Alkyne

Alkynes (formerly acetylenes and acetylene hydrocarbons ) are chemical compounds selected from the group of aliphatic hydrocarbons, having at least one carbon-carbon triple bond in the molecule at any position. Alkynes with triple bonds, only one such form a homologous series of the general total formula C n H 2n -2 ( n = 2, 3, 4, ... ) which starts with ethyne.

Compounds having two or more carbon-carbon triple bonds are called polyynes; in a broader sense acyclic hydrocarbons can be counted with more carbon- carbon triple bonds to alkynes.

Cyclic compounds containing a carbon-carbon triple bond, however, are counted among the Cycloalkynes and called aromatic hydrocarbons with formal triple bond in the ring arynes.

• 7.1 hydrogenation 7.1.1 hydrogenation catalyst with normal
• 7.1.2 Hydrogenation of poisoned catalyst

• 7.3.1 halogenation
• 7.3.2 hydrohalogenation
• 7.3.3 hydration

History

For the first time acetylene ( ethyne ) is the simplest alkyne, in 1836 by Edmund Davy was obtained, a chemistry professor at the Royal Dublin Society and cousin of the famous English chemist Humphry Davy. When trying to represent potassium in metallic form, he received ethyne by heating potassium salts such as potassium acetate or potassium carbonate with carbon, followed by the reaction of Kaliumcarbids formed with water. However, the discovery was forgotten. In 1862, the German chemist and physician Friedrich Wöhler succeeded in the representation of acetylene by the reaction of water with calcium carbide. In 1863 the French chemist Marcelin Berthelot succeeded in the representation of the elements on the arc between graphite electrodes in a hydrogen atmosphere. He gave the name of the gas acetylene. 1895 saw Henry Le Chatelier that acetylene burns with oxygen with a very hot flame. Thus the basis of the acetylene welding and cutting - was laid.

Homologous series

The most important of alkynes, ethyne ( C2H2) (acetylene ) to Decin ( C10H18 ) with names and Molecular Formula:

• Acetylene: C2H2
• Propyne: C3H4
• Butin: C4H6
• Pentyne: C5H8
• Hexyne: C6H10
• Heptyn: C7H12
• Octin: C8H14
• Nonin: C9H16
• Decin: C10H18

Nomenclature

The naming of alkynes according to the IUPAC rules based on the name for the alkanes. As root name for the alkyne to Choose the root word of the alkane with the same number of carbon atoms and replaces the suffix -an by - in. In branched alkynes the longest possible carbon chain is with the triple bond the root name. Alkynes have a higher priority than alkenes. The first two representatives of the homologous series of this group of ethyne and propyne are. The position of the triple bond is described in an imaginary number, so that it is as small as possible, for example, 1-butyne and 2 -butyne. Contains the carbon chain more triple bonds, one adds the name in front of the syllable in the syllable di, tri, tetra, etc.. So an alkyne with five (Greek penta ) carbon atoms and two triple bonds obtained after the first and fourth carbon atom the IUPAC name 1,4- penta.

Electronic Structure

The triple bond of the alkyne comprises a sp hybrid bond, and two orthogonal P bonds. These two orthogonal p- bonds ( orthonormal basis ) form two rotationally invariant orbitals, which can be detected eg by IR spectroscopic investigations on W2 (CO) 6. Due to the higher proportion of s- orbitals in the sp hybrid orbitals, the probability of the electrons of the CH bond in the vicinity of the carbon core is greater than that of alkenes (sp2 ) and alkanes (sp3 ), whereupon the CH- acidity of terminal alkynes is based. Therefore, the pKa of ethyne 25 is many times smaller than that of ethylene ( 44) and ethane (50). With strong bases, a terminal C- atom may be deprotioniert and replaced by a metal atom, for example, at lower alkynes. Salts of alkynes are called acetylides. Apart from the low - polarity due to shortened CH CH bond, alkynes are relatively non-polar. From the electronic structure of the carbon-carbon triple bond follows that the carbon atoms of the triple bond and the two directly linked to these carbon atoms atoms in a line ( linearly ) aligned. The C- C distance of a triple bond is 120 pm and is thus shorter than the distance of a CC double bond.

Spectroscopic properties

1H -NMR

In the 1H NMR alkynyl hydrogen atoms are not as strongly deshielded as Alkenylwasserstoffatome as induced in the rotationally symmetric triple bond by the external magnetic field, a ring current whose magnetic field is the outer opposite. Since the triple bond spin -spin coupling transmits good, the 4J couplings have four bonds and the 5J couplings over five bonds usually coupling constants 1-3 Hz for remote couplings which is a relatively high value, so it is for the alkynyl often characteristic.

Infrared spectroscopy

Infrared spectroscopy is next to the 13C -NMR spectroscopy is a useful method for identification of a terminal alkyne, because at about 2100 cm -1 shows a strong characteristic band for the C-C stretching vibration.

Production and representation

• Extraction and subsequent hydrolysis of calcium carbide

• Dimerization and dehydrogenation of methane

• Partial oxidation of methane

Reactions

Hydrogenation

Hydrogenation catalyst with normal

Alkynes can be hydrogenated using conventional catalysts such as platinum or palladium with hydrogen to alkanes.

Hydrogenation with a poisoned catalyst

With a poisoned catalyst, Lindlar's catalyst, the alkyne reacts only to the alkene. This produces exclusively (Z)- alkenes ( cis), because coming close to the hydrogen atoms from the same side to the alkyne and react.

Formation of alkynylidene

Since terminal alkynes, as already mentioned, CH- acids, they can respond to the so-called alkynylidene. With alkali metals to form the Alkalialkinylide in liquid ammonia. In aqueous solutions of silver (I) - and copper ( I) salts in doing so the insoluble Schwermetallalkinylide. These are, however, in the dry state, highly explosive!

Electrophilic Addition

The addition to a triple bond is stronger as compared to the exothermic addition to a double bond.

Nevertheless, alkynes are less reactive than alkenes towards electrophilic reagents. The apparent contradiction can be explained by the stability of the intermediate carbenium cations:

The alkyl cation is much more stable than a vinyl cation, the addition reaction of electrophiles to alkynes is thus kinetically inhibited ( = higher activation energy).

Halogenation

After halogenation hallabschen method is applied as follows: The carbon-carbon triple bond is less nucleophilic than the carbon-carbon double bond, which is why the halogens do not add spontaneously to the triple bond. This is done only by a Aufpolarisierung the halogen -halogen bond using a Lewis acid, such as FeCl3 or AlCl3. This results, stereoselective, the (E ) -1,2- Dihalogenalken.

Therefore, the carbon-carbon double bond can be selectively halogenate in the presence of a carbon-carbon triple bond, provided that no Lewis acid is added.

Hydrohalogenation

Alkynes of hydrogen halide can be added in the first stage to vinylogous haloalkene. However, these react only under drastic conditions continue to dihalogens, as these are less reactive. This produces, according to the Markovnikov rule 1.1 dihaloalkanes.

The Hydrochlorination of ethyne to vinyl chloride was of great industrial importance, since vinyl chloride is the monomeric building block for the production of PVC.

Hydration

Water added in the presence of catalysts in an acidic environment of ethyne to ethenol. This tautomeric form is finally superimposed by the acetaldehyde ( keto -enol tautomerism ).

Terminal alkynes store, however after hydration (again, according to the Markovnikov rule) to the methyl ketones to.

Natural occurrence

Alkynes are in nature not very common ( only 1,000 compounds are known ), and these are just a few of physiologically active in their own organism. The rest usually act as fungicides or as a defense or poison mucosa irritation fabric. The skin of poison dart frogs secrete histrionicotoxin example, a substance containing two alkyne groups and the Frog protects against mammals and reptiles. A large group of biologically active alkynes form the enediyne antibiotics. Natural substances that have the enediyne moiety as a structural motif often act cytotoxic to human tumor cell lines and thus represent potential chemotherapeutic agents represents a example of this is secreted by Streptomyces carcinostaticus neocarzinostatin.

Importance

Of industrial importance are only ethyne (common name acetylene) and propyne, they are used among other things as a welding gas, as the flames of this extremely hot (up to 3100 ° C). Acetylene is in the chemical industry is of great importance for the production of many other compounds such as acrylic acid or acrylamide. And 2- butyne-1 ,4 -diol is a precursor for the production of tetrahydrofuran ( THF).