Alkene

The simplest alcheno of all is etheno.

The alkenes are unsaturated hydrocarbons that have a carbon-carbon double bond in their molecule. It can be said that an alkene is an alkane that has lost two hydrogen atoms, resulting in a double bond between two carbons. Cyclic alkenes are called cycloalkenes.

Formerly they were called olefins given the properties that their simplest representatives, mainly ethene, had to react with halogens and produce oils.

Formulation and nomenclature of alkenes

The general formula for an open-chain alkene with a single double bond is C_nH_2n. For each additional double bond there will be two fewer hydrogen atoms than indicated in this formula.

Traditional names

As with other organic compounds, some alkenes are still known by their non-systematic names, in which case the systematic -ene ending is replaced by -ylene, as is the case with ethene which is sometimes called ethylene, or propene for propylene.

Systematic nomenclature (IUPAC)

1. Name the main hydrocarbon: Find the longest carbon chain containing the double bond, not necessarily the largest, by placing the lowest numbered locants on the double bonds, numbering the carbon atoms in the chain starting at the end closest to the double bond. NOTE: If when enumerating from left to right and from right to left, the locants of the unsaturations are the same, it is sought that the double bonds have a lower position or lower locator.

2. If the main chain has the same substituents on the same carbon atom, separating the locating numbers that are repeated on the atom by commas, these are separated by a dash from the prefixes: di, tri, tetra, etc. Respectively to the number of times that the substituent is repeated.

3. Substituents are written in alphabetical order with their respective chemical locant.

4. If there are several equal branched substituents in the main chain, the locator number is placed in the main chain, separated by a hyphen, and the prefix corresponding to the number of times it is repeated with the prefixes is written: bis, tris, tetrakis, pentakis, etc.. Followed by a parenthesis within which the complex substituent with the ending -IL is named.

5. Once all of the above has been done in relation to the substituents, the double bond locator number is placed in the main chain separated by a hyphen, followed by the name according to the number of carbon atoms, replacing the ending -ane with the suffix -ene.

6. If more than one double bond is present, it is named indicating the position of each of the double bonds with its respective locating number, the root of the name of the alkene from which it comes is written, followed by a quantity prefix: di, tri, tetra, etc. and using the suffix -ene. Ex: -diene, -triene and so on.

Formula	Recommendations IUPAC-1979	Recommendations IUPAC-1993
	locator - prefix number C atoms (finished in -en)	prefix number atoms C - locator -no
1-pent	1-pent	pent-1-no
cyclehex-1-no numbered	1-cyclohexeno	cyclehex-1-no
but-2-eno	2-butno	but-2-eno
hept-3-eno	3-heptene	hept-3-eno
buta-1,3-dieno	1.3-butadieno	buta-1,3-dieno
octa-1,3,6-triene	1,3,6-octatriene	octa-1,3,6-triene
cyclocta-1,3,5,7-tetraene	1,3,5,7-cycloctatetraene	cyclocta-1,3,5,7-tetraene
3-methyl -but-1-no	3-methyl-1-butene	3-methyl-but-1-no

Electronic structure of the C=C double bond

We will use ethene as an example of a compound with a C=C double bond. The double bond has two components: the σ bond and the π bond. The two carbon atoms that share the bond have a sp² hybridization, hybridization resulting from the mixing of one 2s orbital and two 2p, which leads to the formation of three sp² orbitals of trigonal planar geometry. When these sp² orbitals combine, the shared electrons form a σ bond, located between both carbons.

In the first figure you can see the methyl radical, with an sp² orbital that bonds a hydrogen atom to carbon. The second figure shows the formation of the π bond (line of points); which is formed by the overlap of the two 2p orbitals perpendicular to the plane of the molecule. In this type of bond, the electrons are delocalized around the carbons, above and below the molecular plane.

Binding energy

Energetically, the double bond is formed by editing two types of bond, the σ and the π. The energy of these bonds is obtained from the calculation of the overlap of the two constituent orbitals, and in this case the overlap of the sp² orbitals is much greater than the orbitals p (the first creates the σ bond and the second the π) and therefore the σ component is much more energetic than the π. The reason for this is that the density of the electrons in the π bond are further away from the nucleus of the atom. However, even though the π bond is weaker than the σ bond, the combination of the two makes a double bond stronger than a single bond.

Summary

Alkenes can be synthesized by the following reactions:

• By changing functional group

Dehydrohalogenation

CH3CH2Br + KOH → CH2=CH2 + H2O + KBr

Dehydration

Water removal from alcohol, for example:

CH3CH2OH + H2SO4 → CH₃CH₂OSO₃H + H₂O → H₂C=CH₂ + H₂SO₄ + H₂O

Also by the reaction of Chugaev and the reaction of Greek.

Disabling

BrCH2CH2Br + Zn → CH₂=₂ + ZnBr₂

Pyrolysis (heated)

CH3(CH2)4CH3 → CH₂=₂ + CH3CH2CH2CH3

Bamford-Stevens Reaction

Barton-Kellogg reaction

• By carbon-carbon bond formation

Wittig reaction

Julia's Olefination

Olefining of Horner-Waddsworth-Emmons

• Pericyclical reactions

• By metal coupling reactions:

Heck reaction

Suzuki reaction

Hiyama coupling

Stille coupling

Physical properties

The presence of the double bond slightly modifies the physical properties of alkenes compared to alkanes. Of these, the boiling temperature is the one that changes the least. The presence of the double bond is most noticeable in aspects such as polarity and acidity.

Polarity

Depending on the structure, a weak dipole moment may appear. The alkyl-alkenyl bond is polarized in the direction of the atom with sp² orbital, since the component s of an sp² orbital is larger than that of an sp³ orbital (this could be interpreted as the ratio of s to p in the molecule, being 1:2 in sp² and 1:3 in sp³, although that idea is merely intuitive). This is due to the fact that the electrons located in hybrid orbitals with a greater s component are more tied to the nucleus than the p ones, therefore the sp^{2 orbital} is slightly attracting electrons and there is a net polarization towards it. Once we have net bond polarity, the geometry of the molecule must allow a net dipole moment to appear in the molecule, as shown in the figure below.

The first molecule is cis and we have a net dipole moment, but the second trans, despite having two slightly polarized bonds, the net dipole moment is zero when both are canceled dipole moments.

Acidity

Alkenyl carbon is more acidic than alkanes, also due to the polarity of the bond. Thus, ethane (alkane) has a pK_a of 50 (or a K_a of 10^-50) versus pK_a = 44 for ethene. This fact is easily explained considering that, when a proton is detached from the molecule, a remaining negative charge remains, which in the case of ethene is more easily delocalized in the π and σ bond than in the simple σ bond that exists in an alkane. In any case, its acidity is lower than that of alcohols or carboxylic acids.

Reactions

Alkenes are more reactive than alkanes. Its characteristic reactions are those of addition of other molecules, such as hydrogen halides, hydrogen and halogens. They also undergo polymerization reactions, which are very important industrially.

1. Hydrohalogenation: refers to the reaction with hydrogen halures forming halogened alkanes in the CH mode₃-CH₂=₂ + HX → CH₃CHXCH₃. For example, halogenation with HBr acid:

These reactions must follow the Markovnikov Rule of double bonds.

1. Hydrogenation: refers to catalytic hydrogenation (using Pt, Pd, or Ni) forming alcanos in CH mode₂=₂ + H₂ → CH₃CH₃.
2. Halogenation: refers to the reaction with halogens (represented by the X) of the CH mode₂=₂ + X₂ → XCH₂CH₂X. For example, bromine halogenation:

1. Polymerization: Form polymers mode n CH₂=₂ → (-CH₂-CH₂-)_n polymer, (polyethylene in this case).