Saturday, September 1, 2007

TMH-JEE-chemistry-Study Guide - ch 23 Alkynes Part 2

Chemical Properties

The alkynes have at least one triple bond in them, therefore, they are quite reactive chemically.

They readily take part in addition reactions and can also be easily oxidized.

I. Addition reactions (specially given in jee syllabus)

1. Addition of Hydrogen

If the triple bond is not present at the end of the chain of the molecule (it is not a terminal alkyne), its reduction (addition of hydrogen) produces either a cis alkene or a trans alkene depending upon the choice of reducing agent.

2. Addition of halogens

chlorine and bromine add on alkali

3. Addition of hydrogen halides

This addition takes place in accordance with Markonikov's rule(do you remember the rule?).
Peroxides have the same effect on addition of the HBr to acetylenes (alkynes) as they have on alkenes (do you remember the effect?).

4. Addition of water (hydration of alkynes) (specially given in jee syllabus)
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K. The catalyst in this reaction is HgSO4 (Mercuric sulphate). The final products of this reaction are carbonyl compounds aldehydes and ketones.

Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess. Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).

Addition of water to Ethyne or acytelene: Acetylene is passed into water (at about 330K) containing 60% H2SO4 and about 1% mercuric sulphate (HgSO4) as a catalyst, acetaldehyde is formed.

In the first step 'ethenol' is formed and in the second step the rearrangement of it takes place and its isomer 'acetaldehyde' is formed.

The conversion of enol form into keto form is termed tautomerism

5. Addition of hypohalous acid (HOX)

Alkynes react with two molecules of hypohalous acids in two stages.
For example take ethyne or acytelene and HOCl.
In the first stage HO gets added to one carbon and Cl adds to the other carbon.
In the second stage one more HOCl gets added to the intermediate product which has a double bond. The addition now follows markonikov's rule. OH gets added to HC-OH and Cl gets added to CH-Cl. Two OHs create instability and H2O molecule gets removed.
An aldehyde 2,2-Dichloroethanal (Dichloroacetaldehyde) is formed.

6. Addition of H2SO4 (Ref: Standard XI Chemistry by Khan et al, Uttam Prakashan)
Acetylene adds two molecules of concentrated H2SO4 in two stages and forms ethylidene hydrogen sulphate as the final product.

In the first stage Vinylhydrogen sulphate H2C=CH-OSO3H is formed. (H gets added to one CH and OSO3H gets added to the other CH).
The addition of second molecule follows markownikov's rule. H gets added to CH2 and OSO3 gets added to the other carbon. Thus two functional groups OSO3H gets added to one carbon.
The final product is Ethylidine hydrogen sulphate.

II. Oxidation

a) Combustion

b) Oxidation with alkaline potassium permanganate

c) Ozonolysis

Alkynes react with ozone to form ozonides. These ozonides on decomposition with water in the presence of zinc give diketones(two carbonyl groups).

Ethyne gives glyoxal on reacting with ozone. Glyoxal also have two carbonyl groups.

III. Polymerization

a) Acytylene dimerizes (two molecules come together as one molecule) in the presence of cuprous chloride and ammonium chloride to give vinyl acetylene which on reacting with HCl gives chloroprene.

The later polymerizes to give neoprene - a synthetic rubber.

b) Cyclic polymerization

This takes place when alkyne is passed through red hot iron tube at 400 degress celsius.

Occurs in two stages. Acetylene becomes mesitylene.

IV. Isomerization

1-alkyne gets converted to 2-alkyne and vice versa.

V. Formation of metal acetylides (specially given in jee syllabus)

Acytelene reacts with Na and LI liberating H-2 gas and forming metal acetylide. Therefore acetylene has chemical behaviour similar to acids.

Heavy metal ions mainly, Ag+ and Cu+ react with acetylinic hydrogen (hydrogen atom in acetylene) to form insoluble acetylides.

VI. Reaction with Grignard reagent or alkyl-lithium

The acytylinic hydrogen on reacting with R'MgBr or R'Li produces the alkane R'H and metal acetylide

Acidic property of acetylene (specially given in jee syllabus)

Acetylene and other terminal alkynes (1-alkynes) are weakly acidic in character.
They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.

Acetylides react with alkyl halides to give higher alkynes.

Explanation for the acidic character of alkynes

the acidic character of 1-alkynes can be explained on the basis of sp hybridisation state of the carbon atoms in alkynes. In sp hybridisation, s-character is 50% and due to this large s-character, the electons in sp hybrid orbitals are held more tightly by thenucleus and areq uite electronegative. Consequently the eletron pair of H-C≡C bond gets displaced more towards the carbon atom and helps in release of H+ ion.

In the case of nonterminal alkynes, hydrogen atom is not attached directly to the triple bonded carbon atom and hence it is not released easily.

Alkynes are weakly acidic but alkenes and alkanes do not show acidic behaviour.

acidic charater
HC≡HC > CH2=CH2 > CH3-CH3

The relative acidity of acetylene is more than that of ammonia but less than that of water.

Dissolving Metal Reduction of Alkynes (specially given in jee syllabus)

Reaction Type: Addition


Alkynes can be reduced to trans-alkenes using Na in NH3 (l)
This reaction is stereospecific giving only the trans-alkene via an anti addition.
Note that the stereochemistry of this reaction complements that of catalytic hydrogenation (syn)
The reaction proceeds via single electron transfer from the Na with H coming from the NH3
These reaction conditions do not reduce alkenes, hence the product is the alkene.

Step 1:
Sodium transfers an electron to the alkyne giving a radical anion.
Step 2:
The radical anion removes a proton from the ammonia in an acid / base reaction.
Step 3:
A second atom of sodium transfers another electron to the alkyne giving an anion.
Step 4:
The anion removes a proton from the ammonia in an acid / base reaction.
Source for reduction

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