•  
agra,ahmedabad,ajmer,akola,aligarh,ambala,amravati,amritsar,aurangabad,ayodhya,bangalore,bareilly,bathinda,bhagalpur,bhilai,bhiwani,bhopal,bhubaneswar,bikaner,bilaspur,bokaro,chandigarh,chennai,coimbatore,cuttack,dehradun,delhi ncr,dhanbad,dibrugarh,durgapur,faridabad,ferozpur,gandhinagar,gaya,ghaziabad,goa,gorakhpur,greater noida,gurugram,guwahati,gwalior,haldwani,haridwar,hisar,hyderabad,indore,jabalpur,jaipur,jalandhar,jammu,jamshedpur,jhansi,jodhpur,jorhat,kaithal,kanpur,karimnagar,karnal,kashipur,khammam,kharagpur,kochi,kolhapur,kolkata,kota,kottayam,kozhikode,kurnool,kurukshetra,latur,lucknow,ludhiana,madurai,mangaluru,mathura,meerut,moradabad,mumbai,muzaffarpur,mysore,nagpur,nanded,narnaul,nashik,nellore,noida,palwal,panchkula,panipat,pathankot,patiala,patna,prayagraj,puducherry,pune,raipur,rajahmundry,ranchi,rewa,rewari,rohtak,rudrapur,saharanpur,salem,secunderabad,silchar,siliguri,sirsa,solapur,sri-ganganagar,srinagar,surat,thrissur,tinsukia,tiruchirapalli,tirupati,trivandrum,udaipur,udhampur,ujjain,vadodara,vapi,varanasi,vellore,vijayawada,visakhapatnam,warangal,yamuna-nagar

Preparation of Alkenes – Introduction, Preparation, Wilkinson’s Catalyst, Practice Problems and FAQ

Preparation of Alkenes – Introduction, Preparation, Wilkinson’s Catalyst, Practice Problems and FAQ

Fragrances are a great way to uplift our mood, especially if it is in the form of essential oils extracted naturally. They have a significant impact on mood upscaling besides having several kinds of healing properties.

Recently, essential oils have gained huge impetus and have an increasing demand in the global market.

The market for essential oils was estimated to be worth USD 18.6 billion in 2020, and from 2021 to 2028, it is anticipated to rise at a CAGR (Compound Annual Growth Rate) of 7.4% in terms of revenue. A rise in demand from key end-use sectors including food and beverage, personal care and cosmetics, and aromatherapy is anticipated to propel the market.

Please enter alt text

Essential oils (which consist of terpenes– naturally derived from the alkene isoprene) have a pleasant scent. It lowers stress levels, cures fungus infections, and promotes sleep. They also have analgesic; anti-fungal; anti-cancer; anti-relaxant; and antimicrobial properties. The "essence" of a plant is transformed into a liquid form through a process called distillation for a variety of therapeutic and recreational applications. Essential oils are available in a huge range and alkenes of various types form the basis of their ‘essence’.

Alkenes (in the form of terpenes), which are plentiful in trees and other plants, affect the distinctive scent of essential oils. The broad class of chemicals (who are basically alkenes) known as terpenes is produced physiologically from isoprene units. Isoprene, a five-carbon building block, serves as the basis for terpenes, which are organic molecules. They are widespread in plants and frequently have unique smells and scents. They are frequent ingredients in essential oils, which get their name from the distinctive "essence" or aroma they possess.

Let us find out more about such techniques of preparation of alkenes.


TABLE OF CONTENTS

  • Alkenes – Introduction
  • Alkenes – Structure and Bonding
  • Alkenes – General Properties
  • Alkenes – Industrial Methods of Preparation
  • Alkenes – Preparation from Alkynes
  • Alkenes – Preparation from Alkyl Halides
  • Alkenes – Preparation from Vicinal Dihalides
  • Alkenes – Preparation from Alcohols
  • Alkenes – Wittig Reaction
  • Wilkinson’s Catalyst
  • Challenges in Using Wilkinson Catalyst for Controlled Hydrogenation of Alkynes to Alkene
  • Practice Problems
  • Frequently Asked Questions – FAQ

Alkenes – Introduction

An alkene is an unsaturated hydrocarbon with a carbon-carbon double bond denoted by the general formula is CnH2n. It forms an integral category of compounds in organic chemistry.

Alkene, which refers to any hydrocarbon with one or more double bonds, is frequently used as a synonym for olefin which means “oil-forming”.

Alkyl groups connected to the sp2 hybridised carbon atoms of alkenes alter the stability of the double bonds. Alkenes' chemical reactivity is affected by the number of alkyl groups attached to the sp2 hybridised carbon atoms and hence they can be classed based on how many alkyl groups are attached to the C=C structural unit.

A single alkyl group is attached to the sp2 hybridised carbon atom of the double bond in monosubstituted alkenes. At the end of the carbon atom chain, a terminal alkene bears a double bond. Two, three, or four alkyl groups are linked to the carbon atoms of the double bond in disubstituted, trisubstituted, and tetrasubstituted alkenes, respectively.

The International Union of Pure and Applied Chemistry (IUPAC) suggests using the name "olefin" for the general class—cyclic or acyclic, with one or more double bonds—rather than "alkene" for acyclic hydrocarbons with just one double bond, alkadiene, alkatriene, etc., or polyene for acyclic hydrocarbons with two or more double bonds, cycloalkene, cyclooctadiene.

Alkenes – Structure and Bonding

A sigma bond and a pi bond make up a carbon-carbon double bond. Compared to a single covalent bond, this double bond is stronger (611 kJ mol-1 for C=C vs. 347 kJ mol-1 for C-C). The average bond length for C=C double bonds is 1.33 pm compared to 1.53 pm for a typical C-C single bond.

Sigma bonds are formed between three atoms by each carbon atom in the double bond using its three sp2hybrid orbitals (the other carbon atom and two hydrogen atoms). The π-bond is made up of the unhybridised 2p atomic orbitals that are parallel to the plane made by the axes of the three sp2 hybrid orbitals. The pi bond's strength is much lower than the sigma bond at 65 kcal mol-1.

The bond angle between the sp2 hybrid orbitals of a carbon atom in a double bond is about 120°, as predicted by the VSEPR model of electron pair repulsion. Steric strain is brought on by nonbonded interactions between functional groups linked to the double bond's carbon atoms might cause the angle to change.

Bredt's rule for bridging alkenes states that unless the rings are sufficiently massive, a double bond cannot happen at the bridgehead of a bridged ring system.

Alkenes – General Properties

Physical state: Alkenes exist in all three states i.e., solid, liquid and gaseous. The first three alkene group members are gaseous, the following fourteen are liquids, and the following ones are solids. They burn in air with a bright, smoky flame.

Density: Alkenes are lighter than water in terms of density.

Solubility: Alkenes are soluble in organic solvents like benzene but are insoluble in water.

Boiling point: As the molecular mass or chain length of an alkene increases, the boiling point gradually rises, indicating that the intermolecular forces get stronger as the molecule gets bigger. Similar to alkanes, straight-chain alkenes have a higher boiling point than branched-chain alkenes.

Polarity: Similar to alkanes, alkenes are weakly polar, but because they include double bonds, they are somewhat more reactive. Since they are just loosely bound together, the electrons that make up double bonds can simply be added or subtracted. Alkenes thus display greater dipole moments than alkanes.

Melting point: Alkenes and alkanes both have comparable melting points. However, since the alkene molecules are arranged in a U-bend configuration, cis-isomer molecules have a lower melting point than trans-isomers.

Alkenes – Industrial Methods of Preparation

Cracking of saturated hydrocarbons is industrially utilised in preparing alkene. Supplied all over from the US, the Middle East, and Asia, the main raw materials are naphtha and the natural gas condensate components, namely ethane and propane. In the presence of a zeolite catalyst, alkanes are split apart at high temperatures to create a combination of mostly aliphatic alkenes and lower molecular weight alkanes. Fractional distillation is used to separate the mixture, which is dependent on the feedstock and temperature. This is mostly employed in the production of tiny alkenes (up to six carbons).

Catalytic dehydrogenation, in which an alkane loses hydrogen at high temperatures to form an equivalent alkene, is related to this. This is the opposite of how alkenes are catalytically hydrogenated.

Reforming is another name for this procedure. At high temperatures, entropy drives both endothermic reactions toward the alkene.

In the presence of nickel, cobalt, or platinum, ethylene can react with the organometallic compound triethylaluminum to produce higher α-alkenes (of the form RCH=CH2) that can then be synthesised catalytically.

Alkenes – Preparation from Alkynes

Using Lindlar’s Catalyst

Reagent: H2Pd, CaCO3, with quinoline

  • Palladium-coated calcium carbonate powder that has been poisoned with quinoline to decrease its catalytic activity and prevent complete hydrogenation of alkyne (so that alkane is not obtained) makes up the poisoned palladium catalyst.

  • The partial reduction of alkyne to an alkene is carried out using Lindlar's catalyst.
  • The catalytic activity is somewhat diminished by poisoning, restricting the reduction of the alkyne to the production of an alkene. Only cis-isomers are formed as products using Lindlar’s catalyst.

Birch reduction

Reagent: Naliq. NH3 in solvent t-BuOH

  • This can be a suitable method for obtaining trans-alkene from alkynes (But internal alkynes alone).

  • The mechanism of Birch reduction follows the below-given steps.

  • Due to the acidic nature of terminal hydrogen, terminal alkyne exhibits redox reaction rather than Birch reduction and liberates H2.

  • Birch reduction of the benzenoid ring produces 1,4- cyclohexadiene having 2 H-atoms attached at opposite ends of the product obtained.

Alkenes – Preparation from Alkyl Halides

  • Alkyl halides are heated with alcoholic potash to produce alkenes. This reaction is called the dehydrohalogenation of alkyl halides.
  • Potassium hydroxide is dissolved in alcohol to produce alcoholic potash.
  • Dehydrohalogenation, or the removal of one halogen acid molecule, occurs in this process. The kind of the connected halogen group and the alkyl group affect the pace of reaction.
  • Dehydrohalogenation, or the removal of HX, can be used to produce alkenes from alkyl halides.
  • Alcoholic KOH is the reagent used.
  • The β-elimination reaction is named after the removal of β-hydrogen.
  • The hydrogen atom opposite to the halogen atom attached to the β-carbon (carbon atom next to the halogen atom) is removed. As a result, it is referred to as anti-elimination or β-elimination.

Mechanism: Alcoholic KOH produces the alkoxide ion, which acts as a strong base. It attacks the mildly acidic alkyl halide molecule and dissociates the β-Hydrogen atom from it. The α-carbon atom, which has a minor electron deficiency as a result of its connection to the halogen atom, pulls the electrons shared by the broken hydrogen carbon bond. As these electrons reach the α-carbon atom, the halogen atom leaves, resulting in the creation of the double bond.

  • This is in accordance with the Zaitsev or Saytzeff Rule. In most elimination reactions with two or more possible products, the one with the highly substituted double bond will be the major product.

  • It is a concerted mechanism. Concerted means the bond breaking and new bond formation occur simultaneously.

  • Dehydrohalogenation, or the removal of one halogen acid molecule, occurs in this process. The kind of the connected halogen group and the alkyl group affect the pace of reaction.
  • Reactivity: R−I > R−Br > R−Cl > R−F
  • Greater the size of halide ion, weaker the H-X bond and greater its reactivity. Hence, RI is the most reactive. This is because iodide is a better leaving group due to its large size.
  • The rate of elimination reaction directly depends upon the stability of the alkene.
  • Rate of elimination: Tertiary alkyl halide > Secondary alkyl halide > Primary alkyl halide
  • This is because tertiary alkyl halides have more β-hydrogens as compared to secondary and primary alkyl halides.

Alkenes – Preparation from Vicinal Dihalides

  • In this method, the dehalogenation (removal of -X2) occurs from vicinal dihalides for preparation of alkenes.
  • NaI in acetone or Zn in the presence of acetic acid, ethanol, or ZnΔ are the reagents used.

  • This reaction is carried out via the E2 mechanism.

Alkenes – Preparation from Alcohols

  • Acid catalysed dehydration, or the E1-elimination of H2O in the presence of an acid, can be used to produce alkenes from alcohol.
  • Concentrated H2SO4 and also concentrated H3PO4 and heat is used as the reagents.
  • The reaction is carried out via the E1 mechanism via the formation of an intermediate carbocation. Hence, rearrangements to gain a product through a more stable carbocation intermediate pathway are possible. It is a β-elimination reaction.

Mechanism

Step 1: Protonation of alcohol

  • Alcohols are weak bases due to the presence of two lone pairs of electrons on oxygen.
  • As a result, they combine with strong mineral acids (H2SO4) to form protonated alcohol.
  • Alcoholic oxygen protonation aids in the elimination of water molecules.

Step 2: Formation of intermediate Carbocation

  • The C−O bond is broken with the elimination of a water molecule in this step to form a carbocation.
  • This corresponds to the slowest step and hence is the rate-determining step.

Step 3: Production of alkene

  • Water thereafter attacks the proton of the carbon atom next to the carbocation, breaking the existing C−Hbond and forming C=C.

Dehydration of Alcohols - Reactivity Order

  • Less stable carbocations will rearrange to more stable carbocation (primary to tertiary or secondary carbocation) to give the most suitable product accordingly.
  • Rearrangement occurs through a 1,2-methyl shift.

Carbocation rearrangement and Its importance with respect to the formation of alkenes

Using various structural rearrangements and hydride or methyl group shifts within the molecule, carbocation rearrangements are "the migration of the carbocation from an unstable state to a more stable state."

Since dehydration of alcohol proceeds through a carbocation intermediate, it is possible for hydride or alkyl shifts to take place that move the carbocation to a more stable location. Therefore, a combination of alkenes, both with and without carbocation rearrangement, are produced after the dehydration reaction. The end product that results from a more stable carbocation intermediate is the main one.

Generally, Saytzeff products are the major products in the dehydration of alcohols.

Alkenes – Wittig Reaction

A triphenyl phosphonium ylide and an aldehyde or ketone undergo a chemical process known as the Wittig reaction, also known as the Wittig olefination. Aldehydes and ketones are most frequently converted to alkenes by Wittig reactions.

Step 1: Wittig reaction follows a three step mechanism. The phosphorus ylide's negatively charged carbon acts as a nucleophile. This carbon then launches a nucleophilic attack on the aldehyde or ketone's carbonyl carbon. As a result, a betaine, a charge separated (and dipolar) intermediate, is created. The following is an illustration of this step:

Step 2: A new oxygen-phosphorus bond must now form between the betaine intermediate from step 1 in order to produce another intermediate with a four-membered ring structure. Here is an illustration of this stage.

Step 3: The carbon-oxygen link and the carbon-phosphorus bond are both cleaved in the four-membered ring intermediate. With the phosphorus, which had lost its bonding pair of electrons to the carbon atom, the oxygen accepts both of the bonding electrons and establishes a new double bond. This electron pair also creates a new carbon-carbon double bond, resulting in the production of the necessary alkene product. Below is an illustration of this stage.

Wilkinson’s Catalyst

Wilkinson's catalyst is a coordination complex denoted by the IUPAC name Chlorotris(triphenylphosphine)rhodium(I), and has the formula [RhCl(PPh3)3]. It is solid with a reddish-brown tinge at ambient temperature and is soluble in chlorinated solvents like dichloromethane (CH2Cl2) and tetrahydrofuran (THF), as well as hydrocarbon solvents like benzene.

Wilkinson’s catalyst performs selective catalytic hydrogenation of unsaturated hydrocarbons (alkenes and alkynes) majorly.

It is famously used in catalytic hydrogenation of alkenes. It is a homogeneous hydrogenation catalyst. But for preparation of alkenes through partial hydrogenation of alkynes, it faces difficulties.

Challenges in Using Wilkinson Catalyst for Controlled Hydrogenation of Alkynes to Alkene

Alkynes tend to get fully saturated readily if reduced by Wilkinson catalyst. They can be reduced to alkanes through the intermediary of the cis-alkene, making it difficult to control the hydrogenation of alkynes. RhCl(C2H4)(PPh3)2 is obtained when ethylene combines with [RhCl(PPh3)3] i.e., Wilkinson's catalyst, however it is not a substrate for hydrogenation.

Therefore, the general tendency of alkynes is to undergo full saturation using Wilkinson’s catalyst as with the substrate obtained it is difficult to control and terminate the reaction.

Alkynes are more prone to additions than alkenes because of the two procurable bonds that set them apart from alkenes. These catalysts modify the placement of substituents on the newly generated alkene molecule in addition to converting them into alkenes.

Because hydrogenation involves a number of stages and is interruptible, it can be halted using modified catalysts during the transitional alkene stage, such as Lindlar's Catalyst. Lead acetate, quinoline, and palladium-calcium carbonate makeup Lindar's catalyst.

Sodium dissolved in an ammonia solvent can be used to convert alkynes to trans-alkenes.


Recommended Video

https://www.youtube.com/watch?v=sbltpQvehWU


Practice Problems

Q1. The correct composition of Lindlar’s Catalyst is

A. RhCl(C2H4)(PPh3)2B. NaI in CH3COCH3C. ZnCH3COOHD. H2Pd, CaCO3 with quinoline 

Answer: D)

Solution: Palladium-coated calcium carbonate powder that has been poisoned with quinoline is called Lindlar’s catalyst. It is poisoned, to decrease its catalytic activity and prevent complete hydrogenation of alkyne (so that alkane is not obtained).

So, option D is the correct answer.

Q2. Wilkinson’s catalyst is essentially important for

A. Hydrohalogenation of Alkene
B. Hydrogenation of Olefins
C. Dehydration of Alcohols
D. Dehydrogenation of alkenes

Answer: B)

Solution: Wilkinson’s catalyst undergoes selective catalytic hydrogenation of unsaturated hydrocarbons, especially alkenes (olefins). It is a complex that is vulnerable to binding substrates (alkenes and H2) because it is coordinatively unsaturated.

So, option B is the correct answer.

Q3. Mention the Zaitsev rule.

Solution: Alkenes can be produced through an alcohol dehydration process in which hydrogen atoms are lost from two separate carbons on the carbocation. The Zaitsev (or Saytzeff) rule states that the more highly substituted alkene, or the alkene with the most substituents on the double bond's carbon atoms, is always the primary product.

Alternatively, it can be said that as per this rule, the elimination of hydrogen from β-carbon with a greater number of substituents occur.

Q4. Dehydrohalogenation of alkyl halides to produce alkene is a β-elimination reaction. Justify.

Answer: Alcoholic KOH produces the alkoxide ion, which acts as a strong base. It attacks the mildly acidic alkyl halide molecule and dissociates the β-Hydrogen atom from it.

The α-carbon atom, which has a minor electron deficiency as a result of its connection to the halogen atom, pulls the electrons shared by the broken hydrogen carbon bond. As these electrons reach the α-carbonn atom, the halogen atom leaves as a halide ion, resulting in the creation of the C=C double bond.

The hydrogen atom opposite to the halogen atom attached to the β-carbon (carbon atom next to the halogen atom) is removed. As a result, it is referred to as anti-elimination or β-elimination.


Frequently Asked Questions – FAQ

Q1. What are vicinal dihalides and geminal dihalides?

Vicinal Dihalides: Dihalides in which two halogen atoms are attached to two adjacent carbon atoms are known as vicinal dihalides.

Geminal Dihalides: Dihalides in which two halogen atoms are attached to the same carbon atom are known as geminal dihalides.

Q2. Which alkene is responsible for the ripening of fruit?
Answer: Along with other hormones and signals, the gaseous plant hormone ethylene is crucial in triggering the ripening of many fruits. Typically, an immature fruit has less ethylene present. Ethylene is released as the fruit ages as a signal to cause fruit ripening. According to fssai , ethylene is essential for the ripening of fruits naturally. It is a hormone that fruits naturally make to speed up the ripening process.

For artificial ripening, ethylene is allowed as long as the concentration doesn't go over 100 ppm (parts per million)

Q3. How is polyethylene prepared?
Answer: Ethene is heated to 473K under a pressure of 1500 atm to create polyethylene. This polymerisation happens as a result of a free radical process that oxygen starts.

The main applications of polyethylene include packaging film, trash and grocery bags, wire and cable insulation, agricultural mulch, bottles, toys, and home goods. Trays, milk, juice, and food packaging goods, as well as crates and containers for fruit and drink, all employ polythene.

Talk to Our Expert Request Call Back
Resend OTP Timer =
By submitting up, I agree to receive all the Whatsapp communication on my registered number and Aakash terms and conditions and privacy policy