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The
After completing this section, you should be able to
- describe the bonding and geometry of the carbon-carbon triple bond in termssp-hybridization of the carbon atoms involved.
- explain the reactivity of alkynes based on the known strengths of carbon-carbon single, double and triple bonds.
- write the equations for the reaction of an alkyne with one or two equivalents of a halogen (chlorine or bromine) or a haloacid (HCl, HBr, or HI).
- Draw the structure of the product formed when an alkyne reacts with one equivalent of the halogens and halogen acids listed in Objective 3.
- identify the alkynes that must have been used in the addition reaction with a halogen or a halogen acid, given the product of such a reaction.
Study note
Before you start working with this chapter, it might be a good idea to review section 1.9. If necessary, construct a molecular model of a simple alkyne. Note the similarity between the behavior of alkenes and alkynes. In the laboratory, you will notice that alkynes readily discolor a solution of bromine in dichloromethane. Section 9.7 describes a test that distinguishes between terminal alkynes (ie where the triple bond is between the last two carbon atoms in the chain) and non-terminal alkynes and alkenes.
Alkyne triple bond
As discussed in§ 1-9, carbon-carbon triple bonds of alkynes are formed by overlapping orbitals on two sp-hybridized carbon atoms. The acetylene molecule (HCCH) is said to contain three sigma bonds and two pi bonds. The C-C sigma bond of acetylene is formed by overlapsphybrid orbital from each of the carbon atoms. Two C-H sigma bonds are formed by overlapping the othersporbital on each carbon atom with 1Shydrogen orbital. Each carbon atom still has two half-filled P orbitals that are perpendicular both to each other and to the line formed by the sigma bonds. These two orthogonal pairsPthe orbitals form two pi bonds between carbon atoms, resulting in a triple bond overall (one sigma bond plus two pi bonds). The electrostatic potential map of acetylene shows that the pi electrons in the triple bond form a negative band (shown in red) around the center of the molecule.
Acetylene is linear as predicted by VSEPR with all four atoms lying in a straight line and both HC bond angles being 180o. The triple bond in acetylene is the shortest (120 pm) and strongest (964 kJ/mol) carbon-carbon bond known.
Electrophilic addition of HX to alkynes
Alkynes undergo electrophilic addition in the same way as alkenes, but the presence of two pi bonds allows for two additions. The addition of an equivalent of hydrogen chloride or hydrogen bromide converts alkynes to haloalkenes. The addition of two or more equivalents of HCl or HBr converts alkynes to geminal dihalides via a haloalkene intermediate. These additions are regioselective and follow the Markovnikov rule. Double bonds formed during reaction with internal alkynes usually have Z stereochemistry, but not always.
HBr addition to the final alkyne
HCl addition to symmetrical internal alkyne
-4-chlorohex-3-ene%253B_hex-3-yne_reacts_with_2_equivalents_of_HCl_in_acetic_acid_to_give_3%252C3-dichlorohexane.svg?revision=3&size=bestfit&width=611&height=254 "Figure 9.3: Alkyne reactions - HX and X₂ addition (4)")
HBr addition to an asymmetric internal alkyne
Addition of HX to an asymmetric internal alkyne tends to produce a mixture of isomers as products.
-3-bromopent-2-ene_and_(Z)-2-bromopent-2-ene%253B_pent-2-yne_reacts_with_2_equivalents_of_HBr_in_acetic_acid_to_give_3%252C3-dibromopentane_and_2%252C2-dibromopentane.svg?revision=3&size=bestfit&width=795&height=253 "Figure 9.3: Alkyne reactions - HX and X₂ addition (5)")
Mechanism
The mechanism of the electrophilic addition of HX to an alkyne is analogous to the addition of HX to an alkene. The presence of two pi bonds in an alkyne allows HX to be added twice. Appendix H+for the alkyne species, the vinyl cation will preferentially form on the more substituted side of the alkyne according to Markovnikov's rule. Later addition no-forms a haloalkene which undergoes electrophilic addition to the other H+. The carbocation form will preferably be formed on the carbon that is attached to the already present halogen. The carbocation is stabilized by the halogen by forming a resonance structure according to the octet rule. This stabilizing effect ensures that the geminal dihalide is the only product and that no vicinal dihalide is formed.
Electrophilic X addition2to alkynes
Alkynes undergo the same type of electrophilic addition with chloride and bromine as alkenes. For alkynes, however, halogen addition may occur once or twice, depending on the molar equivalents of halogen used in the reaction. If a molar equivalent of halogen is used, a dihaloalkene is formed. The anti addition of the reaction mechanism makes the halogens trans in the resulting alkene. The addition of two or more molar equivalents of halogen converts the alkyne to tetrahaloalkane via a dihaloalkene intermediate.
-1%252C2-dibromopent-1-ene%252C_and_pent-1-yne_reacts_with_2_equiv_of_Br2_in_dichloromethane_to_give_1%252C1%252C2%252C2-tetrabromopentane.svg?revision=2&size=bestfit&width=525&height=249 "Figure 9.3: Alkyne reactions - HX and X₂ addition (7)")
Mechanism
The alkyne undergoes electrophilic addition with bromine to form the bromonium ion in the three-membered ring. The expelled bromide ion performs SN2 reaction with bromonium ion, causing ring opening and bromine in the resulting alkenetransconfiguration. The process is repeated with a second pi bond to form a tetrahaloalkane product.
Relative reactivity of alkynes and alkenes to electrophilic reagents
When studying the addition reactions of electrophilic reagents such as strong Brønsted acids and halogens to alkynes, an interesting paradox emerges. Alkyne reactions are even more exothermic than alkene additions, but the rate of alkyne addition is 100 to 1000 times slower. This concept is illustrated by the reaction of one equivalent of bromine with 1-penten-4-yne to produce 4,5-dibromopent-1-yne as the major product.
Why are the reactions of alkynes with electrophilic reagents slower than the corresponding reactions of alkenes? Typically, addition reactions to alkynes are more exothermic than additions to alkenes, and there appears to be a higher density of π electrons around the triple bond (two π bonds to one). Two factors are important in explaining this apparent paradox. First, although more π electrons are attached to the triple bond, sp hybridized alkyne carbons are more electronegative than sp2-hybridized alkene carbons. The alkyne carbons strongly attract their π-electrons, which are consequently more tightly bound to the functional group than the π-electrons in the double bond. This is evident in the ionization potentials of ethylene and acetylene. Rememberionization potentialis the minimum energy needed to remove an electron from a compound molecule. Since the initial interaction between the electrophile and the alkene or alkyne involves electron donation, the relatively slower reactions of the alkynes become understandable.
| Acetylene | HC≡CH+Energy→ [HC≡CH •(+)+ e(–) | ΔH = +264 kcal/mol |
|---|---|---|
| ethylene | H2C=CH2+Energy→ [H2C=CH2] •(+)+ e(–) | ΔH = +244 kcal/mol |
The second factor is believed to be the stability of the carbocation intermediate generated by the sigma bond of a proton or other electrophile to one of the triplet carbon atoms. Comparing the mechanism of addition of HBr to alkene and alkyne, the reaction of the alkyne forms a vinyl carbocation, which is less stable than the alkyl carbocation formed during the reaction of the alkene.
In fact, we can modify our previous carbocation stability ordering to include these vinyl cations as shown below.
| Substitution | Methyl | 1°-vinyl | 1° | 2°-vinyl | 2° | 1°-Allil | 3° | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Stability | CH3(+) | ≈ | RCH=CH(+) | < | RCH2(+) | ≈ | RCH=CR(+) | < | R2CH(+) | ≈ | CH2=CH-CH2(+) | < | R3C(+) |
Application of Hammond's postulate indicates that the activation energy to produce the vinyl cation intermediate would be higher than that of the lower energy intermediate. Electrophilic reactions with alkenes thus have a lower activation energy and proceed faster than the corresponding reaction with alkyne. They illustrate this for alkenes versus alkynes with the following energy diagrams. Despite these differences, electrophilic additives to alkynes have proven to be extremely useful synthetic transformations.
Electrophilic addition of HX with peroxides to alkynes
When 1 equivalent of HBr is reacted with alkynes in the presence of peroxides, anti-marker addition occurs. The use of peroxides causes the reaction to occur via a free radical mechanism. Bromine is added to the less substituted alkyne carbon, while hydrogen is added to the more substituted one to form a haloalkene. Typically, H and Br are added in a syn or anti fashion to form a mixture of products.
_and_(E)-1-bromopent-1-ene.svg?revision=1&size=bestfit&width=761&height=105 "Figure 9.3: Alkyne reactions - HX and X₂ addition (12)")
Exercise \(\PageIndex{1}\)
Draw the structure and give the IUPAC name of the product formed in each of the following reactions:
- Response
FAQs
What is alkyne addition of HX? ›
The addition of HX to terminal alkynes occurs in a Markovnikov-manner in which the halide attaches to the most substituted carbon. The pi electrons react with the hydrogen and it bonds to the terminal carbon. The bromide reacts with the resulting carbocation intermediate to form the vinyl halide.
What is the addition reaction of an alkyne? ›Alkynes and halogens undergo addition reaction to form halogenated alkenes which further react with halogens to give halogen substituted alkanes. The reddish orange coloured solution of bromine and carbon tetrachloride gets decolorized as a result of the addition reaction. This is used as a test for unsaturation.
What is the addition of HX to an alkene? ›The electrophilic addition of HX to an alkene is said to follow Markovnikov's rule. Markovnikov's rule: During the electrophilic addition of HX to an alkene, the H adds to the carbon of the double bond with the fewest number of alkyl substitutent.
When two equivalents of HX are added to an alkyne the halogen atoms add to? ›Addition Of A Second Equivalent Of HX To An Alkyne
Indeed, if we add a second equivalent of H-Cl, it adds to either side of the C-C π bond, giving us the product where two chlorine atoms are on the same carbon. By the way, we call this a “geminal” dichloride (think Latin – “gemini” = twins).
Alkenes undergo electrophilic addition reactions.
What is the order of reactivity of HX with alkene? ›So, the addition of hydrogen halide with alkene will be in order HI>HBr>HCl>HF.
What is the formula for alkyne reaction? ›Their general formula is CnH2n-2 for molecules with one triple bond (and no rings). Alkynes undergo many of the same reactions as alkenes, but can react twice because of the presence of the two p-bonds in the triple bond.
What is an example of an addition reaction? ›Addition reactions are limited to chemical compounds that have multiple bonds, such as molecules with carbon-carbon double bonds (alkenes), with triple bonds (alkynes) or with carbonyl (C=O) groups can undergo addition as they too have double bond character. For example, CH2=CH2 + Cl2 → CH2Cl−CH2Cl.
What is the formula for an alkyne? ›The alkynes are unsaturated hydrocarbons that contain one triple bond, the general formula of alkynes CnH2n-2 and the triple bond is known as the 'acetylenic bond'.
What is addition of x2 in alkene? ›Reaction proceed through the formation of 3- membered cyclic halonium ion. Nucleophile X− attacks from backside of cyclic halonium ion hence total reaction is anti addition reaction.
What is HX in chemistry? ›
The hydrogen halides (HX) make up one of the most important classes of inorganic compounds containing the halogens in the −1 oxidation state.
What is the name of HX in chemistry? ›In chemistry, hydrogen halides (hydrohalic acids when in the aqueous phase) are diatomic, inorganic compounds that function as Arrhenius acids. The formula is HX where X is one of the halogens: fluorine, chlorine, bromine, iodine, or astatine.
What type of product is generated when an alkyne is reacted with one equivalent of HX? ›When treated with 1 equivalent of HX, alkynes form vinyl halides. Hydrogen halide reactivity order : HI > HBr > HCl > HF (paralleling acidity order). H+ adds from one HX at the same time as X- adds to the other side from another HX.
What is the additional reaction of alkenes and alkynes? ›Addition reactions to alkenes and alkynes are sometimes called saturation reactions because the reaction causes the carbon atoms to become saturated with the maximum number of attached groups.
What is the addition reaction of alkenes with H2? ›Ch 6 : Alkenes + H2. Alkenes can be reduced to alkanes with H2 in the presence of metal catalysts such as Pt, Pd, Ni or Rh. The two new C-H σ bonds are formed simultaneously from H atoms absorbed into the metal surface. The reaction is stereospecific giving only the syn addition product.
What is an example of an alkene addition reaction? ›What is an example of alkene addition reactions? Alkene addition reactions include the bromine test and production of ethanol. These reactions occur through hydration, halogenation, and halohydrin reactions.
What are 2 examples of alkenes? ›| IUPAC Name | Molecular Formula | Boiling Point (°C) |
|---|---|---|
| ethene | C 2H 4 | –104 |
| propene | C 3H 6 | –47 |
| 1-butene | C 4H 8 | –6 |
| 1-pentene | C 5H 10 | 30 |
Scope of Reaction
The order of reactivity of the hydrogen halides is HI > HBr > HCl (HF is generally unreactive).
Electrophilic Addition of X2 to Alkynes
If one molar equivalent of halogen is used, a dihaloalkene is formed. The anti addition of the reaction mechanism causes the halogens to be trans in the resulting alkene.
Reactivity of alkenes towards HX decreases in the order
Reaction intermediate of electrophilic addition is carbocation. Therefore, reactivity of alkene will depend on the stability of carbocation.
What is the formula of alkyne alkene? ›
Alkenes have the general formula CnH2n. The general formula for alkynes is CnH2n-2. Acetylene is the simplest alkyne with the formula as C2H2.
What is the first formula of alkyne? ›Alkynes are organic molecules made of the functional group carbon-carbon triple bonds, with the general formula;CnH2n−2. They are unsaturated hydrocarbons. The first member of alkyne family is CH≡CH; Ethyne.
What is alkyne alkene? ›An alkene is a hydrocarbon with one or more carbon-carbon double covalent bonds. An alkyne is a hydrocarbon with one or more carbon-carbon triple covalent bonds.
How do you find addition reactions? ›See if you can recognize the bonds formed and broken. Here's the basic pattern: break a C-C multiple bond (also called a π bond) and form two new single bonds (“σ-bonds” to carbon. This reaction is called an “addition reaction” and it's an extremely common type of reaction, especially in Org 1.
What are basic addition reactions? ›Addition reaction occurs when two or more reactants combine to form a product without the loss of any atoms present in the reactants. Addition reaction is common in compounds that have unsaturated C-C bond, like double (alkene) and triple (alkyne) bonds. The weaker π bond is converted into two new stronger σ bonds.
What is an example of an alkyne? ›...
Introduction.
Hydrocarbons having at least one triple bond between two carbon atoms are known as alkyne. General formula of alkyne is CnH2n−2. As in case of alkene, minimum two carbon atoms are required to form alkyne.
What are the first 5 alkynes? ›- Ethyne (C2H2)
- Propyne (C3H4)
- Butyne [1-Butyne and 2-Butyne] (C4H6)
- Pentyne [1-Pentyne and 2-Pentyne] (C5H8)
Alkene molecules can undergo different types of addition reactions. Alkene molecules can undergo hydrogenation, hydrohalogenation, halogenation, and hydration-type addition reactions.
Can an alkene have 2 double bonds? ›Dienes are alkenes with 2 double bonds. IUPAC: Same as alkene, but change -ene to -adiene and use two numbers to locate the two double bonds (number from the end of the chain which makes the smaller of these numbers smaller). Double bonds separated by more than one single bond are isolated.
What is the formula for alkene double bond? ›
Alkenes are hydrocarbons which contain carbon-carbon double bonds. Their general formula is CnH2n for molecules with one double bond (and no rings). Alkenes are also known as olefins, after the original name for ethene, olefiant gas.
What is a HX stand for? ›Hx (uncountable) (medicine) Abbreviation of history. quotations ▼
What is used of HX? ›What is it used for? Hx 25mg Tablet belongs to a group of medicines called antihistamines, which means it is an anti-allergy medicine. It is used to treat various allergic skin conditions (e.g., eczema, dermatitis, allergies, rash).
What is the full meaning of HX? ›Acid HX is a strong acid, while HY is a weak acid.
What is H+ and H * in chemistry? ›H+ = Proton
The H+ hydrogen ion is the basis of the pH scale. H:– = Hydride. Hydride is a hydrogen atom which has an extra electron. This means that it is a negatively charged ion, or anion.
Chemical Formulas
For example, "H" represents one atom of hydrogen and "O" represents one atom of oxygen. If we want to represent two atoms of hydrogen, instead of writing H H, we write H2. The subscript "2" means that two atoms of the element hydrogen have joined together to form a molecule.
Addition reactions to alkenes and alkynes are sometimes called saturation reactions because the reaction causes the carbon atoms to become saturated with the maximum number of attached groups.
What is the addition reaction of triple bonds? ›The primary reaction of alkynes is addition across the triple bond, which results in the formation of alkanes. Similar to the addition reactions of alkenes, these addition reactions occur in the presence of alkenes. Hydrogenation.
What is the addition reaction of alkanes? ›Addition reactions provide a way to add new substituents to a hydrocarbon chain and thereby produce new derivatives of the parent alkanes. In theory, two products can form when an unsymmetric reagent such as HBr is added to an unsymmetric C=C double bond. In practice, only one product is obtained.
What is the reaction of alkyne to alkene? ›
Hydrogenation of an Alkyne to a Trans-Alkene
The anti-addition of hydrogen to an alkyne pi bond occurs when reacted with sodium or lithium metal dissolved in ammonia. This reaction, also called dissolving metal reduction, involves radicals in its mechanism and produces a trans-alkene as it product.
Alkenes have double bonds; alkynes have triple bonds. Both undergo addition reactions. No; a triply bonded carbon atom can form only one other bond.
Does alkene show addition reaction? ›All alkenes undergo addition reactions with the hydrogen halides. A hydrogen atom joins to one of the carbon atoms originally in the double bond, and a halogen atom to the other.
How do you find the addition reaction? ›When an atom is added to a combination with a double or triple bond, an addition reaction occurs. Addition reactions are linked to unsaturated molecules. These are hydrocarbons with two or three double or triple bonds.
What is 4.3 3 addition reaction? ›4.3. 3 Addition Reaction Unsaturated hydrocarbons add hydrogen in the presence of catalysts such as palladium or nickel to give saturated hydrocarbons. Catalysts are substances that cause a reaction to occur or proceed at a different rate without the reaction itself being affected.
What happens in an addition reaction? ›An addition reaction occurs when two or more reactants combine to form a single product. This product will contain all the atoms that were present in the reactants. Addition reactions occur with unsaturated compounds.
How many types of addition reactions are there? ›There are two main types of addition reactions: nucleophilic addition and electrophilic addition.
How many bonds does alkyne have? ›Molecules that contain a triple bond between two carbon atoms are known as alkynes. The triple bond is made up of one σ bond and two π bonds.
What is an example of an alkene and alkyne? ›Examples: Methane (CH₄), ethane (C₂H₆) is an examples of alkanes. Propane (C₃H₈), and ethene are examples of alkene. Ethyne and propyne are examples of alkyne.