Alkenes Flashcards
Naming of alkenes (3)
1: identify longest C chain with C=C
2: no. C, C=C should have the lowest no. (thro both Cs)
3: 2 C=C: dienes; 3 C=C: trienes
Cis-alkene less/more stable than trans due to ___
less stable due to steric strain b/w 2 aklyl groups in cis
Defn of an electrophile & what can they be (3)
Electrophile is an e- pair acceptor that is attracted to e- rich site
It is e- deficient
- +ve charge
- partial +ve charge
- incomplete orbit
Electrophilic addition mechanism
for CH2=CH2 + HBr ——> CH3CH2Br
Steps? (7)
Step 1: Addition of electrophile (𝛿+H of HBr cuz Br > electro-ve than H)
slow step … (look at notes)
Step 2: Nucleophilic attack by Br-
fast step
- Name mechanism
- Indicate 𝛿- & 𝛿+ if any
- Use full arrows to show movement of e-
- Show lp of e- on nucleophile
- indicate slow/fast step
- draw formula for any intermediate
- draw prod (identify major n minor)
Markovnikorv’s rule :Determining major/minor prod when an unsymmetrical alkene reacts
Major prod: when electrophile is added to produce more stable carbocation intermediate
For HX: add to C with more H
For others: add to C with most R
When drawing a mechanism, draw more stable one
What makes a carbocation more stable and why
More R - more stable - form faster
Reason: R is e- donating -> disperses the +ve charge on C -> stabilises carbocation
vs e- withdrawing -> intensifies the +ve charge on C -> destabilises the carbocation
All reactions involving adding some halide (3)
Addition of hydrogen halides (HX)
C=C + HX —-DRY HX (g) —–> C - C
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H X
* must be (g), if it’s (aq), it’ll form alcohol
Reactions with halogens (in CCl4)
Conditions: dark, rm temp (prevent FRS)
C=C + X2 —-CCl4 —–> C - C
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X X
*CCl4 is inert
Reactions with halogens (in H2O) —————- (CHECK NOTES for electrophilic addition mechanism)
C=C + X2 + H2O ———> C - C + HX
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X OH
Reagents: X2 in water OR X2(aq)
Conditions: dark, rm temp
What does the +ve/-ve charge mean n for an ion what’s the _ or -
positive charge where an electron or two is absent
negative charge because we have an extra electron or two
Addition of water (lab method) - *mechanism too
Addition of water (industrial method)
Addition of water (lab method) - *mechanism too
C=C + H2O —- 1. cold conc H2SO4 2. H2O, heat —> C - C
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H OH
Mechanism:
Step 1a: addition of e-phile (H in H2SO4)
Step 1b: nucleophilic attack by -OSO2OH
Step 2: Hydrolysis
Addition of water (industrial method)
C=C + H2O(g) —- H3PO4 on silica, high temp, high pressure —> C - C
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H OH
Oxidation of alkenes (3) + observations
mild oxi
C=C + H2O + [O] —– KMnO4(aq), NaOH(aq), cold ——> C - C
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OH OH
Observations: purple KMnO4 decolourised + brown-black ppt of MnO2 observed
strong oxi
- C=C —– KMnO4(aq), H2SO4(aq), heat ——> ketones OR/& carboxylic acid OR/& CO2 & H2O
Rule:
~ Cleave C=C and put O on C=
~ Replace H directly attached to C= with OH
~ anything formed that is not ketones/carboxylic acid, just convert to CO2 & H2O
Observations: purple KMnO4 decolourised + CO2 gas evolved
- C=C —– KMnO4(aq), NaOH(aq), heat ——> ketones OR/& ‘carboxylic acid’ just that OH is O-Na+ instead OR/& Naa2CO3 & H2O
Same rule just instead of putting OH, replace with O-Na+
Reduction & what does no. of mol of the reactant tell u
C=C + H2(g) — Ni, heat /Pd/Pt —-> C - C
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H H
- amt of H2 reactant per mole of alkene is a measure of the no. of C=C bonds present cuz 1 mol of C=C bond requires 1 mol of H2 to reduce
Addition polymers
Addition polymers are formed with monomer containing C=C
eg poly(ethene)
monomer struc: C=C
polymer struc/formula -[- C - C -]- n
Preparation of alkenes (2) - elimination rctns
- Dehydrohalogenation of halogenoalkanes
C - C —KOH in ethanol, heat—-> C=C + HX
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H X
Reagents: KOH in ethanol/ethanolic KOH/alcoholic KOH/NaOH in ethanol
- Elimination of H2O from alcohols
C - C —excess conc H2SO4, heat—-> C=C + H2O
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OH H
Reagents: excess conc H2SO4, heat OR Al2O3, heat OR H3PO4, heat
Rule for major product when it comes to elimination reactions (preparation of alkenes)
Saytzeff’s rule: preferred prod - C=C with greater no. of alkyl grp (most sub. alkene)
Distinguishing tests
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