Alkenes – Definition, Nomenclature, Preparation, Properties

In organic chemistry, a hydrocarbon is an organic molecule consisting entirely of hydrogen and carbon. Hydrocarbons are an example of group 14 hydrides. Hydrocarbons are colourless and hydrophobic, with a slight odour. Because of their different chemical structures, it’s hard to generalise anymore. The bulk of anthropogenic hydrocarbon emissions come from the burning of fossil fuels, which includes both fuel production and combustion. Ethylene, isoprene, and monoterpenes are all-natural hydrocarbons found in plant emissions.

What are Alkenes?

Unsaturated hydrocarbons with a double bond between the carbon atoms are known as alkenes. Between carbon-carbon atoms, there is at least one double bond. Alkenes have the generic formula C_nH_2n. Alkenes are frequently used interchangeably with the term olefin.

The word olefin comes from the Greek phrase olefin gas, which meaning oil-forming gas. Alkenes are extremely reactive chemically due to the presence of double bonds.

We’re all familiar with saturated hydrocarbons like alkanes, but there’s another class of unsaturated hydrocarbons present in crude oils that’s more reactive. Let’s learn everything there is to know about Alkenes, which are unsaturated hydrocarbons with at least one double bond between carbon-carbon atoms.

Nomenclature of Alkenes

Alkenes have the generic formula CnH2n, where n-2, 3, 4, 5, so on. Alkenes contain a suffix-ene in their names, according to the IUPAC system, and the prefix is determined by the number of carbon atoms. The following are some examples of alkene names:

Number of carbon atoms	Formula	IUPAC name
2	C2H4	Ethene
3	C3H6	Propene
4	C4H8	Butene
5	C5H10	Pentene
6	C6H12	Hexene
7	C7H14	Heptene
8	C8H16	Octene
9	C9H18	Nonene
10	C10H20	Decene

Isomerism in Alkene

Stereoisomerism and structural isomerism are both present in alkenes.

Stereoisomerism and structural isomerism are both presents in alkenes: Only one structure exists in ethene and propene, while alkenes with four or more carbon atoms display both chain and position isomerism.

• The location of the double bond is not altered in chain isomerism. The chain isomerism in Butene, for example, will be as follows:

Chain Isomerism

• In position isomerism, the isomers differ in their double bond location. Two isomers of butene, for example, are as follows:

Positional Isomerism

Stereoisomerism or Geometrical Isomerism: Geometrical isomers are isomers with the same structural formula that differ in the relative spatial arrangement of atoms or groups of atoms around the double bond. Geometrical isomerism is the name given to this occurrence. Cis-isomer is an isomer in which a similar group of atoms is present on the same side of the double bond. Trans-isomerism, on the other hand, refers to an isomer with comparable bonds on the opposite side.

Example of Stereoisomerism

General Methods of Preparation of Alkenes

Alkenes are prepared using the methods listed below:

• By partial reduction of alkynes- Alkenes are made by hydrogenating alkynes in the presence of a particular catalyst, palletised charcoal, which is mildly deactivated using quinoline or sulphur compounds.

Preparation of Alkenes by partial reduction of alkynes

• From alkyl halides- Alkyl halides are dehydrogenated into alkenes when heated with a strong base such as sodium ethoxide or a concentrated alcoholic solution of potassium hydroxide.

Preparation of Alkenes from Alkyl Halides

• From vicinal halides- Vicinal halides are alkane di-halogen derivatives in which two halogen atoms are present on neighbouring carbon atoms. By heating appropriate dihalides with zinc dust in methanol or ethanol, alkenes can be made.

Preparation of Alkenes from Vicinal Halides

• From alcohols- Alcohols lose a water molecule when heated with mineral acids such as strong sulphuric acid, forming alkenes.

Preparation of Alkenes from Alcohols

Physical Properties of Alkenes

1. Physical State- The initial three individuals are dry gases; the following fourteen (C5-C18) individuals are fluids, while the higher ones are solids.
2. Shading and smell-Except ethene which has a wonderful smell, any remaining alkenes are vapid and scentless.
3. Dissolvability- Alkenes are insoluble in water however genuinely solvent in non-polar solvents like benzene, petrol, ether, and so forth.
4. Boiling Point-The edges of boiling over increment with an increment in the atomic mass. Straight-chain alkenes have higher limits than isomeric extended chain alkenes.
5. Alkenes are all lighter than water in terms of density. Alkene densities grow as molecular mass increases until the limiting value of 0.8gcm^-3 is achieved.

Chemical Reactions of Alkenes

• Addition of Hydrogen- At room temperature, alkenes quickly add hydrogen to create alkanes in the presence of either the catalyst nickel, palladium, or platinum. As an example,

• Addition of halogens- Halogen derivatives are formed when alkenes react with halogens in an inert solution such as CCl₄.

• Addition of halogen halides- Alkenes react with halogen halides (HCl, HBr, HI) to produce alkyl halides, which are mono haloalkanes.

Uses of Alkenes

• Lower individuals from alkenes are utilized as energisers and illuminants.
• Alkenes and subbed alkenes upon polymerization from numerous valuable mixtures like polythene, PVC, Teflon, orlon, and so on
• Alkenes are utilized for the counterfeit maturing of organic products.
• Ethylene is utilized in the assembling of ethyl liquor and ethylene glycol.
• Alkenes are utilized for making hostile to thump for motors.
• Ethylene is utilized in making oxygen-ethylene fire for cutting and welding.

Sample Questions

Question 1: What is the function of alkenes?

Answer:

Alkenes are used to artificially ripen fruits, make anti-knock for vehicle engines, create ethyl alcohol and ethylene glycol, and a variety of other useful chemicals such as polythene, PVC, Teflon, and orlon.

Question 2: What is an alkene in chemistry?

Answer:

Unsaturated hydrocarbons with a double bond between the carbon atoms are known as alkenes. Between carbon-carbon atoms, there is at least one double bond. Alkenes have the generic formula C_nH_2n. Alkenes are frequently used interchangeably with the term olefin. The word olefin comes from the Greek phrase olefin gas, which meaning “oil-forming gas.” Alkenes are extremely reactive chemically due to the presence of double bonds.

Question 3: What are the characteristics of alkenes?

Answer:

Except for ethene, which has a pleasant odour, all other alkenes are colourless and odourless. Alkenes are insoluble in water but relatively soluble in non-polar solvents such as benzene, petroleum, ether, and others. The boiling point of straight-chain alkenes is greater than that of isomeric branched-chain alkenes.

Question 4: How can we identify alkenes?

Answer:

Bromine water can be used to evaluate alkenes. The solution is decolonized by alkenes

Question 5: What are the two uses of alkenes?

Answer:

1. Alkenes and substituted alkenes are used in the polymerization of several important chemicals such as polythene, PVC, Teflon, and orlon.
2. Alkenes are used to make fruits mature artificially.

Question 6: What are Geometrical isomers?

Answer:

Geometrical isomers are isomers with the same structural formula that differ in the relative spatial arrangement of atoms or groups of atoms around the double bond. Geometrical isomerism is the name given to this occurrence. Cis-isomer is an isomer in which a similar group of atoms is present on the same side of the double bond. Trans-isomerism, on the other hand, refers to an isomer with comparable bonds on the opposite side.