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Chemistry of Alkynes
(I) Definition of Alkynes
Alkynes are aliphatic unsaturated
hydrocarbons which
possess a triple bond (two p bonds) between carbon atoms in the molecule. Alkynes have four hydrogen atoms less than the corresponding alkanes. Thus,
the triple bond introduces two degree of
unsaturation.
(II) Characteristics of Alkynes
1. In alkynes, at least two
carbon atoms are sp hybridized while rest of the carbon atoms may be sp3-hybridized.
2. They form homologous series with general formula CnH2n–2 [n = 2–a].
3. They can undergo addition
reactions, oxidation reactions and substitution reactions (only 1-alkynes).
4. They can add two molecules of different reagents.
(III) Important Members of Alkynes
C2H2 ; HC≡CH Ethyne
/ Acetylene
C3H4 ; H3C–C≡CH Propyne/ Allylene/ Methyl acetylene
C4H6 ; H3C–CH2–C≡CH 1-butyne/Crotonylene/ Ethyl acetylene
C4H6 ; H3C–C≡C–CH3 2-butyne/dimethyl acetylene
(IV) Physical Properties of Ethyne
(V) Uses of Ethyne
Ethyne is used:
1. To produce oxy-acetylenic flame which is used for welding and cutting metals.
2. To prepare acetaldehyde, acetic
acid, ethanol etc.
3. To prepare synthetic fibre and plastics.
4. For the artificial ripening of fruit.
5. To prepare acetylene dichloride and acetylene tetrachloride which is used as solvent for varnishes and rubber.
(VI) Identification of Acetylene by Laboratory Tests
Acetylene can be identified by following tests:
1. It gives white ppt. of silver acetylide with an aqueous
ammonical silver nitrate solution.
2. It gives red ppt. of copper acetylide with an aqueous ammonical
cuprous chloride solution.
terminal alkyne (terminal acetylene) and Internal alkyne
(internal acetylene)
Many of an
alkyne’s chemical properties depend on whether there is an acetylenic hydrogen
(H–CºC), i.e.
whether the triple bond comes at the end of a carbon chain. Such an alkyne is
called a terminal alkyne or a terminal acetylene. If the triple bond is located somewhere other than the end of the
carbon chain, the alkyne is called an internal
alkyne or an internal acetylene.
Terminal alkynes have the triple bond
at the end of the carbon chain so that a hydrogen atom is directly bonded to a
carbon atom of the triple bond.
Physical Properties
The physical properties of alkynes are
similar to those of alkanes and alkenes of similar molecular weights.
1. Alkynes are colourless and odourless gases with the exception of acetylene which has garlic like smell. Many alkynes have characteristic, mildly offensive odors. Alkynes are relatively non-polar and nearly insoluble in water. They are quite soluble in most organic solvents, including acetone, ether, methylene chloride, chloroform, benzene, carbon tetracholirde and alcohols.
2. The first three members are colourless gases and next five to six members are liquids while higher alkynes are solids. Ethyne, propyne, and the butynes are gases at room temperature, just like the corresponding alkanes and alkenes.
3. The branched alkynes will have low boiling points than linear alkynes. The boiling points of alkynes is slightly greater than the corresponding alkanes and alkenes. In fact, the boiling points of alkynes are nearly the same as those of alkanes and alkenes with similar carbon skeletons.
4. They are comparatively more denser than alkanes and alkenes.
5. The H/C ratio is least in alkynes and hence they will burn with sooty flame due to incomplete combustion.
6. The alkynes having triple bond in the last carbon atom are
called as terminal alkynes. The hydrogen atom attached with carbon in terminal alkyne is
polarized due to high electronegative nature of carbon. The hydrogen gets partial
positive charge and can be removed easily by strong bases like soda amide (NaNH2).
Hence alkynes are more acidic than alkenes
and alkanes.
Skeletal formula of Alkynes
The triple bond cannot bend, so the bonds adjacent to a triple bond
must be straight.
General Methods of Preparation of Alkynes
Following are the general methods of preparation of alkynes:
1. By Double
Dehydrohalogenation of vicinal dihalide (Action of Alkalis on dihalides)
2. By Double dehalogenation of tetrahaloalkane (Vicinal tetrahaloalkane)
3. By alkylation of
monosodium acetylide with alkyl halide.
4. Preparation of alkynes
from alkenes by its halogenation followed by dehydrohalogenation
Specific Methods of Preparation of Acetylene
1. By the action of water
on calcium carbide (Hydrolysis of CaC2).
2. By heating methane
3. By hydrolysis of
monosodium acetylide
1. By Double Dehydrohalogenation of Vicinal Dihalide (Action of
Alkalis on dihalides)
Vicinal Dihalide
The compounds that contain two same halogen atoms on adjacent
carbon atoms are called vicinal dihalides or vic-dihalides (which are isomeric
with gem-dihalides having two same halogen atoms on the same carbon atom).
Vicinal dihalides are readily prepared from corresponding alkenes by their
halogenation (addition of halogens).
Dehalogenation
Dehydrohalogenation (de-HX) is the elimination of hydrogen as well
as halogen atom from adjacent carbon atoms of a compound.
Reagent Used for Dehalogenation
Dehydrohalogenation is usually carried out by alcoholic solution of
KOH called alcoholic potash (aqueous KOH leads to SN reaction to
produce alcohol).
NaNH2 is preferred over alcoholic KOH in the second step
as alcoholic KOH finds it difficult to fetch a hydrogen halide molecule as the
two leaving groups are attached to sp2-hyrbidezd carbon atoms.
Reagent Used for Dehydrohalogenation
Alcoholic KOH is commonly used reagent for dehydrohalogenation. But
alcoholic KOH fails remove a hydrogen halide molecule from inert vinylic
halides formed in the first step of preopation of alkyne from vicinal dihalide as the two leaving groups are attached to sp2-hyrbidezd
carbon atoms.NaNH2 in liquid ammonia is preferred over alcoholic KOH
in the second step as alcoholic
Method
Vicinal dihalide on treatment with alcoholic solution of KOH
undergoes dehydrohalogenation losing two molecules of HX from adjacent carbon
atoms forming alkyne.
When vicinal dihalide is treated with alcoholic KOH followed by
NaNH2 in liquid ammonia form ethyne. Molten KOH or alcoholic KOH at
200oC favours an internal alkyne.
Mechanism
Dehydrohalogenation
of vicinal dihalide produces vinyl halide (olefinic halide) in the first step
which is unreactive and a stronger base like sodium amine (NaNH2) is
used to remove the second molecule of HX. The vicinal dihalides may be directly
treated with NaNH2 to give alkynes.
Preparation of alkene from gem dihalides
A compound
containing two halogen atoms on the same carbon atom is called gem dihalide
(Latin word 'Gemini' means twins). On heating with alcoholic KOH, gem dihalides
give alkynes.
2. By Double Dehalogenation of tetrahalides or tetrahaloalkane
Dehalogenation
The removal of halogen atoms from adjacent carbon atoms is called
dehalogenation. Dehahlogenation is carried out by zinc dust or sodium iodide.
Reagent Used for Dehalogenation
1,1,2,2-tetrahaloalkanes or tetrahalides on heating with zinc dust
in alcohol losing 2 molecules of halogens forming alkyne.
Drawback
This method is not of great utility for preparing alkynes because
the tetrahalides are themselves made by halogenation of alkynes.
3. Preparation of Higher
Alkynes by the action of Monosodium Acetylide or alkynides on Alkyl Halide
3. Preparation of Higher Alkynes by the action
of Monosodium Acetylide or alkynides on Alkyl Halide
1. Monosodium acetylide is a sodium salt
of acetylene or 1-alkynes prepared
by treating ethyne or 1-alkyne with sodium in liquid ammonia.
2. Higher alkyne can be
prepared by the action of sodium alkynides on primary alkyl halides. This reaction is known as alkylation of sodium
alkynides.
3. The main advantage of this method is that it can be used to convert
lower alkynes into higher alkynes. Yields
are poor with higher alkyl halides.
Preparation of alkynes from alkenes:
This process involves two steps:
(i) Halogenation of alkenes to form
vicinal dihalides
(ii) Dehalogenation of vicinal dihalides
to form alkynes.
4. Preparation of Alkynes from
Electrolysis of Salts Of Unsaturated dicarboxylic Acids. (Kolbe’s Electrolytic
Method)
Electrolysis of sodium or potassium salt
of maleic or fumaric acid yields alkynes.
7.9.B Specific Methods of Preparation of Acetylene
1.By the action of water
on calcium carbide (Hydrolysis of CaC2).
2. By heating methane
3. By hydrolysis of
monosodium acetylide
4. Preparation
of Ethyne from Hydrolysis of Monosodium Acetylides
5. Preparation
of Ethyne by Acidification of Monosodium Acetylides
1. By the Hydrolysis Calcium Carbide (By the action of water on
calcium dicarbide)
Calcium dicarbide wrongly called calcium carbide on treatment with
water, hydrolyze to give ethyne. Calcium dicarbide is made by heating lime and
anthracite in an electric furnace.
2. By Heating Methane to 1300°C
Today,
ethyne is normally prepared by the pyrolysis of methane. In this procedure, a
stream of methane gas is briefly heated to 1500°C in an airless chamber. Two
molecules of methane are dehydrogenated at 1300°C giving ethyne.
Air must be
excluded from the reaction or oxidation (combustion) will occur.
3. By Partial Combustion of Methane at high temperature
(Industrial Method)
4. Preparation of Ethyne from Hydrolysis of Monosodium
Acetylides with Water
5. Preparation of Ethyne by Acidification of Monosodium
Acetylides with HCl
7.10 Chemical
Reactions of Alkynes (Ethyne)
Reactivity of Alkynes
Alkynes are unsaturated compounds with two pi bonds. It is expected
to be more reactive than alkenes. But in practice, alkynes are less reactive than alkenes. The rate of addition to alkynes is
slower by a factor of 100 to 1000 than addition to equivalently
substituted alkenes.
Two factors are significant in
explaining this inertness of alkynes:
1. The two pi bonds in alkynes merge to form single big
electrons cloud. Since electrons per unit volume decrease and the availability
of pi electrons for the electrophile decreases, so the rate of
electrophilic addition decreases.
2. The carbon atom in alkynes is sp-hybridized which shows 50%
s-character. While in alkenes, it is 33%. Greater the s-characters of an
orbital the more tightly bound the electrons to the nucleus and greater
is the stability.
Following types of reactions occur in alkynes:
(A) Addition Reactions.
(B) Oxidation Reactions.
(C) Substitution Reactions
7.10.A Addition
Reactions
Definition
Addition Reaction involves the combination of two (or more)
molecules of substances to form a single molecule of product. In organic
chemistry, addition reactions are those in which two or more atoms or groups
are simply added across a double or triple (multiple) bond without the
elimination of any atom or molecule. These
reactions proceed by the addition of reacting molecules along the triple (or
double) bond.
1. In these reactions, at least
one p-bond is lost while two new σ-bonds are formed. During addition reactions double bonds become saturated to single
bond and triple bonds are converted into double bond or may become saturated by
further addition.
2. These reactions are characteristic of unsaturated organic
compounds containing C=C or C-C bonds like alkenes, alkynes, benzenes and
carbonyl compounds containing carbonyl group (>C=O) like aldehydes and
ketones.
3. Addition reactions are of following types:
(i). Electrophilic
Addition Reactions (EAR)
(ii). Nucleophilic addition reactions
(NAR) Or Nucleophilic Carbonyl
addition (NCA)
(iii). Free Radical addition (Addition
Polymerization)
Electrophilic
Addition Reactions (EAR)
Definition
The type of addition reactions in which the species that first
attack the multiple bond is an electrophile (electron seeker) are called
Electrophilic Addition Reactions (EAR).
Reason for calling Electrophilic Addition Reactions
It is called Electrophilic Addition because the reaction is
triggered by the attack of an electrophile (an acid or a proton) on the p–electrons
of the C-C multiple bonds of alkynes or alkenes (which are weak bases or
nucleophiles).
Reason of Alkynes to undergo Electrophilic Addition Reactions
Alkynes (or Alkenes) are nucleophilic in character due to presence
of readily available p–electrons. Thus Alkynes (or Alkenes) undergo EAR by providing p–electrons
to electrophilic reagents.
Alkynes are nucleophilic in character due to presence of readily
available and much exposed p-electrons. Thus they undergo addition reactions by providing p-electrons
to electrophilic reagents. [Alkynes are chemically more reactive than the
corresponding alkenes because of their higher degree of unsaturation. In addition, electron-rich triple bond makes
the alkynes very prone to addition reactions].
Addition of Two molecules of attacking
reagent in Alkynes
Alkynes undergo EAR adding two molecules of
reagent by providing to electrophilic reagents. Two sequential addition
reactions take place in alkynes; addition of one equivalent of reagent forms an
alkene, which can then add a second equivalent of reagent to yield a saturated
product having four new bonds. However the addition of HCN and water are
initiated by nucleophilic attack and only one molecule of these reagents can be
added.
Application of Markownikoff’s rule
The addition of first reacting molecule may or
may not be governed by Markownikoff’s rule but the addition of second reacting
molecule is always almost dictated by Markownikoff’s rule.
General
Representation
OR
General Pattern of
Mechanism of Electrophilic Addition Reactions (E.A.R.)
Electrophilic Addition Reactions (E.A.R.) proceeds through two
steps mechanism. The first step, which is normally the rate-determining step,
involves the transfer of the two p–electrons of alkene to an electrophile of unsymmetrical reagent to
generate a carbocation called carbonium ion as a reactive intermediate which
react with nucleophile of unsymmetrical reagent to form final addition product.
Step I (Slow Reversible
Formation of Carbonium ion by the attack of Electrophilic Hydrogen of Reagent on p–electrons of Alkene)
Step II (Fast Irreversible Attachment of Carbonium ion and
Leaving Nucleophilic Group of Reagent Giving Addition Product)
Examples of Electrophilic Addition Reactions (E.A.R)
1. Hydrogenation (Addition
of Hydrogen; H – H ) [Alkane or
Alkene]
2. Halogenation (Addition of Halogen; X – X ) [tetrahalide]
3.Hydrohalogenation (Addition of Hydrogen halide; H – X ) [Gem dihalide]
4 Hydration(Addition of water; H – OH
) [Aldehyde or Ketone]
5. Hydronitrilation (Addition of Hydrogen Cyanide; H – CN) [Alkanenitrile]
6. Self-addition/Cyclic Polymerization [Benzene]
8. Ozonolysis (Addition of O3
followed by decomposition through hydrolysis) [Ozonide]
1. Hydrogenation
Alkynes add two molecules of H2 in the presence of Ni
catalyst at 250–300°C or Pt/Pd catalyst at room temperature under high pressure
and thus hydrogenated first into corresponding alkenes and finally alkanes. The
reduction can be stopped at the alkene stage by using Pd poisoned with BaSO4
+ quinolone (Lindlar’s catalyst).
Lindlar’s catalyst
The reduction can be stopped at the alkene
stage by using Pd poisoned with BaSO4 + quinoline (Lindlar’s
catalyst). To stop the reaction at alkene, catalyst is inactivated partially by
poisoning it with salts of heavy metals.
Lindlar’s catalysts can be broken down into three primary
components –
▶ Palladium (accounts for only 5% of the total weight)
▶ Calcium
Carbonate or barium sulphate (over which the palladium is
deposited)
▶ Catalyst poisons (typically lead salts like lead
acetate, lead(II) oxide, and quinoline)
2. Halogenation
1. Alkynes add two molecules of halogens in an inert solvent in the
presence of metallic halide as a catalyst first forming Olefinic dihalides
(dihaloalkene) and finally tetrahalides (tetrahaloalkane).
2. Order of reactivity of halogens is Cl2 > Br2
> I2. Thus chlorine reacts
explosively (to prevent explosion, reactants are mixed in retort with
Keiselguhr, SiO2 and iron fillings to absorb the heat of reaction),
bromine reacts less violently but least reactive iodine add with difficulty
only in the presence of ethanol to give dihalide only.
3. Hydrohalogenation
Hydrohalogenation
The addition of halogen acids or hydrogen halides i.e. HX to an
unsaturated compound is called Hydrohalogenation.
Reagent used for Hydrohalogenation
Alkynes (Ethyne) add two molecules of hydrogen halides (HX) in the
presence of light or metal halide catalyst (HgCl2 or CuCl) either in
the gas-phase or in the solution (e.g. in benzene) in two stages first forming
corresponding unsaturated halide (alkenyl halide/ haloalkene) which further
reacts with halogen acid to form dihaloalkane or gem-dihalide according to
Markownikoff’s Rule.
Markownikoff’s Rule
The addition of second molecule of HX in alkenyl halides is obeyed
by Markownikoff’s Rule which states that the negative part of the adding
substance will be attached to that carbon atom of the unsymmetrical molecule,
which has least number of hydrogen atoms.
Addition in unsymmetrical
alkynes
The addition of both molecules of HX to unsymmetrical alkynes take
place in accordance with Markownikoff’s Rule. Thus both the halogens become
attached to the same carbon atom.
Application of Reaction
Haloalkenes or alkenyl halides are peculiarly unreactive and can be
isolated and then polymerized to polyvinyl halides.
order
of reactivity of different halogen acids
The order of reactivity of different halogen acids is HI > HBr
> HCl.
Hydrogen chloride being least reactive among halogen acids, adds
very slowly to acetylene in the absence of catalysts giving vinyl chloride or monochloroethylene
as the main product (which can then be polymerized to polyvinyl chloride).
Further addition of HCl is difficult and technically, of course, undesirable,
since the vinyl compounds are the required products. Presumably, ethylidene
dichloride would be formed. HBr and HI add more readily to acetylene than HCl
yielding ethylidene haldies as the final product.
General Reaction
MECHANISM
1st Step (Attack of p-Electrons on Electrophilic Part of Reagent)
2nd Step (Combination of Carbonium Ion and
Nucleophile)
3rd Step
The further addition of HCl takes place in the same manner, but
addition proceeds in accordance with Markownikoff’s Rule which states that
negative part of adding molecule must attach to that carbon atom which has
least hydrogen atom.
4. Hydration
Definition
The addition of water across multiple bond
is called hydration.
Reagent Used and Product of Hydration of Alkynes
Alkyne adds a molecule of water via acid-catalyzed reaction in
the presence of a mixture of mercurous sulphate
and sulphuric acid as catalysts at 75-100°C,
forming an unstable intermediate enol (alkenol/
alkenyl alcohol) which then itself readily undergoes tautomerization or molecular rearrangement giving an aldehyde or ketone. Ethyne gives aldehyde while higher alkynes
give ketone e.g. propyne give acetone. Markownikoff’s rule is
followed in hydration of higher alkynes.
Reaction Mechanism
Hydration of alkynes is initiated by
nucleophilic attack.
General Reaction
Mechanism of
Hydration
1. Alkenyl alcohol is an enol which is
far too unstable to be isolated, but exists in dynamic equilibrium with ketone,
the equilibrium lying very much towards the right-hand side.
2. This phenomenon where a pair of
isomers are in dynamic equilibrium with each other is termed as tautomerism or
keto-enol tautomerisation.
3. Since tautomerism involves the
migration or shift of proton (H+), it is also known as prototropy or
prototropic isomerism.
5. Addition of Hydrogen Cyanide
1. The catalyzed addition of hydrogen cyanide to alkyne in the presence of cuprous chloride and ammonium chloride under pressure and at 80°C gives alkenyl cyanide or alkenenitrile.
2. Ethyne gives vinyl cyanide or propenenitrile (acrylonitrile, which is used in making polypropenenitrile or Acrilan).
3. The reaction is initiated by nucleophilic attack.
6. Cyclic Polymerization
Three moles of acetylene (ethyne) when passed passing through
copper-iron tube over heated organo-nickel catalyst at 300-450°C, polymerizes
to give benzene.
7. Addition of NH3
5. Ozonolysis/Ozoniation
(Ozonide Reaction)
Ozonolysis is an organic redox chemical reaction involving oxidative cleaving of the unsaturated bonds of alkenes, alkynes
and azo compounds (compounds with the diazenyl functional group; −N=N−) using ozone (O3; a reactive allotrope of
oxygen). The process allows for carbon-carbon double or triple bonds to be
replaced by double bonds with oxygen.
Definition of Ozonolysis or Ozoniation
The addition of ozone or trioxygen (O3) to an alkene or
alkyne to give an adduct called ozonide is known as ozoniation. The hydrolysis
of ozonide (product of ozoniation) is called ozonolysis. In other words, the
process of preparing the ozonide by the addition
of ozone involving the cleavage of an
alkene or alkyne followed by its subsequent decomposition by
hydrolysis in which the pi bonds
between carbon-carbon atoms are broken down
in the presence of reducing agent such as zinc
or oxidizing agent such as hydrogen peroxide
is called ozonolysis. Ozonolysis is a
chemical reaction in which the pi bonds between carbon-carbon atoms are broken
down by the addition of ozone (O3).
Products
of Ozoniation of Alkynes
The hydrolysis of ozonide in
presence of reducing agents such as Zinc dust or dimethyl sulfide is known as reductive
ozonolysis. In this type, aldehydes and ketones are formed as
products.
The hydrolysis of ozonide in
presence of oxidizing agent such as H2O2 is
known as oxidative ozonolysis. In this type, already-formed
aldehydes and ketones are converted to carboxylic
acids.
Terminal
alkynes undergo oxidative cleavage to yield one
carboxylic acid and CO2. (If the alkyne is terminal i.e.
contains R≡CH, then the products are RCO2H and CO2 since
unstable carbonic acid, HOC(=O)OH, is formed).
Internal
alkynes undergo oxidative cleavage to yield two
carboxylic acids. For a symmetrical, internal alkyne this
would mean two equivalents of the same
carboxylic acid. For an asymmetrical, internal alkyne this
would mean two different carboxylic acids.
Note that each of the CC bonds in the C≡C becomes
a CO bond
General
Reaction
ozone (O3) adds across the triple
bond of an alkyne to form an unstable intermediate called ozonide which on
decomposition with hydrogen peroxide to
form glyoxal.
Alkyne reacts with ozone to form ozonide. The ozonide may react with
water in presence of zinc dust to give ketone (1,2-dicarbonyl
compound) which are finally oxidized to acids by H2O2
produced in the reaction.
