Alkenes chemistry Assignment Help




In organic chemistry, an Alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond.[1] The simplest acyclic alkenes, with only one double bond and no other functional groups, form an homologous series of hydrocarbons with the general formula CnH2n.

Alkenes are unsaturated hydrocarbons containing a carbon double bond (C=C) in their molecule. They were earlier known as olefins.

Formula

Alkenes are described by the general formula CnH2n where

n = 1, 2, …..

Alkenes are represented by the general formula,

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R = alkyl group or H

The chemical behavior of alkenes is largely determined by the C = C functional group.

Various members of alkene family are:

C2H4 Ethene (Ethylene) H2C=CH2

C3H6 Propene (Propylene) CH3-CH=CH2

C4H8 Butene (Butylene) CH3-CH3-CH=CH2 or CH3-CH=CH-CH3

Classification

Depending upon whether alkenes have an open chain or cyclic structure, they may be classified as acyclic or cyclic. A few examples of each class are:

Acyclic alkenes

  • Ethylene Ethene CH2 = CH2
  • Propylene Propene CH3CH = CH2
  • 2-Butylene 2-Butene CH3CH=CHCH3

Cyclic alkenes

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Structure of Alkenes:--

Methane

Methane is the simplest carbon. Here, the four half filled orbitals belonging to the valence shell of carbon atom undergoes hybridization, resulting in the formation of four sp3 hybrid orbitals which are directed towards the corners of tetrahedron. Each of the sp3 hybrid orbitals of carbon overlaps with 1s orbital of hydrogen to form carbon-hydrogen sigma bonds. The carbon atom lies at the centre of the tetrahedron while the four hydrogens occupy the four corners or vertices of the tetrahedron.

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Fig 14.1 - Spatial formula of methane

Ethane

In ethane there are two carbon atoms and six hydrogen atoms. Each carbon atom is sp3 hybridized. Thus, there are in all eight sp3 hybrid orbitals on the two carbon atoms. Two hybrid orbitals (one from each carbon) overlap with each other and form a C-C single bond. Other six hybrid orbitals (three on each carbon atom) form six C-H single bonds due to the overlap with the s orbitals of hydrogen atoms as shown.

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Formation of C-C and C-H bonds in ethane

The overlapping of two sp3 hybrid orbitals of the two carbon atoms, or of sp3 hybrid orbital of C and one s orbital of H atom, gives rise to a single bond called as sigma (s) bond.

Structure of Alkenes

In alkenes, two carbon atoms are bonded through a double bond in the parent chain. The formation of a double bond in alkenes is illustrated below by taking the simplest alkene, ethene, as a typical example.

Structure of Ethene

Ethene can be described by the molecular formula C2H4. Each carbon atom in this molecule has three points of attachment and therefore, each carbon atom in ethene is sp2 hybridized.

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The sp2 hybridization of carbon orbital

The three sp2 hybrid orbitals lie in a plane and are inclined to each other at an angle of 120° (see above figure). One of the p orbitals (say 2pz) on each carbon atom is left unhybridized.

The three sp2 hybrid orbitals form three sigma (s) bonds: two with H-atoms and one with the adjacent carbon atom. This results in a planar structure. The two partially filled unhybridized p orbitals of the two carbon atoms overlap side-ways to form a pi bond as shown:

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Fig 14.2 - Side-ways overlap of unhybridized pz orbitals of the two carbon atoms leading to the formation of a p bond

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Simplified representation of ethene molecule

Therefore, in ethene,the carbon-carbon double bond (=) is a combination of a sigma (s) and a pi bond(p). The simple structural formula of ethene is written as shown above.

Structure of Alkynes

In alkynes, the two carbon atoms in the parent chain are bonded through a triple bond. The formation of triple bond in alkynes is illustrated below by taking ethyne (acetylene) as a typical example.

Structure of ethyne (acetylene)

Ethyne (acetylene) can be described by the molecular formula C2H2. As each carbon atom in this molecule has two points of attachments, each carbon atoms in ethyne (acetylene) is sp hybridized.

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sp hybridization of carbon orbitals

The two 'sp' hybrid orbitals are oriented at an angle of 180° to each other, while the two p orbitals (py and pz) on each carbon are left unhybridized. These orbitals are at right angles to each other as well as to the sp hybrid orbitals.

One of the 'sp' hybrid orbitals overlaps with the 'sp' hybrid orbital of the other carbon atom to form a sp-sp sigma bond. The remaining two sp hybrid orbitals of the two carbon atoms overlap with the s orbitals of two hydrogen atoms to form two sigma bonds. The unhybridized py and pz orbitals of two carbon atoms overlap laterally to form two p bonds. The combination of one (sp-sp) sigma bond, and two (p-p) p bonds gives rise to the triple ( img) bond. Two p-electron clouds merge into each other to form a cylindral p-p electron cloud.

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Fig 14.3 - Formation of a triple bond between two carbon atoms due to sideways overlap of the unhybridized orbitals

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