the concept of hard and soft acids and bases (the principle

GMCCO). In accordance with this concept, related acids and bases interact most effectively: a soft acid with a soft base, a hard acid with a hard base.

Signs of hard acids and bases (Table 8): 1) small size of an ion or molecule; 2) high electronegativity; 3) localized charge; 4) low polarizability; 5) the lowest vacant orbitals (HVO) of acids have high energy; 6) the highest filled orbitals (HOO) of the bases have low energy.

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Conjugate acids and bases. Acids and bases in organic chemistry. Conjugate acid and conjugate base. Acid-base balances, examples. Effect of substituents in a molecule on acidity and basicity. Modern ideas about acids and bases

Modern ideas about acids and bases. Bronsted and Lowry

Bronsted-Lowry theory. Conjugate acids and bases.