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Home , Alkyne, Butyne, Geminal, Vinylacetylene

CHEMICAL REACTIONS OF

04-04-2020 SKS 1. Electrophilic Addition Reactions a. Electrophilic Addition of Bromine and Chlorine to Alkynes Alkynes show the same kind of addition reactions with chlorine and bromine that do.  With alkynes the addition may occur once or twice, depending on the number of molar equivalents of we employ:

Anti-addition takes place: mechanism is same as the

addition of Cl2 / Br2 takes place on alkenes.

04-04-2020 SKS b. Addition of Halides to Alkynes

 Alkynes react with one molar equivalent of or to form haloalkenes, and with two molar equivalents to form dihalides.

 Both additions are regioselective and follow Markovnikov’s rule:

04-04-2020 SKS Mechanism of Electrophilic Addition of Hydrogen Halides to Alkynes:

R1 R1 H - R1 R2 + H X or + X H R R 1 mole 2 2 A vinylic cation (Search for the more stable carbocation)

X R X R1 1 X H R1 X or R1 H H H H R2 H R2 H R2 X R2 Resonance stabilized + X-

R1 X H X H R2

04-04-2020 SKS Q1: The addition of HCl to 3,3-dimethyl-1- gives the following products: 2,2-dichloro-3,3- dimethylbutane (44 %), 2,3-dichloro-2,3-dimethylbutane (18 %) and 1,3-dichloro-2,3-dimethylbutane (34 %). Account in detail for the formation of these products.

Q2. The addition of 1.0 equivalent and excess HBr to 1-butyne gives 2-bromo-1- and 2,2- dibromobutane (a geminal dihalide), respectively. Account in detail for the formation of these products.

Q3. Write the structure of the product along with the when 1.0 equivalent of HCl is reacted with 2-butyne. Give explanation.

Q4. Give the mechanism for the formation of 1-bromo-1-butene, when 1 equivalent of HBR is reacted with 1-butyne in the presence of .

04-04-2020 SKS c. Addition of H2O  Like alkenes, alkynes can also be hydrated.

 This is an example of electrophilic .

 Alkynes undergo acid-catalysed addition of H2O.

 Terminal alkynes are less reactive than the internal towards the addition of H2O. Terminal alkynes will add water if Hg2+ is added to the acidic mixture. The Hg2+ acts as a catalyst to increase the rate of the addition reaction.  give , whereas other alkynes (both internal and terminal) give .

04-04-2020 SKS Mechanism

Practice Problem: Q. Write the mechanism of hydration of ethyne, 1- butyne and 2-butyne.

H H H H Ketone 04-04-2020 SKS 2. Nucleophilic addition reactions:

 The sp-hybridised of the triple-bond render alkynes more electrophilic than similarly substituted alkenes because the electronegativity of sp-hybridized orbital is more than sp2, hence pi- electrons will be more close to the carbon nucleus.

- + C2H5O K HC CH H2C C OC2H5 in C2H5OH H at high temp HCN Cat. NaCN

H2C C CN H Practice problem: Write the involved when acetylene is

reacted with C2H5S-Na+ in . 04-04-2020 SKS 3. Reduction of Alkynes

Reduction using Na/ Liq. NH3 (Birch Reduction):  Na / Liq. NH3 or Li / Liq. NH3 is used to carry out reduction of alkynes into alkenes.  Alkynes are selectively converted into trans-alkenes.

 Na / liq NH3 is the source of ammoniated electrons.  Na in ethanol can also be used.

2-Butyne

trans-2-Butene

04-04-2020 SKS Mechanism

trans-Vinylic radical anion is more stable as the –ve charge and radical are at the anti- position, so no electronic repulsion.

OR Mechanism

04-04-2020 SKS Reduction using Lindlar’s catalyst H2/ Pd(BaSO4)  H2/ Pd(BaSO4) partially deactivated (poisoned) by quinoline is used to carry out reduction of alkynes into cis-alkenes.  Alkynes are selectively converted into cis-alkenes.

 syn-addition of H2 takes place.

04-04-2020 SKS Mechanism:

The quinoline serves to prevent complete of the alkyne to an .

N N H 04-04-2020Quinoline SKS 1,2-Dihydroquinoline -oxidation of alkynes:

Hydroboration of internal alkynes is not a particularly useful procedure because a mixture of products will often be obtained, unless the triple-bond is symmetrically substituted.

O 1. BH3 in THF H3C C C CH3 2. H2O2 / NaOH is preferred over BH3 to 04-04-2020 SKS avoid the possibility of further addition Oxidative Cleavage of Alkynes Treating alkynes with hot basic followed by acid, leads to cleavage at the carbon– carbon . The products are carboxylic acids. Whereas under neutral condition it gives 1,2-diketo compounds (in case of internal alkyne), b-keto acids (in case of terminal alkynes), respectively.

Alkaline condition

O O - hot R1 C O R1 C OH - O O - + KMnO / OH KMnO / OH + H3O + R C C R 4 4 1 2 R1 C C R2 O O - R2 C O R2 C OH

hot - O O - O O KMnO4 / OH KMnO4 / OH R C C H R C C H R C C O-

- KMnO4 / OH

O + O H3O R C OH + CO2 R C O- + CO2 04-04-2020 SKS O of alkynes KMnO4 /H2O R C C R R R Neutral Condition O O R C OH O 1 3 + O R1 C C R2 KMnO4 /H2O dil. CH3COOH O R C C H OH R R C OH Neutral Condition or H2O 2 O

O O3 H3C C C CH3 H3C C OH dil. CH3COOH 2 moles or H2O

O O3 H3C C C H H3C C OH + CO2 + H2O dil. CH3COOH

or H2O

04-04-2020 SKS Distinction between terminal and internal alkynes

 Terminal alkynes react with Tollens reagent (ammoniacal solution) to form silver alkynides as a white precipitate, whereas internal alkynes do not react with Tollens reagent.

+ - R1 C C H + [Ag (NH3)2] OH R1 C C Ag + 2NH3 + H2O Tollens reacgent Silver alkynide White ppt + - R1 C C R2 + 2 [Ag (NH3)2] OH No Reaction Tollens reacgent

+ - H C C H + 2 [Ag (NH3)2] OH Ag C C Ag + 4NH3 + 2H2O Tollens reacgent Silver alkynide White ppt

04-04-2020 SKS + - R1 C C H + [Cu (NH3)2] OH R1 C C Cu + 2NH3 + H2O Tollens reacgent Silver alkynide Red ppt + - R1 C C R2 + [Cu (NH3)2] OH No Reaction Tollens reacgent

+ - H C C H + 2 [Cu (NH3)2] OH Cu C C Cu + 4NH3 + 2H2O Tollens reacgent Silver alkynide Red ppt

04-04-2020 SKS Reactions of terminal alkynes with sodamide, Sodium , n-BuLi and :

NaNH2 / liq. NH3 R1 C C Na R1 C C Na + NH3

NaH R1 C C Na + H2 R1 C C H acidic hydrogen n-BuLI R1 C C Li + n-

RMgX R1 C C MgX + RH

04-04-2020 SKS Conversion of terminal alkynes into internal alkynes

NaNH2 / liq. NH3 R1 C C H R1 C C Na + NH3 acidic hydrogen Act as nucleophile Terminal alkyne R -X Nucleopholic substitution reaction 2

R1 C C R2 + NaX Internal alkyne

Practice problems: 1. Convert 1-butyne into 2-. 2. Convert into 2-butyne. 3. Convert ethyne into n-butane.

04-04-2020 SKS Reactions of ethyne (Acetylene)

1. Addition of Arsenic trichloride

HgCl 2  Learn about British H C C H + ASCl3 Cl C C AsCl2 H H anti-lewisite (BAL). b-Chlorovinyldichloroarsine (trans)

Lewisite

2. Dimerisation of Acetylene

CuCl + NH4Cl H2 H C C H H2C C C C H H C C C CH H 2 2 Pd-BaSO4 H H 1,3-

Practice Problems: 1. What happen when ozonolysis of acetylene is carried out ? 2. What happen when acetylene is heated with hot alkaline potassium permanganate solution? 3. How will you distinguish between , ethene and ethyne ? Explain. 04-04-2020 SKS