Alkenes

Question 1

Why do alkenes exhibit cis-trans isomerism?

Answer 1

• There is RESTRICTED ROTATION about the C=C double bond due to presence of π bonds
• each carbon atom of the double bond is bonded to TWO DIFFERENT GROUPS

Question 2

Physical properties of alkenes

Answer 2

Refer to alkanes

Question 3

Why are alkenes more reactive than alkanes?

Answer 3

The double bond in alkenes is composed of a STRONG σ BOND and a relatively WEAKER π BOND. The π electrons are exposed and more loosely held than σ electrons, therefore more susceptible to reaction. Loosely held π electrons serve as a source of electrons —> Alkenes act as LEWIS BASES

Question 4

Why are alkenes susceptible to reaction with electrophilic reagents or electrophiles?

Answer 4

C=C is electron rich

Question 5

Describe the mechanism of an electrophilic addition reaction

Answer 5

Question 6

Reagent and condition for the addition of X₂ (Halogens)

State expected observation

Answer 6

Reagent and conditions
- X₂ room temperature
- X₂ dissolved in CCl₄, room temperature

Observation:
- Decolorisation of Br (reddish-brown for (l) and orange red for CCl₄) to form a colourless solution
- Decolorisation of Cl (yellowish-green) to form a colourless solution

Question 7

Reagent and conditions for the addition of HX

Answer 7

HX (g) or HX dissolved in CCl₄, room temperature

Question 8

What affects the stability of the carbocation intermediate in electrophilic addition?

Answer 8

• Relative stability of carbocation depends on how well it DISPERSES THE POSITIVE CHARGE ON THE CENTRAL CARBON ATOM
• The stability of a charged system is INCREASED by an increase in the number of ELECTRON DONATING GROUPS which DISPERSES the positive charge.
• Tertiary carbocation is most stable and primary carbocation is least stable
• ELECTRON WITHDRAWING GROUPS will INTENSIFY the positive charge on the central carbon atom hence destabilising the carbocation.

Question 9

Why is a racemic mixture formed from electrophilic addition reactions?

Answer 9

• In step 2 of the mechanism, there is PLANAR ARRANGEMENT about the carbon with the positive charge.
• The carbocation can be ATTACKED BY THE ELECTROPHILIE FROM EITHER THE TOP OF THE PLANE OR THE BOTTOM OF THE PLANE WITH EQUAL PROBABILITY.
• This leads to the formation of TWO MIRROR IMAGES WHICH ARE NON-SUPERIMPOSABLE
• Since EQUAL quantity of each enantiomer is formed, an OPTICALLY INCATIVE RACMIC MIXTURE is obtained.

Question 10

What is the reagent and condition for the addition of water (Direct hydration) to alkenes

Answer 10

Reagent: H₂O (g)
Condition: Conc. H₃PO₄, high temperature and pressure

Question 11

What is the reagent and condition for the addition of water in presence of sulfuric acid (indirect hydration) to alkenes

Answer 11

Reagent and Condition: Use cold, concentrated sulfuric acid, followed by H₂ (l), heat

Question 12

What is the reagent and condition for the catalytic addition of hydrogen to alkenes (reduction)

Answer 12

Reagent: H₂ (g)
Condition: Pt or Pd catalyst, room temperature and pressure
OR
Ni catalyst, high temperature and pressure

Question 13

What is the reagent and condition of mild oxidation (partial bond cleavage of >C=C<) to form diols?

State the observations

Answer 13

Reagent and Condition: KMnO₄(aq), NaOH(aq), cold
OR
Cold dilute acidified KMnO₄

Observations: Decolorisation of purple KMnO₄ (and formation of brown ppt of MnO₂ in alkaline conditions)

Question 14

What is the reagent and condition for strong oxidation (Total bond cleavage of >C=C<)

State the observations

Answer 14

Under Acidic conditions
-Reagent and Condition: KMnO₄(aq), dil. H_{2/sub>SO4, heat
-Observation: Decolorisation of purple KMnO4 to form a colourless solution (effervescence of CO2 observed if terminal alkene is present)}

Under Alkaline Conditions
Reagent and Condition: KMnO₄(aq) NaOH(aq), heat
Observation: Decolorisation of purple KMnO₄ and formation of brown ppt of MnO₂

Question 15

What is the reagent and conditions for the elimination of water from alcohols

Answer 15

Reagent and Conditions: EXCESS concentrated H₂SO₄, 170°C
OR Al₂O₃ at 400°C

Question 16

What is the reagent and condition of elimination of HX from a halogenoalkane

Answer 16

Reagent and Condition: Alcoholic KOH, heat