Name: why terminal alkynes form acetylide ions
Uploaded: 2023-04-30T18:28:38.000Z
Duration: 2 min 42 s
Channel: JoVE

In comparison to alkanes and alkenes, 1-alkynes or terminal alkynes have lower pKa values.
Recall that the lower the pKa, the stronger the acid. Thus, terminal alkynes are more acidic than other hydrocarbons. 
Among ethane, ethylene, and acetylene, acetylene is 1026 more acidic than ethane, and 1019 stronger than ethylene.
What makes terminal alkynes more acidic than alkanes or alkenes? The answer lies in the relative stability of the carbanions formed by deprotonation of the corresponding hydrocarbon.
This stability depends on the nature of the hybridized orbital on the negatively charged carbon. In ethane, the lone pair resides in an sp3 orbital, while in ethylene it occupies the sp2 orbital, and an sp orbital in acetylene.
An sp3 orbital has 25% &#34;s&#34; character, compared to 33% in an sp2 orbital and 50% in an sp orbital.
Since s orbitals are closer to the positively charged nucleus than p orbitals, electrons in a hybrid orbital with a higher &#34;s&#34; character will experience stronger electrostatic attraction.
Therefore, the acetylide anion will be the most stable and readily formed in the presence of a suitable base.
How does one choose a suitable base?
For a base to deprotonate an acid, the pKa of the base&#8217;s conjugate acid must be at least 10 units greater than the acid&#39;s pKa.
Terminal alkynes have a pKa of 25. Here, the base chosen is such that its conjugate acid has a pKa of at least 35.
With sodium amide as the base, terminal alkynes form sodium acetylide and ammonia as the conjugate acid. Since the pKa of ammonia is greater than 35, the equilibrium favors the formation of sodium acetylide, making sodium amide a suitable base for the deprotonation reaction.
In contrast, sodium hydroxide forms water as its conjugate acid. Since the pKa of water is less than 35, the equilibrium favors the reactants. Therefore, terminal alkynes cannot be deprotonated using sodium hydroxide.

acidity of 1-alkynes

Acidity of 1-Alkynes

<h4></h4>
The acidic strength of hydrocarbons follows the order: Alkynes &#62; Alkenes &#62; Alkanes. The strength of an acid is commonly expressed in units of pKa &#8212; the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.
<table style="margin-left: auto; margin-right: auto;">
	<tbody style="border: 1px solid black;">
		<tr>
			<td style="border-style: solid; border-color: black; color: black; text-align: center;"><img alt="Ethane molecule diagram, C2H6 structure with covalent bonds between carbon and hydrogen atoms." src="/files/ftp_upload/11833/11833_PT_Figure_1_left.svg" style="text-align: center; height: 75px;" data-altupdatedat="1761830439000" data-alt="Figure1left" /> 
			Ethane 
			(pKa = 51)</td>
			<td style="border-style: solid; border-color: black; color: black; text-align: center;"><img alt="Ethene molecular structure; chemical compound diagram; double bond; carbon, hydrogen atoms." src="/files/ftp_upload/11833/11833_PT_Figure_1_Middle.svg" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Figure1middle" /> 
			 
			Ethene 
			(pKa = 44)</td>
			<td style="border-style: solid; border-color: black; color: black; text-align: center;">
			
			<img alt="Ethylene chemical structure H-C&#8801;C-H, structural molecular formula, organic chemistry diagram." src="/files/ftp_upload/11833/11833_PT_Figure_1_Right.svg" style="text-align: center; height: 20px;" data-altupdatedat="1761830439000" data-alt="Figure1right" /> 
			 
			 
			Ethyne 
			(pKa = 25)
			</td>
		</tr>
	</tbody>
</table>
With a pKa difference of 26 units, acetylene is 1026 times more acidic than ethane. Similarly, a difference of 19 pKa units makes it 1019 times stronger than ethene.
<h4>Hybridization effect</h4>
The pronounced increase in the acidity of terminal alkynes relative to the other hydrocarbons can be explained by considering the stability of the corresponding carbanions formed by deprotonation. Note that, in the nomenclature of organic compounds, the suffix "-ide" indicates that the molecule is a negatively charged ion.
<table style="margin-left: auto; margin-right: auto;">
	<tbody style="border: 1px solid black;">
		<tr>
			<td style="border-style: solid; border-color: black; color: black; text-align: center;"><img alt="Alkene reaction mechanism, proton removal equation, chemical structure diagram, organic chemistry." src="/files/ftp_upload/11833/11833_PT_Figure_2_top.svg" style="text-align: center; height: 80px;" data-altupdatedat="1761830439000" data-alt="Figure2top" /> 
			ethanide (an alkyl anion) 
			sp3 lone pair, 25% &#8220;s&#8221; character</td>
		</tr>
		<tr>
			<td style="border-style: solid; border-color: black; color: black; text-align: center;"><img alt="Hydrogen atom removal from ethylene, electrophilic mechanism, chemical reaction diagram." src="/files/ftp_upload/11833/11833_PT_Figure_2_middle.svg" style="text-align: center; height: 60px;" data-altupdatedat="1761830439000" data-alt="Figure2middle" /> 
			ethenide (a vinylic anion) 
			sp2 lone pair, 33% &#8220;s&#8221; character</td>
		</tr>
		<tr>
			<td style="border-style: solid; border-color: black; color: black; text-align: center;"><img alt="Alkyne ionization process, chemical reaction formula diagram with acetylide ion formation." src="/files/ftp_upload/11833/11833_PT_Figure_2_bottom.svg" style="text-align: center; height: 40px;" data-altupdatedat="1761830439000" data-alt="Figure2bottom" /> 
			acetylide (an acetylenic anion) 
			sp lone pair, 50% &#8220;s&#8221; character</td>
		</tr>
	</tbody>
</table>
The stability of the carbanion depends on the nature of the hybridized orbital occupied by the lone pair of electrons. As shown above, in ethane the lone pair resides in an sp3 orbital, while in ethene it occupies the sp2 orbital and an sp orbital in the case of acetylene. The sp3, sp2, and sp orbitals have 25%, 33%, and 50% "s" character, respectively. Since "s" orbitals are closer to the positively charged nucleus, a hybrid orbital with a higher "s" character will effectively stabilize the negative charge. Thus, the acetylide ions will be the most stable and readily formed in the presence of a suitable base.
<h4>Choosing a suitable base</h4>
In general, for a base to deprotonate an acid, the pKa of the base's conjugate acid must be at least 10 pKa units greater than that of the acid.
<img alt="Acid-base equilibrium; HA + B&#8315; &#8652; A&#8315; + HB formula; chemical reaction diagram." src="/files/ftp_upload/11833/11833_Figure_3v2.svg" style="text-align: center; height: 80px;" data-altupdatedat="1761830439000" data-alt="Figure3" />
Terminal alkynes have a pKa of 25. Therefore, an appropriate base would be one where the conjugate acid has a pKa that is at least 35. Recall that for an acid-base reaction, the equilibrium favors the formation of weaker acids and bases from stronger acids and bases.
stronger acids + stronger bases <img alt="Static equilibrium diagram: &#931;Fx=0 for a ladder setup demonstrating balance and forces analysis." src="/files/ftp_upload/11833/arrow.png" style="text-align: center; height: 13px;" data-altupdatedat="1761830439000" data-alt="arrow" />weaker acids + weaker bases
With sodium amide as the base, terminal alkynes form sodium acetylide and ammonia as the conjugate acid. Since the pKa of ammonia is greater than 25, the equilibrium favors the formation of sodium acetylide, making sodium amide a strong enough base for the deprotonation reaction.
<img alt="Chemical reaction mechanism; alkyne formation via sodium amide elimination diagram." src="/files/ftp_upload/11833/11833_Figure_4.svg" style="text-align: center; height: 100px;" data-altupdatedat="1761830439000" data-alt="Figure4" />
Apart from sodium amide, sodium hydride, butyllithium, and lithium diisopropylamide (LDA) are other commonly used bases to form acetylide ions.
<table style="margin-left: auto; margin-right: auto;">
	<tbody style="border: 1px solid black;">
		<tr>
			<td style="border-style: solid; border-color: black; color: black; text-align: center;">
			<img alt="Sodium hydride (NaH) chemical structure, displaying ion formation." src="/files/ftp_upload/11833/11833_PT_Figure_5_left.svg" style="height: 25px;" data-altupdatedat="1761830439000" data-alt="Figure5left" />
			Sodium hydride
			</td>
			<td style="border-style: solid; border-color: black; color: black; text-align: center;"><img alt="Allylic anion structure with Li+ in chemical bonding, organic chemistry formula." src="/files/ftp_upload/11833/11833_PT_Figure_5_middle.svg" style="text-align: center; height: 60px;" data-altupdatedat="1761830439000" data-alt="Figure5middle" /> 
			Butyllithium (n-BuLi)</td>
			<td style="border-style: solid; border-color: black; color: black; text-align: center;"><img alt="Tetraalkylammonium ion structure; chemical formula; molecular diagram; organic chemistry concept." src="/files/ftp_upload/11833/11833_PT_Figure_5_right.svg" style="text-align: center; height: 60px;" data-altupdatedat="1761830439000" data-alt="Figure5right" /> 
			Lithium diisopropylamide (LDA)</td>
		</tr>
	</tbody>
</table>
In the presence of sodium hydroxide as the base, terminal alkynes form sodium acetylide and water as the conjugate acid. However, since the pKa of water is less than 25, the equilibrium favors the reactants. Therefore, sodium hydroxide is not a suitable base to form acetylide ions.
<img alt="Alkyne deprotonation process diagram, chemical reaction, alkoxide ion formation." src="/files/ftp_upload/11833/11833_Figure_6.svg" style="text-align: center; height: 110px;" data-altupdatedat="1761830439000" data-alt="Figure6" />
<h4>Synthesis of organometallic reagents </h4>
The relative acidity of terminal alkynes finds application in the synthesis of organometallic compounds when treated with Grignard or organolithium reagents. These are examples of transmetalation reactions involving the transfer of a metal atom from one carbon to another, thereby forming new metal&#8211;carbon bonds. However, they can also be interpreted as acid&#8211;base reactions that favor the formation of weaker acids and bases.
<img alt="Organic synthesis diagram; alkyne reaction with Grignard reagent, showing chemical equations." src="/files/ftp_upload/11833/11833_Figure_7v3.svg" style="text-align: center; height: 225px;" data-altupdatedat="1761830439000" data-alt="Figure7top" />
<div></div>


The acidic strength of hydrocarbons follows the order: Alkynes &#62; Alkenes &#62; Alkanes. The strength of an acid is commonly expressed in units of pKa ...

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Organic Chemistry

Unit 1

Alkynes

KEY TERMS AND CONCEPTS

why terminal alkynes form acetylide ions

Terminal alkynes are more acidic than alkenes and alkanes. Acid strength is usually measured with pKa, and a lower pKa means a stronger acid. Ethane has a pKa of 51, ethene has a pKa of 44, and acetylene has a pKa of 25. That means acetylene is far more acidic than the other two hydrocarbons.

This difference comes from the stability of the carbanion formed after deprotonation. A carbanion is a negatively charged carbon species. In ethane, ethene, and acetylene, the lone pair sits in an sp3, sp2, or sp orbital, respectively. These orbitals have 25%, 33%, and 50% s character. Because s orbitals hold electrons closer to the nucleus, the sp-hybridized acetylide ion is the most stable.

That stability helps explain which base can remove the terminal alkyne hydrogen. In general, the conjugate acid of the base should have a pKa at least 10 units higher than the acid being deprotonated. Since terminal alkynes have a pKa of 25, the base should have a conjugate acid with a pKa of at least 35. Sodium amide works because it forms sodium acetylide and ammonia, and ammonia is a weaker acid than the alkyne. Sodium hydride, butyllithium, and lithium diisopropylamide, or LDA, are also commonly used.

Sodium hydroxide is not strong enough to form acetylide ions. When NaOH is used, the products would be sodium acetylide and water, but water has a pKa below 25. The equilibrium then favors the reactants instead of product formation. This acid-base behavior is also useful in making organometallic reagents from terminal alkynes with Grignard or organolithium compounds. These reactions transfer a metal from one carbon to another, so they are transmetalation reactions as well as acid-base reactions.

Why Terminal Alkynes Form Acetylide Ions

structure and physical properties of alkynes

structure-and-physical-properties-of-alkynes

Alkyne Bonds, Shape, and Physical Traits

Alkynes are hydrocarbons with carbon-carbon triple bonds. They are unsaturated compounds and are written with the general formula CnH2n-2. This group includes simple molecules like ethyne, also called acetylene, which is a colorless gas used as a fuel for welding because it burns at high temperatures. Alkynes also appear in natural and synthetic compounds. Some natural alkynes are found in the poison from South American tree frogs. Synthetic alkyne-containing compounds are found in medicines...

nomenclature of alkynes

Naming Alkynes with IUPAC Rules

Alkynes are unsaturated hydrocarbons with carbon-carbon triple bonds. Their general formula is CnH2n-2, and their names use the suffix "-yne" instead of "-ane" or "-ene." Alkynes can be named with IUPAC names or common names. The IUPAC system uses a step-by-step method based on the molecule’s structure. First, find the longest carbon chain that contains the triple bond. Add the suffix "-yne" to the parent chain name. In the example shown, the longest chain has seven carbon atoms, so the parent...

Terminal alkynes are more acidic than alkenes and alkanes. Acid strength is usually measured with pKa, and a lower pKa means a stronger acid. Ethane has a pKa of 51, ethene has a pKa of 44, and acetylene has a pKa of 25. That means acetylene is far more acidic than the other two hydrocarbons. This difference comes from the stability of the carbanion formed after deprotonation. A carbanion is a negatively charged carbon species. In ethane, ethene, and acetylene, the lone pair sits in an sp3,...

preparation of alkynes: alkylation reaction

preparation-of-alkynes-alkylation-reaction

Alkyne Chain Extension by Alkylation

Terminal alkynes can be made into longer carbon-chain alkynes by alkylation with primary alkyl halides. This reaction uses a strong base such as sodium amide. One example is 1-propyne, which reacts with sodium amide and then ethyl bromide to give 2-pentyne. The reaction happens in two steps. First, the strong base removes the acidic hydrogen from the terminal alkyne and forms an acetylide ion. An acetylide ion is the carbon-based species that then takes part in the next step. Next, the...

preparation of alkynes: dehydrohalogenation

preparation-of-alkynes-dehydrohalogenation

Making Alkynes from Dihalides

Alkynes can be made from vicinal or geminal dihalides by dehydrohalogenation. This reaction uses a strong base, such as sodium amide, in liquid ammonia. Two equivalents of hydrogen halide are lost through two successive E2 elimination reactions. In a vicinal dihalide, the two halogen atoms are on neighboring carbon atoms. The first E2 step removes a proton that is anti to the leaving group. Because E2 reactions are concerted, the proton is removed and the halide leaves at the same time. This...

electrophilic addition to alkynes: halogenation

electrophilic-addition-to-alkynes-halogenation

Alkyne Halogenation and Anti Addition

Alkyne halogenation is an electrophilic addition reaction where a halogen molecule adds across a triple bond. Because alkynes have two π bonds, they can add two equivalents of bromine or chlorine. The first addition usually gives a trans-dihaloalkene as the major product and the cis isomer as the minor product. A second equivalent of halogen then forms the tetrahalide. The reaction begins when a π bond in the alkyne acts as a nucleophile and attacks the electrophilic end of the polarized...

electrophilic addition to alkynes: hydrohalogenation

electrophilic-addition-to-alkynes-hydrohalogenation

Alkyne Reactions with HX and HBr Peroxides

Hydrohalogenation of alkynes adds hydrogen halides, HX, across a carbon-carbon triple bond. In this reaction, X can be chlorine, bromine, or iodine. The first addition gives a vinyl halide, and the second addition gives a geminal dihalide. When HCl reacts with an alkyne, one pathway begins with proton transfer to the triple bond. The pi electrons attack the hydrogen in hydrogen chloride and push out chloride ion. This forms a secondary vinylic carbocation, and chloride then attacks to make a...

alkynes to aldehydes and ketones: acid-catalyzed hydration

alkynes-to-aldehydes-and-ketones-acid-catalyzed-hydration

Keto-Enol Tautomerism in Alkyne Hydration

Alkyne hydration uses acid, water, and a mercuric salt to convert alkynes into aldehydes and ketones. It is similar to alkene hydration, but the product type is different. A mercuric salt such as mercuric sulfate, HgSO4, is usually added because the reaction is much slower than alkene hydration. Terminal alkynes follow Markovnikov’s rule during hydration. In contrast, internal alkynes do not add water in a regioselective way. That means the product outcome depends on the alkynes structure. <br...

alkynes to aldehydes and ketones: hydroboration-oxidation

alkynes-to-aldehydes-and-ketones-hydroboration-oxidation

Alkyne Hydration via Borane and Peroxide

Alkyne hydration can make aldehydes and ketones through hydroboration-oxidation. In this indirect hydration reaction, an alkyne first reacts with borane, then with alkaline peroxide. The alkyne forms an enol, and that enol quickly changes into a carbonyl product. Terminal alkynes give aldehydes, while internal alkynes give ketones. The reaction happens in two steps. The hydroboration step is a concerted syn addition, which means the boron and hydrogen add to the same side of the carbon-carbon...

alkynes to carboxylic acids: oxidative cleavage

alkynes-to-carboxylic-acids-oxidative-cleavage

Triple-Bond Cleavage in Alkynes

Alkynes can be broken apart by oxidative cleavage. This reaction uses strong oxidizing reagents such as potassium permanganate and ozone. It cuts the triple bond, which has one sigma bond and two pi bonds, and turns the alkyne into carboxylic acids. With warm, basic aqueous potassium permanganate, the alkyne first forms an unstable alpha-diketone intermediate. The reaction then gives carboxylate salts. A mild acid treatment protonates these carboxylate anions and produces the free carboxylic...

reduction of alkynes to cis-alkenes: catalytic hydrogenation

reduction-of-alkynes-to-cis-alkenes-catalytic-hydrogenation

Stopping Alkyne Hydrogenation at cis-Alkenes

Alkyne hydrogenation can stop at a cis-alkene instead of going all the way to an alkane. Transition metal catalysts such as Pt, Pd, or Ni add hydrogen in two syn steps, which means both hydrogens add to the same side. The cis-alkene is the key intermediate in this process. Hydrogenation also helps compare the thermodynamic stability of hydrocarbons. Acetylene has a heat of hydrogenation of −176 kJ/mol, while ethylene has a value of −137 kJ/mol. The larger heat released for acetylene shows that...

reduction of alkynes to trans-alkenes: sodium in liquid ammonia

reduction-of-alkynes-to-trans-alkenes-sodium-in-liquid-ammonia

Dissolving Metal Reduction of Alkynes

Dissolving metal reduction turns alkynes into trans-alkenes with sodium or lithium in liquid ammonia. This reaction gives the alkene with anti addition across the carbon-carbon triple bond, which leads to the trans product. The reaction must be kept very cold because ammonia is a gas at room temperature. It is run with a dry ice and acetone bath, which holds the mixture at low temperature. When sodium dissolves in liquid ammonia, it forms sodium ions and free electrons. Ammonia molecules...

key terms and concepts

Learn why terminal alkynes are more acidic, how pKa and s character affect acetylide ion formation, and which bases can deprotonate them.

Why Terminal Alkynes Form Acetylide Ions | JoVE K12

organic chemistry

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Why Terminal Alkynes Form Acetylide Ions

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