Alkenes Flashcards
Alkenes General formula
CnH2n
First 4 in Homologous series
- Ethene
- Propene
- But-1-ene
- Pent-1-ene
Why BE(C=C) < 2BE(C-C)
π bond weaker than σ bond
=> Side-on overlap less effective than head-on overlap of orbitals
Isomerism in Alkenes - Why Cis-Trans?
C=C bond restricts rotation, each C atom in C=C bonded to 2 different groups
Relative Stabilities of Cis vs Trans Isomer
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
Trans vs Cis
Boiling point
B.P Cis > B.P Trans
=> Cis is slightly polar hence both id-id & pd-pd interactions between molecules
=> Link to energy required & conclude
Trans vs Cis
Melting point
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
Reactivity of Alkenes
HIGH reactivity
=> Presence of C=C (electron-rich region)
=> Able to attract electrophiles
Reactions of Alkenes
- Electrophilic Addition
- Oxidation (Strong/ Mild)
- Reduction
- Combustion
4 types of Electrophilic Addition
- E.Addition of H-X
- E.Addition of X2 in CCL4
- E.Addition of X2 in H2O
- E.Addition of water/ steam (complicated one)
E.Addition of H-X
RNC + special outcome & reason
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
E.Addition of water/ steam (complicated one)
RNC
- Lab method
- Industrial method
- Lab method
- Cold conc H2SO4
- H2O, heat - Industrial method
- H3PO4 on silica
- High T & P (700 atm, 300 deg C)
E.Addition of X2 in CCL4
RNC + 2 Unfavourable halogens
X2 in CCL4, dark
F 2 (Explosive), I 2 (Thermodynamically unfeasible) => Seldom used
Step 1 of E.A mechanism
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
E.Addition of X2 in H2O
RNC + Major/Minor Pdt + Reason
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)