5.12 alkenes Flashcards
what are the 4 reactions that alkenes can undergo?
combustion, addition, reduction, oxidation
state the reagents, conditions, products and observations for addition of halogens
Br2 in CCl4 at room temperature to produce dihalogenoalkane
orange-red bromine decolourises
state and briefly outline the mechanism for addition reactions
electrophillic addition.
1) electron-rich C=C bond induces poles on Br2, making it an electrophile. Nucleophile attacks electrophile, generating an unstable carbocation and Br- anion (slow)
2) Br- anion attacks carbocation to form dihalogenoalkane (fast)
state the reagents, conditions, products and observations for addition of halogens in water
Br2 in water at room temperature to produce halohydrins
orange bromine decolourises
what’s the difference from addition with and without water?
in water, H2O with lone pair on O is in excess, acting as the electrophile to attack carbocation in step 2.
Heterolytic fission leaves -OH on carbon and H3O+ as products
state the reagents, conditions, products and observations for addition of hydrogen halides
dry HX at room temperature to produce halogenoalkanes
no observation
explain, using Markovnikov’s Rule why 2-chloropropane is the major product in the addition of HCl to alkene instead of 1-chloropropane
2-chloropropane is major because it is formed from a secondary carbocation intermediate while 1-chloropropane is formed from a primary carbocation intermediate.
The secondary intermediate has two electron-donating alkyl groups bonded to the positively charged carbocation while that in the primary intermediate is only bonded to one electron-donating alkyl group.
Since the intermediate is more stable, 2-chloropropane forms at a faster rate
state the reagents, conditions, products and observations for indirect hydration
cold concentrated H2SO4 followed by H20 and heat to produce alcohol
state the reagents, conditions, products and observations for direct hydration
H2O (gas) steam, heat with concentrated H3PO4 catalyst at high temperature and pressure
state the reagents, conditions, products and observations for reduction / hydrogenation
H2 (g), nickel / platinum catalyst at room temperature to from alkanes
no observations
state the reagents, conditions, products and observations for mild oxidation
cold KMnO4(aq) in KOH(aq) to produce diols
purple KMnO4 decolourises, brown MnO2 is formed
state the formula for mild oxidation of ethene
C2H4 + [O] + H2O -> C2H4(OH)2
state the reagents, conditions, products and observations for strong oxidation
KMnO4(aq) in H2SO4(aq), heat under reflux / in water bath to produce carbon dioxide / carbonyls / carboxylic acid
purple KMnO4 decolourises, effervescence of CO2 gives white ppt in limewater (if applicable)
what are the two special cases in strong oxidation?
terminal alkene -> CO2 + H2O
ethanedioic acid -> 2CO2 + H2O
what is the mechanism and two reactions that alkenes can be produced from?
elimination: dehydration of alcohols and dehydrohalogenation
