CHAPTER 3 | ALKENES, ALKYNES AND ALKADIENES | ALKYNES B.Sc. 1st Year Page : 2 Organic Chemistry
Alkynes or acetylenes are acyclic unsaturated hydrocarbons containing a triple bond. Their general molecular formula is CnH2n-2 when n = 2, 3, 4, etc. These are isomeric with dienes. The simplest member is acetylene. The higher members have generally named derivatives of acetylene. In I.U.P.A.C. System, acyclic compounds containing a triple bond are called `alkynes’ (by replacing -ane of alkane by -yne). The longest continuous chain will determine the word root ‘calk’. For examples :
Structure of Alkyne
In alkynes, triple bonded carbon atoms are in sp hybridization. A triple bond is made up of two – bonds and: ne a – bond. The fraction of the triple bonded molecule is linear in nature. The sigma bond is formed by the axial overlapping of sp hybrid orbitals of two carbons. For example, the overlapping in ethyne is as follows-
Fig. 3.01: Overlapping of one half-filled sp-orbital of each carbon atom with s-orbital of H-atoms. sp-sp orbital overlapping between two carbon atoms and sidewise overlapping of unhybridized 2py and 2pz orbital of one C-atom with the same orbitals of another carbon resulting in the formation of three 6-and two n-bonds in CH=CH.
Each carbon forming a triple bond has two unhybridized 2p – orbitals containing unpaired electrons. These orbitals are oriented perpendicular to the axis of the 6 – bond. Sidewise overlapping of these orbitals result :n the formation of two IC – bonds. The carbon-carbon bond distance in acetylene is 1.20A: and the bond angle is 180° (since the molecule possesses a linear geometry).
General Methods of Preparation of Alkynes :
it may be prepared by the following methods :
1- From halogen derivatives of alkanes
Alkynes can be prepared from di- and tetra-halogen derivatives of alkanes.
(a) From dihaloalkanes :
Alkynes may be prepared by dehydrohalogenation (elimination) of haloalkanes, preferably Bromo derivatives.
(b) From alkylene halides (vicinal-dihalides) :
When a vicinal dihalide is boiled with alcoholic potash, it loses 2 molecules of hydrogen halide to form the corresponding alkyne.
For example 1,2-dibromoethane on boiling with alcoholic KOH gives ethyne KOH(alcoholic) KOH(alcoholic)
sodamide in liquid ammonia can be used instead of alcoholic KOH. It gives a better yield as compared to earlier since there is little tendency to form by-products.
(ii) From alkylidene halides (gem-dihalides): When a gem-halide is boiled with alcoholic KOH corresponding alkyne is obtained. For example,
(i) An alkylene dihalide has two halogen atoms, one of each of the two adjacent carbon atoms is called `vicinal dihalide’
(ii)The dihalo derivatives in which both the halogen atoms are attached to the same carbon atoms are known as ‘gem-dihalides’.
(b) From tetrahaloalkanes :
Alkynes can be obtained from tetrahaloalkanes by heating with zinc.
2- From Acetylene :
Acetylene reacts with sodmide (a mixture of Na in ammonia) to form a salt called as `sodium acetylide’. This salt on treatment with an alkyl halide forms a substituted acetylene.
Ethyne or Acetylene is the first important member of alkynes. Following methods are uses for the :reparation of acetylene.
(a) By the action of water on calcium carbide : Calcium carbide obtained by heating irnestone with :oke when reacts with water forms acetylene.
b) By heating a haloform with silver powder :
Physical Properties Alkynes :
(i)Acetylene is a colorless gas with an ethereal smell when pure. It is sparingly soluble Br is readily soluble in acetone.
(ii) When compressed or liquefied acetylene explodes.
(iii) It burns with luminous sooty flame(due to the high carbon content) and is used for lighting purposes.
(iv) It is also used for the preparation of several compounds such as CH3CHO, C2H5OH, C1-1300OH, etc.
(v) Higher acetylenes generally have greater b.p’s. then the corresponding alkenes because acetylenes are linear and more packed as compared to alkenes.
Chemical Properties Alkyne:
 The acidity of acetylene and 1-alkynes :
Acetylene and 1 -alkynes(or terminal acetylenes or terminal a:kynes) are acidic in nature because the hydrogen attached to the triple bonded carbon is acic5c while -hydrogen atom of alkenes and alkanes are not so. It is evident from the following reactors .
[2[ Formation of Alkali metal acetylides with sodium metal and sodamide :
Acetylenes and alkynes react with strong bases such as sodium metal at highly temperature and sodamide in quid ammonia to form sodium acetylides with the liberation of hydrogen gas.
Similarly, with Fehling solution (ammoniacal cuprous chloride) alkynes form copper acetylides.
 Addition of Hydrogen :
Addition of hydrogen on alkynes take place in two stages as they possess two it – bonds. In the first stage an alkene is formed. The subsequent addition results in the formation of an alkane. Thus, the course of the reaction depends upon the amount of reagent used. If the addition is carried out with one mole of the reagent, alkene would be the product. If excess of reagent is used, ultimately an alkane is produced. In this way addition can be controlled for obtaining a desired product.
Electrophilic Addition :
Alkynes are less reactive towards electrophile than alkenes even though they contain more it – bonds. Hence, electrophilic addition in alkynes are slower than in alkene. It is because of the following two reasons-
(i) The sp hybridized carbon atom is more electronegative due to more s- character (50%) than sp2 hybridized carbon atom of alkene (33% s- character). The greater s- character draws 1E- electrons in alkynes closer to the nuclei. In other words, it- electrons of alkynes are more tightly held by the carbon atom than that of alkenes. Consequently, it- electrons of alkynes are less available for reaction with electrophiles and hence less reactive.
(ii) Two π- electron cloud perpendicular to each other as well as with the C-C a- bond can delocalized to attain a cylindrical ii- electron cloud. This more delocalized cylindrical 7E- electron cloud of the triple bond gives more stability and thus 71- electrons become less available for addition.
A possible contributing factor responsible for decreased reactivity of a triple bond to electrophile may be the fact that the bridged carbonium ion intermediate formed from acetylene. This intermediate is more strained the bridged carbonium ion obtained from ethylene.
Some important electrophilic addition reactions are discussed here. Many addition rectkx: require a .7:a.’_alyst like mercuric(Hg++) ion or barium(Ba++) ion. :
(3) Addition of Halogens :
Due to the presence of a triple bond, acetylene is more urts=mat-d than f-Tylene and forms additional product more easily with two or four monovalent atoms are grows. e.g. -a:ogens like chlorine and bromine readily add to alkynes first forming trans- 1. 2 -dthair-…ars.enes and –sen 1,1,2,2 – tetrahaloalkanes so called geminal dihalide(Latin; geminus = twins). general ,
It can be summarised through the following steps :
Br–Br® → Br +Br
 Addition of Halogen acids :
Halogen acids add to alkynes in presence of light or mercury salts to =orm alkenyl halides first and then alkylidene dihalides( or gem – dihalide). These, addition of HX take place in accordance with Markonikov’s rule. For example :