NCERT Handwritten Notes Alkynes [ Pdf Download ]

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What are Alkynes?

An alkyne is an unsaturated hydrocarbon with at least one carbon-carbon triple bond in organic chemistry.

alkynes
alkynes

 

Alkynes are unsaturated hydrocarbons with one triple bond; their usual formula is CnH2n+2, and the triple bond is referred to as the “acetylenic bond.” Alkynes can be found in abundance in nature. With two carbon atoms bonded by a triple bond, ethyne (C2H2) is the first member of the alkyne family.

An alkyne is an unsaturated hydrocarbon with at least one carbon-carbon triple bond in organic chemistry. Alkynes, like other hydrocarbons, are hydrophobic in nature. The word acetylene is most usually used to refer to ethyne. It is the most basic of the alkynes, with two carbon atoms joined by a triple bond and each carbon atom capable of bonding to one hydrogen atom.

Isomerism in Alkynes

Alkynes show three types of isomerism

  1. Chain isomerism
  2. Position isomerism
  3. Functional isomerism

1. Chain isomerism

The difference between a straight chain and a branched chain is due to the varied arrangement of carbon atoms in the chain.

Example: 4-methylpent-2-yne and hex-2-yne

alkynes
alkynes

2. Position isomerism

It’s due to a change in the triple bond’s placement.

Example: Pent-1-yne and pent-2-yne

 

alkynes
alkynes

3. Functional isomerism

Alkynes and alkadienes are isomeric, with both having the general formula CnH2n-1.

Example: But-1-yne and buta-1,3-diene

 

alkynes
alkynes

Alkynes have a functional group that is made up of three bonds. The triple bond determines a lot of the properties of alkynes. The triple bound carbon atoms in ethyne show sp hybridization. As a result, the ethylene molecule is a linear one.

Alkynes Homologous Series

A homologous series is a collection of carbon compounds in which the hydrogen atom is replaced by the same functional group. These compounds have comparable chemical characteristics due to the presence of the same type of functional group in the chain.

A homologous series is a collection of hydrocarbons with similar chemical characteristics and the same general formula. They are chemical compounds with identical structural and functional groups. The members of the homologous series have a range of physical qualities.

The chemical formulas and names of the first 10 carbon straight chain alkynes are listed below.

Name  Molecular Formula
Ethyne C2H2
Propyne C3H4
1-Butyne C4H6
1-Pentyne C5H8
1-Hexyne C6H10
1-Heptyne C7H12
1-Octyne C8H14
1-Nonyne C9H16
1-Decyne C10H18

Alkynes with a carbon-carbon triple bond have a linear or straight shape as a key feature. A single-dimensional straight line runs through a portion of the molecule. Ethyne is a chemical that can be used to make a range of different chemicals. A few examples of these applications are as follows: Organic molecules such as ethanol, ethanoic acid, and acrylic acid are all made from ethyne.

Tests for the Presence of a Triple Bond

Any hydrocarbon that has a triple bond is unsaturated, and the chemical passes Baeyer’s test. Alkynes, on the other hand, are identified by a series of assays, which are detailed below.

  1. With ammoniacal silver nitrate: With ammoniacal silver nitrate, alkynes produce a white precipitate of silver acetylide.
  2. With ammoniacal cuprous chloride: Alkynes produce a red cuprous acetylide precipitate when combined with ammoniacal cuprous chloride.

Uses of Alkyne

  • Ethyne is often used in oxyacetylene gas welding and oxyacetylene gas cutting because it has a very hot flame. When ethyne is combined with oxygen, the ensuing flame has a temperature of approximately 3600 Kelvin.
  • Millions of kilogrammes of acetylene are produced annually through fractional oxidation of natural gases, with the dominant alkyne being used as a fuel. Some of these alkynes are used to make chemical compounds including ethanoic acid, acrylic acid, and ethanol.
  • Organic molecules such as ethanol, ethanoic acid, and acrylic acid are all made from ethyne. It’s also utilised to create polymers and polymer raw materials.
  • Acetylene is decomposed into two parts: carbon and hydrogen. Even if there is no air or oxygen present, this reaction produces a lot of heat, which might cause the gas to ignite.
  • Alkynes are commonly utilised as starting materials in the production of a wide range of industrially important organic compounds such as chloroprene, vinyl chloride, and others.

 

FAQs

 

What is the structural formula of Ethyne?

The formula for ethyne condensed structure is HC≡CH. The chemical formula for ethyne is C2H2. That means ethyne is made up of two carbon atoms and two hydrogen atoms, with the two carbon atoms linked by triple bonds.

How are alkynes named?

Higher alkenes and alkynes are identified by prefixing the stem name of the unbranched alkane with that number of carbons with a -ene (alkene) or -yne (alkyne) suffix and counting the number of carbons in the longest continuous chain that includes the double or triple bond.

What is an alkyne group?

Alkynes are organic compounds with the functional group carbon-carbon triple bonds and have the empirical formula CnH2n2. Unsaturated hydrocarbons are what they are. When there is only one alkyne in the molecule, the suffix –yne is used, similar to how alkenes have the suffix –ene.

How are alkynes prepared?

Chlorine or bromine is mixed with an inert halogenated solvent like chloromethane to make a vicinal dihalide from an alkene. To make an alkyne, the produced vicinal dihalide is heated and reacted with a strong base.

How can we reduce alkyne to alkene?

Alkynes can be transformed to trans-alkenes by dissolving sodium in an ammonia solution. A Na radical transfers an electron to one of the P bonds in a carbon-carbon triple bond. This produces an anion in an ammonia solution, which can be protonated by hydrogen.

 


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