14.2 Alkenes

Question 1

Producing alkenes

Answer 1

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

Question 2

Elimination of HX from a halogenoalkane by ethanolic NaOH and heat

Answer 2

Halogenoalkane + NaOH (ethanol) with heat= Alkene + water + NaX

Rxn Conditions:
- strong base (NaOH, KOH)
- Ethanol solvent
- High temp

Question 3

2 Different reactions depending on NaOH

Answer 3

1) NaOH (ethanol) - elimination reaction to form alkene + NaX + H2O
2) NaOH (aq)- nucleophilic substitution in which an alcohol + NaX is produced

Question 4

Dehydration of an alcohol, by using a heated catalyst (AI2O3) or a concentrated acid/pieces of porous pot or pumice

Answer 4

Alcohol = Alkene + Water
- the smaller alkenes are all gases at room temp and can be collected over water

Question 5

Reactions of alkenes

Answer 5

Electrophilic addition
1) Hydrogenation
2) Steam
3) Hydrogen halide (HX)
4) Halogens (X2)

• reactions shown in notes

Question 6

Oxidation

Answer 6

• 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

Question 7

Addition polymerisation

Answer 7

• in notes

Question 8

Test for unsaturation

Answer 8

• 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

Question 9

Electrophilic addition of HBr, Br2

Answer 9

in notes

Question 10

Inductive effect

Answer 10

• 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

Question 11

Markovnikov’s rule

Answer 11

• 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

Question 12

Major and minor product

Answer 12

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

Question 13

Commonly asked example with HBr

Answer 13

in notes