Chemistry PREPARATION, PROPERTIES AND REACTIONS OF ALKYNES

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PREPARATION, PROPERTIES AND REACTIONS OF ALKYNES

Introduction :

(i) Alkynes are hydrocarbons that contain carbon-carbon triple bond.

(ii) The general formula is : `C_nH_(2n-2)` (one triple bond)

(iii) The hybridization of carbon `(C equiv C)` in alkynes is carbon `sp`

(iv) Overlapping of these `sp` hybrid orbitals with each other and with the hydrogen `s` orbitals gives the sigma bond framework which is linear `(180^o)` structure.

(v) Two `pi` bonds result from overlap of the two remaining unhybridized `p` orbitals on each carbon atom. These orbitals overlap at right angles `(90^o)` to each other, forming one pi bond with electron density above and below the `C - C` sigma bond, and the other with electron density in front and in back of the sigma bond. This result in a cylindrical `pi` electron cloud around `sigma` bonded structure. See fig.

`text(Note :)` Any type of stereoisomerism does not arise in acetylenic bond due to linearity of `C equiv C` bond.

Laboratory test of alkynes :

(i) `text(Bayer's reagent test)`

Cold, dilute alkaline `KMnO_4` solution is Bayer's reagent. It is pink in colour.

`CH equiv CH +4O oversettext(Bayer's reagent)-> HOOC - COOH`

Pink colour disappears

(ii) `text(Bromine water test)`

`Br_2// H_2O` is reddish brown

See fig.

Laboratory test of terminal alkynes :

When triple bond comes at the end of a carbon chain. The alkyne is called a terminal alkyne. See fig.

(i) Ammoniacal silver nitrate test

`HC equiv CH + 2AgNO_3 + 2NH_4OH -> undersettext[(white ppt)](Ag - C equiv C - Ag) + 2NH_4NO_3 + 2H_2O`

(ii) Ammonical `Cu_2Cl_2` test

`HC equiv CH + Cu_2Cl_2 + 2NH_4OH -> undersettext[(Red ppt)](Cu - C equiv C - Cu) + 2NH_4Cl + 2H_2O`

(iii) `Na` in ether test

`HC equiv CH + 2Na -> undersettext(Colourless gas)(Na - C equiv C- Na) + H_2`

Nomenclature :

`CH equiv CH` Ethyne

`CH_3 -C equiv CH` Propyne

`CH equiv C-CH_2-CH_3` But-1-yne

`CH-C equiv C -CH_3` But -2 - yne

6-Bromo-2-methylpent-3-yne But-3-yn-2-ol

Isomerism :

(i) Chain isomerism `: CH_3 - CH_2 - CH_2 - C equiv CH & CH_3--C equiv CH`

(ii) Position isomerism `: CH_3 - CH_2 - CH_2 - C equiv CH & CH_3 - CH_2 - C equiv C - CH_3`

`CH_3-CH_2-C equiv C -CH_3 & CH_3 -CH = C CH -CH_3`

(iii) Functional group isomerism

`CH_3CH_2-C equiv C -CH_3 & CH_3 -CH = C = CH -CH_3`

Physical Properties of Alkynes :

(i) Alkynes are relatively nonpolar (w.r.t. alkyl halides and alcohols) and are nearly insoluble in water (but they are more polar than alkenes and alkynes). They are quite soluble in most organic solvents, (acetone, ether, methylene chloride, chloroform and alcohols).

(ii) Acetylene, propyne, and the butynes are gases at room temperature, just like the corresponding alkanes and alkenes. In fact, the boiling points of alkynes are nearly the same as those of alkanes and alkenes with same number of carbon atoms.

`text(Table : Comparative study of alkanes, alkenes, alkynes)` : See Table.

General methods of preparation :

(i) `text(By dehydrohalogenetion of geminal and vicinal dihalide :)`

`text((Anti elimination reaction))` See fig.1.

mechanism : See fig.2.

(ii) `text(By dehalogenetion of tetrahaloalkane :)` See fig.3.

(iii) `text(By alkylation of terminal alkynes :)` See fig.4.

`text(Example)`

(a) `H -C equiv C -H underset(NaNH_2)overset(2 e.q)-> Na^(+) c^(-) equiv C^(-) Na^(+) overset(2 text(moles) (ch_3X))-> CH_3 -C = C CH_3`

(b) `R-C equiv C-H overset(R'MgX)->R -C equiv C Mg X overset(R'X)-> R-C equiv C- R'+ R' H`

(iv) `text(By Kolbes electrolylic synthesis)` See fig.5.

(v) `text(By hydrolysis of carbides)`

`CaC_2 + 2HOH -> C_2H_2 + Ca(OH)_2`

`MgC_2 + 2HOH -> C_2H_2 + Mg(OH)_2`

`Mg_2C_3 + 4HOH -> CH_3 - C equiv CH + 2Mg(OH)_2`

Chemical reactions :

`(i)` `text(Formation of alkylide anions (alkynides)`

Sodium, lithium, and magnesium alkynide

`R-C equiv C-H +NaNH_2 -> R-C equiv C^(-) NA^(+) +NH_3 `

`R -C - C -H + R' Li -> R -C equiv C Li + R' H`

`R-C - C-H + R' MgX -> R-C equiv CMgX + R'H`

`text(Alkylation of alkylide ions)`

`R-C equiv C^(-) +R' -X -> R-C equiv C-R'`

(`R'-X` must be an unhindered primary halide or tosylate)

`text(Example)`

`undersettext(Sodiumbutymide)(CH_3CH_2-C eqiuv CH^(-) NA^(+)) + undersettext(1- Bromopropane)(CH_3ZCH_2CH_2-Br) -> undersettext(Hept - 3- yne)(CH_3CH_2- C equiv C - CH_2CH_2CH_3)`

`text(Reactions with carbonyl groups)` : See fig.1.

`text(Example)` : See fig.2.

(ii) `text(Oxidation to) alpha-text(diketones)` : See fig.3.

`text(Example)` : See fig.4.

(iii) `text(Oxidative cleavage)` : See fig.5.

`text(Example)` : See fig.6.