Topic 2.

Chapter 3. Alkenes, alkadienes, alkynes (unsaturated hydrocarbons)

Unsaturated (unsaturated) hydrocarbons include alkenes (CnH2n), alkadienes (CnH2n-2) and alkyne (CnH2n-2) containing multiple carbon-carbon bonds.

Alkyne contains a triple bond in the molecule.

2.3.1. The structure of the carbon atom of alkenes

Alkenes contain a C = C double bond, the carbon atoms at the double bond are in the sp2 hybridization state. Hybridization scheme:

The location and shape of molecular orbitals of sp2-hybridized carbon alkenes in space is as follows:

Hybrid orbitals are located in the same plane and equidistant from each other, located at an angle of 120 degrees. The non-hybridized p-orbital is perpendicular to the plane of the hybrid orbitals.

The double bond is formed by overlapping (the orbital model of the ethylene molecule is shown):

a) sp2-hybridized orbitals of the carbon atom (s-bond) and

b) overlapping unhybridized p-orbitals (p-bond).

As a result of this structure of the multiple bond, there are some important consequences regarding the structure and reactivity of alkenes:

- there is unsaturation, therefore the addition of reagents is possible;

- there is an electron density accessible to the action of electrophilic reagents, formed by overlapping p-orbitals;

- rotation (for obvious reasons) around the multiple bond at ordinary temperature is limited, which is the reason for the appearance of spatial isomerism in the series of alkenes.

2.3.2. The structure of the carbon atom of alkadienes

Alkadienes contain TWO double bonds in a molecule. Depending on the relative position of multiple bonds one relative to the other, there are:

a) cumulated dienes:

CH3-CH=C=CH-CH2-CH2-CH3 (heptadiene-2,3) or CH3-CH2-CH2-CH=C=CH2 (hexadiene-1,2) (multiple bonds are adjacent);

Allene (prop-1,2-diene) is one of cumulated dienes and shown at right.

It can be noted that in cumulated dienes, the carbon atom with two double bonds has sp hybridization, while the adjacent carbons have the usual sp2 hybridization. The chemical properties of cumulated dienes differ sharply from those of both alkenes and other types of dienes.

b) conjugated alkadienes (or just dienes, multiple bonds are located through one simple one):

CH2 = CH-CH = CH2 (butadiene-1,3), CH3-CH=CH-CH=CH-CH2-CH3 (hept-2,4-diene)

In conjugated dienes, there is an overlap of adjacent p-orbitals (between carbon atoms, "formally" separated by a simple bond). This overlap takes place in reality, although it is not reflected in the structural formulas. In fact, in the 1,3-butadiene molecule, for example, there are no “pure” single or double bonds, but all bonds are “averaged” and represent something in between a single and a double bond. This averaging is not absolute. The length of the outer bonds is 1.37 A, and the intermediate bond is 1.46 A. Recall that the length of the isolated double bond C = C is 1.32 A, and the length of the simple C-C bond is 1.54 A.

As a result of this interaction of the orbitals, even some stabilization (resonance) energy appears, which reduces the internal energy of the molecule, albeit by a small, but noticeable amount of 3-4 kcal/mol (measured by comparing the heats of hydrogenation of isolated and conjugated dienes). It also has some chemical effects, which will be discussed below.

c) isolated dienes (multiple bonds are separated by more than one simple):

CH2 = CH-CH2-CH = CH2 (pent-1,4-diene)

Isolated dienes behave like ordinary alkenes because multiple bonds do not interfere with each other in their molecules.

2.3.3. The structure of the carbon atom of alkynes

Alkines are hydrocarbons containing a C = C triple bond, the carbons at which are in the sp-hybridized state. Scheme of sp-hybridization of carbon atom orbitals:

The spatial configuration of the molecular orbitals of the carbon atom in alkynes:

The triple bond is formed by overlapping two sp-hybridized orbitals (from each of the carbons) and four unhybridized p-orbitals (two from each carbon). Two unhybridized p-orbitals for each of the carbons are located in mutually perpendicular planes:

The figure shows the orbital model of the acetylene molecule.

2.3.4. Homologous series of alkenes

Ethylene is the parent of the homologous series of alkenes. The rest of the members of the series each differ from the previous one by the methylene unit –CH2-.

2.3.5. Alkenes nomenclature

The nomenclature of alkenes is very similar to the nomenclature of alkanes, with the exception that the presence of a multiple bond causes the appearance of isomers of its position, as well as isomers of the carbon skeleton. In a number of cases, geometric spatial isomerism appears. All these features are taken into account when compiling the names of alkenes.

When naming alkenes, the longest chain of carbon atoms is selected as a basis, INCLUDING a MULTIPLE bond, the chain is numbered from the edge to which the multiple bond(s) is located closer.

If there are two variants of chains of the same length, choose a chain with more substituents.

 

In this case, the spatial arrangement of the main chain does not matter. The name is formed by listing the numbers of the carbon atoms of the main chain and the substituents present with them. The ending in the name of the main carbon chain for alkenes is replaced by –EN, alkadienes - by DIEN, the position of multiple bonds is indicated through a dash (of the two carbon numbers between which the multiple bond is located, choose the LOWEST). In the example above, the compound would be named: trans-3,6-dimethyl-4-propylheptene-3.

Due to the presence of a multiple bond in a molecule, around which rotation is impossible, alkenes and alkadienes may (or may not) have spatial isomers. To determine the possibility of the presence of spatial isomers, it is necessary to determine whether there are DIFFERENT SUBSTITUTES AT EACH of the two carbons. If this requirement is met, then such a compound has spatial isomers. If at least one of the carbons at a multiple bond has the SAME substituents, spatial isomerism is impossible:

The figure above shows 3 alkenes, the first two of which cannot have cis-trans isomers, since one of the carbon atoms has the same substituents at a multiple bond. The third compound may have steric isomerism, since each of the carbon atoms in the multiple bond has different substituents.

Since each carbon atom located at a multiple bond (in the figure above) has DIFFERENT substituents, cis-trans isomerism is possible, and the figure shows cis-3-methylhept-2-ene.

If the main chain of carbons is located ONE SIDE of the axis of the multiple bond, then such an isomer is the CIS isomer, if the multiple bond crosses the axis of the multiple bond, it is the TRANS isomer.

The rational nomenclature of alkenes implies their naming as substituted ethylenes. When compiling the name, the substituents at the ethylene unit are listed. The name is based on ethylene. (For example, trimethyl-ethylene or the same as 2-methylbutene-2).

2.3.6. Alkyne nomenclature

Follows the same rules as alkenes nomenclature, except that the -in ending is assigned to the backbone hydrocarbon:

IUPAC: 6-methylhept-3-yne.

The chain is numbered from the edge where the multiple link is much closer.

When naming alkynes according to a rational nomenclature, they are called acetylene derivatives, listing the substituents at the triple bond carbons. Alkynes do not possess spatial isomerism.

2.3.7. General properties of unsaturated (unsaturated) hydrocarbons

A common place for unsaturated hydrocarbons is the presence of multiple bonds, that is, a certain degree of unsaturation, as a result of which the possibility of attaching a number of reagents, accompanied by the rupture of p-bonds.

Since p-bonds are regions of increased electron density, moreover, they are easily accessible, they become easy "prey" for electron-deficient reagents (electrophiles). Therefore, unsaturated hydrocarbons easily react with a variety of compounds even in the cold (that is, without heating).

IUPAC: 6-methylhept-3-yne.

The detailed mechanism of reactions of electrophilic addition to multiple bonds is described separately. It also describes the regularities of the addition of unsymmetrical reagents to unsymmetrical and substituted alkenes and alkynes. For alkyl-substituted ethylenes and acetylenes, when writing the addition products, one should use the Markovnikov’s rule, which states that hydrogen is attached to the most hydrogenated carbon atom (with a multiple bond).

This rule, which is valid only for reactions of electrophilic addition to alkenes, has a modern rationale, which makes it possible to explain the anomalous addition in the case of electron-withdrawing substituents with multiple bonds:

Along with the cyano group -CN, the nitro group -NO2, the nitroso group -N = O, the carbonyl group -C = O, the sulfo group –SO3H, as well as other groups, under the CONDITION that the atom directly bonded to the carbon atom with a multiple bond, DOES NOT HAVE lone pairs of electrons.

Otherwise, Markovnikov's rule is not violated:

2.3.8. Other reactions of alkenes and alkynes

1. Oxidation reactions

When interacting with KMnO4 solutions in the cold, dihydric alcohols (glycols) are formed from alkenes:

When interacting with hot KMnO4 solutions, as well as with energetic oxidants, the molecule breaks at the site of the multiple bond and carboxylic acids are formed, regardless of whether it was a double or a triple bond:

Ozonation followed by treatment of products (ozonides) with water leads to the formation of aldehydes and ketones (in the case of alkenes) or carboxylic acids (alkynes):

If there is more than one multiple bond in a molecule, they are ozonized at the same time.

Epoxy formation

Acid hydroperoxides quantitatively convert alkenes into their oxides, called epoxides (N.A. Prilezhaev):

The same result is obtained by oxidizing alkenes with hydrogen peroxide (in acetonitrile).

2. Polymerization reactions

Alkenes polymerize by different mechanisms, under different conditions, with the formation of long-chain polymer molecules (different molecular weights or degrees of polymerization). Polymerization of propylene:

Polymers of ethylene and propylene are very widely used in practice.

Conjugated dienes can be polymerized in two types. Depending on the structure of the repeating unit of the polymer, the types of 1,2- and 1,4-polymerization are distinguished. (Monomeric units in the chain are linked, respectively, through 1-2 and 1-4 carbon atoms):

Conjugated dienes in any type of polymerization have a multiple bond in the structure of the structural unit, which allows further modification of the polymers in order to impart the desired properties to them. So, for example, if finely ground sulfur is added to the mass of isoprene polymer (polyisoprene, an analogue of natural polymer - rubber) and the mixture is heated, crosslinking by covalent bonds of individual polymer chains occurs, as a result of which the structural and mechanical properties of the polymer are dramatically improved. The degree of "crosslinking" is related to the amount of elemental sulfur added to the polymer mass. At low sulfur contents, elastic polymers called rubbers are produced. If sulfur is added in excess, solid polymers are obtained (due to an increased degree of crosslinking) called ebonites.

In addition to sulfur, when making rubbers, soot and additives are also added to the mass to improve the performance properties of rubbers - antioxidants, plasticizers, light stabilizers, etc.

3. Cycling reactions (Diels-Alder reaction)

The reaction between conjugated dienes and alkenes with the formation of cyclic compounds, called the Diels-Alder reaction, has acquired an important synthetic significance. It consists in the formation of 6-membered rings in the reaction between a conjugated diene and an alkene. In the simplest example, such a reaction can occur between ethylene and 1,3-butadiene, with the formation of cyclohexene:

The figure shows a diagram of the redistribution of electrons with the formation of new bonds and the disappearance of old multiple bonds.

However, the reactions between alkene and alkadiene do not proceed very readily - at high temperatures, for a long time and with low yields of the target product. Reactions are much softer and more successful in cases where an alkene (usually called a dienophile) has electron-withdrawing substituents instead of hydrogen atoms (groups –C = O, NO2, etc.), and the diene has electron-donating groups (alkyl groups). Reactions are especially successful if the configuration of the diene is inhibited in the cisoid form, as shown by the example of the reaction between cyclohexadiene-1,3 and maleic anhydride (activated dienophile):

The resulting reaction products are often referred to as adducts.

Their spatial configuration is rather complicated:

4. Radical substitution in alkenes (Allyl substitution)

At a high temperature, or low concentration of halogen, the reaction of substitution of a hydrogen atom with a halogen at a saturated carbon atom located in the vicinity of a double bond (the so-called allylic carbon) occurs. The multiple link is not affected. The process was first developed by DuPont in the 50s of the last century:

A similar effect is obtained by the use of N-bromosuccinimide (NBS), which is characterized by its ability to slowly generate halogen radicals (atoms), maintaining their low concentration in the reaction mixture:

Therefore, in laboratory conditions, it is possible to carry out allyl halogenation without reaching high temperatures.

2.3.9. Distinctive reactions of alkynes

Due to the fact that the sp-hybridized carbon of alkynes is more electronegative, hydrogen at a carbon atom with a triple bond has significantly more acidic properties than hydrogen at an sp2-hybridized carbon in alkenes and even more significant than hydrogen at an sp3-hybridized carbon of alkanes.

In this regard, alkynes with a terminal triple bond are characterized by reactions:

Reaction of terminal hydrogen substitution:

If substitution for alkali metal atoms occurs only under the action of very strong bases (NaNH2, NaH, sodium butyl, etc.), then substitution with heavy metals (Cu, Ag, Hg) proceeds very easily, in aqueous solutions at room temperature. Acetylenides of heavy metals are unstable in the dry state and decompose explosively.

Reaction of water addition (reaction of M.G. Kucherov)

Water is added via a triple bond in the presence of divalent mercury salts (most often sulfate, MG Kucherov). In the case of acetylene, acetaldehyde is obtained:

In the case of other alkynes, only ketones are obtained. Water is connected according to Markovnikov's rule:

The intermediate formed unsaturated alcohol is unstable and quickly rearranges into an aldehyde or ketone.

Addition of alcohols and carboxylic acids

In the presence of caustic potassium (KOH) and under pressure, acetylene and alkynes with a terminal triple bond add alcohols to form alkyl vinyl ethers:

Under the conditions of heterogeneous catalysis with phosphoric acid or boric anhydride (B2O3), acetylene adds acetic acid to form vinyl acetate (a stable compound, vinyl alcohol ester and acetic acid):

Vinyl alcohol, which does not exist in a free state, is quite stable in the form of esters and ethers. Polymerization of vinyl acetate gives polyvinyl acetate (PVA). The hydrolysis of the latter makes it possible to obtain polyvinyl alcohol (PVA), the synthesis of which is impossible in any other way.

2.3.10. Alkenes production

In industry, an important source of unsaturated compounds is the products of cracking and pyrolysis of petroleum fractions, as well as coking gases (ethylene, propylene).

Methods for the dehydrogenation of alkanes on catalysts are often used for the same purpose:

CH3-CH2-CH2-CH3 --> CH2=CH-CH2-CH3 + CH3-CH=CH-CH3 + H2

The catalyst for this process is usually a specially prepared chromium oxide Cr2O3 at 300 ° C.

The usual method for obtaining alkenes in laboratory conditions is the dehydration of alcohols:

CH3-CH2-CH2-OH --> CH3-CH=CH2 + H2O

Mineral acids serve as catalysts for dehydration. The dehydration process proceeds according to the following mechanism:

At the stage of elimination of the water molecule, a carbocation is formed. The resulting carbocation very often undergoes isomerization (as a result of hydride or alkyl shift), and the carbon skeleton of the molecule may even change.

The carbcation splits off a proton and turns into an alkene. Usually, there are two possible options for the elimination of a proton (from one of the positions adjacent to the charged carbon atom), so two different products are obtained. The most stable alkene is formed as the main product (Zaitsev's rule). The most stable are trans-isomers of maximally substituted alkenes, with minimal steric hindrances created by neighboring groups.

Alkenes are obtained by dehydrohalogenation (elimination of hydrogen halide) of alkyl halides:

CH3-CHCl-CH2-CH3 --> CH2=CH-CH2-CH3 + CH3-CH=CH-CH3 + HCl

The elimination of hydrogen halide is carried out with an alcoholic alkali solution (KOH, NaOH) upon heating and also proceeds with the formation of the most stable alkene (Zaitsev's rule). Thus, the main product of the above reaction will be trans-butene-2. The mechanism for the elimination of hydrogen halide depends on a number of factors and is not considered here.

The method of dehalogenation of vicinal ("adjacent") dihaloalkyls is a method of obtaining pure alkenes. The resulting product contains a multiple bond in the place where the halogens were:

СН₂Cl-СНCl-СН₂-СН₃ + Zn à СН₂=СН-СН₂-СН₃ + ZnCl₂

In rare cases, alkenes are obtained by hydrogenation of more accessible alkynes:

СН₃-С≡С-СН₃ + Н₂ --> СН₃-СН=СН-СН₃

2.3.11. Methods for obtaining alkynes

Most often, alkynes are obtained by dehydrohalogenation of dihalide alkanes (vicinal or geminal):

CH2Cl-CHCl-CH2-CH3 --> CH≡C-CH2-CH3 + 2HCl

As in the case of alkenes, the reaction is carried out in alcoholic solutions of alkalis. First, one molecule of hydrogen halide is split off. The resulting halogen-substituted alkene cleaves the second molecule of hydrogen halide with much greater difficulty than the first. To obtain alkynes, the elimination of the second hydrogen halide molecule requires the use of a solid alkali with heating or a stronger base such as sodium amide NaNH2.

Alkylation of acetylene can be obtained by many other alkynes (homologues of acetylene). The reaction is carried out in two stages. First, an alkali metal acetylenide is obtained:

HC≡CH + NaNH2 (or CH3-Mg-J) --> HC≡CNa + NH3 (or HC≡C-Mg-J + CH4)

The obtained organometallic derivatives of acetylene are alkylated with halogenated alkyls:

HC≡СNa (or HC≡С-Mg-J) + СН3-J --> HC≡C-CH3 + NaJ (MgJ2)

The operations can be repeated and thus replaced the second hydrogen atom of acetylene, but usually it is impossible to obtain monoalkylacetylenes in pure form, since a mixture of mono- and disubstituted acetylenes is obtained.

2.3.12. Production of alkadienes

Obtaining of butadiene from ethanol (Lebedev S.V., 1932).

Ethanol is passed at 400-500 C over a catalyst of magnesium and zinc oxides. Such a catalyst both has the property of dehydrogenating (oxidizing) alcohols to aldehydes:

and it can cause a condensation reaction:

and dehydratation:

The versatile action of Lebedev's catalyst made it possible to obtain butadiene in one pass of alcohol vapor over it with a good yield. However, the reactions that took place were, of course, more complex.

Obtaining butadiene by condensation of acetylene with formaldehyde (Repe's synthesis)

The method developed by Repeat is based on the condensation of acetylene with formaldehyde

reduction of the obtained butyne-2-diol-1,4 in butanediol-1,4 and dehydration of the latter into butadiene-1,3:

END

Alkanes

Alkenes