Alkenes

Question 1

What are the reagents needed to turn an ethene into a dichloroethane

Answer 1

Cl2 in CCL4, rtp and dark

Question 2

What are the reagents needed to turn ethene into chloroethanol

Answer 2

Cl (aq), rtp and dark

Question 3

Ethene into chloroethane

Answer 3

HCl (g), rtp

Question 4

Alkene into alchohol

Answer 4

(i) Concentrated H2SO4, cold
(ii) H20 (l), heat

Or

H2O (g), conc. H3PO4 cat., high temp. and high pressure

Question 5

How to get diol from alkene

Answer 5

KMnO4, dil. H2SO4, cold

Question 6

Oxidative cleavage of alkene

Answer 6

KMnO4, dil. H2SO4, heat

Question 7

What are the possible products of oxidative cleavage

Answer 7

(i) ketone
(ii) carboxylic acid
(ii) CO2

Question 8

Reduction of alkene reagents and conditions

Answer 8

H2 (g), Ni, high heat and high pressure

Or

H2 (g), Pt, rtp

Question 9

What type of isomerism can alkenes display?

Answer 9

(i) chain isomerism
(ii) positional isomerism
(iii) functional group isomerism
(iv) Cis-Trans isomerism

Question 10

Why can Cis-Trans isomerism be displayed in alkenes

Answer 10

(i) There is a restricted rotation about the C atoms due to the presence of pie bond
(ii) There can be 2 different functional groups bonded to each C atom

Question 11

The major product formed during the elimination of an alchohol and a halogenalkane into alkene would be

Answer 11

The more highly substituted alkene which is the more stable alkene

Question 12

Why is the more substituted alkene the more stable alkene.

Answer 12

Saytzeff’s rule - proton is removed from the carbon with least number of hydrogen substituents

Question 13

How to produce alkene from alchohol

Answer 13

Al2O3 (s), heat

Or

Excess concentrated H2SO4, heat

Question 14

How to produce alkene from halogenalkane

Answer 14

KOH/NaOH in ethanol, heat

Question 15

What types of reactions do alkene undergo

Answer 15

(i) reduction
(ii) oxidation
(iii) electrophilic addition
(iv) combustion

Question 16

In Cis-Trans isomerism displayed in alkenes, which is the major product

Answer 16

Trans-isomer is more stable than cis-isomer due to reduced steric hindrance

Question 17

What is the basis of Markovnikov’s rule

Answer 17

Stability of carbocations

Question 18

What is the rate of formation of products of electrophilic addition dependant on

Answer 18

Stability of carbocation (direct proportional r/s)

Question 19

Why do carbocation stability increases from primary to tertiary

Answer 19

As alkyl groups are electron-donating, the electron deficiency of the carbocation is minimised as the positive charge of the carbocation is dispersed to a greater extent.

Question 20

Why do some functional groups lead to a slower rate of formation

Answer 20

Some functional groups are electron-withdrawing, intensifying the positive charge and destabilising the carbocation

Question 21

The thinking process of polar reaction mechanisms

Answer 21

1. Electron flow (Nu: to E)
2. Nu: can be negatively charged or neutral (LP availability)
3. E can be positively charged or neutral (Electron-deficient)
4. Octet rule obeyed for period 2 elements (H duplet rule)

Question 22

Electrophilic addition

Answer 22

1. RDS. Heterolytic fission occurs involving the reagent that is not the alkene producing Nu:
   (Note: highest activation energy step is the RDS)
2. Nu: attracted to carbocation formed and C-Nu bond formed

Question 23

Why can electrophilic addition involving alkenes produce a racemic mixture

Answer 23

This is because the carbocation intermediate is trigonal planar and thus the Nu: can be attracted from either side of the planes with equal probabilities. If the carbon is chiral, the resulting mixture is racemic as two enantiomers, of equimolar amounts, of opposing optical activities are formed. Thus the optical activities of the two enantiomers cancels each other exactly and hence the product does not rotate plane polarised light.

Question 24

How to distinguish alkenes from other functional groups

Answer 24

Br2 in CCl4, rtp and dark
Decolourises orange-red soln
Br2(aq) 
Decolourises orange Br2(aq)
Br2(l) 
Decolourises reddish-brown soln