The Chemical Reaction of Alkynes - Acidic Nature, Hydrogenation, Halogenation, Addition of Water, Ozonolysis, Practice Problems and FAQs

We do react differently depending on what happens to us. The reaction is happiness on are doing well in studies. The reaction is fear of falling behind the scheduled covering of the syllabus. When you lose a cricket match, you feel sad and disgusted. If your friends throw you a surprise party, you are surprised.

Similarly, alkynes react differently, based on the chemical situation, which includes the reacting species and the reaction conditions. So, let us go over them in depth.

Table of Contents

• Chemical reaction of alkyne
• Acidic nature of alkynes
• Catalytic Hydrogenation of Alkyne
• Halogenation to alkyne
• Addition of water to alkynes
• Addition of water in internal unsymmetrical alkynes
• Combustion of Alkynes
• Ozonolysis of alkynes
• Practice Problems
• Frequently Asked Questions

Chemical reaction of alkyne

The alkyne molecule is sp hybridised with two pi bonds. Due to the presence of two weak pi (π) bonds, alkynes undergo addition reaction. Alkynes add up to two molecules of dihydrogen, halogens, hydrogen halides, etc.

An illustration of orbital overlap in alkynes representing the weak pi (π) electron cloud is given in

Acidic nature of alkynes

Sodium metal (Na) and sodamide (; strong base) react with terminal alkynes like ethyne to form sodium acetylide () with the liberation of dihydrogen () and ammonia gas (), respectively.

Example 1:

Ethyne Monosodium ethynide

Disodium ethynide

Example 2:

Propyne Sodium propynide

These reactions are not observed in the case of ethane (alkane) or ethene (alkene), indicating

that ethyne (terminal alkyne) is acidic in nature when compared to ethane and ethene.

 Catalytic Hydrogenation of Alkyne

An addition of dihydrogen to alkynes in the presence of Ni/Pd/Pt gives alkanes. 

For example,

Partial hydrogenation (reduction) of alkynes

•  While adding to alkynes, if the reduction is to be stopped at the alkene stage, then the

catalyst needs to be poisoned.

• Palladium catalyst () deliberately poisoned with lead or sulphur compounds or quinoline, etc., is known as Lindlar’s catalyst.

Reduction of propyne to propane and propene

In the presence of a palladium catalyst, propyne is reduced to propane. When the reduction is carried out with palladium poisoned with quinoline, propyne is reduced to propene.

Halogenation to alkyne

• Alkynes react with a molecule of (in ) to form 1,2-dihaloalkenes.
• Further, the reaction of one more molecule of to 1,2-dihaloalkene produces tetra halogenated products.

Addition of (in ) to propyne

When the first molecule of in is added to propyne, an anti-addition reaction happens since the two substituents are added on the opposite sides of the pi bond.

Test for unsaturation with in

The reddish-orange colour of bromine solution in carbon tetrachloride is discharged when

bromine adds up to an unsaturated site.

• Hence, this reaction is used as a test for unsaturation.
• Alkenes and alkynes decolourise the reddish-brown colour of /

We have hexane, hexene, and hexyne in three test tubes. When / is added to hexane, the reddish brown colour of / is retained. When / is is added to hexene, the reddish-brown colour is discharged (decolourised) and 1,2-dibromohexane is formed. Similarly, when / is added to hexyne, the reddish-brown colour is discharged (decolourised) and 1,1,2,2-tetrabromohexane is formed.

Addition of HZ to alkyne:

Let us consider the addition of any compound (HZ) to an alkyne.

Step 1: Addition of electrophile

When we add one equivalent of an electrophile (let’s say ) to an alkyne, it forms a vinyl carbocation. This is an electrophilic addition reaction. When a protic acid (HX) is added to an asymmetric alkene, the π electrons shift in such a way that the positive charge is present on the carbon attached to the alkyl group so that the carbocation is stabilized by the +I effect of the alkyl group and also by the hyperconjugation of respective ⍺-hydrogen atoms.

Step 2: Addition of nucleophile () to vinyl carbocation

(i) Addition of hypohalous acid to alkynes

• Since the electronegativity of oxygen is higher than that of halogen except for fluorine, HOX breaks into and .
• Two molecules of HOX (HOCl, HOBr or HOI) get added to alkynes and form geminal diols.
• This geminal diols form ⍺,⍺-dihalo ketones (two halogen groups are present at ⍺-position with respect to carbonyl group) on the elimination of . 
• The geminal diols are unstable. Therefore, they are easily converted to their respective ketones or aldehydes by dehydration (loss of one water molecule).

Generally, the -OH group is added to that carbon atom of alkyne, where more alkyl groups are present.

When HOX is added to the given alkyne, the pi electrons shift in such a way that the positive charge

is present on the carbon attached to the alkyl (-R) group so that the positive charge is stabilized

by +I effect of the alkyl group. When the second molecule of HOX is added to the alkene formed

previously, the pi electrons shift in such a way that a positive charge is present on the carbon

attached to the -OH group as the positive charge is stabilized by the +M effect of the -OH group.

(ii) Addition of hydrogen halides to alkynes

When two molecules of HX (HCl, HBr, or HI) are added to the alkynes, geminal dihalides (in which two halogen atoms are attached to the same carbon atom) is formed.

According to Markovnikov's rule, the addition of HX to unsymmetrical alkynes takes place.

Adding HBr to ethyne according to the Markovnikov rule

When the second molecule of HBr is added to bromoethene, two possible carbocations can be

formed. The carbocation labeled as (A) is more stable than (B) due to the +M effect of the -Br

group and also by the hyperconjugation of three ⍺-hydrogen atoms. Hence, the major product is

obtained from carbocation (A).

Adding HBr to propyne according to the Markovnikov rule

On adding the first molecule of HBr to propyne, we obtain 2-bromopropene and

1-bromopropene as the major and minor products, respectively due to the Markovnikov rule.

When the second molecule of HBr is added to 2-bromopropene, we get the final product as 2,2-dibromopropane.

(iii) Addition of water to alkynes

Like alkanes and alkenes, alkynes are also immiscible in water. However, one molecule of water adds to an alkyne on heating with and diluted at 333 K to form a carbonyl compound.

The addition of water to alkynes happens according to the Markovnikov rule.

Adding to ethyne

An addition of to ethyne gives an enol compound. This enol compound tautomerizes to the aldehyde (i.e., acetaldehyde is formed).

Adding to propyne

When is added to propyne, the π electrons in propyne shift in such a way that the positive charge is present on the carbon attached to the group so that the carbocation formed is stabilized by the +I effect of the group and also by the hyperconjugation of three ⍺-hydrogen atoms. Thus, attacks the carbocation. We obtain an enol compound that tautomerizes to the keto compound (propanone).

Addition of water in internal unsymmetrical alkynes

Adding to Pent-2-yne

When is added to pent-2-yne, the π electrons shift in such a way that the positive charge is present on the carbon attached to the alkyl group so that the carbocation formed is stabilized by the +I effect of the alkyl group and also by the hyperconjugation of respective three ⍺-hydrogen atoms. Thus, attacks the carbocation. We obtain an enol compound that tautomerizes to the keto compound.

Combustion of Alkynes

The general reaction for the combustion of alkynes is given as follows:

Also, the combustion of propyne is given as follows:

Ozonolysis of alkynes

Alkynes contain 2 pi bonds. The triple bond of alkynes, in the presence of an ozone molecule, undergoes oxidative cleavage. Alkynes undergo oxidation resulting in the formation of end products such as diketones and acid anhydrides. In the presence of water, the acid anhydride gives rise to two carboxylic acids with the help of hydrolysis.

Mechanism of ozonolysis reaction of alkyne

The reaction mechanism of ozonolysis involves three steps.

Step 1: Attack of the ozone molecule 

Step 2: Formation of ozonide intermediate

Formation of Molozonide and Ozonide 

Step 3: Formation of carbonyl compounds

It can be performed by two types of mechanisms, which are given as follows:

1. Reductive ozonolysis
2. Oxidative ozonolysis

Reductive ozonolysis

This type of ozonolysis of alkyne involves the addition of an ozone molecule to an alkyne to form an ozonide. Ozonide is unstable and decomposes easily by reduction into . The reagent that is used in reductive ozonolysis is given as follows:

(i)

(ii)

(iii)

The formation of can be given as:

Oxidative Ozonolysis

This type of ozonolysis of alkynes involves the addition of an ozone molecule to an alkyne to

form an ozonide, followed by the oxidation of the ozonide to smaller oxidised molecules like acid. The reagent which is used in oxidative ozonolysis is given as follows:

(i)

(ii)

The formation of carboxylic acids can be given as:

In oxidative ozonolysis, if formic acid will form, it will further decompose into carbon dioxide and water

Practice Problems

1. What are the products formed in the following reaction?

Solution: In the absence of Zn, ozonolysis followed by hydrolysis produces acid. Hence, ethanoic acid and propanoic acid are formed by oxidative ozonolysis of pent-2-yne.

Hence, option (B) is the correct answer.

1. 1 mol of 1,2-dibromopropane on treatment with X mol of followed by the treatment with ethyl bromide gave pent-2-yne. What is the value of X?
2. 1
3. 2
4. 3
5. 4

Solution: When 1,2-dibromopropane reacts with one mole of , it produces propenyl bromide, which on further reaction with produces alkyne. Alkyne on further reaction with forms a conjugate base (acts as a nucleophile) that can react with ethyl bromide to forms a higher alkyne. So, 1 mol of 1,2-dibromo propane will react with 3 mol of , which on treatment with ethyl bromide will give pent-2-yne.

Therefore, option (C) is the correct answer.

1. In the following reaction, what are X and Y?

1. X = 1-Butyne; Y = 3-Hexyne
2. X = 2-Butyne; Y = 3-Hexyne
3. X = 2-Butyne; Y = 2-Hexyne
4. X = 1-Butyne; Y = 2-Hexyne

Solution: Ethyne, HC≡CH, on reaction with sodamide, , forms (conjugate

base, acts as a nucleophile) which on reaction with ethyl bromide, forms 1-butyne, . So, X is 1-butyne.

Now, 1-butyne on reaction with produces a conjugate base, , which on reaction with ethyl bromide forms , i.e., 3-hexyne. So, Y is 3-hexyne.

Hence, the correct option is (A).

1. Which of the following is product B?

Solution: The given reactant (terminal alkyne substrate) on reaction with sodamide, , is deprotonated and gives the conjugate base. The reaction of the conjugate base (which acts as a nucleophile) with methyl bromide, , produces higher alkyne. The product A is shown below:

Now, A on reaction with Lindlar’s catalyst converts triple bond (−C≡C−) into cis double bond

(>C=C<). Syn-addition would happen in this conversion. So, product B is as follows:

v

Therefore, option (C) is the correct answer.

Frequently Asked Questions

1. What type of reaction is very common for alkynes?

Alkenes and alkynes have very similar reactivity. They undergo electrophilic additions such as halogenation and hydrohalogenation. They can also be reduced with the help of a heterogeneous catalyst or oxidized using a variety of methods.

1. Do alkynes undergo substitution?

They do not prefer to substitute because it takes more energy to break existing bonds than it does to form new ones.

1. Why are alkynes called acetylene?

The extra electrons in hydrogen atoms are exchanged by two carbon atoms that form double bonds because the compound is unsaturated. ACETYLENES are alkynes derived from the first compound in the sequence.

1. Is an alkyne polar or nonpolar?

Alkynes are nonpolar, unsaturated hydrocarbons with physical properties similar to alkanes and alkenes. Alkynes dissolve in organic solvents, have little solubility in polar solvents, and are insoluble in water. In comparison to alkanes and alkenes, alkynes have significantly higher boiling points.

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