Alcènes Flashcards
Carbon-carbon double bond signifies what functional grp?
Alkene
If more than 1 C=C, add what to the stem (eg ‘but’ in butene) ?
letter ‘a’
eg 2 C=C in butene at C1, C3, so
buta-1,3-diene
where ‘a’ means more than 1 C=C bond, 1,3 means location of double bond (w smallest no.) & di means 2 C=C bonds
Define electrophile
e- deficient species that can act as e- pair acceptors
δ+ δ-
eg. Br+, NO2+ and δ+H of H-Cl
(anions) (+ve dipole)
Since e- philes r e- deficient, they r readily attracted to & react w/attack e-rich species
Define nucleophiles
e-rich species that can act as e- pair donors
..
eg. OH-, Br- and δ-N of NH3
(anion) (δ- atom w l.p.)
Since nucleophiles r e- rich, they r readily attracted to & attack/react w e- deficient species
Describe electrophilic addition mechanism using bromine (liquid) and ethene as example
(Remember to draw full displayed formula)
Step 1:
CH2=CH2 ,full arrow from C=C to Br-Br + full arrow from δ+Br to δ-Br
(slow)
—-> +
CH2BrCH2 + :Br-
Step 2:
+
CH2BrCH2 + :Br-
, full arrow from :Br- to carbocation
(fast)
—->
CH2BrCH2Br
Describe electrophilic addition mechanism using bromine (aq) and ethene
(diff main pdt)
overall eqn
CH2=CH2 + Br2(aq) + H2O
—-> CH2 - CH2 + HBr
| |
Br OH
*CH2BrCH2 still formed but minor pdt
Step 1: same slow step as that for Br(l)
Step 2:
CH2 - CH2 + H2O
| +
Br
, full arrow from l.p. from O to carbocation + full arrow from 1 O-H bond to O in H2O
——> CH2 - CH2 + H+
| |
Br OH
(the H+ & Br- fr step 1 form HBr)
What are the reagent, condition, observation of electrophilic addition of halogen
Type of rxn: Electrophilic Add n
R: X2 in CCl4 (*other inert organic solvent oso can) OR Br2(l)
C: room temp, dark (prevent FRS)
O:
- Br2 (in CCl4): decolour n of orange-red bromine
- Br2 (l): decolour n of reddish-brown bromine
- Cl2 (in CCl4): decolour of pale yellow sol n
(colour oso in data booklet)
What are the reagent, condition of electrophilic addition of hydrogen halide?
Type of rxn: Electrophilic Add n
R: dry HX(g)
C: room temp
What are the reagent, condition of electrophilic addition of steam?
Type of rxn: Electrophilic add n
R: Steam/H2O(g)
C:
- (lab method) cold concentrated H2SO4 followed by heating w water
OR (must memorise BOTH)
- (industrial method) heat at high Pa w concentrated H3PO4 as catalyst
What is the important concept in Markonikov’s Rule?
Major pdt is the one formed when e- phile attaches itself to unsaturated C atom w LARGER no of H atoms
What are the reagent, condition of catalytic hydrogenation
type of rxn: reduction
R: H2 gas
C:
- Ni catalyst in presence of high temp & high Pa
OR
- Pd/Pt catalyst at r.t.p.
What are the reagent, condition and observation of mild oxidation (partial C=C bond cleavage)?
type of rxn: oxidation
R & C: cold alkaline/acidified KMnO4
Observation: decolour n of purple KMnO4 sol n & form n of brown ppt (MnO2)
What are the reagent, condition and observation of strong/vigorous oxidation (complete C=C bond cleavage)?
Type of rxn: oxidation
R & C: KMnO4, dilute H2SO4, heat
Observation: decolour n of purple KMnO4 sol n (+ effervescence of CO2(g) if terminal alkene present)
What is terminal alkene?
When there is C=C bond at the ‘end’ of molecule
What are the steps to obtain pdt fragments formed by strong oxidat n of alkene?
- Cleave =C=/=C= to form =C=O + O=C=
- Add O to all H atoms DIRECTLY attached to C=C
- If unstable intermediates below form,
-OH // O
O=C and HO - C
-OH \ C= O
HO /
it decomposes to CO2 + H2O
What are the steps to deduce structure of alkene after oxidative cleavage?
- Remove all O atoms
- Recombine fragments by ‘joining at double bond’
What are the chemical tests for alkene functional grp? What are the observations?
- Add Br2 in CCl4 at rtp in the dark
O: Decolour n of orange-red bromine sol n
NOTE: Br2(aq) can oso b used. Decolour n of orange Br2(aq) is observed - Add COLD alkaline KMnO4 sol n
O: Decolour n of purple KMnO4 sol n & brown ppt (MnO2) formed - Add HOT acidified KMnO4 sol n (strong oxidation)
O: Decolour n of purple KMnO4 sol n (and effervescence of CO2 if terminal alkene present)
What are the relative stability of carbocations? Why?
tertiary>secondary>primary>methyl carbocation
bcos need to be as stable as possible (can b attained by dispersion of charge)
greater no of e- donating alkyl grp means greater:
- e- donating effect
- extent of dispersal of +ve charge on carbocation/reduct n of intensity of +ve charge on carbocation
- stability of carbocation formed => form major pdt
What rxn is used for preparing alkenes?
Elimination
What are the reagent, condition of elimination of alcohols?
R & C:
- excess concentrated H2SO4, heat
OR
- Al2O3 catalyst, heat
OR
- excess concentrated H3PO4, heat
What are the reagent, condition of elimination of halogenoalkanes
R & C:
- alcoholic KOH/alcoholic NaOH, heat
Explain how markonikov’s rule works
- secondary, tertiary carbocation more stable than primary carbocation
(Or tertiary more stable than secondary)
Bcos
- more e- donate grp to disperse positive charge, so more stable carbocation
- (if got adjacent benzene) positive charge dispersed due to delocalisation into benzene ring, so more stable carbocation
