Alkenes

Question 1

[ALKENES] Alkane to alkene from petroluem oil

Answer 1

Cracking of alkanes by strong heating
industrial method

Question 2

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

Answer 2

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

Question 3

[ALKENES] Alcohol to alkenes (reagents)

Answer 3

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

Question 4

[ALKENES] preparation Position to eliminate

Answer 4

ADJACENT!

Question 5

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

Answer 5

Elimination of HOH
Give alkenes

Question 6

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

Answer 6

Elimination of HX
Give alkenes

Question 7

[ALKENES] Order of stability

Answer 7

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

Question 8

[ALKENES] How to predict the major product

Answer 8

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

Question 9

[ALKENES] Combustion eqn of alkenes

Answer 9

CxHy + (x+y/4)O2 —>
xCO2 + y/2H2O

Question 10

[ALKENES] Hydrogenate/reduce alkene to alkane

(Reagents)

Answer 10

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

Question 11

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

Answer 11

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

Question 12

[ALKENES] What is an electrophile

Answer 12

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

Question 13

[ALKENES] What is nucleophile

Answer 13

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

Question 14

[ALKENES] Reactivity of alkenes towards electrophiles

Answer 14

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

Question 15

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

Answer 15

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

Question 16

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

Answer 16

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

Question 17

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

Answer 17

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

Question 18

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

Answer 18

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

Question 19

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

Answer 19

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

Question 20

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

Answer 20

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

Reddish brown Br/ Orange Br water
decolourises

Question 21

[ALKENES] Test for C=C/unsaturation?

Answer 21

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

Question 22

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

Answer 22

Question 23

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

Answer 23

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

Question 24

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

Answer 24

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

Question 25

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

Answer 25

Generally: Alkene + H20 --\> Alcohol with OH concentrated H3PO4 , high temp, high pressure H3PO4 is a catalyst in this reaction **NO 2 stage process!!!**

Question 26

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

Answer 26

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

Question 27

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

Answer 27

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

Question 28

**[ALKENES]** Mild oxidation of alkene tests for?

Answer 28

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

Question 29

**[ALKENES]** Vigourous oxidation of alkene tests for?

Answer 29

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

Question 30

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

Answer 30

_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

Question 31

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**?

Answer 31

_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 CO_3^2-**, bubbling through Ca(OH)2 is good indicator.

Question 32

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

Answer 32

K2Cr2O7

Question 33

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

Answer 33

``` Temperature used (warm/cold) NOT THE MEDIUM!! ```

Question 34

**[ALKENES]** ALKENE REACTIONS PRODUCTS: How do I know if a ppt will be formed or what colour change will occur?

Answer 34

**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 a**_ddition_** 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 ```

Question 35

**[ALKENES]** saturation

Answer 35

unsaturated

Question 36

**[ALKENES]** general formula

Answer 36

C_nH_2n

Question 37

**[ALKENES]** Hybridisation structure in alkenes

Answer 37

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

Question 38

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

Answer 38

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

Question 39

**[ALKENES]** Types of isomerism of alkenes and cause

Answer 39

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

Question 40

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

Answer 40

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 H_c more exo, more energy, less stable)

Question 41

**[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

Answer 41

_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

Question 42

**[ALKENES]** bp and mp of straight-chain alkenes

Answer 42

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

Question 43

**[ALKENES]** bp and mp of branched alkenes

Answer 43

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

Question 44

**[ALKENES]** bp and mp of cis/trans alkenes

Answer 44

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

Question 45

**[ALKENES]** Solubility of alkene in water/polar solvent

Answer 45

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

Question 46

**[ALKENES]** Solubility of alkene in non-polar solvent

Answer 46

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