14.2 Alkenes Flashcards
Producing alkenes
1) Elimination of HX from a halogenoalkane by ethanolic NaOH and heat
2) Dehydration of an alcohol, by using a heated catalyst (AI2O3) or a concentrated acid/pieces of porous pot or pumice
3) Cracking of a longer chain alkane
Elimination of HX from a halogenoalkane by ethanolic NaOH and heat
Halogenoalkane + NaOH (ethanol) with heat= Alkene + water + NaX
Rxn Conditions:
- strong base (NaOH, KOH)
- Ethanol solvent
- High temp
2 Different reactions depending on NaOH
1) NaOH (ethanol) - elimination reaction to form alkene + NaX + H2O
2) NaOH (aq)- nucleophilic substitution in which an alcohol + NaX is produced
Dehydration of an alcohol, by using a heated catalyst (AI2O3) or a concentrated acid/pieces of porous pot or pumice
Alcohol = Alkene + Water
- the smaller alkenes are all gases at room temp and can be collected over water
Reactions of alkenes
Electrophilic addition
1) Hydrogenation
2) Steam
3) Hydrogen halide (HX)
4) Halogens (X2)
- reactions shown in notes
Oxidation
- Acidified potassium permanganate (KMnO4) [0]
Cold, dilute
- pale purple to colourless solution
- diol produced (2 OH groups)
Hot, concentrated
- harsh conditions, therefore C=C completely breaks
- OH groups in diol are further oxidised to ketones, aldehydes, carboxylic acids and ketones
- product depends on what is bonded to C=C
Addition polymerisation
- in notes
Test for unsaturation
- unknown compound is shaken with bromine water
- if the compound is unsaturated (contains C=C) addition rxn will occur and solution will go from orange to colourless
Electrophilic addition of HBr, Br2
in notes
Inductive effect
- primary carbocation, secondary and tertiary
- primary carbocation is the least stable
- tertiary carbocation is most stable as they have 3 electron-donating alkyl groups which energetically stabilize the carbocation
- due to positive charge on the C atom they are electrophiles
Markovnikov’s rule
- in a electrophilic addition reaction of a hydrogen halide (HX) to an alkene, the halogen ends up bonded to the most substituted carbon atom, the hydrogen to the least substituted
- applies to unsymmetrical alkenes
Major and minor product
Major- favours Markovnikov’s rule as halogen is bonded to most substituted carbon atom
Minor- favours Anti-Markovnikov’s rule as halogen is bonded to least substituted carbon atom
Commonly asked example with HBr
in notes