Alcènes

Question 1

Carbon-carbon double bond signifies what functional grp?

Answer 1

Alkene

Question 2

If more than 1 C=C, add what to the stem (eg ‘but’ in butene) ?

Answer 2

letter ‘a’
eg 2 C=C in butene at C1, C3, so
buta-1,3-diene

where ‘a’ means more than 1 C=C bond, 1,3 means location of double bond (w smallest no.) & di means 2 C=C bonds

Question 3

Define electrophile

Answer 3

e- deficient species that can act as e- pair acceptors
δ+ δ-
eg. Br+, NO2+ and δ+H of H-Cl
(anions) (+ve dipole)
Since e- philes r e- deficient, they r readily attracted to & react w/attack e-rich species

Question 4

Define nucleophiles

Answer 4

e-rich species that can act as e- pair donors
..
eg. OH-, Br- and δ-N of NH3
(anion) (δ- atom w l.p.)
Since nucleophiles r e- rich, they r readily attracted to & attack/react w e- deficient species

Question 5

Describe electrophilic addition mechanism using bromine (liquid) and ethene as example

Answer 5

(Remember to draw full displayed formula)
Step 1:
CH2=CH2 ,full arrow from C=C to Br-Br + full arrow from δ+Br to δ-Br
(slow)
—-> +
CH2BrCH2 + :Br-

Step 2:
+
CH2BrCH2 + :Br-
, full arrow from :Br- to carbocation
(fast)
—->
CH2BrCH2Br

Question 6

Describe electrophilic addition mechanism using bromine (aq) and ethene

Answer 6

(diff main pdt)
overall eqn
CH2=CH2 + Br2(aq) + H2O
—-> CH2 - CH2 + HBr
| |
Br OH

*CH2BrCH2 still formed but minor pdt

Step 1: same slow step as that for Br(l)

Step 2:
CH2 - CH2 + H2O
| +
Br
, full arrow from l.p. from O to carbocation + full arrow from 1 O-H bond to O in H2O

——> CH2 - CH2 + H+
| |
Br OH
(the H+ & Br- fr step 1 form HBr)

Question 7

What are the reagent, condition, observation of electrophilic addition of halogen

Answer 7

Type of rxn: Electrophilic Add n

R: X2 in CCl4 (*other inert organic solvent oso can) OR Br2(l)

C: room temp, dark (prevent FRS)

O:
- Br2 (in CCl4): decolour n of orange-red bromine
- Br2 (l): decolour n of reddish-brown bromine

• Cl2 (in CCl4): decolour of pale yellow sol n
  (colour oso in data booklet)

Question 8

What are the reagent, condition of electrophilic addition of hydrogen halide?

Answer 8

Type of rxn: Electrophilic Add n

R: dry HX(g)

C: room temp

Question 9

What are the reagent, condition of electrophilic addition of steam?

Answer 9

Type of rxn: Electrophilic add n

R: Steam/H2O(g)

C:
- (lab method) cold concentrated H2SO4 followed by heating w water
OR (must memorise BOTH)
- (industrial method) heat at high Pa w concentrated H3PO4 as catalyst

Question 10

What is the important concept in Markonikov’s Rule?

Answer 10

Major pdt is the one formed when e- phile attaches itself to unsaturated C atom w LARGER no of H atoms

Question 11

What are the reagent, condition of catalytic hydrogenation

Answer 11

type of rxn: reduction

R: H2 gas

C:
- Ni catalyst in presence of high temp & high Pa
OR
- Pd/Pt catalyst at r.t.p.

Question 12

What are the reagent, condition and observation of mild oxidation (partial C=C bond cleavage)?

Answer 12

type of rxn: oxidation

R & C: cold alkaline/acidified KMnO4

Observation: decolour n of purple KMnO4 sol n & form n of brown ppt (MnO2)

Question 13

What are the reagent, condition and observation of strong/vigorous oxidation (complete C=C bond cleavage)?

Answer 13

Type of rxn: oxidation

R & C: KMnO4, dilute H2SO4, heat

Observation: decolour n of purple KMnO4 sol n (+ effervescence of CO2(g) if terminal alkene present)

Question 14

What is terminal alkene?

Answer 14

When there is C=C bond at the ‘end’ of molecule

Question 15

What are the steps to obtain pdt fragments formed by strong oxidat n of alkene?

Answer 15

1. Cleave =C=/=C= to form =C=O + O=C=
2. Add O to all H atoms DIRECTLY attached to C=C
3. If unstable intermediates below form,
   -OH // O
   O=C and HO - C
   -OH \ C= O
   HO /

it decomposes to CO2 + H2O

Question 16

What are the steps to deduce structure of alkene after oxidative cleavage?

Answer 16

1. Remove all O atoms
2. Recombine fragments by ‘joining at double bond’

Question 17

What are the chemical tests for alkene functional grp? What are the observations?

Answer 17

1. Add Br2 in CCl4 at rtp in the dark
   O: Decolour n of orange-red bromine sol n
   NOTE: Br2(aq) can oso b used. Decolour n of orange Br2(aq) is observed
2. Add COLD alkaline KMnO4 sol n
   O: Decolour n of purple KMnO4 sol n & brown ppt (MnO2) formed
3. Add HOT acidified KMnO4 sol n (strong oxidation)
   O: Decolour n of purple KMnO4 sol n (and effervescence of CO2 if terminal alkene present)

Question 18

What are the relative stability of carbocations? Why?

Answer 18

tertiary>secondary>primary>methyl carbocation

bcos need to be as stable as possible (can b attained by dispersion of charge)
greater no of e- donating alkyl grp means greater:
- e- donating effect
- extent of dispersal of +ve charge on carbocation/reduct n of intensity of +ve charge on carbocation
- stability of carbocation formed => form major pdt

Question 19

What rxn is used for preparing alkenes?

Answer 19

Elimination

Question 20

What are the reagent, condition of elimination of alcohols?

Answer 20

R & C:
- excess concentrated H2SO4, heat
OR
- Al2O3 catalyst, heat
OR
- excess concentrated H3PO4, heat

Question 21

What are the reagent, condition of elimination of halogenoalkanes

Answer 21

R & C:
- alcoholic KOH/alcoholic NaOH, heat

Question 22

Explain how markonikov’s rule works

Answer 22

• secondary, tertiary carbocation more stable than primary carbocation
  (Or tertiary more stable than secondary)

Bcos
- more e- donate grp to disperse positive charge, so more stable carbocation
- (if got adjacent benzene) positive charge dispersed due to delocalisation into benzene ring, so more stable carbocation