(Fischer projection), a way of depicting spaces on the plane. structures org. Comm., having one or several. chiral centers. When projecting a molecule onto a plane (Fig.) asymmetric. the atom is usually omitted, retaining only the crossed lines and substituent symbols; in this case, the substituents in front of the plane are located on the right and left, and behind the plane - at the top and bottom (dashed line).

F-ly Fisher for a molecule with one asymmetric. atom (I), as well as a scheme for constructing such a f-ly for compounds with two asymmetric. atoms (P) HAWORS FORMULA

HAWORS FORMULA

(Hewers f-ly), image on the plane of spaces. cyclic structures. conn. When building X. f. the cycle is conditionally considered flat (in fact, the molecule can be in the conformation of a chair or bath) and is projected onto a plane at a certain angle; in this case, the part of the ring closest to the observer in the drawing is located below and is usually distinguished by a thicker line (Fig.). In monosaccharides, the oxygen atom of the cycle is usually located on the max. away from the observer (in the case of the pyranose cycle, on the right).

Fisher's formulas (a) and Haworth's (b) formulas of monosaccharides - -D-glucopyranose (I) and -L-galactofuranose (II).

Atoms or groups of atoms depicted in Fischer f-lakhs on the left and on the right, in X. f. located respectively. above and below the cycle plane. Side chains at the C-5 atom in pyranoses or at C-4 in furanoses are shown above the ring plane in the D-configuration of the carbon atom and below the plane in the L-configuration (see Fig. Stereochemical nomenclature).
Proposed by W. Haworth (Heworth) in 1926.

55. Compounds with a hydroxyl group.

Hydroxyl group (hydroxyl) is the functional group OH of organic and inorganic compounds, in which the hydrogen and oxygen atoms are linked by a covalent bond. In organic chemistry, it is also called " alcohol group».

Phenols are derivatives of aromatic hydrocarbons whose molecules contain one or more hydroxyl groups directly connected to the benzene ring.

The names of phenols are made taking into account the fact that the trivial name "phenol" is preserved for the parent structure according to the rules of IUPAC. The numbering of the carbon atoms of the benzene ring starts from the atom directly bonded to the hydroxyl group (if it is the highest function), and continues in such a sequence that the substituents present receive the smallest numbers.

The simplest representative of this class is phenol itself, C 6 H 5 OH.

The structure of phenol. One of the two unshared electron pairs of the oxygen atom is drawn into the -electron system of the benzene ring (+ M-EFFECT of the OH group). This leads to two effects: a) the electron density in the benzene ring increases, and the electron density maxima are in the ortho and para positions with respect to the OH group; b) the electron density on the oxygen atom, on the contrary, decreases, which leads to a weakening of the O–H bond. The first effect is manifested in the high activity of phenol in electrophilic substitution reactions, and the second - in the increased acidity of phenol compared to saturated alcohols.

Monosubstituted phenol derivatives, such as methylphenol (cresol), can exist in the form of three structural isomers - ortho-, meta- and para-cresols:

physical properties. Phenols are mostly crystalline substances (meta-cresol - liquid) at room temperature. They have a characteristic odor, are rather poorly soluble in cold water, but well - in hot and especially in aqueous solutions of alkalis. Phenols form strong hydrogen bonds and have fairly high boiling and melting points. So, phenol itself is a colorless crystals with t pl = 41 ° C and t bp = 182 ° C. Over time, the crystals turn red and darken.

56. Five-membered heterocyclic compounds.

Five-membered heterocycles- organic cyclic compounds, which include at least one heteroatom.

The most famous representatives:

Representative	Structural formula	Related compounds
Furan		Furfural, Pyrosmucus acid, Cumparone, Isobenzfuran, Tetrahydrofuran, 1,3-dioxolane
thiophene		Thionaften, Thioften, Tetrahydrothiophene, Thiolane, Thiolandoxide, Biotin
pyrrole		Indole, Oxindol, Indoxyl, Isatin, Carbazole, Pyrrolidine, 2-pyrrolidone, N-methylpyrrolidone, Proline
Oxazol		Benzoxazole, 2-oxazoline
Isoxazole

57.alcohols aliphatic compounds containing a hydroxyl group are called (alkanols, alkenols, alkynols); hydroxyarenes or aromatic hydroxy derivatives are called phenols . The name of an alcohol is formed by adding the suffix -ol to the name of the corresponding hydrocarbon or based on a hydrocarbon radical. Depending on the structure of the hydrocarbon radical, alcohols are distinguished:

primary:

secondary:

tertiary:

Monatomic phenols:

Phenols are characterized by stronger acidic properties than alcohols; the latter do not form carbonium ions AIk - O - in aqueous solutions, which is associated with a lower polarizing effect (electroacceptor properties of alkyl radicals compared to aromatic ones).

Alcohols and phenols nevertheless readily form hydrogen bonds, so all alcohols and phenols have higher boiling points than the corresponding hydrocarbons.

If the hydrocarbon radical does not have pronounced hydrophobic properties, then these alcohols dissolve well in water. The hydrogen bond determines the ability of alcohols to pass during hardening into a glassy rather than a crystalline state.

Compounds with two or more hydroxyl groups are called polyhydric alcohols and phenols:

58. Lewis acids and bases.

J. Lewis proposed a more general theory of acids and bases.

Lewis foundations these are electron pair donors (alcohols, alcoholate anions, ethers, amines, etc.)

Lewis acids - they are electron pair acceptors , those. compounds having a vacant orbital (hydrogen ion and metal cations: H +, Ag +, Na +, Fe 2+; halides of elements of the second and third periods BF 3, AlCl 3, FeCl 3, ZnCl 2; halogens; tin and sulfur compounds: SnCl 4 , SO 3).

Thus, the Brønsted and Lewis bases are the same particles. However, Bronsted basicity is the ability to attach only a proton, while Lewis basicity is a broader concept and means the ability to interact with any particle that has a low-lying free orbit.

According to Lewis, the acid-base interaction is a donor-acceptor interaction, and any heterolytic reaction can be represented as the interaction of a Lewis acid and a Lewis base:

There is no single scale for comparing the strength of Lewis acids and bases, since their relative strength will depend on which substance is taken as the standard (for Bronsted acids and bases, this standard is water). To assess the ease of acid-base interaction according to Lewis, R. Pearson proposed a qualitative theory of “hard” and “soft” acids and bases.

Rigid bases have high electronegativity and low polarizability. They are difficult to oxidize. Their highest occupied molecular orbitals (HOMO) are of low energy.

Soft grounds have low electronegativity and high polarizability. They oxidize easily. Their highest occupied molecular orbitals (HOMO) are high energy.

Hard acids have high electronegativity and low polarizability. They are difficult to recover. Their lowest free molecular orbitals (LUMO) have low energy.

Soft acids have low electronegativity and high polarizability. They are easy to recover. Their lowest free molecular orbitals (LUMOs) are high energy.

The hardest acid is H +, the softest is CH 3 Hg +. The hardest bases are F - and OH - , the softest ones are I - and H - .

59. .Ethers.

Ethers- organic substances having the formula R-O-R 1 where R and R 1 are hydrocarbon radicals. However, it should be taken into account that such a group may be part of other functional groups of compounds that are not ethers.

• According to Williamson

Under laboratory conditions, esters are obtained according to Williamson by the interaction of halogen derivatives capable of reacting with Sn2 and alkoxide and phenoxide ions. The reaction proceeds smoothly with halomethane and primary haloalkanes. In the case of secondary haloalkanes, the reaction can be complicated by a side elimination reaction.

Physical Properties

Ethers are mobile low-boiling liquids, sparingly soluble in water, very flammable. They exhibit weakly basic properties (they attach a proton at the O atom).

Ethers form peroxide compounds under the action of light:

As a result, when distilling ethers under laboratory conditions, it is forbidden to distill them to dryness, since in this case a strong explosion will occur as a result of the decomposition of peroxides.

The most important esters

Name	Formula	Melting temperature	Boiling temperature
dimethyl ether	CH 3 OCH 3	-138.5°C	-24.9°C
diethyl ether	CH 3 CH 2 OCH 2 CH 3	-116.3°C	34.6°C
Diisopropyl ether	(CH 3) 2 CHOCH(CH 3) 2	-86.2°C	68.5°C
Anizol		-37°C	154°C
Oksiran		-111.3°C	10.7°C
Tetrahydrofuran		-108°C	65.4°C
dioxane		11.7°C	101.4°C
Polyethylene glycol	HOCH 2 (CH 2 OCH 2) n CH 2 OH	−	−

biological significance

Aryl ethers - preservatives, antioxidants, are used in the perfume industry. Some ethers are insecticidal.

60. Substitutive nomenclature (IUPAC) for organic compounds.

In the substitutional IUPAC nomenclature, the name of an organic compound is determined by the names of the main chain (the root of the word), in which carbon atoms are numbered in a certain order, as well as substituents and functional groups (denoted as prefixes or suffixes). Any atom or group of atoms substituting for hydrogen is considered as a substituent. A functional group is an atom or a group of non-hydrocarbon atoms that determine whether a compound belongs to a particular class. If there are several groups, then the older one is distinguished:

IUPAC is the generally accepted nomenclature, now it is the standard in chemistry.

61. Oxidation of C-H and C = C bonds.

62. Covalent bonds. Hybridization.

covalent bond(atomic bond, homeopolar bond) - a chemical bond formed by the overlap (socialization) of a pair of valence electron clouds. The electronic clouds (electrons) that provide communication are called common electron pair. A covalent bond is formed by a pair of electrons shared between two atoms, and these electrons must occupy two stable orbitals, one from each atom.

A + B → A: B

As a result of socialization, electrons form a filled energy level. A bond is formed if their total energy at this level is less than in the initial state (and the difference in energy will be nothing more than the bond energy).

According to the theory of molecular orbitals, the overlap of two atomic orbitals leads in the simplest case to the formation of two molecular orbitals (MOs): binding MO And antibonding (loosening) MO. Shared electrons are located on a lower energy binding MO. Hybridization of orbitals- a hypothetical process of mixing different (s, p, d, f) orbitals of the central atom of a polyatomic molecule with the appearance of identical orbitals, equivalent in their characteristics.

Types of hybridization

Projection of a three-dimensional molecule onto a plane

Fisher projection (Fisher projection formula, Fisher formula) is a way of depicting a three-dimensional molecule in the form of a projection, in which the vertical bonds are removed beyond the projection plane, and the horizontal bonds protrude in front of this plane. These formulas were proposed by E. Fisher in 1891 to depict the structures of carbohydrates. The use of Fischer projections for non-carbohydrate molecules can be misleading and is not recommended by IUPAC.

Building

In the Fisher projection, chemical bonds are depicted as horizontal and vertical lines, at the crossroads of which there are stereocenters. The carbon skeleton is depicted vertically, with the carbon atom at the top, from which the numbering of the skeleton begins (for example, the aldehyde atom for aldoses). In addition, in the Fisher projection, all horizontal links are directed towards the observer, and vertical ones are removed from the observer. This condition is important for the correct construction of the Fisher projection, as well as for restoring the three-dimensional structure of a molecule from its projection. For this reason, the Fisher projection cannot be rotated 90° or 270°, as this will change the configuration of the stereocenters. Hydrogen atoms should be depicted explicitly according to IUPAC guidelines, but structures without hydrogen atoms are also considered acceptable.

Recovery of 3D Recording

To restore the spatial form of the molecule from the Fisher projection, it is necessary to depict the horizontal bonds directed towards the observer (bold wedges), and the vertical ones - going beyond the image plane (dashed wedges). Next, you can depict the molecule in any three-dimensional representation.

Usage

Fischer projections are most widely used to construct the structural formulas of monosaccharides, as well as amino acids. They also form the basis of the d/l nomenclature used to distinguish between the enantiomers of these natural compounds.

Substances that can rotate the plane of polarization of light passing through them are called optically active. This phenomenon itself is called optical activity. Optically active substances exist in the form of pairs optical antipodes or enantiomers, which differ (ceteris paribus - the same concentration, the same path length of the light beam in the substance) by the sign of rotation of the plane of polarization of light.

Molecules of optically active substances have the property chirality- enantiomers relate to each other as the original and its mirror image (incompatible under any rotation). Most often, for the occurrence of chirality, the presence in the molecule is necessary. chiral carbon atom ( chiral or asymmetric center) - in a state of sp 3 hybridization and having four different substituents:

An equimolar mixture of enantiomers has no optical activity. Such a mixture is called racemic mixture or racemate.

If a molecule contains several chiral centers, it is very difficult to depict it in a projection similar to the previous figure. In this case, use the projection formulas E. Fisher.

The number of stereoisomers in the case of several chiral centers can be determined by the formula 2 n where n is the number of chiral carbon atoms. In the case of aldotetroses, in which there are two chiral centers, there are 4 stereoisomers:

Molecules 1 and 2, 3 and 4 are enantiomers. Molecules 2 and 4, 1 and 3, 2 and 3 are not enantiomers, however, they are stereoisomers.

Stereoisomers that are not enantiomers are called diastereomers.

Diastereomers differ in chemical and physical properties and can be separated by conventional chemical methods.

The number of stereoisomers may be less than 2n if there is mesoforms. The mesoform occurs if the molecule has internal planes of symmetry. For example, tartaric acid has three stereoisomers:

If isomers 1 and 2 are a pair of enantiomers, then 3 and 4 are the same thing - the molecule has an internal symmetry plane, shown by a dotted line. The meso form is essentially an intramolecular racemate. Indeed, the top 3 (above the dotted line) is a mirror image of the bottom. The optical activity of the mesoform does not possess.

Nomenclature of optical isomers

The first substances for which the phenomenon of optical isomerism was discovered and studied were carbohydrates and amino acids. Therefore, it has historically developed so that the stereoisomers of these compounds are determined by belonging to one or another steric series and to erythro-threo isomers. For compounds of other classes, the concept is used absolute chiral center configuration.

Fisher projection formulas

Fisher's formulas are one of the ways to represent the three-dimensional structure of a chiral center on a plane. Let's take a pair of enantiomers and build the Fisher projection for the right molecule:

Let's choose the direction from which we will consider the molecule - it is shown by an arrow:

In this case, the links C-A and C-E are directed towards us, they, in accordance with the rules for writing the Fisher formula, are depicted by a horizontal line. Links C-B and C-D are directed away from us, they are depicted by a vertical line. As a result, the Fisher projection will look like (1):

Currently, both the vertical and horizontal lines are drawn as solid, the carbon atom is not drawn - the intersection of the lines and implies a chiral center, as a result, the projection (2) is generally accepted.

If we consider the same molecule from the other side, then we can get another Fischer projection:

In general, twelve Fischer projections can be drawn for a given molecule. In order to compare the obtained projections with each other, it is necessary to take into account that the Fisher projections allow a number of transformations over themselves.

Transformations that preserve the original formula

1. An even number of permutations. By permutation is meant the exchange of places of any two deputies. For example, in the formula 2b, you can first change D and A (the first permutation), and then E and D (which now stands in place of A) - this will be the second permutation, as a result, 2b has been transformed into 2. It is noticeable that this is the same thing.

2. Projection rotation in the drawing plane by 180, 360, 540, etc. degrees:

3. Cyclic permutation: one substitute (any) is left in place, the remaining three are rearranged in a circle - clockwise or counterclockwise. This operation is equivalent to two permutations, but is sometimes more convenient.

Transformations leading to an enantiomer

1. An odd number of permutations - swap D and E - one permutation, using a mirror depicted by a vertical dotted line, it is easy to verify that these are enantiomers.

2. Rotation in the plane of the drawing by 90, 270, 450, etc. degrees. Rotate 2b 90 o counterclockwise:

In the resulting formula, we will make an even number of permutations - swap B and E, A and D. Comparing 2b and what happened, we observe that this is an enantiomer.

3. Reflection in the mirror or viewing "in the light."

Fisher standard projection

In the standard notation of the Fisher projection, the main chain or cycle is depicted as a vertical line, the numbering of carbon atoms (according to IUPAC) in the chain goes from top to bottom.

Aldehydes and ketones are known to react with alcohols to form hemiacetals and ketals. Cyclic hemiacetals are formed especially easily. For this, the necessary conditions are: 1) the hydroxyl and carbonyl group must be parts of the same molecule; 2) when they interact, a five- or six-membered ring can be formed.
For example, 4-hydroxypentanal forms a five-membered cyclic hemiacetal. This creates a new stereocenter at carbon C-1 (all four substituents at C-1 are different):

Similarly, 5-hydroxyhexanal forms a six-membered cyclic hemiacetal, which also generates a new stereocenter at C-1:

Hydroxyl and carbonyl groups are contained in one molecule of monosaccharides, so monosaccharides exist almost exclusively in the form of cyclic hemiacetals.
Fisher's cyclic projections. The size of the hemiacetal ring of a monosaccharide is compared with heterocyclic molecules - pyran and furan:

Six-membered hemiacetal rings are designated by the word "pyran", and five-membered - "furan".
When crystallized from ethanol, D-glucose gives -D-glucopyranose, t pl = 146 °С, specific optical rotation D = +112.2°. Crystallization from aqueous ethanol gives -D-glucopyranose, t pl \u003d 150 ° С, D \u003d + 18.7 °. These - and -isomers - six-membered cyclic hemiacetals - are formed by the reaction of OH hydroxyl at carbon C-5 with a carbonyl group in position 1. The new stereocenter that arises when obtaining a hemiacetal is called anomeric carbon. The diastereomers formed in this way have a special name - anomers. The configuration of an anomeric carbon is denoted by the prefix when its hydroxyl group is on the same side of the Fischer projection as the OH group at the highest-numbered stereocenter. With the opposite orientation of these hydroxyls, the configuration of the anomeric carbon is .

According to the 13 C NMR method of D-glucose in an aqueous solution, there are: -pyranose (38.8%),
-pyranose (60.9%), -furanose (0.14%), -furanose (0.15%), open linear hydrate (0.0045%).
Here are the - and - forms of glucofuranose in comparison with the cyclic forms of fructose -
-fructofuranose and -fructofuranose.

In aldoses, ring closure is possible due to the 1st (aldehyde) carbon and hydroxyl at the 4th (or 5th) C atom, and in ketoses - due to the 2nd (carbonyl) carbon and hydroxyl in the 5th or 6th -th position of the chain.

Haworth formulas. An alternative way of representing the cyclic structures of monosaccharides is known as Haworth projections and is named after the English chemist Walter Haworth (Nobel laureate, 1937). In Haworth's formulas, five- and six-membered cyclic hemiacetals are represented as flat five- or hexagons located, as it were, perpendicular to the plane of a sheet of paper. The groups attached to the carbons of the ring are placed above or below the plane of the ring and parallel to the plane of the paper. In Haworth's formulas, the anomeric carbon is usually written on the right, and the hemiacetal oxygen is written behind it. The Haworth projections of the - and -pyranose forms of D-glucose are shown below.

EXERCISES.

1. What does the concept of "cyclic forms of carbohydrates" mean?

2. Give the structural and projection formulas of Fisher for: a) triose; b) tetroses;
c) pentoses.

3. How to distinguish by chemical formulas L - And D- isomers (for example, erythrose)?

4. Specify acetal bonds and asymmetric carbon atoms (stereocenters) in the compounds:

5. Write the structural formulas of pyran and furan heterocycles, indicating each atom.

6. Draw schemes for the formation of cyclic hemiacetal forms from:
a) D - threoses; b) D riboses (furanose and pyranose forms).

7. Transform the graphic formulas of compounds a)–c) into Fisher projections and assign these projections to D - or L -glyceraldehyde:

8. How much is ketotetrosis possible? For each draw a Fisher projection.

9. Write Haworth's formulas:

1) -D- glucopyranose; 2) -D- glucofuranoses.

Answers to exercises for topic 2

Lesson 34

1. Cyclic forms of carbohydrates contain a cycle with oxygen in the ring. It is usually a cyclic hemiacetal. There is no free aldehyde group in its molecule, but there is an acetal bond. For example, for erythrosis:

3. In order to distinguish between the D- and L-isomers of erythrose by chemical formulas, they should be presented in the form of Fisher projections. The orientation of the hydroxyl to the right at the highest stereocenter C * -3 means
D isomer. The direction of the HO group to the left of C * -3 is characteristic of the L-isomer:

4. Acetal bonds are marked with an arrow (), and stereocenters are marked with an asterisk (*):

c) two successive permutations of substituents do not change the configuration (D or L) at the stereocenter:

8. Two enantiomeric ketotetroses are possible, for which the Fischer projections are as follows:

9. Haworth formulas:

diastereomers- stereoisomers, the molecules of which are not mirror images of each other.

For the image on the i-plane of molecules with asymmetric carbon atoms, the projections proposed in 18-1 by E. Fisher are often used.

Consider the principle of their construction using the example of a bromofluorochloromethane molecule. The starting point for constructing Fisheoa projections is the spatial model of the molecule or its wedge-shaped projection.
Let us arrange the molecule in such a way that only the carbon atom of the bromfluorochloromethane molecule remains in the plane of the drawing, as shown in the figure:

Let's project all atoms onto the drawing plane (Br and CL from the bottom up, since they are located under the drawing plane, and F and H - from top to bottom). In order for the resulting projection to differ from the structural formula, we agree not to represent the asymmetric carbon atom. He implied in the Fisher projection at the intersection of the vertical and horizontal lines:

As can be seen from the above example, the Fischer projection is constructed in such a way that the bonds of an asymmetric atom with substituents are depicted by vertical and horizontal (but not oblique!) lines.

When using Fisher projections, it is important to remember that the vertical line in them depicts connections moving away from us, and the horizontal line - connections directed towards us. . This implies the rules for using Fisher projections:

The listed operations with Fisher projections cannot be carried out, since they lead to the projection of the antipode.

Examples.
a) An even number of pairwise permutations. Let's swap the F and CI, Br and H atoms in the bromofluorochloromethane molecule:

b) Circular permutation of three deputies. Let's make a circular permutation of the halogen atoms. The hydrogen atom is left untouched:

When constructing Fischer projection formulas for molecules that include several carbon atoms, the molecule is arranged in such a way that the carbon chain is arranged vertically. Placed at the top most oxidized a carbon atom (as a rule, this atom is part of the carbonyl CH \u003d O or carboxyl COOH groups.):