Alkenes
Alkenes are a homologous series of hydrocarbons that contain a carbon-carbon double bond. The variety of hydrogen atoms in an alkene is double the number of carbon atoms, so they have the general formulaCnH2n.
For instance, the molecular formula of ethene is C2H4, while for propene it is C3H6.
Alkenes are unsaturated, implying they consist of a double bond. Alkenes have two hydrogen atoms less than the corresponding saturated hydrocarbons (alkanes). They are likewise known as Olefins (derived from the Latin word olefiant meaning oil forming) because lower members form oily products on treatment with chlorine or bromine.
The most basic olefin is C2H4, ethene. Alkenes having one double bond is called mono-enes with the general formula CnH2n. Alkenes consisting of two double bonds are called dienes.
Structure of Alkenes
Alkenes contain a double bond that is composed of one sigma and one pi bond between two carbon atoms. The sigma bond has similar properties to those found in alkanes, while the pi bond is more reactive. The carbon atoms in the double bond are sp2 hybridized, forming a planar structure.
Rotation around the double bond is disfavored, so alkenes form fairly steady isomers depending on the positioning of substituents on the very same (cis) or opposite (trans) sides of the double bond. These isomers are called diastereomer isomers.
Physical Characteristics of Alkenes
- Physical State
These double-bonded compounds are colorless and odorless in nature. However, ethene is an exception because it is a colorless gas and has a faintly sweet smell.
The very first three members of the alkene group are gaseous in nature, the next fourteen members are liquids and the remaining alkenes are solids.
- Solubility
The alkenes are insoluble in water due to their nonpolar characteristics. However, are completely soluble in nonpolar solvents such as benzene, ligroin, etc.
- Melting Point
The melting points of these double-bonded compounds rely on the positioning of the particles. The melting point of alkenes resembles that of alkanes. However, cis-isomer particles have a lower melting point than trans-isomers as the molecules are crammed in a U-bending shape.
- Boiling Point
The boiling points of the compounds increase as the number of carbon atoms in the substance increases. When alkenes are compared with alkanes, it is observed that the boiling points of both are almost comparable, as if the compounds are made up of the same carbon skeleton. The boiling point of straight-chain alkenes is more than branched-chain alkenes just as in alkanes.
- Polarity
Alkenes are weakly polar similar to alkanes however are slightly more reactive than alkanes due to the existence of double bonds.
The π electrons that make up the double bonds can quickly be removed or included as they are weakly held. For this reason, the dipole moments exhibited by alkenes are more than alkanes. The polarity relies on the functional group attached to the substances and the chemical structures.
Reactivity of a π -bond
In the formation of a π -bond, the partially filled π -orbitals overlap in a parallel style. The probability of finding an electron is hence far from the line joining the two nuclei. Due to this reason π -electrons are less strongly held between the nuclei. A π -bond is, therefore, a weak bond as compared to a σ -bond.
During a reaction, it breaks relatively easily rendering alkenes a reactive group of compounds. Additionally, the loosely held π -electrons are more exposed to attack by the electrophilic reagents. Alkenes, therefore, undergo electrophilic reactions extremely easily.
Uses of Alkenes
Alkenes are incredibly essential in the manufacture of plastics. All plastics remain in some method related to alkenes. The names of some plastics (Polythene or Poly Ethene, Polypropene), resemble their alkene partners. Plastics are used for all types of jobs, from product packaging and wrapping to clothes and outdoor apparel.
- 1) Alkenes discover lots of varied applications in the industry. They are utilized as raw materials in the synthesis of alcohols, plastics, detergents, and fuels.
- 2) Ethene is the most essential organic feedstock in the polymer market. E.g., PVC, Saran’s, and polyethylene. These polymers are used in the manufacture of floor tiles, shoe soles, synthetic fibers, raincoats, pipelines, etc.
- 3)Lower alkenes are utilized as fuel and illuminant. These might be caused by the breaking of kerosene or fuel.
- 4)As raw materials for the manufacture of industrial chemicals such as alcohols, aldehydes, etc.
- 5)Besides, alkenes are likewise used for synthetic ripening of fruits, as a general anesthetic, and for making dangerous mustard gas (War gas) and ethylene-oxygen flame.
Multiple-Choice Questions (MCQs) with Answers
- What is the general formula for alkenes?
- A) CnH2n-2
- B) CnH2n
- C) CnH2n+2
- D) CnH2n+4
- Answer: B
- What is the term used for the carbon-carbon double bond in alkenes?
- A) Sigma bond
- B) Pi bond
- C) Delta bond
- D) Theta bond
- Answer: B
- Which is the simplest alkene among the following?
- A) Methane
- B) Ethene
- C) Propane
- D) Butene
- Answer: B
- What is the term for alkenes having two double bonds?
- A) Mono-enes
- B) Diastereomers
- C) Dienes
- D) Polyenes
- Answer: C
- How are the carbon atoms in the double bond of alkenes hybridized?
- A) sp
- B) sp2
- C) sp3
- D) dsp2
- Answer: B
- What kind of isomers are formed due to the restricted rotation around the double bond in alkenes?
- A) Structural isomers
- B) Geometric isomers
- C) Optical isomers
- D) Diastereomers
- Answer: B
- What is the physical state of the first three members of the alkene group?
- A) Solids
- B) Liquids
- C) Gases
- D) Plasma
- Answer: C
- Which is the exception among alkenes regarding odor?
- A) Propene
- B) Ethene
- C) Butene
- D) Methane
- Answer: B
- In alkenes, which isomer has a lower melting point, cis-isomer, or trans-isomer?
- A) Cis-isomer
- B) Trans-isomer
- C) Both have the same melting point
- D) Neither has a melting point
- Answer: A
- What solvent are alkenes generally insoluble in?
- A) Water
- B) Ethanol
- C) Benzene
- D) Acetone
- Answer: A
- Which bond in alkenes contributes to higher reactivity compared to alkanes?
- A) Sigma bond
- B) Pi bond
- C) Delta bond
- D) Zeta bond
- Answer: B
- What type of reaction do alkenes undergo very easily due to their π -bond reactivity?
- A) Substitution
- B) Addition
- C) Elimination
- D) Redox
- Answer: B
- What is the significance of π -electrons in alkenes?
- A) They form strong bonds
- B) They are weakly held and contribute to reactivity
- C) They have no role in bonding
- D) They stabilize the molecule
- Answer: B
- What are alkenes crucial in the manufacture of in the polymer industry?
- A) Metals
- B) Ceramics
- C) Plastics
- D) Fiberglass
- Answer: C
- Which alkene is the most essential organic feedstock in the polymer industry?
- A) Propene
- B) Butene
- C) Ethene
- D) Pentene
- Answer: C
- In what form are lower alkenes used?
- A) Gaseous fuel
- B) Liquid fuel
- C) Solid fuel
- D) Plasma fuel
- Answer: A
- What is ethylene-oxygen flame used for?
- A) Cooking
- B) Lighting
- C) Welding
- D) Heating
- Answer: C
- What are alkenes used for in synthetic ripening of fruits?
- A) Reducing spoilage
- B) Enhancing flavor
- C) Accelerating the ripening process
- D) Preventing ripening
- Answer: C
- Which of the following is a characteristic of alkenes related to their solubility?
- A) Highly soluble in water
- B) Insoluble in nonpolar solvents
- C) Soluble in both polar and nonpolar solvents
- D) Insoluble in water but soluble in nonpolar solvents
- Answer: D
- What term is derived from the Latin word ‘olefiant’ meaning oil forming, associated with alkenes?
- A) Olfactory
- B) Oligomeric
- C) Olefinic
- D) Osmotic
- Answer: C
- What is the general formula for alkenes consisting of two double bonds?
- A) CnH2n-2
- B) CnH2n
- C) CnH2n+2
- D) CnH2n+4
- Answer: D
- Which structural feature in alkenes results in weakly held π -electrons?
- A) Linear arrangement
- B) Planar structure
- C) Tetrahedral geometry
- D) Diagonal alignment
- Answer: B
- What type of isomers are formed based on the positioning of substituents around the double bond in alkenes?
- A) Structural isomers
- B) Geometric isomers
- C) Optical isomers
- D) Positional isomers
- Answer: B
Summary
The tutorial on Alkenes provides a comprehensive understanding of the structure, physical characteristics, and applications of this important class of hydrocarbons. Alkenes are characterized by the presence of a carbon-carbon double bond, and their general formula is CnH2n. The tutorial discusses the structure of alkenes, emphasizing the planar arrangement resulting from the sp2 hybridization of carbon atoms in the double bond.
The physical characteristics of alkenes, including their colorless and odorless nature, exceptions like ethene, and variations in physical states among different members of the alkene group, are explored. Solubility, melting points, boiling points, and polarity are highlighted, drawing comparisons with alkanes.
The tutorial goes into the reactivity of the π -bond in alkenes, pointing out the weak nature of the π -bond and its susceptibility to electrophilic reactions. The concept of diastereomer isomers due to restricted rotation around the double bond is explained.
Additionally, the practical uses of alkenes are discussed, with a focus on their significant role in the polymer industry, where they serve as essential raw materials for plastics production. Examples such as Polythene and Polypropene are cited. The importance of ethene as a crucial organic feedstock for polymers like PVC, Saran’s, and polyethylene is emphasized. Furthermore, alkenes find applications in the synthesis of alcohols, detergents, and fuels. The tutorial concludes by highlighting diverse applications, including the use of lower alkenes as fuel, synthetic ripening of fruits, and even in the production of dangerous substances like mustard gas and ethylene-oxygen flame for welding purposes.