Alkenes | Organic Chemistry 2
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
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
nC = number of carbons
nN = number of nitrogens
nH = number of hydrogens
nX = number of halogens (F, Cl, Br, I)