Alkenes | Organic Chemistry 2

Alkenes are studied in this chapter: name and properties of alkenes, pi bond in chemistry, geometric isomers E and Z, preparation of alkenes by elimination reactions or alcohol dehydration, Saytzev and Hofmann rules, degree of unsaturation of a chemical compound

Nomenclature of Alkenes

Name: the -ane ending of the corresponding alkane is replaced by -ene

1) Find the longest chain that includes the C=C bond
2) Indicate the position of the C=C bond, starting the numbering with the end closest to C=C
3) Add the name of the substituents and their positions (with the smallest number possible) as prefixes
4) Identify the type of stereoisomers (cis-trans for disubstituted alkenes or E,Z for alkenes with 3 or 4 substituents) and add as a prefix
5) Prioritize the -OH functional group over C=C: alkenols


Pi Bond

C=C bond of alkenes:

1 σ bond ⇒ formed from 2 sp2 orbitals of carbon atoms
1 π bond ⇒ formed from 2 p orbitals of carbon atoms

The π bond is weaker (higher energy) and therefore more reactive than the σ bond
C=C bonds are shorter than C-C bonds



The carbons of the C=C bond of an alkene are sp2 hybridized ⇒ trigonal geometry
The angles H-C-H and H-C-C are ~ 120°. H-C-H angles are slightly smaller than H-C-C angles due to the repulsion of hydrogen atoms by electrons in the π bond

Reactivity of alkenes is 'all about the π bond' (mainly: addition reactions)

Properties of Alkenes

Boiling and melting points: similar to corresponding alkanes
Acidity: extremely weak base (pKa = 40)


​​​​​​​​​​​​​​1H δ ~ 5-7 ppm; 13C δ ~ 100-160 ppm
Each H of the C=C bond has different chemical shift and  coupling constants.They can be cis, trans or geminal
​​​​​​​cis coupling constant: between 6-14Hz
trans coupling constant: between 11-18Hz
geminal coupling constant: between 0-3Hz



E and Z isomers:

There is no interconvertion of the substituents of an alkene by rotation: π bonds are fixed ⇒ Alkenes have geometric isomeric forms called E and Z isomers. If the 2 groups with the highest priorities (Cahn-Ingold-Prelog rules) are on the same side: Z isomer (Zusammen = together in deutsch), if they are on the opposite side: E isomer (Entgegen = opposite)


Preparation of Alkenes

Elimination reactions (generally E2 reactions):

Saytzev rule: non bulky bases form the thermodynamic product (product faster to form)
Hofmann rule: bulky bases form the kinetic product (more stable product)



Alcohol dehydration:


Order of Reactivity: primary < secondary < tertiary
Major product: thermodynamic product

Degree of Unsaturation

Unsaturated molecules: 

Molecules containing a double bond (1 unsat.), a triple bond (2 unsat.) and/or a ring (1 unsat.)

Degree of unsaturation:

Calculation that determines the total number of rings and π bonds

Degree of unsaturation = (2nC + 2 + nN - nH - nX) / 2

nC = number of carbons
nN = number of nitrogens
nH = number of hydrogens
nX = number of halogens (F, Cl, Br, I)