Alkyne

Model in third dimension of acetylene.

The simplest alkyno is the acetylene.

The alkynes are aliphatic hydrocarbons with at least one triple bond (two π pi bonds and one Σ sigma bond) -C≡C- between two carbon atoms. These are metastable acid compounds due to the high energy of the carbon-carbon triple bond. Its general formula is C_nH_2n-2.

In order to name hydrocarbons of the alkyne type, certain rules are followed similar to those of alkenes.

1. It is taken as main chain the longest continuous chain that contains the to the triple links.
2. The string is numbered so that the carbon atoms of the triple link have the lowest possible numbers.
3. This main chain is named with the termination -in, specifying the number of carbon atoms of that chain with a prefix (et-two, prop-three, but-four; pent-; hex-; etc.). e.g.: propino, CH₃-C≡CH.
4. If necessary, the position of the triple link is indicated by the lower number corresponding to one of the carbon atoms of the triple link. That number is located before the termination -no. E.g.: CH₃-CH₂-CH₂-CH₂-C≡C-CH₃Hept-2-ino.
5. If there are several triple links, it is indicated with prefixes di, tri, tetra... e.g.: octa-1,3,5,7-tetraino, CH≡C-C≡C-C-C≡CH.
6. If there are double and triple links, the lowest number is given to double link. e.g.: pent-2-in-4-ino, CH₃-CH=CH-C≡CH
7. Substituents such as halogen atoms or alkylo groups are indicated by name and number, in the same way as for the case of the alcanos. e.g.: 3-chloropropino, CH≡C-CH₂Cl; 2.5-dimetilhex-3-ino, CH₃-C(CH)₃)-C≡C-C(CH)₃)-CH₃.

Examples of alkynes

 CH    ≡ ≡    {displaystyle equiv } CH etino(acethylene) CH3-C    ≡ ≡    {displaystyle equiv } CH propino
CH3-CH2-C    ≡ ≡    {displaystyle equiv } CH 1-butino CH3- C    ≡ ≡    {displaystyle equiv } C-CH3 2-buddy
CH    ≡ ≡    {displaystyle equiv } C- etinilo CH    ≡ ≡    {displaystyle equiv } C-CH2– 2-propinyl
CH3-C    ≡ ≡    {displaystyle equiv } C- 1-propinyl CH3-CH2-CH2-C    ≡ ≡    {displaystyle equiv } CH 1-pentine

Physical properties

1) They are insoluble in water, but quite soluble in usual organic solvents and of low polarity: ligroin, ether, benzene, carbon tetrachloride.

2) They are less dense than water and their boiling points show the usual increase with increasing carbon number and the usual chain branching effect.

3) The boiling points are almost the same as for alkanes or alkenes with the same carbon skeleton.

4) The first three terms are gases; the others are liquid or solid.

5) As the molecular weight increases, the density, melting point and boiling point increase.

6) Acetylene are low polarity compounds, which is why their physical properties are very similar to those of alkenes and alkanes.

7) They are flammable

Keep in mind that acetylenes complete the quartet rule.

Chemical properties

The most frequent reactions are those of addition: hydrogen, halogen, water, etc. In these reactions the triple bond is broken and bonds of lower polarity are formed: double or single.

Hydrogenation in the presence of a catalyst: cis.

Hydrogenation of alkynes

Alkynes can be hydrogenated to give the corresponding cis-alkenes (double bond) by treating them with hydrogen in the presence of a palladium catalyst on barium sulfate or on calcium carbonate (Lindlar catalyst) partially poisoned with lead oxide. If palladium on activated carbon is used, the product obtained is usually the corresponding alkane (single bond).

CH≡CH + H₂ → CH₂= CH₂ + H₂ → CH₃- CH₃

Although the density of electrons and with this the negative charge in the triple bond is high, they can be attacked by nucleophiles. The reason lies in the relative stability of the vinyl anion formed.

In front of sodium or lithium in liquid ammonia, it is hydrogenated producing trans-alkenes.

CH₃-C≡C-CH₃ + 2 Na + 2 NH₃ → CH₃- CH= CH-CH₃ (trans) + 2 NaNHH₂

Halogenation, hydrohalogenation and hydration of alkynes

Just like alkenes, alkynes participate in other addition reactions:

Halogenation

Depending on the conditions and the amount of halogen added (fluorine, F₂; chlorine, Cl₂; bromine, Br₂...), it is possible to obtain halogenated derivatives of the corresponding alkene or alkane.

HC≡CH + Br₂ → HCBr≡CHBr

HC≡CH + 2 Br₂ → HCBr₂-CHBr₂

Hydrohalogenation, hydration, etc.

The triple bond can also add hydrogen halides, water, alcohol, etc., with the formation of double or single bonds. In general, the Markovnikov rule is followed.

HC≡CH + H-X → CH₂=X where X = F, Cl, Br...

HC≡CH + H₂O → CHOH= CH₂

Terminal hydrogen acidity

In some reactions (against strong bases, such as sodium amide Na-NH₂ in ammonia NH₃) they act as weak acids since the terminal hydrogen has some acidity. Acetylides (conjugate base of the alkyne) are formed, which are good nucleophiles and give nucleophilic substitution mechanisms with the appropriate reagents. This allows other longer-chain alkynes to be obtained.

HC≡CH + Na-NH₂ → HC≡C:^- Na⁺

HC≡C:^- Na⁺ + Br-CH₃ → HC≡C-CH₃ + NaBr

In this case, the sodium acetylide formed has reacted with bromomethane to form propyne.

Pericyclic reactions

• Alder-eno reaction
• Reaction of Diels-Alder

Applications

Most alkynes are made in the form of acetylene. In turn, a good part of the acetylene is used as fuel in gas welding due to the high temperatures reached.

In the chemical industry alkynes are important starting products for example in the synthesis of PVC (HCl addition) of artificial rubber etc.

The alkyne group is present in some cytostatic drugs.

The polymers generated from alkynes, the polyalkynes, are organic semiconductors and can be endowed similar to silicon although they are flexible and long materials.

Analysis

Alkynes decolorize an acid solution of potassium permanganate and bromine water. If they are terminal alkynes (with the triple bond at one of the final carbons of the molecule) they form salts with ammonia solutions of silver or copper (these salts are explosive).

Electronic structure

The triple bond between the carbons is formed by two sp orbitals and two p orbitals. Bonds to the rest of the molecule are through the remaining sp orbitals. The distance between the two carbon atoms is typically 120 pm. The geometry of the triple bond carbons and their substituents is linear.