Alkynes, Chemistry tutorial

Introduction:

Ethyne, CH ≡ CH, the simplest alkyne, was burnt in the miners' lamps prior to electric lamps were enlarged. It is utilized in oxyacetylene torches for cutting and welding metals. It is extensively employed as a fuel gas. In industry, it is the starting material for the preparation of many vital chemicals, for example, ethanoic acid, chloroethene (vinyl chloride), propanone, butanol, ethanol, and so on A large number of naturally occurring compounds enclosing triple bonds have been isolated from plant kingdom. For instance, a triyne from safflower has considerable activity against nematodes and evidently forms part of the plant's chemical defiance against infestation. We will see in this chapter that the carbon-carbon triple bond reacts through many of the reagents that react by alkenes. We will as well learn the most unique aspect of the chemistry of alkynes, the acidity of terminal alkynes.

Types of Alkynes:

Alkynes, as well recognized as acetylenes, constitute the homogenous series of open chain unsaturated hydrocarbons that have one or more carbon-carbon triple bond. Alkynes might be of 2 kinds: terminal and internal. In the terminal alkynes, the triple bond lies at the end of the carbon chain and in the internal alkynes, the triple bond lies anywhere except at the terminal position.

CH3C ≡ CH                  CH3CH2C ≡CH2CH3                             CH3C ≡CC ≡CCH3

1-propyne                          3-hexyne                                             2, 4-hexdiyne

(terminal alkyne)           (internal alkyne)                                     (internal alkyne)  

Physical properties:

The physical properties of alkynes are alike to those of analogous alkenes. They are all colourless and odourless (except ethyne). The first 3 members, for example, ethyne, propyne and 1-butyne, are gases at room temperature; the next 8 members are liquids and the higher members are solids. The physical constants similar to melting points, boiling points and densities amplify gradually by the raise in molecular weight. The increase in boiling point through increase in molecular weight is due to increased London forces, as conversed in module 1 of this course. Alkynes contain slightly higher boiling points than the analogous alkenes and alkanes.  Terminal alkynes contain lower boiling points than the isomeric internal alkynes as well as can be divided via suspicious fractional distillation.

Alkynes share through alkanes and alkenes the properties of low density and low water solubility. They are nonpolar and dissolve readily in typical organic solvents, these as diethyl ether, chlorinated hydrocarbons, and so on.

Spectral properties:

The alkynyl chromophore absorbs below 200 nm (π-π* transition) in UV region that is frequently difficult to detect. Conjugation by a multiple bond, though, consequences in a bathochromic shift.

The IR absorption region of compounds through a triple bond based on whether they have alkyne hydrogen or not. Therefore, in terminal alkynes, RC≡CH, there is one absorption band in the region of 3300-3100 cm-1 due to the C≡CH stretching and another in the region of 2140-2100cm-1 due to -C≡C- stretches. In the internal alkynes RC≡CR, there is absorption in the region of 2260-2190 cm-1 analogous to the -C≡C- stretching.  

The internal alkynes, RC≡CR, have no alkynyl hydrogen, therefore, they have no nmr absorption characteristics of alkynyl hydrogen. The terminal alkynes, RC≡CH, provide an absorption signal between ∂ 2 and 3, trait of alkynyl proton. Therefore, value for alkynyl proton is less than the value of alkenyl protons. Let us suggest an explanation for this.

The chemical shift of a particular proton based on the magnetic field felt via it. As we know, the chemical shift as well as the magnetic field felt via a particular proton depends on:

  • The electronegativity of the carbon atom to which the proton is attached,
  • The way the proton is oriented to the neighboring π electron cloud, if any.

The sp hybridised carbon in alkynes is more electronegative than the sp2 hybridised carbon in alkenes. Therefore, we could expect higher ∂ value for alkynyl protons than for alkenyl protons. But the orientation of the alkynyl proton to the π electron cloud is unfavourable for higher ∂ values, as compared to the alkenyl protons.

The mass spectra of an alkyne provide distinct molecular ion peak. Though, the fragmentation is often complex and not easily interpreted.

Preparation:

Organic synthesis makes utilize of 2 major reaction types: one is functional group transformation and the other is carbon-carbon bond forming reaction. Both these strategies are applied to the preparation of alkynes. In this chapter, we shall discuss how alkynes are prepared via elimination reactions and by adding alkyl group(s) to the smaller ethyne unit. Several significant methods of preparation of alkynes are outlined in table.

Table: Preparation of Alkynes

301_Preparation of Alkynes.jpg

Dehydrohalogenation of Dihalides:

An alkyne can be prepared via the elimination of 2 molecules of HX from a dihalide. The dihalide might be of the geminal or vicinal kind.

Geminal dihalide: One in which both the halogen atoms are substituted on similar carbon atom.

Vicinal dihalide: One in which halogen atoms are substituted on adjacent carbon atoms.

1533_Dehydrohalogenation of Dihalides.jpg

Because an alkyne contains a triple bond as compared to the double bond in alkenes, two molecules of HX must be abolished. Consequently, stronger conditions are required to get rid of the second HX molecule. For instance, when 1, 2-dibromopropropane reacts by a strong base, a two-fold abolition occurs and as a consequence a propyne is produced.

         Br  Br

          |      |     KOH/C2H5OH                        NaNH2                                           

CH3CH CH2       581_aro.jpg    CH3CH=CHBr     581_aro.jpg     CH3C≡CH

1, 2-dibromo-1                            1-bromo -                    propane

Propyne                                      1-propene              

We had familiar that dihalides are prepared via the addition of halogen to an alkene. Therefore, the overall sequence of halogenation-dehydrohalogenation gives an excellent process for going from an alkene to an alkyne, for example.

856_1-propyne.jpg

We might remind that alkenes can be prepared through elimination reactions of alkyl halides that again, can be attained from alkanes. Therefore, we can say that alkane can serve as a starting material for the preparations of alkynes.

1674_alkene.jpg

Different bases can be utilized for dehydrohalogenation; sodium amide is desired, because it generally gives a higher yield.

The two-fold dehydrohalogenation follows similar mechanism as the dehydrohalogenation of alkyl halide to alkenes. 

Dehalogenation of Tetrahalides:

Alkyne can as well be prepared via dehalogenation of tetrahalides. For instance, propyne is formed whenever the vapours of 1, 1, 2, 2-tetrabromopropane are passed over heated zine.

167_propyne.jpg

This reaction doesn't contain any synthetic importance, because the tetrahalides themselves are usually prepared from alkynes. Though, it provides a process for the purification of alkynes.

Alkylation of Ethyne:

Reactions that lead to the attachment of an alkyl group to a molecular fragment are termed alkylation.

 In this part, we shall observe how alkynes are prepared via combining smaller chapters to build larger carbon chains. One of such structural chapter's ethyne itself. Through attaching alkyl group to ethyne, more complex alkynes can be prepared. For instance.

2004_1-iodobujtane.jpg

Alkylation is a two-step procedure. In the 1st step, ethyne reacts by sodium amide to provide a sodium ethynide ion, the conjugate base of ethyne. In the 2nd step, sodium ethynide ion attacks the C-I carbon atom of 1-iodobutane and pushes out the iodine ion, yielding the terminal alkyne, 1-hexyne, giving an overall 86% yield. Again, 1-hexyne can itself be converted into an alkynide anion, and can be alkylated a second time to yield an internal alkyne. A different alkyl halide can be used this time.

                                1 - NaNH2  

CH3 (CH2)3C≡ CH      581_aro.jpg          CH3 (CH2)3C≡CCH2CH4

  1-hexyne               2-CH3CH2Br                   3-octyne

This reaction provides good yields of alkyne only with primary alkyl bromoides and iodides.

Alkylation can as well be carried out via reacting ethyne and Grignard reagent, given via the action of an alkyl halide,

2131_alkyl halide.jpg

Acidity of Alkynes:

We have already learnt the Bronsted-Lowry theory of acids and bases in chapter 2 of this course. According to Bronsted and Lowry, an acid is a species that donates H+. In reality, any compound containing a hydrogen atom can act as an acid under suitable situations. Acid strength can be measured via measuring dissociation constants and expressing the consequences as values. Strong acids have lower pKa values than weak acids.

Hydrocarbons are usually not regarded as acids. Nevertheless, we can consider the removal of a proton from a hydrocarbon by a very strong base.

1955_Acidity of Alkynes.jpg

In this equation, the hydrocarbon acts as a Bronsted acid and the conjugate base is a carbon anion or carbanion. Approximate acidities of different types of aliphatic hydrocarbons have been measured and their pKa values are given in Table.

1152_Acidities of some hydrocarbons.jpg

Table: Acidities of some hydrocarbons

From the data followed in Table, we can see that there is a significant difference in the acidity of alkynes and other hydrocarbons. The order of acid strength of these hydrocarbons is:

 CH≡CH >CH2 = CH2 > CH3CH3

Conversely, the decreasing order of basic strength of the conjugate anions resulting from such hydrocarbons should be:

  CH3CH2 >      CH2=CH>     CH≡ C

  alkanide           alkenide       alkynide

  anion                 anion           anion

This decreasing order of acidities and basicities of hydrocarbons can be illustrated as follows:

Conjugated bases of alkanes, alkenes and alkynes have an electron pair in sp3, sp2 and sp orbitals, respectively, for instance,

605_alkanide anion.jpg

As we proceed from the alkanide anion to alkynide anion, the s character of the hybrid orbital amplifies and p character decreases. The alkanide anion (sp3 hybridized) has 25% s character and 75% p character; alkenide anion (sp2 hybridised) has 33.3% s character 64.6% p character; and alkynide anion (sp hybridized) has 50% s character and 50% p character. We have already learnt in our previous classes that electrons in s orbital are closer to the nucleus than those in p orbital. Because s character is maximum in alkynide anion and minimum in alkanide anion, the e- pair should be held most tightly in alkynide anion and most loosely in alkanide anion. Alkenide anion lies in between. In other terms, the electron pair in alkynide anion should be least available for protonation. As we know, the basic strength is more if the electron pair is easily available for protonation. Consequently, in the above series, alkynide anion is the weakest base and alkanide anion is the strongest base. Conversely, alkyne is the strongest acid and alkane the weakest acid.

This can be illustrated in another way as well. On the basis of the above discussion, we can say  that the sp hybridized carbon would attract the electrons pair constituting the C-H bond of  an alkyne more than the sp2 and sp3 hybridised carbons in alkene and alkane; respectively. This implies that C-H bond in an alkyne would be more ionic and has a tendency to donate a proton (H+), therefore, ethyne would be more acidic as compared to ethene or ethane. Let us now compare its acidity with ammonia and water. Addition of ethyne to sodamide in ether yields ammonia and sodium ethynide.

HC≡CH + NH   ↔       H   - NH2 + HC≡ C

stronger     stronger            weaker       weaker

acid           base                       acid            base

Note:

HA    ↔       H+ + A

Acid           conjugate base                      

The stronger the conjugate base, the weaker the acid and vice-versa.

The weaker acid, H - NH2, is displaced from its salt via the stronger acid, HC≡CH. In other terms, the stronger base, NH2 pulls the hydrogen ion way from ethyne to yield a weaker conjugate base. HC≡C-, because NH2 holds the hydrogen ion more tightly than HC≡C-, ammonia must necessarily be a weaker acid than ethyne.

Addition of water to sodium ethynide forms sodium hydroxide and regenerates eithyne.

                  -  +

H-OH+HC≡C: Na   →    HC≡C-H + NaOH

Stronger   stronger          weaker         weak

  acid              base            acid          base     

The weaker acid, ethyne, is displaced from its salt via the stronger acid, H2O. Thus ethyne is a stronger acid than ammonia, but a weaker acid than water, for example the 3 compounds have the given order of acid strength:

H2O>HC≡CH>NH3

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