Organic Chemistry/Alkynes

The triple carbon bonds is formed in alkynes, due to the absence of hydrogens, thus allowing carbon bonds to become stronger, due to the nucleus central force which pulls in nearby atoms

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Alkynes are hydrocarbons containing carbon-carbon triple bond. They exhibit neither geometric nor optical isomerism. The simplest alkyne is ethyne (HCCH), commonly known as acetylene, as shown at right.

Multiple Bonds Between Carbon Atoms

As you know from studying alkenes, atoms do not always bond with only one pair of electrons. In alkenes (as well in other organic and inorganic molecules) pairs of atoms can share between themselves more than just a single pair of electrons. Alkynes take this sharing a step further than alkenes, sharing three electron pairs between carbons instead of just two.

Two π Bonds

As you should know already, carbon is generally found in a tetravalent state - it forms four covalent bonds with other atoms. As you know from the section on alkenes, all four bonds are not necessarily to different atoms, because carbon atoms can double-bond to one another. What this does is create the appearance of only being bound to three other atoms, but in actuality four bonds exist.

Alkenes are molecules that consist of carbon and hydrogen atoms where one or more pairs of carbon atoms participate in a double bond, which consists of one sigma (σ) and one pi (π) bond. Alkynes are also molecules consisting of carbon and hydrogen atoms, but instead of forming a double bond with only one sigma (σ) and one pi (π) bond, the alkyne has at least one pair of carbon atoms who have a σ and two π bonds -- a triple bond.

The carbon-carbon triple bond, then, is a bond in which the carbon atoms share an s and two p orbitals to form just one σ and two π bonds between them. This results in a linear molecule with a bond angle of about 180ﾟ. Since we know that double bonds are shorter than single covalent bonds, it would be logical to predict that the triple bond would be shorter still, and this is, in fact, the case. The triple bond’s length, 1.20Ǎ, is shorter than that of ethane and ethene’s 1.54 and 1.34 angstroms, respectively, but the difference between the triple and double bonds is slightly less than the difference between the single and double bonds.

The chemistry is very similar to alkenes in that both are formed by elimination reactions, and the major chemical reactions that alkynes undergo are addition type reactions.

Index Of Hydrogen Deficiency

If we compare the general molecular formulas for the Alkane, Alkene, and Alkyne families as well as the Cycloalkane and cycloalkene families we see the following relationship:

Family	Molecular Formula
Alkane	CnH2n+2
Alkene	CnH2n
Cycloalkane	CnH2n
Alkyne	CnH2n-2
Cycloalkene	CnH2n-2

We see that for a ring structure or a double bond there is a difference of two hydrogens compared to the alkane structure with the same number of carbons. If there is a ring + double bond (cycloalkene) or a triple bond (alkyne) then the difference is four hydrogens compared to the alkane with the same number of carbons. We say that the Index of Hydrogen Deficiency is equal to the number of pairs of Hydrogens that must be taken away from the alkane to get the same molecular formula of the compound under investigation. Every π-bond in the molecule increases the index by one. Any ring structure increases the index by one. Here is a list of possibilities:

Per molecule	Index of Hydrogen Deficiency
One double bond	1
1 ring	1
1 double bond and 1 ring	2
2 double bonds	2
1 triple bond	2
1 triple bond + 1 double bond	3
3 double bonds	3
2 double bonds + 1 ring	3
2 triple bonds	4
4 double bonds	4

Just remember that double bonds have 1 π bond, triple bonds have 2 π bonds and each π bond is an index of 1. We can use the index and the molecular formula to identify possibilities as to the exact nature of the molecule. For example, determine the molecular formula and speculate on what kind of Pi bonding and/or ring structure the molecules would have if the Index was given to be 3 and it is a 6 carbon hydrocarbon.

Identify the Alkane molecular formula for six carbons. For n = 6 we would have C_nH_2n+2. That would be C₆H₁₄.

Since an index of 3 means that there are 3 pairs(2) of hydrogen atoms less in the compound compared with the alkane we determined in step 1, then we would have C₆H_14-6 or C₆H₈.

In speculating as to what the bonding and structure could be with an index of 3 that could mean:

• • Three double bonds in a non-cyclic structure like hexatriene
  • Two double bonds in a ring structure like a cyclohexadiene
  • One triple bond and one double bond in a non-cyclic structure

Clearly the answer cannot be determined from the formula alone, but the formula will give important clues as to a molecule's structure.

Cycloalkynes

Cycloalkynes are seldom encountered, and are not stable in small rings due to angle strain. Cyclooctyne has been isolated, but is very reactive, and will polymerize with itself quickly. Cyclononyne is the smallest stable cycloalkyne.

Benzyne is another cycloalkyne that has been proposed as an intermediate for elimination-addition reactions of benzene.

Preparation

In order to synthesize alkynes, one generally starts with a vicinal or geminal dihalide (an alkane with two halogen atoms attached either next to one another or across from one another). Adding sodium amide (NaNH₂) removes the halogens with regiochemistry subject to Zaitsev's Rule, resulting in a carbon-carbon triple bond due to the loss of both halogens as well as two hydrogen atoms from the starting molecule. This is called a double dehydrohalogenation.

Dehydrogenation of an alkane or alkene

R-R -(H^2)⇒ CH^2=CH^2

Dehalogenation of a tetrahaloalkane

Dehalogenation of tetrahalides in the presence of Zn yields Alkynes. When the vapors of tetrahalides are passed over heated Zinc. However this reaction is not so much useful for the preparation of Alkynes. Because tetrahalides required for this reaction are prepared from alkynes. So, this reaction can be used as a purification for the alkynes.

Through Kolbe's Electrolysis

The starting compound is a salt already containing a carbon-carbon double bond. One such compound is maleic acid. Mechanism is similar to that of formation of ethane using kolbe's electrolysis.

CH-COONa
||          (sodium maleate) (maleic acid)
CH-COONa

At Anode:                  At cathode:
CH-COO-         2Na+ + 2e- -> 2Na
||
CH-COO-         2Na + H2O -> 2NaOH + H2

-2e- ->

CH-COO*                                    CH*           CH
||         ->    -2CO2              ->     ||     ->     |||
CH-COO*                                    CH*           CH
(free radical formation)

In this manner, alkyne is obtained at anode, while NaOH is formed at cathode and hydrogen gas is liberated.

Vicinal dihalides may be converted into alkynes by using extreme conditions such as sodium amide NaNH2 typically at 150°C or molten/fused potassium hydroxide KOH typically at 200°C.

From Calcium carbide

Calcium carbide is the compound CaC₂, which consists of calcium ions (Ca²⁺) and acetylide ions, C₂^2-. It is synthesized from lime and coke in the following reaction:

CaO + 3C → CaC₂ + CO

This reaction is very endothermic and requires a temperature of 2000^o C. For this reason it is produced in an electrical arc furnace.

Calcium carbide may formally be considered a derivative of acetylene, an extremely weak acid (though not as weak as ammonia). The double deprotonation means that the carbide ion has very high energy. Instant hydrolysis occurs when water is added to calcium carbide, yielding acetylene gas.

CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂

Properties

Physical properties

Most alkynes are less dense than water (they float on top of water), but there are a few exceptions.

Chemical properties

Liquid alkynes are non-polar solvents, immiscible with water. Alkynes are, however, more polar than alkanes or alkenes, as a result of the electron density near the triple bond.

Alkynes with a low ratio of hydrogen atoms to carbon atoms are highly combustible. Carbon-carbon triple bonds are highly reactive and easily broken or converted to double or single bonds. Triple bonds store large amounts of chemical energy and thus are highly exothermic when broken. The heat released can cause rapid expansion, so care must be taken when working with alkynes such as acetylene.

One synthetically important property of terminal alkynes is the acidity of their protons. Whereas the protons in alkanes have pK_a's around 60 and alkene protons have pK_a's in the mid-40's, terminal alkynes have pK_a's of about 25. Substitution of the alkyne can reduce the pK_a of the alkyne even further; for example, PhCCH has a pK_a around 23, and Me₃SiCCH has a pK_a around 19. The acidity of alkynes allows them easily to be deprotonated by sufficiently strong bases, such as butyllithium BuLi or the amide ion NH₂^-. More acidic alkynes such as PhCCH can even be deprotonated by alkoxide bases under the right conditions.

Reactions

Alkynes can be hydrated into either a ketone or an aldehyde form. A (Markovnikov) ketone can be created from an alkyne using a solution of aqueous sulfuric acid (H₂O/H₂SO₄) and HgSO₄, whereas the anti-Markovnikov aldehyde product requires different reagents and is a multi-step process.

Hydrohalogenation of Alkynes

Alkynes react very quickly and to completion with hydrogen halides. Addition is anti, and follows the Markovnikov Rule.

RCCH + H-Br (1 equiv) --> RCBr=CH2 RCCH + H-Br (2 equivs) --> RCBr2CH3

Adding a halide acid such as HCl or HBr to an alkyne can create a geminal dihalide via a Markovnikov process, but adding HBr in the presence of peroxides results in the Anti-Markovnikov alkenyl bromide product.

Halogenation of Alkynes

Adding diatomic halogen molecules such as Br₂ or Cl₂ results in 1,2-dihaloalkene, or, if the halogen is added in excess, a 1,1,2,2-tetrahaloalkane.

Adding H₂O along with the diatomic halide results in a halohydrin addition and an α-halo ketone.

Combustion

Alkynes burn in air with a sooty, yellow flame, like alkanes. Alkenes also burn yellow, while alkanes burn with blue flames. Acetylene burns with large amounts of heat, and is used in oxyacetylene torches for welding metals together, for example, in the superstructures of skyscrapers.

2 C2H2 + 5 O2 --> 4 CO2 + 2 H2O

Reduction

Alkynes can be hydrogenated by adding H₂ with a metallic catalyst, such as palladium-carbon or platinum or nickel, which results in both of the alkyne carbons becoming fully saturated. If Lindlar's catalyst is used instead, the alkyne hydrogenates to a Z enantiomer alkene, and if an alkali metal in an ammonia solution is used for hydrogenating the alkyne, an E enantiomer alkene is the result.

Complete Hydrogenation of Alkynes

As mentioned above, alkynes are reduced to alkanes in the presence of an active metal catalyst, such as Pt, Pd, Rh, or Ni in the presence of heat and pressure.

RCCR' + 2 H2 (Pt cat.)--> RCH2CH2R'

Syn-Hydrogenation of an Alkyne

There are two kinds of addition type reactions where a π-bond is broken and atoms are added to the molecule. If the atoms are added on the same side of the molecule then the addition is said to be a "syn" addition. If the added atoms are added on opposite sides of the molecule then the addition is said to be an "anti" addition. Hydrogen atoms can be added to an alkyne on a one mole to one mole ratio to get an alkene where the hydrogen atoms have been added on the same side of the molecule. Isotopic identification allows chemists to determine when this syn-hydrogenation has occurred.

As mentioned above, alkynes can be reduced to cis-alkenes by hydrogen in the presence of Lindlar Pd, i.e. palladium doped with CaSO₄ or BaSO₄.

RCCR + H2 cis-RCH=CHR

Anti-Hydrogenation of an Alkyne

In regards to the syn-hydrogenation, anti is hydrogenation when one hydrogen is added from the top of the pi bond and the other is added from the bottom. Anti-hydrogenation of an alkyne can be done via metallic reduction, using sodium in liquid ammonia.

Anion formation

Because of the acidity of the protons of terminal alkynes, they are easily converted into alkynyl anions in high yield by strong bases.

Examples

RCCH + NaNH2 -> RCCNa + NH3 C4H9Li + RCCH -> C4H10 +RCCLi

Alkynes are stronger bases than water, and acetylene (ethyne) is produced in a science classroom reaction of calcium carbide with water.

CaC2 + 2 H2O --> Ca(OH)2 + C2H2

Alkynyl anions are useful in lengthening carbon chains. They react by nucleophilic substitution with alkyl halides.

R-Cl + R'CCNa --> RCCR' + NaCl

The product of this reaction can be reduced to an alkane with hydrogen and a platinum or rhodium catalyst, or an alkene with Lindlar palladium.

References

• IIT Chemistry by Dr.O.P.Agrawal and Avinash Agrawal

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