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Alkynes

Building Bridges to Knowledge

Photo of a bridge way to the Delicate Arch near Moab, Utah

The simplest molecule with a triple bond is acetylene (ethyne), C2H2

Following the octet rule, the structure for C2H2 would be:

A sigma, σ, and two pi, π, bonds make up the triple bond.

To understand the nature of the triple bond, let’s revisit the electron configuration of each of the two carbons that comprise the acetylene molecule. The electron configuration of carbon is 1s2 2s2 2p2.

In forming the triple bond, an electron in the 2s orbital of carbon is promoted to the 2p orbital of carbon. Diagram 5.1 is an illustration of this process.

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Diagram 5.1: promotion of an electron from the 2s atomic orbital of carbon to the 2p atomic orbital of carbon

A 2p atomic orbital of carbon containing one electron linearly combines with the 2s atomic orbital to form two degenerate 2sp hybridized atomic orbitals.

Diagram 5.2 illustrates this process.

Diagram 5:.2 Hybridization of a 2s atomic orbital of carbon with a 2p atomic orbital of carbon

A 2sp hybridized atomic orbital of carbon linearly combines with a 1s atomic orbital of hydrogen to form a C-H sigma bond. Two of these

1s

2s

2px 2pz2py 2py 2pz2px

2s

1s

2p

2sp2sp

1s

2p

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C-H bonds are formed in the acetylene molecule. Each C-H single bond is described by Diagram 5.3.

Diagram 5.3: linear combination of a 1s atomic orbital of hydrogen with a 2sp hybridized atomic orbital of carbon

The second 2sp hybridized atomic orbital linearly combines with the 2sp hybridized atomic orbital of the second carbon to form a (2sp + 2sp) σ bond: The remaining two 2p orbitals of carbon linearly combine with the remaining two 2p orbitals of a second carbon atom to form two π bonds. Diagram 5.4 is an illustration of this process.

σ (*2sp+1s)

σ (2sp+1s)

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Diagram 5.4: sigma and pi bonds in the carbon-carbon triple bond

The overall pictorial representation of the acetylene molecule is illustrated in Diagram 5.5.

π (2p - 2p)* π (2p - 2p)

*

π (2p+2p) π (2p+2p)

σ (2sp - 2sp)*

σ (2sp+2sp)

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Diagram 5.5 the triple bond

Two pi bonds are formed from the vertical overlap of four p orbitals and the sigma bond is the horizontal overlap of two 2sp hybridized atomic orbitals of one carbon atom with another carbon atom. Together, the two pi bonds and the sigma bond constitute the C-C triple bond. Each C-H single bond can be described as the horizontal overlap of a 1s atomic orbital of hydrogen with a 2sp atomic orbital of carbon. The H-C-C bond angle is 180o.

The hybridization around the carbon atom of any triple bond of an alkyne can be described in an analogous fashion, i.e., the hybridization around any carbon atom of a triple bond before it participates in the formation of a molecular orbital would be 2sp, and each of the two pi bonds are formed by the linear combination of a 2p atomic orbital of one carbon atom with a 2p atomic orbital of another carbon atom.

Physical Properties and Structure

The general formula for alkynes is CnH(2n-2). As indicated earlier, alkynes have triple bonds:

C

C

H

H

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where two π bonds perpendicular to one another are formed by the vertical combination of two 2p atomic orbitals to form one π bonding molecular orbital and the vertical combination of another two 2p atomic orbitals to form a second π bonding molecular orbital and a σ bonding molecular orbital formed by the horizontal combination of two 2sp atomic orbitals.

In review, acetylene, C2H22, compound I, two electrons (one from the

2sp atomic orbital of carbon and one from the 1s atomic orbital of hydrogen) reside in a σ (2sp + 1s) bonding molecular orbital. These two electrons are closer to the carbon atom than the electrons

in a C=C-H σ (2sp2 + 1s) bond in ethylene or a C-H σ (2sp3 + 1s)

bond in ethane; consequently, hydrogen atoms attached to terminal triple bonds are more acidic, Kaa

≈ 10-26 , than hydrogen atoms attached to the carbon atoms of single or double bonds. The weak acidity, pK

a ≈ 26, of terminal alkynes are designed so that they will

react with strong bases such as NaNH2.

Figure 5.1 describes the bonding in acetylene.

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Figure 5.1: the π bonds and σ bonds in acetylene (ethyne)

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The H-C-C bond angle is 180oo, and spinning the π bonds about the

H-C-C-H molecular orbitals axis generates a cylindrical sheath that is illustrated in Figure 5.2.

Figure 5.2: Cylindrical sheath resulting from spinning the π bonds about the H-C-C-H axis

The formation of the cylindrical sheath as illustrated in Figure 5.2 results from two 2p atomic orbitals vertically combining to form one π bonding molecular and two 2p atomic orbitals perpendicular to the previous two 2p atomic orbitals vertically combining to form a second π bonding molecular orbital. The movement of the four electrons in these two π bonding molecular orbitals generates the cylindrical sheath.

The boiling and melting points of alkynes are a little higher than those of comparable molecular mass alkenes and alkanes. The higher boiling points and melting points are undoubtedly due to the rod-like structures of the alkynes. The rod-like structure, due to the triple bonds, allows alkynes to be more closely packed permitting greater Van der Waals attraction forces between the alkyne molecules.

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The relatively low polarity of alkynes makes them more soluble in solvents of low polarity, e.g., ether and carbon tetrachloride easily dissolves alkynes.

Nomenclature

The IUPAC system for naming alkynes is analogous to the system for naming alkenes, i.e., the longest continuous carbon-carbon chain containing the triple bond is selected. The triple bond is given the lowest possible number, and the compound is identified as a –yne. If the compound is an alkenyne, i.e., it contains a double bond and a triple bond, then the double bond receives the lower number when identifying the longest continuous carbon-carbon chain containing both the double bond and the triple bond.

Table 5.1 lists some examples of alkynes:

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Compound Structure Name C6H10

2-hexyne

C7H12

5-methyl-2-hexyne

C7H10

2-hepten-5-yne

C9H14

3-nonen-6-yne

C10H10

1-pheny-1-butyne

C8H11Br 1-bromo-2-methyl-2-hepten-4-yne

Table 5.1: IUPAC Nomenclature of Some Alkynes

Alkynes can also be named as acetylene derivatives (a common nomenclature). For example, CH3C≡C-H is called methylacetylene (IUPAC propyne); CH3CH2C≡CCH

3 would be ethylacetylene (IUPAC

2-pentyne); and C6H5C≡CCH2CH3 would be ethylphenylacetylene (IUPAC 1-phenyl-1-butyne).

Preparation of Alkynes

The simplest alkyne, acetylene or ethyne, can be prepared from the hydrolysis of calcium acetylide (calcium carbide).

CaC2 (s)2 + 2 H

22O (l) → Ca(OH)2 (aq) + H-C≡C-H (g)

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Calcium acetylide (calcium carbide) can be obtained from calcium carbonate, limestone, by the following sequence of reactions.

CaCO3 (s) → CaO (s) + CO2 (g)

CaO (s) + 3 C (s) 2273 K⎯ →⎯⎯ CaC2 (s) + CO (g)

Acetylene can also be made from the incomplete oxidation of methane.

6CH4 (g) + O2 (g) → 2H-C≡C-H (g) + 2CO (g) + 10 H2 (g)

Alkynes can be synthesized from terminal alkynes. Since the hydrogen atoms of acetylene are slightly acidic, a strong base such as sodium amide, NaNH2, will react with acetylene to yield sodium acetylide, H-C≡C—Na+. The sodium acetylide can act as a nucleophilic reagent in an S

N2 (substitution nucleophilic bimolecular) reaction to attack a primary alkyl halide to produce a substituted ethyne. The remaining hydrogen can be replaced in an analogous manner. If only one alkyl group replaces one hydrogen atom of acetylene (ethyne), then the product would be a terminal acetylene. Substitution nucleophilic reactions will be discussed in Chapter 6. The resulting terminal alkyne can in turn react with NaNH

2 to produce

another acetylide which can undergo an SN2 reaction to yield a larger molecular mass alkyne. These reactions are illustrated in the following synthesis of 3-octyne from ethyne (acetylene).

(1)

H-C≡C-H + NaNH22 → H-C≡C

- Na+ + NH3

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(2)

H-C≡C - Na+ +

3 CH3CH2CH2l → CH3CH2CH22

C≡C-H

(3)

CH3CH2CH22C≡C -H + NaNH22

→ CH3CH2CH22C≡C -

- Na + + NH3

(4)

CH3(CH2)22C≡C -

- Na + + CH3CH2Br → CH3(CH2)22

C≡CCH2CH3 + NaBr

3-octyne

Alkynes can be synthesized from lithium acetylide-ethylenediamine. Lithium acetylide-ethylenediamine (EDA), H-C≡CLi∙NH2CH2CH2NH2

conveniently reacts with primary alkyl chlorides, alkyl bromides and epoxides through an SN2 mechanism to form interesting alkynes. For example, 3-butyn-1-ol can be prepared by reacting lithium acetylide∙EDA with ethylene oxide.

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Nomenclature of alcohols will be discussed in a future paper.

Alkynes can be prepared through dehydrohalogenation of vicinal (vic) dihalides with alcoholic potassium hydroxide followed by a stronger base such as sodium amide. This is illustrated in the following reaction.

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A strong base is required to convert the vinyl halide to an alkyne since a higher energy of activation is required for dehydrohalogenation of a vinyl halide.

Another method for preparing alkynes is the dehalogenation of tetrahalides.

Following is a plausible mechanism for the preparation of alkynes from tetrahaloalkanes.

(1)

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(2)

(3)

(4)

(5)

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(6)

The sum of steps 1-6 gives the stoichiometric amounts of reactants and products.

Reactions of Alkynes

Alkynes, like alkenes, possess centers of unsaturation which are sensitive to addition of halogens; however, alkynes yield tetrahalo adducts rather than dihaloalkanes.

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The addition of halogens to the triple bond is slower than the addition of halogens to alkenes. For example, 1-penten-4-yne, compound II, reacts with one mole of Br2 to yield 4,5-dibromo-1-pentyne, compound III:

This is due to the fact that electrophilic addition to an alkene favors the formation of a stable carbocation, whereas the initial addition of an electrophile to an alkyne forms an unstable vinyl carbocation. Figure 5.3 is a potential energy diagram that illustrates the differences between the Eact for the formation of the vinyl carbocation compared to the Eact for the formation of the alkyl carbocation.

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Figure 5.3: the energy of activation that leads to the formation of the vinyl carbocation is greater than the energy of activation that leads to the formation of the alkyl carbocation

Alkynes react with HX, where X = Cl, Br and I. The addition follows Markovnikov’s rule to form gem dihalides.

(a)

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(b)

Alkynes can undergo an analogous reaction which alkenes experience, i.e., the addition of HBr in the presence of peroxide. Like alkenes, this reactions proceeds through an anti-Markovnikov addition mechanism.

Hydrogen halides, HX, add to alkynes and the resulting vinyl halide can undergo coupling with organocopper lithium reagents to form alkenes.

Following is an illustration of vinyl halides undergoing a coupling reaction with lithium dialkylcuprate reagents to produce an alkene.

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Acidic hydrolysis of non-terminal alkynes in the presence of mercury (II) sulfate results in the formation of ketones.

Acidic hydrolysis of terminal alkynes in the presence of mercury (II) sulfate results in the formation of methyl ketones.

The mechanism for this reaction is not perfectly clear, but the reaction proceeds through the formation of an enol, and the enol undergoes tautomerism to form a ketone. Ketones will form when the triple bond of akynes is sandwiched between two alky groups. If the alkyne has a terminal triple bond, then a methyl ketone will be the major product from the reaction of water with the triple bond in the presence of mercury (II) sulfate and sulfuric acid. The mechanism of the reaction proceeds with mercury forming a

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complex with the triple bond.

Step 1

Step 2

Step 3

Step 4 involves the formation of an enol.

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Step 5 The enol undergoes tautomerism to the keto compound. Keto-enol tautomerism will be discussed in a future paper.

If the alkyne has a terminal triple bond, then the final carbonyl compound is a methyl ketone. Following is the mechanism of the reaction.

(1)

(2)

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(3)

(4)

(5)

Diborane, B2H6 , adds to alkynes to produce vinylboranes which undergo oxidation to ketones or aldehydes.

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If R’ is a hydrogen atom, then the compound produced is an aldehyde. If R’ is an alkyl group, then the compound produced is a ketone.

The difficulty with this reaction is controlling the formation of the vinyl borane. It is easier to add B2H6to substituted acetylenes, because substituted acetylenes slowly add another B2H6 molecule; therefore, providing more control over the formation of the vinyl borane. On the other hand, terminal acetylenes tend to add another B2H6 molecule to form an organoborane complex. Control at the vinylborane stage can be accomplished by using a hindered borane such as bicyclo[3.3.1]-9-borononane (H-BBN), compound IV. H-BBN can be synthesized from 1,5-cyclohexadiene, compound V, in the following manner.

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Compound IV, H-BBN, can react with terminal alkynes to produce aldehydes.

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Dialkylacetylenes will not react with H-BBN because of steric inhibition anticipated in the resulting alkenyl-BBN product.

Catalytic hydrogenation of alkynes leads to cis alkenes or alkanes depending upon the stoichiometric quantities of hydrogen molecules added. The isolation of the alkene is difficult, but the reaction can be controlled by the use of Lindlar’s catalyst (a heterogeneous catalyst

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consisting of Pd deposited on CaCO3 treated with PbO) http://everything2.com/title/Lindlar%2520catalyst

If a trans alkene is desired from an alkyne, then the reduction must be chemical and not catalytic. Chemical reductions of alkynes to form alkenes can be accomplished using Na in liquid NH3 or Li in liquid NH3.

Alkynes will undergo ozonolysis to form carboxylic acids, and terminal alkynes will undergo ozonolysis to produce CO2.

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(1)

(2)

(1)

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(2)

The mechanism of the reaction involves two steps. Step 1 is the formation of the ozonide, and a second step for cleavage involving zinc and water. The mechanism is analogous to the mechanism of ozonolysis for alkenes where molozonides are initially formed followed by the formation of ozonides. Following is a proposed mechanism for the ozonolysis of of alkynes.

(1) Formation of a molozonide

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(1) Formation of a double molozonide

(1) Rearrangement of the molozonide

(4)

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(5)

(6)

(7)

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(8)

(9)

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Problems

Alkynes

1. Draw the structures and give the IUPAC names for all possible

hexenynes.

2. What is the anticipated product(s) obtained from the treatment of

propyne with the following reagents?

a. HgSO4 , H2O and H2SO4

b. HBr

c. NaNH2

d. H2 , Pd, CaCO3 and PbO

e. H-BBN followed by treatment with H2O2 and NaOH

f. Br2 in CCl4

g. BrCCl3 in t-butyl peroxide

h. The product of (c) with n-butylbromide

i. LiNH2 in liquid NH3

j. The product of (c) with ethylene oxide

3. Beginning with CaC2 and any other necessary inorganic and

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organic reagents, suggest syntheses for the following compounds:

a. 1-butyne

b. trans-butene

c. (E)-1-bromopropene

d. 1-deuteropropyne

e. 3-hexyn-1-ol

f. 1-penten-4-yne

g. Butanal (an aldehyde)

h. 1-deuteropropane

i. 2-butyne

j. 1-deuteropropanal (an aldehyde)

k. 1,1,1-trichloro-2-butyne

4. Suggest mechanisms for the following reactions:

a. NaNH2 + CH3C≡CCH2OH → NaOH + CH3C(NH2)=C=CH2

b. CH3C≡CCH2CH3 + HgSO4 + H2O + H2SO4 →

2-pentanone + 3-pentanone

c.

13CH3C≡C-H + NaNH2 in liquid NH3 → H- 13C≡C-CH3

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5. Suggest structures for compounds A, B and C in the following

transformation:

Compound A, C6H100, when treated with ozone followed by

treatment with H2O/Zn yielded compound B, C3H6O2, as the

only product. Compound A reacts with HgSO4, H2SO4 and

H2O to produce compound C, C6H12O.

6. Suggest chemical methods to distinguish between the following

pairs of molecules.

a.

a.

C C

CH3

CH3

H

H

and CH3CH3 C C

C C

CH3

H

H

and CH3CH2 C C

H

C H

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i.

7. When (CH3)3C-C≡C-H is treated with HCl,

(CH3)2C(Cl)C(CH3)C=CH2 is formed as a minor product. Suggest a

mechanism that would account for this transformation.

8. Identify the molecules associated with the following

transformations:

CH3 CH3C CBr2

CCl4A

H2

PdB

alcoholic KOH

CBr2CCl4

313KD

(1) O3

(2) H2OZn

E