Structure and Reactivity of Alkenes: MDCAT Chemistry notes
Structure and Reactivity of Alkenes for MDCAT: the C=C bond, electrophilic addition, Markovnikov's rule, hydration, bromine test and ozonolysis products.
Structure of the double bond
Each carbon of C=C is $sp^2$ hybridized. The double-bond carbons and the four atoms attached to them lie in one plane with angles of about 120°. The double bond is one σ bond plus one π bond, and it is shorter (1.34 Å) than a C–C single bond (1.54 Å). Rotation about it is restricted, which is why geometric isomers exist.
Why alkenes are reactive
The π electrons lie above and below the plane, loosely held and exposed. The π bond is weaker than the σ bond. So alkenes attract electrophiles and undergo electrophilic addition: the π bond breaks and two new σ bonds form. This makes alkenes much more reactive than alkanes.
Markovnikov's rule
When an unsymmetrical reagent HX adds to an unsymmetrical alkene, the H goes to the carbon that already has more hydrogens. The X therefore goes to the more substituted carbon. The reason is that $\mathrm{H^+}$ adds first to give the more stable carbocation, following the order 3° > 2° > 1°.
Worked examples
- $\mathrm{CH_2{=}CH{-}CH_2{-}CH_3 + HBr \to CH_3{-}CHBr{-}CH_2{-}CH_3}$ (2-bromobutane)
- 2-butene $\mathrm{CH_3CH{=}CHCH_3}$ + HBr gives 2-bromobutane. The alkene is symmetrical, so either carbon gives the same product, and 3-bromobutane is not a correct name.
- Isobutylene $\mathrm{(CH_3)_2C{=}CH_2}$ + HBr gives tert-butyl bromide, $\mathrm{(CH_3)_3CBr}$.
- Propene + $\mathrm{H_2O}$ (with $\mathrm{H_2SO_4}$) gives propan-2-ol, via the secondary carbocation.
Other addition reactions
| Reagent | Product | Use |
|---|---|---|
| $\mathrm{H_2}$ / Ni (or Pt, Pd) | Alkane | Hydrogenation |
| $\mathrm{Br_2}$ (in water or $\mathrm{CCl_4}$) | 1,2-dibromide | Test for a double bond: the red-brown colour disappears |
| Cold dilute alkaline $\mathrm{KMnO_4}$ | 1,2-diol (glycol) | Hydroxylation: ethene gives ethylene glycol; also Baeyer's test (the purple colour disappears) |
| $\mathrm{O_3}$, then Zn / $\mathrm{H_2O}$ | Aldehydes and/or ketones | Ozonolysis, which locates the double bond |
Ozonolysis: predicting products
Cut the C=C bond and put =O on each carbon.
- But-2-ene gives 2 $\mathrm{CH_3CHO}$, only one kind of aldehyde.
- 1-pentene gives HCHO + butanal, which are two different aldehydes.
- 2-methylpropene gives acetone + HCHO.
- 2,3-dimethyl-2-butene gives 2 acetone, a ketone only.
Sodium periodate ($\mathrm{NaIO_4}$) splits a 1,2-diol at the C–C bond between the two OH-bearing carbons. This is called oxidative cleavage.
Common MDCAT traps
- Alkenes undergo electrophilic addition. Electrophilic substitution is typical of benzene.
- In Markovnikov addition, the halogen goes to the carbon with fewer H atoms.
- Hydroxylation (cold $\mathrm{KMnO_4}$) gives glycol. Hydration gives an alcohol.
- Only a symmetrical alkene with H on each double-bond carbon gives a single aldehyde on ozonolysis.
- Bromine water tests for unsaturation, not for aldehydes or ketones.
Quick revision
- Alkene carbons are $sp^2$ with a 1.34 Å double bond.
- The π electrons attract electrophiles.
- Propene + $\mathrm{H_2O}$ gives propan-2-ol.
- Isobutylene + HBr gives tert-butyl bromide.
- Decolourizing bromine water shows C=C is present.