A triple bond gives an alkyne four fewer hydrogen atoms than the corresponding alkane. There fore the triple bond contributes two degree of unsaturation (DU).
Alkynes are not as common in nature as alkenes, but some plants do use alkynes to protect themselves against disease or predators. Acetylene is by far the most important commercial alkyne. Acetylene is an important industrial feedstock but its largest use is as the fuel for the oxyacetylene welding torch.
2. Structure and Bonding in Alkynes
(1) Alkynes are hydrocarbons that contain carbon -carbon triple bond.
(2) Alkynes are also called acetylenes because they are derivatives of acetylene.
(3) The general formula is : CnH2n-2. (one triple bond)
(4) In alkyne C≡C bond length is 1.20 Å.
(5) Its bond energy is 192 kcal. mol-1
(6) The hybridization of carbon atoms having triple bond (C≡C) in alkynes is sp
(7) Overlapping of these sp hybrid orbitals with each other and with the hydrogen orbitals gives the sigma bond framework which is linear (180º) structure.
(8) Two p bonds result from overlap of the two remaining unhybridized p orbitals on each carbon atom. These orbitals overlap at right angles (90º) to each other, forming one p 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 p electron cloud around s bonded structure
Note : Any type of stereoisomerism does not arise in acetylenic bond due to linearity of C º C bond.
Ex.1 Cis-trans isomerism is not possible in alkynes because of :
Ans. 180º bond-angle at the carbon-carbon triple bond.
Ex.2 Draw the geometrical isomers of hept -2-en-5-yne?
3. Physical Properties of Alkynes:
(1) 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 alkanes). They are quite soluble in most organic solvents, (acetone, ether, methylene chloride, chloroform and alcohols).
(2) Acetylene, propyne, and the butynes are gases at room temperature, just like the corresponding alkanes and alkenes. In fact, the boiling point of alkynes are nearly the same as those of alkanes and alkenes with same number of carbon atoms.
Relative density (at 20°C)
HC = CH
HC = CCH2CH3
HC = C(CH2)2CH3
ch3c = cch3
ch3c = cch2ch3
HC = CCH(CH3)2
5. TABLE - COMPARATIVE STUDY OF ALKANES, ALKENES, ALKYNES
Ex.3 Which has a longer carbon-methyl bond, 1-butyne or 2-butyne. Explain?
Ans. The bond from the methyl group in 1-butyne is to an sp3-hybridised carbon and so is longer than the bond from the methyl group in 2-butyne, which is to an sp-hybridised carbon.
Ex.4 Arrange the following bond-lengths in increasing order.
Ans. (d) < (b) < (c) < (e) < (a)
Q.3 Arrange C - H bond -lengths (a,b,g) in increasing order as shown : -
6. Laboratory Test of Alkyne
7. Laboratory test of terminal alkynes
When triple bond comes at the end of a carbon chain. The alkyne is called a terminal alkyne.
1-Butyne, terminal alkyne
8. Acidity of Terminal Alkynes:
Terminal alkynes are much acidic than other hydrocarbons due to more electronegative sp hybridized carbon. The polarity (acidity) of a C - H bond varies with its hydridization, increasing with the increase in percentage's character of the orbitals.
sp3 < sp2 < sp
The hydrogen bonded to the carbon of a terminal alkyne is considerably more acidic than those bonded to carbons of an alkene and alkane (see section). The pKa values for ethyne, ethene & ethane illustrate this point
The order of basicity of their anions is opposite to that of their relative acidity:
CH3CH2: > H2C = CH:- > HC≡C:-
pKa 15.7 16-17 25 38 44 50
9. General methods of preparation :
(I) By dehydro-halogenation of gem and vic dihalide:
RCH = CHR + Br2 →
A vic - dibromide
The dehydrohalogenations occur in two steps, the first yielding a bromoalkene and the second alkyne.
e.g. CH3CH2CH = CH2 CH3CH2C≡CH
e.g. [CH3CH2C º CH] CH3CH2C≡C!Na+
CH3CH2C≡C:-Na CH3CH2C≡CH + NH3 + NaCl
Ex.5 Give the structure of three isomeric dibromides that could be used as starting materials for the preparation of 3,3-dimethyl-1-butyne.
Ex.6 Show the product in the following reaction
Q.4. 1,1-dibromopentane on reaction with fused KOH at 470 K gives 2-pentyne
1,1-dibromo pentane 2-pentyne
Give the mechanism of this rearrangement.
(II) By Dehalogenation of Tetrahaloalkane:
(III) Replacement of The Acetylenic Hydrogen atom of terminal Alkynes.
Sodium ethynide and other sodium alkynides can be prepared by treating terminal alkynes with sodium amide in liquid ammonia.
(R or R' or both may be hydrogen)
The following example illustrates this synthesis of higher alkyne homologues.
(R'-X must be an unhindered primary halide or tosylate)
The unshared electron pair of the alkynide ion attacks the back side of the carbon atom that bears the halogen atom and forms a bond to it. The halogen atom departs as a halide ion.
Addition of acetylide ions to carbonyl groups
Sodium Propanal 1-Pentyn-3-ol acetylide
3-Methyl-1-butyne 4-Methyl-1-Phenyl pent-2-yne-1-ol
Ex.7 Show how to synthesize 3-decyne from acetylene along with necessary alkyl halides.
Sol. H - C≡C - H H3C - (CH2)5 - C≡C - H
H3C - (CH2)5 - C≡C - H CH3 - (CH2)5 - C≡C - CH2CH3
Q.5 Show how you would synthesize the following compound, beginning with acetylene and any necessary additional reagents.
(IV) By Kolble's Electrolytic synthesis.
(V) By Hydrolysis of carbides
CaC2 + 2HOH → C2H2 + Ca(OH)2
MgC2 + 2HOH → C2H2 + Mg(OH)2
Mg2C3 + 4HOH→ CH3 - C≡CH + 2Mg(OH)2
10. Chemical reactions of Alkyne
(I) Reduction to alkenes
(a) By Lindlar's reagent
(b) By Brich reduction
General Reaction R - C º C - R' (anti addition)
(c) By hydroboration reduction
General Reaction R - C º C - R'
Ex.8 Identify (X) and (Y) in the following reaction
CH3-CH2 - C º CH
Ans. (X) : (Y) :
(II) Addition of Halogen (X2=Cl2, Br2)
General Reaction R - C º C - R' R - CX = CX - R' (Anti-addition)
Ex.9 Explain why alkynes are less reactive than alkenes toward addition of Br2.
Sol. The three memebered ring bromonium ion fromed from the alkyne (A) has a full double bond causing it to be more strained and less stable than the one from the alkene (B).
(A) less stable than (B)
Also, the C's of A that are part of the bormonium ion have more s-character than those of B, further making A less stable than B.
(III) Addition of Hydrogen halides (Were HX = HCl, HBr, HI)
R - C≡C - R'
e.g. CH3-C≡C - CH2CH3 HBr →
e.g. H - C≡C - CH2CH2CH3