Alkenes Flashcards

1
Q

Alkenes General formula

A

CnH2n

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2
Q

First 4 in Homologous series

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

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

A

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

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4
Q

Isomerism in Alkenes - Why Cis-Trans?

A

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

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5
Q

Relative Stabilities of Cis vs Trans Isomer

A

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

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6
Q

Trans vs Cis
Boiling point

A

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

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7
Q

Trans vs Cis
Melting point

A

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

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8
Q

Reactivity of Alkenes

A

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

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9
Q

Reactions of Alkenes

A
  1. Electrophilic Addition
  2. Oxidation (Strong/ Mild)
  3. Reduction
  4. Combustion
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10
Q

4 types of Electrophilic Addition

A
  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)
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11
Q

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

A

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

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12
Q

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

  1. Lab method
  2. Industrial method
A
  1. Lab method
    - Cold conc H2SO4
    - H2O, heat
  2. Industrial method
    - H3PO4 on silica
    - High T & P (700 atm, 300 deg C)
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13
Q

E.Addition of X2 in CCL4

RNC + 2 Unfavourable halogens

A

X2 in CCL4, dark

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

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14
Q

Step 1 of E.A mechanism

A

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

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15
Q

E.Addition of X2 in H2O

RNC + Major/Minor Pdt + Reason

A

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)

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16
Q

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

Reaction pattern + RNC + observation

A

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

17
Q

Oxidation of Alkenes (STRONG) => Oxidative cleavage

Reaction pattern + RNC + observation

A

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

18
Q

Distinguishing tests tips

A

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

19
Q

Preparation of Alkenes: Elimination reaction

A
  1. Elimination of HX from halogenoalkanes
    - KOH in ethanol, heat
  2. Elimination of H2O from Alcohols
    - Excess conc
20
Q

Preparation of Alkenes: Elimination reaction

A
  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)