Alkene Flashcards
Be familiar with the topic.
Physical properties of alkenes
- Alkenes are insoluble in water and are non polar
- Boiling point increases and number of carbon increases
Difference in BP and MP in alkene isomers
Cis-alkene is more polar than trans-alkene. Net dipole moment about the molecules causes PD-PD interactions between molecules. More energy…
Formation of alkenes: Elimination of HX from halogenoalkanes
Reagent: NaOH/KOH, ethanol (solvent)
Conditions: heat under reflux
Formation of alkenes: Elimination of water from alcohol
Reagent: Excess conc H2SO4
Conditions: 170 degree celsius
Saytzeff’s rule
An alkene with more alkyl substitutents is generally more stable. The more stable alkene is the major product
Why are alkenes reactive
High electron density of the C=C bond. The Pi electron cloud attracts electrophiles/ electron deficient molecules
Reaction of alkenes involves breaking of the weaker PI bond. Alkenes undergo electrophilic addition.
Reaction of alkenes: EA of X2 with alkenes
Reagents: X2 in inert CCL4 solvent
Conditions: Room temp in the dark
Reaction of alkenes: EA of HX with alkenes
Reagents: HX(g)
Conditions Room temperature
An alkyl group is electron donating
Carbon is slightly more electronegative than hydrogen. In -CH3 group, carbon attracts the electron density of the shared electrons. Carbon thus acquires a partial negative charge, and is able to donate increased electron density through sigma bonds/ inductive effect.
Markovnikov’s rule
When an asymmetrical alkene undergoes electrophilic addition, the more stable carbocation intermediate is formed. Stability of carbocation is greater, rate of formation is greater
Reaction of alkenes: EA of X2(aq) with alkenes
Reagent: X2 (aq)
Condition: Room temp, in the dark
Reaction of alkenes: EA of H2SO4, followed by hydrolysis in Formation of alcohols
Step 1: Concentrated H2SO4, cold
Step 2: H2O, warm
Method 2:
Reagent: H2O (g), H3PO4(L) on celite
Condition: 300’c, 70 atm
Stereochemistry involved in EA
Intermediate carbocation is planar. Hence it has an equal likelihood of being attacked from either side.
Therefore, if chiral center is generated, a racemic mixture of enantiomers is expected.
Reaction of alkenes: Reduction
Method 1: H2 Ni catalyst, 150’c
Method 2: H2 Pt catalyst, room temp
Method 3: H2 Pd/C catalyst, heat
Reaction of alkenes: Mild Oxidation
Causes formation of Diols
Reagents: KMnO4 (aq), NaOH (aq)
Conditions: cold
