# 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

\<\< Alkenes \|**Alkynes**\|
Dienes \>\>

![](Ethyne-2D-flat.png "Ethyne-2D-flat.png"){width="250"} **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        C~n~H~2n+2~
  Alkene        C~n~H~2n~
  Cycloalkane   C~n~H~2n~
  Alkyne        C~n~H~2n-2~
  Cycloalkene   C~n~H~2n-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~n~H~2n+2~. That would be C~6~H~14~.

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 C~6~H~14-6~ or C~6~H~8~.

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
(NaNH~2~) 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.

## Dehydrohalogenation of a dihaloalkane

## 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 + H`~`2`~`O -> 2NaOH + H`~`2`~\
\
`-2e`^`-`^` ->`\
\
`CH-COO*                                    CH*           CH`\
`||         ->    -2CO`~`2`~`              ->     ||     ->     |||`\
`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 CaC~2~, which consists of calcium ions
(Ca^2+^) and acetylide ions, C~2~^2-^. It is synthesized from lime and
coke in the following reaction:

CaO + 3C → CaC~2~ + 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.

CaC~2~ + 2H~2~O → Ca(OH)~2~ + C~2~H~2~

## From alkyl or aryl halides

# 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 p*K*~a~\'s
around 60 and alkene protons have p*K*~a~\'s in the mid-40\'s, terminal
alkynes have p*K*~a~\'s of about 25. Substitution of the alkyne can
reduce the p*K*~a~ of the alkyne even further; for example, PhCCH has a
p*K*~a~ around 23, and Me~3~SiCCH has a p*K*~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 NH~2~^-^. 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 (H~2~O/H~2~SO~4~) and HgSO~4~, whereas the
anti-Markovnikov aldehyde product requires different reagents and is a
multi-step process.

## Oxidative Cleavage of Alkynes

## Hydrohalogenation of Alkynes

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

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RCCH + H-Br (1 equiv) \--\> RCBr=CH~2~\
RCCH + H-Br (2 equivs) \--\> RCBr~2~CH~3~

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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 Br~2~ or Cl~2~ results in
1,2-dihaloalkene, or, if the halogen is added in excess, a
1,1,2,2-tetrahaloalkane.

Adding H~2~O 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.

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2 C~2~H~2~ + 5 O~2~ \--\> 4 CO~2~ + 2 H~2~O

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## Reduction

Alkynes can be hydrogenated by adding H~2~ 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.

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RCCR\' + 2 H~2~ (Pt cat.)\--\> RCH~2~CH~2~R\'

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### 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 CaSO~4~ or
BaSO~4~.

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RCCR + H~2~ *cis-*RCH=CHR

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### 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**

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RCCH + NaNH~2~ -\> RCCNa + NH~3~ C~4~H~9~Li + RCCH -\> C~4~H~10~ +RCCLi

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

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CaC~2~ + 2 H~2~O \--\> Ca(OH)~2~ + C~2~H~2~

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

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

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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

\<\< Alkenes \| Alkynes \|
Dienes \>\>

