Alkenes

Question 1

Alkenes General formula

Answer 1

C_nH_2n

Question 2

First 4 in Homologous series

Answer 2

1. Ethene
2. Propene
3. But-1-ene
4. Pent-1-ene

Question 3

Why BE(C=C) < 2BE(C-C)

Answer 3

π bond weaker than σ bond
=> Side-on overlap less effective than head-on overlap of orbitals

Question 4

Isomerism in Alkenes - Why Cis-Trans?

Answer 4

C=C bond restricts rotation, each C atom in C=C bonded to 2 different groups

Question 5

Relative Stabilities of Cis vs Trans Isomer

Answer 5

Trans MORE STABLE than Cis
=> Steric Strain (electron cloud repulsion) arising from crowding between 2 groups (In Cis Isomer)
=> especially so when groups are large

Question 6

Trans vs Cis
Boiling point

Answer 6

B.P Cis > B.P Trans
=> Cis is slightly polar hence both id-id & pd-pd interactions between molecules
=> Link to energy required & conclude

Question 7

Trans vs Cis
Melting point

Answer 7

M.P Trans > M.P Cis
=> Molecules of Cis pack poorly in Solid lattice (2 bulky alkyl on same side)
=> Larger distance between molecules
=> Weaker IMFOA
=> Link to energy required & conclude

Question 8

Reactivity of Alkenes

Answer 8

HIGH reactivity
=> Presence of C=C (electron-rich region)
=> Able to attract electrophiles

Question 9

Reactions of Alkenes

Answer 9

1. Electrophilic Addition
2. Oxidation (Strong/ Mild)
3. Reduction
4. Combustion

Question 10

4 types of Electrophilic Addition

Answer 10

1. E.Addition of H-X
2. E.Addition of X₂ in CCL₄
3. E.Addition of X₂ in H₂O
4. E.Addition of water/ steam (complicated one)

Question 11

E.Addition of H-X
RNC + special outcome & reason

Answer 11

HX (g) dry, rtp, dark

FORMS RACEMIC MIXTURE
=> carbocation in step 1 of mechanism is trigonal planar
=> Nucleophile attacks +charged carbocation from either side of the trigonal plane
=> with equal probability
=> forming 2 enantiomers in equal proportions
=> racemisation

Question 12

E.Addition of water/ steam (complicated one)
RNC

1. Lab method
2. Industrial method

Answer 12

1. Lab method
   - Cold conc H₂SO₄
   - H₂O, heat
2. Industrial method
   - H₃PO₄ on silica
   - High T & P (700 atm, 300 deg C)

Question 13

E.Addition of X₂ in CCL₄

RNC + 2 Unfavourable halogens

Answer 13

X₂ in CCL₄, dark

F ₂ (Explosive), I ₂ (Thermodynamically unfeasible) => Seldom used

Question 14

Step 1 of E.A mechanism

Answer 14

SLOW STEP
Br₂ molecule approaches C₂H ₄ molecule
=> π electron cloud of C₂H ₄ interacts with Br₂ molecule
=> like charges repel
=> Br₂ electron cloud distorted
=> Br-Br bond polarised
=> Partial +ve/-ve formed across Br-Br

Question 15

E.Addition of X₂ in H₂O

RNC + Major/Minor Pdt + Reason

Answer 15

X₂ (aq), dark, rtp
=> Major: Forms Halohydrin (H₂O in excess => more avail for nucleophilic attack)
=> Minor: Forms di-brominated alkane (Br₂ avail in very small proportions)

Question 16

Oxidation of Alkenes (MILD) => distinguishing test for alkenes

Reaction pattern + RNC + observation

Answer 16

KMnO₄ (aq), NaOH (aq), cold
Observation: purple KMnO₄ decolorised + Brown black ppt of MnO₂
Reaction pattern => Add 2 -OH Across C=C

Question 17

Oxidation of Alkenes (STRONG) => Oxidative cleavage

Reaction pattern + RNC + observation

Answer 17

KMnO₄ (aq), H₂SO₄ (aq), HEAT
Observations: Purple KMnO₄ decolorised
Reaction pattern
1. Cleave C=C become C=O
2. Replace any H attached to C=O with OH

Question 18

Distinguishing tests tips

Answer 18

Performed using test tubes, different observable change, NO complicated equipment
1. RnC
2. +ve/ -ve observations
3. Chemical test for gas evolved
4. Eqns if required by qn

Question 19

Preparation of Alkenes: Elimination reaction

Answer 19

1. Elimination of HX from halogenoalkanes
   - KOH in ethanol, heat
2. Elimination of H₂O from Alcohols
   - Excess conc

Question 20

Preparation of Alkenes: Elimination reaction

Answer 20

1. Elimination of HX from halogenoalkanes
   - KOH in ethanol, heat
2. Elimination of H₂O from Alcohols
   - Excess conc H₂SO₄ or H₃PO₄, heat

Elimination follows Zaitsev’s rule
- Preferred pdt is more substituted alkene (More alkyl groups attached to C=C)