Alkenes Flashcards
What are the reagents needed to turn an ethene into a dichloroethane
Cl2 in CCL4, rtp and dark
What are the reagents needed to turn ethene into chloroethanol
Cl (aq), rtp and dark
Ethene into chloroethane
HCl (g), rtp
Alkene into alchohol
(i) Concentrated H2SO4, cold
(ii) H20 (l), heat
Or
H2O (g), conc. H3PO4 cat., high temp. and high pressure
How to get diol from alkene
KMnO4, dil. H2SO4, cold
Oxidative cleavage of alkene
KMnO4, dil. H2SO4, heat
What are the possible products of oxidative cleavage
(i) ketone
(ii) carboxylic acid
(ii) CO2
Reduction of alkene reagents and conditions
H2 (g), Ni, high heat and high pressure
Or
H2 (g), Pt, rtp
What type of isomerism can alkenes display?
(i) chain isomerism
(ii) positional isomerism
(iii) functional group isomerism
(iv) Cis-Trans isomerism
Why can Cis-Trans isomerism be displayed in alkenes
(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
The major product formed during the elimination of an alchohol and a halogenalkane into alkene would be
The more highly substituted alkene which is the more stable alkene
Why is the more substituted alkene the more stable alkene.
Saytzeff’s rule - proton is removed from the carbon with least number of hydrogen substituents
How to produce alkene from alchohol
Al2O3 (s), heat
Or
Excess concentrated H2SO4, heat
How to produce alkene from halogenalkane
KOH/NaOH in ethanol, heat
What types of reactions do alkene undergo
(i) reduction
(ii) oxidation
(iii) electrophilic addition
(iv) combustion
In Cis-Trans isomerism displayed in alkenes, which is the major product
Trans-isomer is more stable than cis-isomer due to reduced steric hindrance
What is the basis of Markovnikov’s rule
Stability of carbocations
What is the rate of formation of products of electrophilic addition dependant on
Stability of carbocation (direct proportional r/s)
Why do carbocation stability increases from primary to tertiary
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.
Why do some functional groups lead to a slower rate of formation
Some functional groups are electron-withdrawing, intensifying the positive charge and destabilising the carbocation
The thinking process of polar reaction mechanisms
- Electron flow (Nu: to E)
- Nu: can be negatively charged or neutral (LP availability)
- E can be positively charged or neutral (Electron-deficient)
- Octet rule obeyed for period 2 elements (H duplet rule)
Electrophilic addition
- RDS. Heterolytic fission occurs involving the reagent that is not the alkene producing Nu:
(Note: highest activation energy step is the RDS) - Nu: attracted to carbocation formed and C-Nu bond formed
Why can electrophilic addition involving alkenes produce a racemic mixture
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.
How to distinguish alkenes from other functional groups
Br2 in CCl4, rtp and dark Decolourises orange-red soln Br2(aq) Decolourises orange Br2(aq) Br2(l) Decolourises reddish-brown soln
