What are Alkenes: Preparation, Properties, Structure, Nomenclature and Many More

Alkenes are a type of unsaturated hydrocarbons that have at least one carbon carbon double bond between their carbon atoms ( $\mathrm{C}=\mathrm{C}$  ). The carbon atoms involved in the double bond are sp² hybridized i.e. they are placed in a trigonal planar geometry shape with each bond angles being 120°. The sp² orbitals overlap with the orbitals of other atoms to form a stronger sigma bond, and the p orbital overlaps side by side to form a weaker π bond.

The general formula used for representation of alkenes is

${\mathrm{C}}_n{H}_{2n}$

where n = number of carbon atoms

Some examples of alkenes include ethane, propene, butane, etc. (those ending with ene).

Alkenes consist of atleast one $\mathrm{C}=\mathrm{C}$ double bond, which consists of a sigma (σ) bond and a pi (π) bond. The carbons in the double bond are sp² hybridized, which means that the formation is a trigonal planar shape with bond angles of 120 degrees each. Each carbon atom is bonded to two hydrogen atoms and a third carbon via double bond, and all of this is in the same plane. It is because of this double bond which makes alkenes more reactive than alkanes since the bond is comparatively weaker and can be further broken easily for reaction mechanisms.

Example: for ethene ( $C_2{H}_4$ ): $H_{2}C=C{H}_2$

Additionally, here are some of the key features for sp² hybridized atoms:

• Geometry: Trigonal Planar Shape.

• Bond angle: ~120° each.

• Hybridized s orbitals: Form σ bonds (stronger).

• Unhybridized p orbital: Forms π bond (weaker).

 

Isomerism in organic chemistry is the phenomenon of two different compounds having the same molecular formula but their structure differs from each other. This is because the atoms are placed differently due to the double carbon bond which allows flexibility.

Isomerism in alkenes is of two major types:

• Structural Isomerism: This exists due to the different positions of the double bond atoms. It can be further classified into :

1. Chain Isomerism: Here, arrangement of the branches in the carbon chain can differ.

   Example: C₄H₈

   • CH₂=CH–CH₂–CH₃ (But-1-ene)

   • CH₃–CH=CH–CH₃ (But-2-ene)

2. Position Isomerism Here, position of the double bond differs.

   Example: C₄H₈

   • CH₂=CH–CH₂–CH₃ (But-1-ene)

   • CH₃–CH=CH–CH₃ (But-2-ene)

• Geometrical (Cis-Trans) Isomerism: In this isomerism, there are restrictions in the rotation of the carbon carbon bonds due to which alkenes have 2 structures: cis and trans forms. Example: Cis-but-2-ene vs Trans-but-2-ene. . For but-2-ene ( ${\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{H}={\mathrm{C}\mathrm{H}\mathrm{C}\mathrm{H}}_3$ ):
  • Cis: $C{H}_3$ groups on the same side.
  • Trans: $C{H}_3$ groups on opposite sides.
• Functional isomerism: These are the types of alkenes which have the same formula as of cycloalkanes.

  Example: C₄H₈

  • alkene (But-1-ene)

  • cyclobutene (cycloalkane)

Nomenclature refers to a certain set of rules and regulations which are to be followed while naming the specific alkene. Naming alkenes follows the IUPAC (International Union of Pure and Applied Chemistry) rules which are specifically designed with the purpose of assigning a unique name to each and every alkene.

Here's a basic procedure of how to apply these rules:

1. First, find the longest carbon chain that includes the double bond. This is known as the parent chain. Use the same alkane name but the only difference will be to change -ane to -ene (e.g., ethene, propene).
2. Next, number the chain to give the lowest possible number to the double bond .
3. Then, if there are branches (alkyl groups like methyl, $-C{H}_3$ ), name them too and don't forget to number them.
4. And suppose if there are multiple double bonds, you can use prefixes like di-, tri- (e.g., buta-1,3-diene for ${\mathrm{C}\mathrm{H}}_2=\mathrm{C}\mathrm{H}-\mathrm{C}\mathrm{H}={\mathrm{C}\mathrm{H}}_2$ ).

like in the case of ${\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{H}={\mathrm{C}\mathrm{H}}_2$ (propene):

• Longest chain with double bond: 3 carbons (prop-).
• Double bond gets the lowest number (position 1).
• So, its propene.

Alkenes can be prepared through various compounds using different techniques. Here are some of the key ones which the candidates can refer to:

1. Dehydration of Alcohols:

Alkenes can be achieved as a product by dehydrating alcohols, which means eliminating water to obtain the desired result. Basically,

Alcohol → Alkene + Water

Example:
CH₃CH₂OH → CH₂=CH₂ + H₂O

Here, ethanol has been converted into ethane by removing H2O

2. Dehydrohalogenation of Alkyl Halides:

General formula for these reactions is = Alkyl halide → Alkene + HX

Reagents like heat and Strong alcoholic KOH/NaOH are required for the desired reaction.

Example:

CH₃CH₂Br + alc. KOH → CH₂=CH₂ + HBr

3. Partial Hydrogenation of Alkynes:

Here, Lindlar’s catalysts (for cis-alkenes) and NA/NH3 (for trans alkenes) are used for conversion of alkynes into alkenes.

General formula is given by: RC≡CR’Lindlar’s​RC=CR’ (cis)

Example:

CH≡CH → CH₂=CH₂

     4. Cracking Alkanes:

Alkanes, when decomposed at higher temperatures in the absence of air can also break down to form lighter weighted alkenes.

​C₁₀H₂₂ → C₂H₄ + C₈H₁₈

     5. From Dihalides:

Using Zn as the reagent while following the process of dehalogenation, alkenes can be prepared easily. Example:
CH₂Br–CH₂Br → (Zn) → CH₂=CH₂

Alkenes possess some unique properties because of their double carbon carbon bond. Let’s break them down:

Physical Properties:

• Alkenes only dissolve in organic solvents such as benzene instead of causing addition reaction with water. This property is what leads to them being called non-polar compounds.
• Due to a strong van der Waals forces, the boiling point of alkenes increases with change in the molecular size (e.g., ethene: -104re, propene: -47 řC).
• Theyre gases $(C_2-C_4)$ , liquids $(C_5-C_{17})$ , or solids ( $C_{18}$ and above) at room temperature.

Chemical Properties:

Alkenes are reactive because of their $\mathrm{C}=\mathrm{C}$ double bond. Here are the key reactions for JEE MAINS:

• Addition Reactions: The double bond breaks, and atoms add across it:

$C{H}_2=C{H}_2+H_2->\left[Pd\right]C{H}_3-C{H}_3$

            Ethene forms ethane (hydrogenation). Alkenes also add halogens:

                       $C{H}_2=C{H}_2+B{r}_2->C{H}_{2}Br-C{H}_{2}Br$

Ethene can also form 1,2-dibromoethane. This reaction is used to test for unsaturation (the red color of Br 2 disappears).

• Addition of HX: Following the Markovnikovs rule, alkenes can also react with hydrogen halides (H adds to the carbon with more hydrogens):

$C{H}_{3}CH=C{H}_2+HBr->C{H}_{3}CHBr-C{H}_3$

            Propene will form 2-bromopropane.

• Combustion: Alkenes can burn in oxygen too. This burning leads to formation of $C{O}_2$ and $H_{2}O$ :

${\mathrm{C}}_2{\mathrm{H}}_4+3{\mathrm{O}}_2\mathrm{}\mathrm{}\mathrm{}->2{\mathrm{C}\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}$

Click Here: Chemistry NCERT Chapter 9: Hydrocarbons

Let us dive into these detail oriented notes which will help ease our preparation and thoroughly understand this dynamic world of chemistry.