Properties of Alkynes

• Physical Properties of Alkynes

The physical properties of alkynes are similar to those of the corresponding alkenes. The lower members are gases with boiling points somewhat higher than those of the corresponding alkenes. Terminal alkynes have lower boiling points than isomeric internal alkynes and can be separated by careful fractional distillation.

The CH₃-C bond in propyne is formed by overlap of a C_sp³-hybrid orbital from the methyl carbon with a C_sp-hydrid from the acetylenic carbon. The bond is C_sp³ -C_sp. Since one orbital has more s-character than the other and is therby more electronegative, the electron density in the resulting bond is not symmetrical. The unsymmetrical electron distribution results in a dipole moment larger than that observed for an alkene, but still relatively small.

• Chemical Properties of Alkynes 

 An alkyne molecule (except ethyne) has three parts                                     

Alkyl part : The alkyl part being inert in alkyne & thus does not show substitution reaction.

Alkyne part : It consists of one σ & 2 Π bonds. Due to more strain than alkene, Π bonds are highly reactive and less stable. The polarization of Π bonds in alkynes leads to addition reaction. Unlike alkene, alkyne reacts with two molecules of additive to form saturated molecule.

Acidic hydrogen part : H atom attached to sp hybridized carbon or triply bonded carbon atom is acidic in nature. The acidic character is due to the fact than an increase in s character of carbon atom give rise to higher electonegativity to it and thus H atom attached on sp hybridized carbon acquires polarity to show acidic nature.

The hybridized carbon atom being more electronegative, is best able to accommodate the electron pair in the anion left after the removal of proton.  

Relative acidic nature :     HOH≈HOR > HC≡CR > HNH₂ > H₂C=CH₂ > CH₃-CH₃

Relative basic nature :      OH^-≈OR^- < C≡CR < NH₂ < CH=CH₂ < CH₂-CH₃

• Acidity of Alkynes

The hydrogens in terminal alkynes are relatively acidic. Acetylene itself has a pK_a of about 25. It is a far weaker acid that water (pK_a 15.7) or the alcohols (pK_a 16-19), but it is much more acidic than ammonia (pK_a  34). A solution of sodium amide in liquid ammonia readily converts acetylene and other terminal alkynes into the corresponding carbanions. RC≡CH + NH₂^–   → RC ≡C^— + NH₃

This reaction does not occur with alkenes or alkanes. Ethylene has a pK_a of about 44 and methane has a pK_a of about 50.

From the foregoing pK'_as we see that there is a vast difference in the stability of the carbanions RC ≡C^—, CH₂ = CH^—, and CH₃^—. This difference may readily be explained in terms of the character of the orbital occupied by the lone-pair electrons in the three anions. Methyl anion has a pyramidal structure with the lone-pair electrons in an orbital that is approximately sp³(1/4 s & ¾ p). In vinyl anion the lone-pair electrons are in an sp²- (1/3 s & 2/3 p).orbital . In acetylide ion the lone pair is in an sp-orbital (1/2 s & 1/2 p).

Electrons in s-orbitals are held, on the average, closer to the nucleus than they are in p-orbitals. This increased electrostatic attraction means that s-electrons have lower energy and greater stability than p-electrons. In general, the greater the amount of s-character in a hybrid orbital containing a pair of electrons, the less basic is that pair of electrons, and the more acidic is the corresponding conjugate acid.

Of course, the foregoing argument applies to hydrogen cyanide as well. In this case, the conjugate base, NºC^—, is further stabilized by the presence of the electronegative nitrogen. Consequently, HCN is sufficiently acidic (pK_a 9.2) that it is converted to its salt with hydroxide ion in water. HCN + OH^— ⇔ CN^— + H₂O

Alkynes are also quantitatively deprotonated by alkyllithium compounds, which may be viewed as the conjugate bases of alkane: CH₃(CH₂)₃CºCH + n-C₄H₉Li → CH₃(CH₂)₃C≡CLi + n-C₄H₁₀

The foregoing transformation is simply an acid-base reaction, with l-hexyne being the acid and n-butyllithium being the base. Since the alkyne is a much stronger acid than the alkane (by over 20 pK units), equilibrium lies essentially completely to the right.

Terminal alkynes give insoluble salts with a number of heavy metal cations such as Ag⁺ and Cu⁺. The alkyne can be regenerated from the salt, and the overall process serves as a method for purifying terminal alkynes. However, many of these salts are explosively sensitive when dry and should always be kept moist. CH₃(CH₂)₃CºCH + AgNO₃ ⇔ CH₃(CH₂)₃C≡CAg + HNO₃

• Addition Reactions of Alkynes

?Nucleophilic pi electrons of alkynes add electrophiles in reactions similar to additions to alkenes. Alkynes can add two moles of reagent but are less reactive (except to H₂) than alkenes.

1. Hydrogenation of Alkynes :

Addition of two molecules of H₂ takes place on alkyne.

However the addition of 2^nd H₂ molecule can be checked if Lindlar catalyst is used.

Dialkyl acetylenes may be catalytically reduced to a mixture of cis and trans alkenes, the former is formed predominantly if Lindlar catalyst is used.

However reduction with sodium in liq. NH₃ or by LiAIH₄ produces trans alkene.

2. Electrophilic addition :

Acetylenic bond in alkyne is a combination of one sigma bond and two Π bonds. Like alkenes, alkynes also show characteristic electrophilic addition reactions which take place in two stages involving the formation of olefinic intermediate. Thus alkynes shows addition of two molecules of addendum.

However the rate of electrophilic addition in acetylene is rather show than that of ethene inspite of the fact that alkynes has excess of pi electron. This fact is also supported than in many of electrophilic addition reaction, presence of catalyst such as Hg²⁺ ions is needed. The low reactivity of acetylene is not yet clear.

3. Addition of Halogens :

1. Western and westrosol are good industrial solvents for rubber, fats and varnishes. Western also have some insecticidal action.

2. The rate of reaction increases in presence of light.

3. The reactivity order for halogens is : CI₂ > Br₂ > I₂

   CH ≡ CH + Br₂ → CHBr = CHBr (only)

   CH ≡ CH + Br₂ → CHBr₂-CHBr₂

                             n CCI₄

     CH≡CH + I₂  CHI=CHI

                 in alcohol

4. Direct combination of acetylene with chlorine may be accompanied with explosions, but it is prevented by the presence of metallic chloride as catalyst.

5. The predominant product during addition of one molecule of halogen on alkyne in trans isomer.

4. Addition of halogen acids :

CH≡CH + HX   CH₂=CHX   CH₃-CHX₂

                                   vinyl halide          ethylidene dihalide

The reactivity order is : HI > HBr > HCI

Acetylene reacts with dil HCI in presence of Hg²⁺ at 65^oC to give vinyl chloride, used in preparation of poly vinyl chloride, a synthetic polymer.

CH≡CH + dil. HCI CHg2 + CH₂=CHCI

Note :

1. Peroxides have the same effect on the addition of HBr to unsymmetrical alkynes as they have on alkenes.
2. Because of -I effect of the bromine atom, the availability of the Π electrons during the second molecule addition becomes much slower than ethylene.

5. Addition of hypohalous acids :

Note : 

1. Presence of two or more OH gp on one carbon atom makes it unstable and the molecules loses H₂O molecule.
2. However two exceptions to this rule; one is chloral hydrate CCI₃CH(OH)₂and the other is carbonic acid  Chloral hydrate is extra stable inspite of two OH gp on one carbon atom due to H-bonding.

6. Addition of AsCI₃ :

 CH≡CH + AsCI₃  CHCI=CHAsCI₂

                                   ^{bchlorovinyl dichloroarsenic (Lewsite),}

                                 ^{A poisonous gas, more poisonous than mustard gas}

7. Addition of HCN :

8. Addition of acetic acid :

9. Addition of CO and H₂O :

10. Polymerisation or self addition : Alkynes undergo polymerization yielding different types of polymeric compounds under different conditions.

(a)  Cyclic polymerization :

(b)  Linear Polymerisation :

Vinyl acetylene on reaction with HCI forms 2-chloro, 1, 3-butadiene (or chloroprene) which on exposure to air polymerizes to give synthetic rubber neoprene

Note : Acetylene on heating with spongy copper or its oxide gives a cork like substance, used in manufacture of linoleum.

11. Addition of H₂SO₄ :

 CH≡CH + H₂SO₄      →       CH₂=CHHSO₄          →        CH₃CH(HSO₄)₂

(cold & conc.)                vinyl hydrogen              Ethylidine dihydrogen

                                              sulphate                       sulphate 

The above reaction can also be made as:

Vinyl alcohol CH₂=CHOH, which is rapidly converted into an equilibrium mixture that is almost CH₃CHO is an example of keto-enol tautomerism.

Note : Only C₂H₂ on addition of H₂O gives aldehyde and rest all alkynes give ketone.

12. H₂O (Hydration to Carbonyl Compounds): When passed into dilute sulphuric acid at 60^oC in the presence of mercuric sulphate as catalyst, acetylene adds on one molecule of water to form acetaldehyde. The mechanism of this hydration takes place via the formation of vinyl alcohol as an intermediate.

The homologues of acetylene form ketone when hydrated, example, propyne gives acetone

Refer to the following video for hydration of alkynes:

13. Addition of Boron Hydride

with  dialkylacetylenes, the products of hydrolysis and oxidation are cis-alkenes and ketones, respectively

• Ozonolysis

Addition of O₃ on alkynes gives their monoozonides which on hydrolysis forms dicarbonyl compounds which are further oxidized to carboxylic acids.

In alkenes two molecules of carbonyl compounds are formed during ozonolysis and in alkyne one molecule of dicarbonyl compound is formed which is further oxidized to two acids.

• Oxidation of Alkynes

1. Combustion : C_nH_2n-2 + (3n-1)/2O₂ → nCO₂ + (n-1)H₂O; ΔH = -ve

2. Oxidation by alkaline KMnO₄ :

R-C≡CH + 4[O] → RCOOH + HCOOH

3. Oxidation by H₂CrO₄ or acidified K₂Cr₂O₇ :

CH≡CH + O + H₂O → CH₃COOH

R-C≡CH + O + H₂O → RCH₂COOH

4. Oxidation by acidified KMnO₄ :

CH≡CH + 3[O] + H₂O → 2HCOOH  H₂O + CO₂

R-C≡CH  RCOOH + HCOOH

5. Oxidation by selenium dioxide :

 CH₃-C≡CH  CH₃COCHO

                                       ^{2-ketopropanal}

• Isomerization of Alkynes

• Action of N2 to Alkynes:

CH≡CH + N₂  2HCN

• Formation of Heterolytic Compounds

• Nucleophilic Addition

Acetylene undergoes nucleophilic addition with CH3OH in presence of CH3ONa.

• Substitution Reaction

Acetylene on passing through sodium hypochlorite solution at 0^oC in absence of light shows substitution of H by chlorine atom.

1. Formation of sodium acetylide or alkynides :

2. Formation of Acetylenic  Grignard reagent :

CH≡CH + R MgX → CH≡CMgX

3. Formation of copper alkynides : on passing alkynes through ammoniacal cuprous chlorides solution, a red precipitate of cuprous alkynide is obtained.

4. Formation of silver alkynides : On passing alkynes through ammoniacal silver nitrate solution (Tollens reagent) a white precipitate of silver alkynides is obtained

CH≡CH + 2AgNO₃ + 2NH₄OH → AgC≡CAg + 2NH₄NO₃ + 2H₂O

                                           silver acetylide

                                              while ppt.)

RC≡CH + AgNO₃ + NH₃OH → RC≡CAg + NH₄NO₃ + H₂O

Note : 

1. These alkynides are ionic in nature

2. Alkynides are generally explosive and unstable when dry.

3. Copper and silver alkynides are very sensitive to shock when dry & may explode

4. These alkynides are easily converted to original alkynes when treated with dilute acids.NaC≡CNa + 2HNO₃ → HC≡CH + 2NaNO₃

5. Acidic nature of alkyne can be utilized to separate, purify and identify alkyne-1 from other hydrocarbons. 

• Uses of Alkynes

1. Among alkynes, acetylene has got wide applications in industries.

2. As oxy-acetylene flame for welding.

3. As illuminating agent in hawker's lamps and light houses.

4. In artificial ripening of fruits.

5. In preparation of monomeric unit (vinyl chloride, vinyl cyanide, vinyl cyanide, vinyl acetate, vinyl acetylene, etc.) to get polymers (PVC, PVA, chloroprene, Buna-S etc.) widely used in textile, plastic, shoe and rubber industries.

6. In preparation of poisonous gas, Lewiste.

7. In preparation of solvents such as westron, westrosol and other useful chemicals e.g., C₆H₆, acetaldehyde, acetone etc.

8. It is used as general anesthetic  under the name Naracylene

You might be interested in referring IIT JEE Organic Chemistry Syllabus, Books and Revision Notes on Organic Chemistry.