Alkenes | Organic Chemistry 2

The alkenes are studied in this chapter: the naming and properties of alkenes, the pi bond in chemistry, the preparation of alkenes with the Saytzev and the Hofmann rules, the degree of unsaturation of a chemical compound.

Naming the Alkenes

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

1) Find the longest chain that includes C=C
2) Indicate the location of C=C by number, starting at the end closer to C=C
3) Add substituents and their positions (with the lowest possible number) as prefixes
4) Identify any stereoisomers (cis-trans for disubstituted alkenes or E,Z for alkenes with 3 or 4 substituents)
5) Give the -OH precedence over C=C: alkenols

 

Pi Bond

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

C=C of alkenes:

1 σ bond ⇒ formed by 2 sp2 orbitals of C atoms
1 π bond ⇒ formed by 2 p orbitals of C atoms

π bond is weaker (higher energy, more reactive) than σ bond.

 

Geometry: sp2 hybridized ⇒ trigonal geometry
⇒ ~ 120° angles (slightly smaller for H-C-H angles and slightly larger for H-C-C angle).

Bond Lengths: C=C are shorter than C-C.

Properties of Alkenes

Boiling and Melting points: similar to corresponding alkanes.

Geometrical isomers: no interconvertion by rotation (π bonds are fixed)
⇒ 2 geometrical isomeric forms: E and Z isomers.
If the two groups with the higher priorities (Cahn-Ingold-Prelog rules) are on the same side ⇒ Z isomer (Zusammen in deutsch). If they are on the opposite side ⇒ E isomer (Entgegen).

 

 

Acidity: extremely weak base (pKa = 40).

NMR: 1H δ ~ 5-7 ppm; 13C δ ~100-160 ppm.
In 1H NMR, each H on the C=C are different ⇒ different characteristic 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

 

Preparation of Alkenes

2 ways can be used to prepare alkenes: elimination reactions or alcohol dehydration.

Elimination reactions (generally E2 reactions)
 


Saytzev rule: Non bulky bases form thermodynamic product
Hofmann rule: Bulky bases form kinetic product

 

 

- Alcohol dehydration
 


 

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

Degree of Unsaturation

Unsaturation = ring or multiple bond (double bond = 1 unsat. / triple bond = 2 unsat.).
 

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

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

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