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Organic Chem Is Killing Me > Alkenes > Flashcards

Alkenes Flashcards

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1
Q

[ALKENES] Alkane to alkene from petroluem oil

A

Cracking of alkanes by strong heating
industrial method

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2
Q

[ALKENES] Halogenalkane/alkyl halide to alkenes (reagents)

A

Eliminate HX
KOH/NaOH in ethanol, heat
(OH)

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3
Q

[ALKENES] Alcohol to alkenes (reagents)

A

Eliminate H2O
excess conc H2SO4, heat
Conc H3PO4, heat (industrial)
Al2O3, heat (industrial)

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4
Q

[ALKENES] preparation Position to eliminate

A

ADJACENT!

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5
Q

[ALKENES] What type of reaction when added to alcohol?
Add excess conc H2SO4, heat
/Conc H3PO4, heat (industrial)
/Al2O3, heat (industrial)

A

Elimination of HOH
Give alkenes

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6
Q

[ALKENES] What type of reactions when added to halogenoalkane?
Add KOH/NaOH in ethanol, heat
(OH)

A

Elimination of HX
Give alkenes

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7
Q

[ALKENES] Order of stability

A

tetra subbed (most) > tri subbed > di subbed > mono subbed (least)

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8
Q

[ALKENES] How to predict the major product

A

Sayteffs rule:
Most substituted alkene is more stable alkene
More alkyl grps to C=C
Major product

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9
Q

[ALKENES] Combustion eqn of alkenes

A

CxHy + (x+y/4)O2 —>
xCO2 + y/2H2O

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10
Q

[ALKENES] Hydrogenate/reduce alkene to alkane

(Reagents)

A

H2 and nickel catalyst, heat
Pt/Pd can be used without heating (more reactive)

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11
Q

[ALKENES] To hydrogenate alkenes to alkanes which RA cannot be used and why?

A

LiAlH4 cannot be used due to hydride H- ions, repelled by e- rich pi e- cloud in C=C, unable to add H atoms.

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12
Q

[ALKENES] What is an electrophile

A

Electron loving
Partial pos S+
Cation>neutral

Formed by accepted pair of lone e from e rich site

Strength depend on size and stability of pos charge

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13
Q

[ALKENES] What is nucleophile

A

Nucleus loving
Contains lone pair of e
Anion>neutral

Forms by donating e pair to e deficient site

Strength depends on avaliability of lone pair of e
so more electroneg, tighter e (less avaliability), weaker nucleophile

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14
Q

[ALKENES] Reactivity of alkenes towards electrophiles

A

Highly reactive due to pi e cloud, which has loosely held pi e cloud that attract electrophiles.

  • Unsaturated can undergo (mainly EA) ddition. Break weaker pi bond and form 2 strong sigma bonds
  • Change of shape from trigonal planar to tetrahedral, sp2 to sp3 hybridisation
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15
Q

[ALKENES] Electrophilic addition mechanism (slow and fast step) HBr

A

1st: slow
heterolytic fission H-Br
pi bond of alkene cleaves
form carbocation intermediate and Br-

2nd: fast
neg charge Br-
donate lone pair e
to carbocation intermediate
product formed

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16
Q

[ALKENES] Electrophilic addition of hydrogen halides HX (hydrohalogenation)

A

Alkene + HX —> halogenoalkane
HI>HBr>HCl
Weaker H-X bond
Greater reactivity

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17
Q

[ALKENES] How to determine major product in EA reaction?

A

Markovnikov’s rule
2 products are possible in asymmetric alkene

major prod when
H atom in HX attaches to C atom with the greatest number of H atoms
X atom in HX attaches to C atom with the least number of H atoms
ie; X attach to C with more R groups –> more stable product

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18
Q

[ALKENES] Why is the product more stable when X or HX attach to more R groups?

A

Alkyl groups are e donating
can disperse + charge on carbocation
hence stabilising it
More substituted carbocation –> more stability –> more products formed

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19
Q

[ALKENES] DIFFERENCES of electrophilic addition of halogen X-X
in INERT vs NUCLEOPHILIC solvent

A

Inert: CCl4

  • must be DARK to avoid FRS
  • product is X and X

Nucleophilic: H2O

  • product is OH and X, additional HX
  • mixture of halohydrin and HX is produced
20
Q

[ALKENES] What can electrophilic addition of halogens test for and what reagents do we use?

A

Test for unsaturation/ presence of C=C
Halogen in CCl4 (inert) or H2O (nucleophilic)

Reddish brown Br/ Orange Br water
decolourises

21
Q

[ALKENES] Test for C=C/unsaturation?

A

EA of halogens in inert/nucleophilic solvent
add Br in CCl4
should decolourise if C=C was present

Mild oxidation/cleavage of C=C pi bond
KMnO4, H2SO4 , cold
Purple KMnO4 is decolourised

22
Q

[ALKENES] Mechanism for EA addition of halogens in nucleophilic solvent

A
23
Q

[ALKENES] Electrophilic addition of water in ALKENES (Hydration)
NON INDUSTRIAL METHOD!

A

Generally:
Alkene + H20 –> Alcohol with OH

Reagent:
1) cold concentrated H2SO4
2) warm H2O
H2SO4 is a catalyst in this reaction

2 stage process

1) Alkene + cold conc H2SO4 –> alkyl hydrogen sulfate OSO3H
2) Alkyl Hydrogen sulfate + warm H2O –> Alcohol + H2SO4

24
Q

[ALKENES] Why do we have to add H2SO4 in Electrophilic addition of water in ALKENES? (Hydration)
NON INDUSTRIAL METHOD!

A

Water is not a good electrophile, have to be acid catalysed

25
Q

[ALKENES] Electrophilic addition of water in ALKENES (Hydration)
INDUSTRIAL METHOD!

A

Generally:
Alkene + H20 –> Alcohol with OH
concentrated H3PO4 , high temp, high pressure
H3PO4 is a catalyst in this reaction

NO 2 stage process!!!

26
Q

[ALKENES] DIFFERENCE between mild and vigorous oxidation of alkenes?

A

Mild: cleavage of pi bond NOT C=C bond

  • diol is produced (2 OHs)
  • test PRESENCE of C=C
  • *-COLD**
  • Different products in acidic vs alkaline medium

Vigorous: cleavage of C=C bond

  • produce ketone, carboxylic acid or CO2
  • test for POSITION of C=C
  • *-HEATED**
  • Different products in acidic vs alkaline medium
27
Q

[ALKENES] What does it mean when you add cold KMnO4 and NaOH, purple KMnO4 decolourises and brown ppt is formed?

A

Mild oxidation of alkene to alcohol
C=C is present
diol is formed (2 OHs)
Alkaline medium

28
Q

[ALKENES] Mild oxidation of alkene tests for?

A

C=C is present
Mild oxidation of alkene to alcohol
when KMnO4 decolourised
Brown ppt formed in alkaline
No ppt for acidic

29
Q

[ALKENES] Vigourous oxidation of alkene tests for?

A

C=C location (TEST MUST BE Acidic)
Vig oxidation of alkene to form either
1) ketone
2) carboxylic acid or salt
3) CO2 and H2O

C=C present
when KMnO4 decolourised
Brown ppt formed in alkaline
No ppt for acidic

Terminal C=C at end
CO2
white ppt bubbled through Ca(OH)2

30
Q

[ALKENES] What does it mean when you add heated KMnO4 and H2SO4 in an acidic medium, and the purple KMnO4 decolourises with NO PPT?

A

When CO2 and H2O is formed:
C=C was at the end of the chain H+H (terminal alkene)
Intermediates are methanal (2H) -> methanoic acid (H+OH) -> carbonic acid (OH+OH)
Since final product is CO2, bubbling through Ca(OH)2 is good indicator.

When Carboxylic acid is formed:
C=C was bonded to R+H
Intermediate is aldehyde (R+H)

When Ketone is formed:
C=C was bonded to (R+R’)
No intermediates formed

31
Q

What does it mean when you add heated KMnO4 and NaOH in an alkaline medium, and the purple KMnO4 decolourises with brown ppt formed, and effervescence of a gas that forms white ppt in limwater?

A

When CO2 and H2O is formed:
C=C was at the end of the chain H+H (terminal alkene)
Intermediates are methanal (2H) -> methanoic acid (H+OH) -> carbonic acid (OH+OH)
Since final product is CO32-, bubbling through Ca(OH)2 is good indicator.

32
Q

[ALKENES] What oxidising agent is not strong enough for oxidation of alkenes?

A

K2Cr2O7

33
Q

[ALKENES] What are the factors deciding if the oxidation of alkene is mild or vigorous?

A 
Temperature used (warm/cold)
NOT THE MEDIUM!!
34
Q

[ALKENES] ALKENE REACTIONS PRODUCTS:

How do I know if a ppt will be formed or what colour change will occur?

A

3 types can happen in alkenes, elimination, addition and oxidation

Elimination is either dehydration of alcohols (remove H20, add ACID)
or dehydrohalogenation of alkyl halides (remove HX, add BASE)

In addition reactions of hydrogen halides H-X final product is just H and X lor

Then for halogens the X-X is always decolourised but depends on the final form (gas or water)
Type of solvent (inert/nucleophilic) only affects whether both are X or 1 X and 1 OH in the final product

BASICALLY in Oxidation of alkenes_,_ brown ppt will be formed in alkaline mediums (ie NaOH is added)

In both hot/cold, _alkaline/acid oxidations_, purple KMnO4 should always decolourise when C=C is formed, CO2 effervescence for terminal C=C is only seen in acidic oxidations. 
Ketones formed in both medium vigourous [O} if both R to C
carboxylic acid(acidic)/carboxylic salt(alkaline) if 1H 1R
35
Q

[ALKENES] saturation

A

unsaturated

36
Q

[ALKENES] general formula

A

CnH2n

37
Q

[ALKENES] Hybridisation structure in alkenes

A

generally: sp2 hybridised, trigonal planar 120
Total orbitals in one C:
3 sp2 hybridised orbitals + 1 unhybridised 2p orbital

  • *2 hybridised overlap head-on** with 1s of H to form 2C-H
  • *last hybridised 1 overlap head-on** with the other sp2 hybrid orbital to form C-C
  • *last unhybridised 2p** finds the other 2p to form a pi bond, converting the C-C to C=C
38
Q

[ALKENES] Energy to break a C=C vs a C-C bond

A

More energy for C=C than C-C
more e hared btween the C atoms
stronger e foa
BUT energy of C=C is not 2x of C-C bond because the pi bond is weaker than sigma due to side on overlap (not head on)
C=C is also shorter in length than C-C because of side on 2p unhybridised orbital overlap, C atoms are closer tgt

39
Q

[ALKENES] Types of isomerism of alkenes and cause

A

Constitutional isomerism and cis-trans

Cis-trans happens because of C=C pi bond restricted rotation
each C joined to 2 diff groups

40
Q

[ALKENES] Diff in stability of cis-trans molecules in alkenes

A

Cis: both same side in a C shape
Trans: Opposing sides

Trans more stable than cis.
Cis have greater steric strain due to inter electronic repulsion between e clouds
(can be quantified by Hc
more exo, more energy, less stable)

41
Q

[ALKENES] Physical properties of Alkenes

  • boiling/melting points
  • Solubility in polar/non-polar solvents
  • density compared to H2O
  • Type of molecular attraction
  • colour of flame
A

Straight chain:
increased no of C, increase e, increased bp/mp
Greater degree of branching, lower bp

Cis-trans:
Cis>trans for bp
BUT trans>cis for mp

alkenes insoluble in H2O
but soluble in non-polar solvnts

42
Q

[ALKENES] bp and mp of straight-chain alkenes

A

non polar molecules
increased C atoms
increased e
bigger more polarisable e cloud
increased idid
More energy to overcome stronger e foa
increased bp/mp

43
Q

[ALKENES] bp and mp of branched alkenes

A

greater degree in branching
alkene more spherical
less SA of contact
weaker idid
lower bp

44
Q

[ALKENES] bp and mp of cis/trans alkenes

A

Cis>trans for BP
cis slightly polar, trans not polar
cis pdpd+idid stronger than trans idid only

Trans>cis for MP
trans more symmetrical
pack into crystal lattice better
closer tgt, more attractive force
higher mp

45
Q

[ALKENES] Solubility of alkene in water/polar solvent

A

No
energy from forming pd-id btwn water/polar and alkene
insufficient to overcome
idid btwn alkene and H bonds/pdpd btwn H2O

46
Q

[ALKENES] Solubility of alkene in non-polar solvent

A

Yes
energy formed by id-id btwn non-polar and alkene
sufficient to overcome
idid btwn alkene and idid btwn non-p

Organic Chem Is Killing Me (7 decks)

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