13 Alkenes

Question 1

Electrophilic Addition

Answer 1

1. Electrophilic attack on electron-rich alkene by partial positive end of electrophile (X2)
2. X radical forms covalent bond with the carbocation
   Reagents and conditions: HX, room temp OR X2 dissolved in CCl4, room temperature

Question 2

General formula of alkenes

Answer 2

C(n) H(2n)

Question 3

Cis-trans isomerism

Answer 3

Restricted rotation about C=C and 2 different groups/atoms attached to each carbon atom of C=C bond
Cis-isomer: identical groups on same side
Trans-isomer: different sides

Question 4

Stability of alkenes

Answer 4

1. More alkyl groups attached to C=C, more substituted and more stable
2. Trans-isomer is more stable than cis-isomer

Question 5

Boiling/melting points of alkenes

Answer 5

1. Number of C atoms increases -> number of electrons increases -> stronger id-id interactions -> increase in boiling/melting point
2. Straight chain higher boiling points than branched isomers (larger surface area -> more extensive id-id interactions)
3. Cis-isomer higher boiling points than trans-isomer (cis-isomers have polarity)
4. Cis-isomer lower melting point than trans-isomer (cis-isomer have lower symmetry, poor packing of cis-isomer -> intermolecular forces not as extensive)

Question 6

Elimination of H2O

Answer 6

1. Heat over Al2O3 (industrial method)
2. Heat with excess conc H2SO4 (lab method)

Question 7

Elimination of HX

Answer 7

Heat under reflux
Ethanolic KOH/NaOH

Question 8

Saytzeff rule

Answer 8

More alkyl groups attached to C=C, more substituted and more stable the alkene

Question 9

Reduction of alkene

Answer 9

Ni catalyst, H2 (g)
Heat

Question 10

Electrophilic addition of HX

Answer 10

HX (g)
Room temperature
Order of reactivity: HCl<HBr<HI (due to bond strength of HX, stronger bond ->less reactive)

Question 11

Markonikov’s rule

Answer 11

For electrophilic addition, electrophile adds to less substituted C atom in the C=C
Only for unsymmetrical alkenes

Due to stability of carbocation intermediate
Alkyl groups are weakly electron-donating, which spreads of the positive charge of the carbocation

Question 12

Electrophilic addition of halogen

Answer 12

X2 dissolved in CCl4, room temperature
When dissolved in water, H2O molecules react with carbocation intermediate

Question 13

Electrophilic addition of steam

Answer 13

H2O (g), H3PO4 catalyst
Heat at high pressure

Question 14

Oxidation

Answer 14

1. cold, alkaline KMnO4 (forms diol)
2. hot, acidified KMnO4 (cleavage of C=C)