Name: alkyne bonds, shape, and physical traits
Uploaded: 2023-04-30T18:28:38.000Z
Duration: 2 min 37 s
Channel: JoVE

Unsaturated hydrocarbons containing carbon&#8211;carbon triple bonds are called &#34;alkynes&#34; and are structurally represented as CnH2n-2.
Ethyne or acetylene is the simplest alkyne. It is a colorless gas that undergoes combustion at high temperatures and is used for welding.
The alkyne group is of significance as a bioactive moiety in drugs used to treat Parkinson&#39;s disease, such as selegiline, and in oral contraceptives like ethynylestradiol.
The carbon&#8211;carbon triple bond of acetylene is composed of one &#963; and two &#960; bonds. The carbon&#39;s sp orbitals overlap to create a &#963; bond, while the 2py and 2pz orbitals&#39; lateral overlap forms two &#960; bonds. The second sp orbital of each carbon participates in a &#963; bond with the 1s orbital of the hydrogen, generating the carbon&#8211;hydrogen bonds.
Thus, alkynes are sp hybridized with a linear geometry and a bond angle of 180&#176;, resulting in the molecules&#39;&#160;structural rigidity.
Alkynes show a fifty percent s character given their sp hybridization. As electrons in the s orbital are closer to the nucleus than p orbitals, their energies are lower, and they are more tightly bound to the nucleus. Hence, an increased s character results in stronger and shorter bonds.&#160;
This is reflected in acetylene&#39;s carbon&#8211;carbon triple bond length of 1.21 &#197; and the carbon&#8211;hydrogen bond of 1.06 &#197;. Both bonds are shorter compared to their corresponding alkenes and alkanes.
Aside from the higher s character, the increased number of bonds between the two carbons contributes to the shorter and stronger bonds as well.&#160;
Hence, the bond dissociation energy of alkynes is significantly higher than that of alkenes or alkanes.
Alkynes are nonpolar compounds that are insoluble in water but are soluble in nonpolar organic solvents like benzene.&#160;
Lower molecular weight alkynes such as acetylene are gases; those with medium molecular weights, such as 1-octyne are liquids. In contrast, higher molecular weight alkynes exist as solids at room temperature.

structure and physical properties of alkynes

Structure and Physical Properties of Alkynes

<h4>Introduction:</h4>
In nature, compounds containing both carbon and hydrogen are known as &#34;hydrocarbons&#34;. Aliphatic hydrocarbons are compounds whose molecules contain saturated single bonds (i.e., alkanes) or unsaturated double or triple bonds. Alkenes contain carbon&#8211;carbon double bonds and have a structural formula CnH2n. Unsaturated hydrocarbons containing carbon&#8211;carbon triple bonds are called &#34;alkynes&#34; and are structurally represented by the formula CnH2n-2.
The simplest alkyne is ethyne, or acetylene, a colorless gas, which undergoes combustion at high temperatures and is used as a fuel for welding. Alkynes are found in a variety of substances of both natural and synthetic origin. For example, natural alkynes can be found in the poison obtained from South American tree frogs. On the other hand, synthetic alkyne-containing compounds play an important role in drugs like oral contraceptives such as ethynylestradiol. Other alkyne-based drugs, such as selegiline, are used in combination with synthetic dopamine or L-dopa to treat Parkinson&#39;s disease.&#160;
<h4>Hybridization of Alkynes:</h4>
The triple bond in alkynes occurs due to the overlap of the&#160;carbon&#8211;carbon (C&#8211;C) bond&#39;s sp orbitals forming a sigma (&#963;) bond, and the lateral overlapping of the 2py&#160;and 2pz&#160;orbitals forming the two pi (&#960;) bonds, respectively. The overlap of the carbon sp orbital with the 1s orbital of the hydrogen atom results in the carbon&#8211;hydrogen (C&#8211;H) sp&#8211;1s sigma bond of the alkyne. Alkynes show&#160;an sp&#160;hybridization with a 50%&#160;s&#160;character. As electrons in the s&#160;orbital show lower energies than the p&#160;orbitals, an increase in the atom&#39;s s character increases its electronegativity.
<h4>Molecular Geometry: Bond length, Bond angles, and Bond Strengths</h4>
Owing to the sp&#160;hybridization involving the lateral overlap of the orbitals, alkynes have a linear geometry with the bond angle being 180&#176;. The C&#8211;C triple bond&#39;s length in acetylene is measured to be 121 pm or 1.21 &#197;, which is lesser than that of alkenes (134 pm or 1.34 &#197;) and alkanes (153 pm or 1.53 &#197;) due to the increased number of bonds holding the two carbons together.
The C&#8211;H bond in alkynes (1.06 &#197; in acetylene) is also shorter compared to that in alkenes and alkanes due to the increased s-character of the sp hybridized carbon orbital forming the sp-1s &#963; bond with the hydrogen. In addition to having lower energies, electrons in the s-orbital are closer to the atomic nucleus and are bound more tightly than those in the p orbitals. As a result, a greater s-character increases the strength of the bond. Hence, C-C triple bonds and C-H bonds in alkynes are shorter and stronger than those of alkenes and alkanes, mainly due to their increased s character. The bond dissociation energy of alkynes is 966 kJ or 231 kcal/mol, which is higher than that of alkanes and alkenes&#160;due to the presence of these shorter and stronger bonds.
<h4>Physical Properties of Alkynes:</h4>
Alkynes show similar physical properties as their parent alkane or alkene. They are nonpolar compounds having a lower density than water and are insoluble in water and polar solvents. However, they show good solubility in nonpolar organic solvents. The lower-molecular weight alkynes such as ethyne and propyne exist as gases at room temperature, while higher molecular weight alkynes such as 1-octyne and 1-decyne are liquids.

Introduction:
In nature, compounds containing both carbon and hydrogen are known as &#34;hydrocarbons&#34;. Aliphatic hydrocarbons are compounds whose ...

structure-and-physical-properties-of-alkynes

Education

JoVE Core

Organic Chemistry

Unit 1

Alkynes

KEY TERMS AND CONCEPTS

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 such as ethynylestradiol, which is used in oral contraceptives, and selegiline, which is used with synthetic dopamine or L-dopa to treat Parkinson’s disease.

The triple bond in an alkyne comes from sp hybridization. One sigma bond forms from overlap of sp orbitals on the two carbon atoms, and two pi bonds form from side-by-side overlap of the 2py and 2pz orbitals. The carbon-hydrogen bond also forms a sigma bond between an sp orbital and the hydrogen 1s orbital. Because sp hybridization has 50% s character, the carbon is more electronegative than in alkenes or alkanes.

This bonding gives alkynes a linear shape with a 180° bond angle. In acetylene, the C-C triple bond is 121 pm, or 1.21 Å, which is shorter than the C=C bond in alkenes and the C-C bond in alkanes. The C-H bond in alkynes is also shorter than in alkenes and alkanes. More s character means electrons are closer to the nucleus, so the bonds are stronger. As a result, alkynes have a bond dissociation energy of 966 kJ, or 231 kcal/mol.

Alkynes have physical properties similar to those of related alkanes and alkenes. They are nonpolar, less dense than water, and insoluble in water and other polar solvents. They do dissolve well in nonpolar organic solvents. Lower-molecular-weight alkynes such as ethyne and propyne are gases at room temperature, while larger alkynes such as 1-octyne and 1-decyne are liquids.

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...

acidity of 1-alkynes

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,...

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

Explore how alkynes form triple bonds, why they are linear, and how bond strength affects their physical properties, solubility, and state at room temperature.

Alkyne Bonds, Shape, and Physical Traits | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Alkyne Bonds, Shape, and Physical Traits

Description

Transcript

Tags