Acetylenes, Chemistry tutorial

Introduction:

Alkynes are the compounds which include carbon-carbon triple bonds. The common formula for an acyclic, mono-alkyne is CnH2n-2. There are two major categorization of alkynes: terminal (RC≡CH) and internal (RC≡CR). The alkyne group is extremely reactive and few molecules having a triple bond are found in nature. Acetylene (HC≡CH) is the most broadly used alkyne and is perhaps best acknowledged for its utilization in oxyacetylene torches.

Nature of -C≡C-H Bond:

The simplest member of the alkyne family is acetylene, C2H2. This is a linear molecule, all four atoms lying all along a single line. Both the carbon-hydrogen and carbon-carbon bonds are cylindrically symmetrical about a line joining the nuclei, and are thus σ-bonds. The carbon-carbon 'triple bond' is made up of one strong σ-bond and two weaker π-bonds.

The triple bond (-C≡C-) is drawn by three identical lines recommending all three bonds are similar. This is not the case though; as one bond is an σ bond and the other two bonds are π bonds. Each and every carbon atom in a triple bond is sp hybridized according to the hybrid atomic orbital theory. The alkyne carbon atom consists of two sp orbitals which are 180o apart that minimizes the repulsion between electrons in these two orbitals. Each and every carbon atom as well consists of two unhybridized p orbitals. The p orbitals on one carbon atom (px and pz) are in a common plane and perpendicular to one other. Two sp orbitals, one from each carbon atom, overlap to make a σ bond. The two p orbitals on one carbon atom overlap by the two p orbitals on the adjacent carbon atom to make two π bonds. The overlapping p orbitals should be in the similar plane for maximum overlap and bond strength.

Acidity of Alkynes:

Very strong bases (like sodium amide) deprotonate terminal acetylenes to make carbanions termed as acetylide ions (or alkynide ions). Hydroxide ion and alkoxide ions are not strong adequate bases to deprotonate alkynes. Internal alkynes don't encompass acetylenic protons, in such a way that they don't react.

CH3CH2 - C ≡ C - H   +   NaNH2 → CH3CH2 - C ≡ C. Na+ + NH3

1-butyne, a terminal        sodium     sodium butynide

alkyne                              amide

The hydrogens in terminal alkynes are comparatively acidic; acetylene itself consists of a pKa of around 25. It is a far weaker acid as compare to water (pKa 15.7) or the alcohols pKa (16 to 19), however is much more acidic as compare to ammonia (pKa 35). Amide ion in liquid ammonia transforms acetylene and other terminal alkynes to the corresponding carbons. Terminal alkynes act as acids in the presence of the strong base. The amide anion, -NH2, the conjugate base of ammonia (pKa = 35), is a strong enough base to entirely eliminate a proton from a terminal alkyne. The given equation illustrates this reaction and the corresponding pKa values. This is an acid-base reaction where RC≡CH is the acid on the left side of the equilibrium expression and ammonia, NH3, is the conjugate acid on the right side of the expression. In acid-base reactions, the rule is a survival of the weakest. As ammonia is a weaker acid (pKa = 35) as compare to the alkyne (pKa = 25), the reaction is shifted strongly to the right. The larger the pKa value, the weaker the acid. The conjugate base of this reaction is acetylide anion, RC≡C-.

RC ≡ CH + NH2- ↔ RC ≡ C- + NH3

pKa = 25               acetylide    pKa= 35

This reaction doesn't take place with alkenes or alkanes. Ethylene consists of a pKa of around 44 and methane consists of a pKa of around 50.

Electrons in the s-orbitals are held, on the average, closer to the nucleus than they are in p-orbitals. This increased electrostatic attraction signifies that s-electrons encompass lower energy and greater stability than p-electrons. In common, the greater the amount of s-orbital in a hybrid orbital having a pair of electrons, the less basic is that pair of electrons. Lower basicity corresponds to the higher acidity of the conjugate acid.

Alkynes are quantitatively deprotonated via alkyllithium compounds.

CH3(CH2)3C ≡ CH + n-C4H9Li → CH3(CH2)3 ≡ CLi +n-C4H10

The prior transformation is simply an acid-base reaction, by 1-hexyne being the acid and n-buthyllithium being the base. Terminal alkynes provide insoluble salts by a number of heavy metal cations like Ag+ and Cu+. The formation of salts serves up as a helpful chemical diagnosis for the RC≡CH function, however most of these salts are explosively sensitive whenever dry and must always be kept moist.

Preparation of Alkynes:

Acetylene or ethyne is the most significant member of this series, and it might be prepared via any of the given methods.

1) By the action of water on calcium carbide:

CaC2 + 2H2O → HC≡ CH + Ca(OH)2

This process of preparation is employed industrially.

2)  By the action of ethanolic potassium hydroxide on ethylene dibromide:

In principle, a triple bond can be introduced to a molecule via removal of two molecules of HX from either a germinal (twin), or a vicinal (near) dihalide.

BrCH2CH2Br + KOH + (ethanol) → H2C = CHBr + KBr + H2O

H2C=CHBr + KOH + (ethanol) → HC ≡ CH + KBr + H2O

Synthesis of Alkynes:

The two different ways are generally employed for the synthesis of alkynes. In the first, a suitable electrophile experiences nucleophilic attack via an acetylide ion. The electrophile might be the unhindered primary alkyl halide (undergoes SN2), or it might be a carbonyl compound (that is, undergoes addition to provide an alcohol). Either reaction combines two fragments and provides a product by a lengthened carbon skeleton. This approach is employed in most of laboratory syntheses of alkynes. The second approach makes the triple bond via a double dehydrohalogenation of a dihalide. This reaction doesn't broaden the carbon skeleton. Isomerization of the triple bond might take place; therefore dehydrohalogenation is helpful only if the desired product consists of the triple bond in a thermodynamically favored position.

Alkylation Reaction (Synthetic application):

Organic chemistry comprises the study of synthesizing molecules which are not readily available from the natural sources. The acetylide anion is a helpful synthetic reagent. Such anions are strong bases and good nucleophiles. They encompass a nonbonding electron pair they are willing to share by an electrophile. Acetylide anions react readily by methyl compounds (CH3X) and primary alkyl compounds (RCH2X); here 'X' is a good leaving group similar to a halide or tosylate anion. An illustration of this reaction is illustrated in the equation below. The C-X bond is polar whenever the X atom/group is more electronegative as compare to C. Bond polarity makes C partially positive and electrophilic. The reaction illustrated below is known as a nucleophilic substitution reaction. The X group is replaced (or substituted) by the acetylide group.

1289_Nucleophilic substitution reaction.jpg

Fig: Nucleophilic substitution reaction

Typical reaction conditions for the formation of alkynes comprise the use of molten KOH, solid KOH moistened by alcohol, or concentrated alcoholic KOH solutions at temperatures of 100 to 200oC. In practice, such conditions are so severe that the process is merely helpful for the preparation of certain type of alkynes. Under such highly basic conditions the triple bond can migrate all along a chain.

342_Formation of alkynes by use of molten KOH.jpg

Fig: Formation of alkynes by use of molten KOH

Disubstituted alkynes are thermodynamically more stable as compare to terminal alkynes (as the preference for s-character in C-C bonds). As a result such conditions might be employed only where such rearrangement is not possible.

Sodium amide is the efficient strong base which is employed in producing an acetylide anion for the synthesis of acetylenes. A nucleophilic substitution reaction is a suitable manner of making larger molecules having an alkyne function. The equation below exhibits a reaction scheme starting by acetylene.

2412_Nucleophilic substitution by alkyne function.jpg

Fig: Nucleophilic substitution by alkyne function

In the very first step, a proton is eliminated from acetylene via the amide anion (-NH2). The resultant acetylide anion reacts by a primary alkyl halide to provide a terminal alkyne. In the subsequent step, amide anion is again employed to eliminate the remaining acetylenic proton. The resultant acetylide anion reacts by the other alkyl halide molecule to provide a disubstituted internal alkyne. Only methyl and primary alkyl halide reagents can be employed in such reactions. Secondary and tertiary alkyl halide compounds tend to provide elimination reactions (that is, alkene formation), not substitution reactions.

Addition of Acetylide Ions to Carbonyl groups:

Similar to other carbanions, acetylide ions are strong nucleophiles and strong bases. Moreover to displacing halide ions in SN2 reactions, they can add to carbonyl (C = O) groups. As oxygen is more electronegative as compare to carbon, the C = O double bond is polarized. The oxygen atom consists of a partial negative charge balanced via an equivalent amount of positive charge on the carbon atom.

The acetylide ion can serve up as the nucleophile in the nucleophilic addition to a carbonyl group. The acetylide ion adds to the carbonyl group to make an alkoxide ion; addition of dilute acid (in a separate step) protonates the alkoxide to provide the alcohol.

2453_Addition of Acetylide Ions to Carbonyl Groups.jpg

Fig: Addition of Acetylide Ions to Carbonyl Groups

Synthesis of Alkynes by Elimination Reactions:

In several cases, we can produce a carbon-carbon triple bond via removing two molecules of HX from a dihalide. Dehydrohalogenation of a geminal or vicinal dihalide provides a vinyl halide. Under strongly basic conditions, a second dehydrohalogenation might take place to form an alkyne.

=> Conditions for Elimination: We are familiar with the various illustrations of dehydrohalogenation of alkyl halides. The second step is new, though, because it comprises dehydrohalogenation of a vinyl halide to provide an alkyne. This second dehydrohalogenation takes place only under extremely fundamental conditions for illustration, fused (molten) KOH or alcoholic KOH in a sealed tube, generally heated to temperatures close to 200°C. Sodium amide is as well employed for the double dehydrohalogenation. As the amide ion (-:NH2) is a much stronger base as compare to hydroxide, the amide reaction occurs at a lower temperature. The following reactions are carefully selected to form products which don't rearrange (see below).

639_Synthesis of Alkynes by Elimination Reactions.jpg

Fig: Synthesis of Alkynes by Elimination Reactions

Coupling Reactions:

Carbon-carbon and carbon-heteroatom bonds are mainly found in numerous compounds which show significant biological, pharmaceutical and materials properties. Because of the significance of these bonds, there has been a requirement to build up mild and general techniques for their synthesis. Classically, the synthesis of such bonds comprised nucleophilic aromatic substitution reactions that required the utilization of electron-deficient aryl halides or N2 as a leaving group. The discovery of transition-metal mediated reactions for the synthesis of carbon-carbon and carbon-heteroatom bonds was a significant discovery for synthetic chemists. Cuprous acetylides experience oxidative coupling to aryl halides in the Castro-Stephens Coupling.

190_Castro-Stevens coupling reaction.jpg

Fig: Castro-Stevens coupling reaction

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