Dear Zhang,

 This is because double and triple bonds are formed by lateral overlapping of p orbitals whereas single bonds are formed by head on collision of orbitals so single bonds are more stable and does not allow addition across them but the double and triple bonds are weak and allow additions across them and try to become stable like single bonds.The re activity of multiple carbon-carbon bonds are relatively greater than single bonded carbons like alkanes. Its because electrons on multiple carbon-carbon bonds are more exposed and unstable.  Multiple carbon-carbon bonds are characteristic carbon-carbon bonding to alkenes-with double bonds and alkynes-with triple bonds.  In the early days of organic chemistry, alkenes were described as “unsaturated” because, in contrast to the “saturated” alkanes, they were found to react readily with substances such as halogens, hydrogen halides, oxidizing agents, and so on. Therefore, the “chemical affinity” of alkenes was regarded as unsatisfied or “unsaturated.”

Why is ethyne so much less stable than ethene or ethane? First, C-C bonds are not as strong as C-H bonds. Therefore a gain in stability usually is to be expected when C-H bonds are made at the expense of C-C bonds; ethene and ethane each have more C-H bonds than ethyne has.  Second, ethyne has six electrons held between the two carbons and these electrons experience considerable mutual interelectronic repulsion. This accounts for the fact that the average C—C bond strength for the triple bond of an alkyne is 200/3 = 67 kcal, compared to 146/2 ==73 for the double bond of an alkene and 83 kcal for a normal single bond of an alkane. The relative bond strength of a multiple carbon-carbon bonds  such us alkyne and alkanes is smaller than normal single bond of an alkene thus making it less stable and reactive.

molecular antibonding orbitals for pi systems are lower in energy than sigma antibonding orbitals. this is resultant from less overlap between the interacting orbitals. A C-H bond is harder to cleave than a pi bond between two carbons. in oxidation reactions, you'll find that the more unsaturated a molecule is, the less stable it will be. carbon-carbon single and double bonds are too strong (for ethane, 140 kcal/mol BDE versus 90 kcal/mol BDE exact; for ethylene, 211 kcal/mol BDE versus 170 kcal/mol BDE exact). Carbon-hydrogen bonds are too weak (for ethane, 76 kcal/mol BDE versus 100 kcal/mol BDE exact; for ethylene, 82 kcal/mol BDE versus 113 kcal/mol BDE exact). The differences in CC and CH bond dissociation energies between ethane, and ethylene, and acetylene however are close to the exact values (with the exception of the CC triple bond energy), suggesting that the energy differences stem from a systematic bias in the energetics of CC versus CH versus radical electrons.But the Nitrogen triple bonds so stable and double bonded nitrogen’s are so unstable.,The three triple bonds you mention (N≡NN≡N, C≡NC≡N and C≡CC≡C) all have very high bond dissociation energies (from various sources such as this one they are ≈950,940 and 960kJ/mol≈950,940 and 960kJ/mol) so all of these triple bonds require a large amount of energy to break.

Regards,

Prem Baboo

Dear Zhang Chunhui,

We should distinguish between terminal and internal alkynes. The terminal have a relatively acidic proton which allows substitution of many groups. The fact that alkynes have two pi bonds allows the addition of groups to the triple bond to furnish double and single bonds. Terminal alkynes are more suspicious to polymirization than internal alkynes beacuse of their structures. It is difficult to protect triple bond from addition reactions or reduction reactions. If you convert terminal alkyne to internal alkyne you may reduce the posibility of polymirization, however, the other reactions such as addition or reduction will still take place.

The following text illustrates some characteristics and reactions of alkynes:

Alkynes are characteristically more unsaturated than alkenes. Thus they add two equivalents of bromine whereas an alkene adds only one equivalent in the reaction. In some reactions, alkynes are less reactive than alkenes. For example, in a molecule with an -ene and an -yne group, addition occurs preferentially at the -ene. They show greater tendency to polymerize or oligomerize than alkenes do. The resulting polymers, called polyacetylenes (which do not contain alkyne units) are conjugated and can exhibit semiconducting properties.

In acetylene, the H–C≡C bond angles are 180°. By virtue of this bond angle, alkynes tend to be rod-like. Correspondingly, cyclic alkynes are rare. Benzyne is highly unstable. The C≡C bond distance of 121 picometers is much shorter than the C=C distance in alkenes (134 pm) or the C-C bond in alkanes (153 pm). The triple bond is very strong with a bond strength of 839 kJ/mol. The sigma bond contributes 369 kJ/mol, the first pi bond contributes 268 kJ/mol and the second pi-bond of 202 kJ/mol bond strength. Bonding usually discussed in the context of molecular orbital theory, which recognizes the triple bond as arising from overlap of s and p orbitals. In the language of valence bond theory, the carbon atoms in an alkyne bond are sp hybridized: they each have two unhybridized p orbitals and two sp hybrid orbitals. Overlap of an sp orbital from each atom forms one sp-sp sigma bond. Each p orbital on one atom overlaps one on the other atom, forming two pi bonds, giving a total of three bonds. The remaining sp orbital on each atom can form a sigma bond to another atom, for example to hydrogen atoms in the parent acetylene. The two sp orbitals project on opposite sides of the carbon atom. Internal alkynes feature carbon substituents on each acetylenic carbon. Symmetrical examples include diphenylacetylene and 3-hexyne. Terminal alkynes have the formula RC2H. An example is methylacetylene. Terminal alkynes, like acetylene itself, are mildly acidic, with pKa values of around 25. They are far more acidic than alkenes and alkanes, which have pKa values of around 40 and 50, respectively. The acidic hydrogen on terminal alkynes can be replaced by a variety of groups resulting in halo-, silyl-, and alkoxoalkynes. The carbanions generated by deprotonation of terminal alkynes are called acetylides. Alkynes characteristically undergo reactions that show that they are "doubly unsaturated", meaning that each alkyne unit is capable of adding two equivalents of H2, halogens or related HX reagents (X = halide, pseudohalide, etc.). Depending on catalysts and conditions, alkynes add one or two equivalents of hydrogen. Hydrogenation to the alkene is usually more desirable since alkanes are less useful:

RC≡CR' + H2 → cis-RCH=CR'H

The largest scale application of this technology is the conversion of acetylene to ethylene in refineries. The steam cracking of alkanes yields a few percent acetylene, which is selectively hydrogenated in the presence of a palladium/silver catalyst. For more complex alkynes, the Lindlar catalyst is widely recommended to avoid formation of the alkane, for example in the conversion of phenylacetylene to styrene.  Similarly, halogenation of alkynes gives the vinyl dihalides or alkyl tetrahalides:

RC≡CR' + 2 Br2 → RCBr2CRBr2

The addition of nonpolar E-H bonds across C≡C is general for silanes, boranes, and related hydrides. The hydroboration of alkynes gives vinylic boranes which oxidize to the corresponding aldehyde or ketone. In the thiol-yne reaction the substrate is a thiol. Hydrohalogenation gives the corresponding vinyl halides or alkyl dihalides, again depending on the number of equivalents of HX added. The addition of water to alkynes is a related reaction except the initial enol intermediate converts to the ketone or aldehyde. Illustrative is the hydration of phenylacetylene gives acetophenone, and the (Ph3P)AuCH3-catalyzed hydration of 1,8-nonadiyne to 2,8-nonanedione:

PhC≡CH + H2O → PhCOCH3

HC≡CC6H12C≡CH + 2H2O → CH3COC6H12COCH3

References:

Rosser & Williams (1977). Modern Organic Chemistry for A-level. Great Britain: Collins. p. 82. ISBN 0003277402.

 Bloch, Daniel R. (2012). Organic Chemistry Demystified (2 ed.). McGraw-Hill. p. 57. ISBN 978-0-07-176797-2.

Peter Pässler, Werner Hefner, Klaus Buckl, Helmut Meinass, Andreas Meiswinkel, Hans-Jürgen Wernicke, Günter Ebersberg, Richard Müller, Jürgen Bässler, Hartmut Behringer, Dieter Mayer (October 15, 2008). "Acetylene". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a01_097.pub3.

 Kenneth N. Campbell and Barbara K. Campbell (1963). "Phenylacetylene". Org. Synth.; Coll. Vol. 4, p. 763

H. Lindlar and R. Dubuis (1973). "Palladium catalyst for partial reduction of acetylenes". Org. Synth.; Coll. Vol. 5, p. 880.

Fukuda, Y.; Utimoto, K. (1991). "Effective transformation of unactivated alkynes into ketones or acetals with a gold(III) catalyst". J. Org. Chem. 56 (11): 3729. doi:10.1021/jo00011a058..

 Mizushima, E.; Cui, D.-M.; Nath, D. C. D.; Hayashi, T.; Tanaka, M. (2005). "Au(I)-Catalyzed hydratation of alkynes: 2,8-nonanedione". Org. Synth. 83: 55.

Hoping this will be helpful,

Rafik

This is because double bonds and triple bonds have pie bond which is a weaker bond than sigma bond.