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Stabilities of Alkenes, sp2 Hybridization in Alkenes

The reaction begins with a mixture of hydrogen gas and an alkene (a double bond between carbon and carbon).

Stabilities of alkenes

The reaction begins with a mixture of hydrogen gas and an alkene (a double bond between carbon and carbon). Alkenes have a pi bond acting as nucleophiles, which is formed by electrons in the alkene joining a hydrogen atom in H2. The carbon with a negative formal charge is the one that did not receive hydrogen after the pi bond breaks. This intermediate is called carbocation. As two electrons remained in the H-H sigma bond, the (unreacted) hydrogen now forms a hydride anion. A bond is formed with the positively charged carbon by the electrons of the negatively charged hydride ion. Exothermic results from this reaction. Without a catalyst, the reaction is slow.

The catalyst

A catalyst reduces the activation energy of a reaction to increasing the reaction rate. However, the catalyst is used to speed the reaction up sufficiently to be observed within a reasonable time frame, despite not being consumed in the reaction. Most commonly, platinum, palladium, and nickel are used as catalysts for alkene hydrogenation. These metal catalysts serve as surfaces on which reactions occur. By placing them near each other, facilitates interaction between the reactants. Due to this catalyst, the two hydrogen atoms bond to the metal instead of forming a sigma bond with H2. As the bond between the alkene and metal weakens, it also weakens.

As long as both carbons are bound to the metal catalyst, hydrogen atoms can easily attach to the already double-bonded carbons. The hydrogen atoms are exposed to only one side of the alkene because both reactants are bonded to the catalyst. Adding hydrogen atoms to a molecule on the same side of the molecule is syn-addition.

Heats of hydrogenation

The energy associated with hydrogenating an alkene molecule can be used to determine the stability of that molecule. This reaction releases the energy contained in the molecule's double bond due to the breaking of the double bond. Very accurate measurement of the heat of hydrogenation is possible with this tool. Alkenes usually have a calorific value of -30 kcal/mol. Energy is simply measured by stability. Less energetic molecules tend to be more stable. Alkenes with more substitutes tend to be more stable than those with fewer due to hyperconjugation. Their hydrogenation heat is lower. Various substituents show differing levels of stability for alkenes:

Because of steric hindrance, trans isomers of disubstituted alkenes are more stable than cis isomers. Internal alkenes also have greater stability than terminal alkenes.

Sp2 hybridization in alkenes

An sp2 hybridization produces three half-filled sp2 orbitals, resulting in a trigonal planar structure. The shape of alkene molecules, such as ethene (H2C=CH2), is determined by the three orbitals of the molecule. Hydrogen atoms form strong bonds with carbon atoms through a half-filled orbital of the 1s electron. Because sp2 hybridized orbitals from each carbon overlap, a strong σ bond can also form between ethene's two carbon atoms.

An ethene bonding pattern is shown in this diagram. Each carbon of ethene is arranged in a trigonal planar arrangement. As a result of sp2 hybridization, carbon atoms have trigonal planar shapes, but the reason why they are rigid and planar is not yet clear. It would be impossible for ethene to remain planar if only σ bonds were present since rotation could occur round the C–C σ bonds. Therefore, further bonding must take place to 'lock' the alkene into the planar shape.

One lobe above and one lobe below the plane of the molecule results in a pi (π) bond between the two-remaining half-filled 2py orbitals on each carbon. It would be necessary to break this π bond to allow rotation around the C–C bond, so this bond prevents rotation. In the case of a σ bond, there occurs a weaker overlap of the 2py orbitals than in the case of a π bond. Alkenes are also more likely to react than alkanes because their π bonds are easier to break and are more likely to participate in reactions.

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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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