slide1 n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Alkenes and Electrophilic Addition PowerPoint Presentation
Download Presentation
Alkenes and Electrophilic Addition

Loading in 2 Seconds...

play fullscreen
1 / 146

Alkenes and Electrophilic Addition - PowerPoint PPT Presentation


  • 332 Views
  • Uploaded on

Alkenes and Electrophilic Addition. Preparation of Alkenes. A. Industrial preparation. Cracking. Prepared by the cracking of alkanes of high molecular masses Give alkenes of low molecular masses. 600 o C. 2CH 3 CH 3  CH 2 = CH 2 + 2CH 4.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Alkenes and Electrophilic Addition' - dotty


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1

Alkenes

and

Electrophilic Addition

New Way Chemistry for Hong Kong A-Level 3A

slide2

Preparation of Alkenes

New Way Chemistry for Hong Kong A-Level 3A

slide3

A. Industrial preparation

Cracking

  • Prepared by the cracking of alkanes of high molecular masses
  • Give alkenes of low molecular masses

New Way Chemistry for Hong Kong A-Level 3A

slide4

600 oC

2CH3CH3 CH2 = CH2 + 2CH4

2CH3CH2CH3 CH3CH = CH2 + CH2 = CH2 + CH4 + H2

600 oC

Cracking

e.g.

New Way Chemistry for Hong Kong A-Level 3A

slide5

B. Synthetic preparation

Elimination Reactions

  • Involve removal of atoms or groups of atoms from adjacent carbon atoms in the reactant molecule
  • Formation of a double bond between carbon atoms

New Way Chemistry for Hong Kong A-Level 3A

slide6

1. Intramolecular Dehydration of Alcohols

  • Removal of a water molecule from a reactant molecule
  • By heating the alcohols in the presence of a dehydrating agent.
  • E.g. Alumina(Al2O3), conc. H2SO4,
  • conc. H3PO4
  • Give alkenesand water as the products

New Way Chemistry for Hong Kong A-Level 3A

slide7

1. Intramolecular Dehydration of Alcohols

New Way Chemistry for Hong Kong A-Level 3A

slide8

1. Intramolecular Dehydration of Alcohols

  • Experimental conditions (i.e. temperature and concentration of concentrated sulphuric acid)
  • is closely related to the structure of the individual alcohol.

New Way Chemistry for Hong Kong A-Level 3A

slide9

1. Intramolecular Dehydration of Alcohols

  • Primary alcohols generally required concentrated sulphuric acid and a relatively high temperature

New Way Chemistry for Hong Kong A-Level 3A

slide10

1. Intramolecular Dehydration of Alcohols

  • Secondaryalcohols are intermediate in reactivity
  • Tertiary alcohols dehydrate under mild conditions (moderatetemperature and dilute sulphuric acid)

New Way Chemistry for Hong Kong A-Level 3A

slide11

>

>

Tertiary alcohol

Secondary alcohol

Primaryalcohol

1. Intramolecular Dehydration of Alcohols

  • The relative ease of dehydration of alcohols generally decreases in the order:

New Way Chemistry for Hong Kong A-Level 3A

slide12

Intramolecular vs intermolecular

Substitution

New Way Chemistry for Hong Kong A-Level 3A

slide13

Intramolecular dehydration is favoured at higher temperatures because it involves breaking of strong C – H bonds.

New Way Chemistry for Hong Kong A-Level 3A

slide14

Q.29(a)

New Way Chemistry for Hong Kong A-Level 3A

slide15

Q.29(b)

New Way Chemistry for Hong Kong A-Level 3A

slide16

Q.29(c)

New Way Chemistry for Hong Kong A-Level 3A

slide17

1. Intramolecular Dehydration of Alcohols

  • Secondary and tertiary alcohols may dehydrate to give a mixture of alkenes
  • The more highly substituted alkene is formed as the major product

New Way Chemistry for Hong Kong A-Level 3A

slide18

2. Dehydrohalogenation of haloalkanes

  • Elimination of a hydrogen halide molecule from a haloalkane
  • By heating the haloalkane in an alcoholic solution of KOH

New Way Chemistry for Hong Kong A-Level 3A

slide19

C2H5OH is a co-solvent for both RX and OH

New Way Chemistry for Hong Kong A-Level 3A

slide20

2. Dehyhalogenation of haloalkanes

e.g.

New Way Chemistry for Hong Kong A-Level 3A

slide21

2. Dehyhalogenation of haloalkanes

  • Dehydrohalogenation of secondary or tertiary haloalkanes can take place in more than one way
  • A mixture of alkenes is formed

New Way Chemistry for Hong Kong A-Level 3A

slide22

Q.30(a)

New Way Chemistry for Hong Kong A-Level 3A

slide23

Q.30(b)

New Way Chemistry for Hong Kong A-Level 3A

slide24

>

>

Primaryhaloalkane

Tertiary haloalkane

Secondary haloalkane

2. Dehyhalogenation of haloalkanes

  • The ease of dehydrohalogenation of haloalkanes decreases in the order:

New Way Chemistry for Hong Kong A-Level 3A

slide25

The relative stabilities of alkenes decrease in the order:

New Way Chemistry for Hong Kong A-Level 3A

slide26

Relative Stability of Alkenes in Terms of Enthalpy Changes of Hydrogenation

  • Hydrogenation of alkenes is exothermic
  • From enthalpy changes of hydrogenation
  •  predict the relative stabilities of alkenes

New Way Chemistry for Hong Kong A-Level 3A

slide27

Enthalpy changes of hydrogenation of but-1-ene, cis-but-2-ene and trans-but-2-ene

New Way Chemistry for Hong Kong A-Level 3A

slide28

Relative Stability of Alkenes in Terms of Enthalpy Changes of Hydrogenation

  • The pattern of the relative stabilities of alkenes determined from the enthalpy changes of hydrogenation:

New Way Chemistry for Hong Kong A-Level 3A

slide29

Addition Reactions

Hydrogenation of alkynes

  • Alkenes can be prepared by hydrogenation of alkynes
  •  Depend on the conditions and the catalyst employed

New Way Chemistry for Hong Kong A-Level 3A

slide30

Hydrogenation

  • Lindlar’s catalyst is metallic palladium(Pd) deposited on calcium carbonate
  • further hydrogenation of the alkenes formed can be prevented

New Way Chemistry for Hong Kong A-Level 3A

slide31

Reactions of Alkenes

An Introduction

New Way Chemistry for Hong Kong A-Level 3A

slide32

Alkenes are more reactive than alkanes

  • Undergoes addition reaction rather than substitution

New Way Chemistry for Hong Kong A-Level 3A

slide33

Presence of C=C double bond

  • C=C double bond is made up of a  bond and a  bond
  • Addition reactions only involve breaking of weaker  bonds of alkenes

New Way Chemistry for Hong Kong A-Level 3A

slide34

The electrons of the  bond are

  •  diffuse in shape
  •  less firmly held by the bonding carbon nuclei

Susceptible to the attack by electrophiles

New Way Chemistry for Hong Kong A-Level 3A

slide35

Electrophiles : -

Electron-deficient species

Attack electron-rich center e.g. C=C bond

Examples :

Cations : H+, Br+, R+,… (lead to heterolysis)

Free radicals : H, Cl, R,…(lead to homolysis)

New Way Chemistry for Hong Kong A-Level 3A

slide36

All have lone pairs for donating to the reaction sites

Nucleophiles : -

Electron-rich species

Attack electron-deficient site

e.g. carbonyl carbon, C=O

Examples :

anions : OH, Br, RO,…

molecules : H2O, ROH, NH3

All lead to heterolytic fissions

New Way Chemistry for Hong Kong A-Level 3A

slide37

Reactions of Alkenes

Examples

New Way Chemistry for Hong Kong A-Level 3A

slide38

Catalytic Hydrogenation

  • Alkenes react with hydrogen in the presence of metal catalysts (e.g. Ni, Pd, Pt) to give alkanes

Lower temperatures can be used with Pd or Pt

New Way Chemistry for Hong Kong A-Level 3A

slide39

Catalytic Hydrogenation

e.g.

cis-addition, refer to notes on ‘chemical kinetics’, pp.36-37)

New Way Chemistry for Hong Kong A-Level 3A

slide40

Catalytic Hydrogenation

Under mild conditions, C=O and benzene ring are unaffected.

New Way Chemistry for Hong Kong A-Level 3A

slide41

Q.31

New Way Chemistry for Hong Kong A-Level 3A

slide42

A / B

New Way Chemistry for Hong Kong A-Level 3A

slide43

A / B

New Way Chemistry for Hong Kong A-Level 3A

slide44

*

*

*

*

*

*

C

New Way Chemistry for Hong Kong A-Level 3A

slide45

Partial hydrogenation

Complete hydrogenation

Application : - hardening of plant oils

Plant oil (polyunsaturated liquid with low m.p.)

Margarine (soft unsat’d solid with higher m.p.)

Animal fat (hard sat’d solid with still higher m.p.)

New Way Chemistry for Hong Kong A-Level 3A

slide46

Catalytic Hydrogenation

  • Fats and oils are organic compounds called triglycerides
  •  triesters formed from glycerol and carboxylic acids of long carbon chains

New Way Chemistry for Hong Kong A-Level 3A

slide47

Catalytic Hydrogenation

  • Saturated fats
  •  solids at room temp
  •  usually come from animal sources
  •  long carbon chains are zig-zag and easily packed

New Way Chemistry for Hong Kong A-Level 3A

slide48

Catalytic Hydrogenation

  • Unsaturated oils
  •  liquids at room temp
  •  usually come from plant sources
  •  lower m.p. due to cis-arrangement (kinked shape)

New Way Chemistry for Hong Kong A-Level 3A

slide49

Catalytic Hydrogenation

  • Fats are stable towards oxidation by air
  • More convenient to handle and store

New Way Chemistry for Hong Kong A-Level 3A

slide50

Catalytic Hydrogenation

  • Advantages:
  • higher m.p.  ideal for baking
  • turning rancidmuch less readily than unsaturated oils

New Way Chemistry for Hong Kong A-Level 3A

slide51

Partial hydrogenation

Complete hydrogenation

Application : - hardening of plant oils

Plant oil (polyunsaturated liquid with low m.p.)

Margarine (soft unsat’d solid with higher m.p.)

Animal fat (hard sat’d solid with still higher m.p.)

New Way Chemistry for Hong Kong A-Level 3A

slide52

150°C, 5 atm

H2 / Ni

trans-fat  coronary heart disease

New Way Chemistry for Hong Kong A-Level 3A

slide53

Catalytic Hydrogenation

Hydrogenation of vegetable oils produces margarine

New Way Chemistry for Hong Kong A-Level 3A

slide54

Catalytic Hydrogenation

  • Margarine and butter do not have sharp m.p. because they are NOT pure substances.
  • They are mixtures containing different triesters.

New Way Chemistry for Hong Kong A-Level 3A

slide55

Electrophilic Addition Reactions(AdE)

  • Addition of electrophiles to the C=C double bond of alkenes

New Way Chemistry for Hong Kong A-Level 3A

slide56

Electrophiles that attack the C=C double bond include

  •  protons (H+)
  •  neutral species in which the molecule is polarized, e.g. bromine

New Way Chemistry for Hong Kong A-Level 3A

slide57

CH3CCl3

(a) Addition of halogens in non-aqueous solvents

X = Cl, Br or I

Occurs with or without light

Addition is preferred to substitution

Reaction mechanism is not required

New Way Chemistry for Hong Kong A-Level 3A

slide58

+ Br

+



bromonium ion

New Way Chemistry for Hong Kong A-Level 3A

slide59

trans-addition

New Way Chemistry for Hong Kong A-Level 3A

slide60

(a) Addition of halogens in non-aqueous solvents

New Way Chemistry for Hong Kong A-Level 3A

slide61

(a) Addition of halogens in non-aqueous solvents

e.g.

New Way Chemistry for Hong Kong A-Level 3A

slide62

The reddish brown colour of bromine is decolourized

A drop of bromine dissolved in 1,1,1-trichloroethane is added to an alkene

(a) Addition of halogens in non-aqueous solvents

  • The decolourization of bromine in 1,1,1-trichloroethane is a useful test for unsaturation

New Way Chemistry for Hong Kong A-Level 3A

slide63

(b) Addition of halogens in aqueous solutions

-OH comes from H-OH which is in excess.

Reaction mechanism is not required.

New Way Chemistry for Hong Kong A-Level 3A

slide64

(b) Addition of halogens in aqueous solutions

e.g.

  • The consequent decolourization of the reddish brown colour of bromine water is also a test for unsaturation

New Way Chemistry for Hong Kong A-Level 3A

slide65

+ Br

+



bromonium ion

New Way Chemistry for Hong Kong A-Level 3A

slide66

+ H3O+

bromohydrin

New Way Chemistry for Hong Kong A-Level 3A

slide67

+

+

Q.32

NaCl(aq)

New Way Chemistry for Hong Kong A-Level 3A

slide68

Cl

New Way Chemistry for Hong Kong A-Level 3A

slide69

+

+

Q.32

NaI(aq)

New Way Chemistry for Hong Kong A-Level 3A

slide70

I

New Way Chemistry for Hong Kong A-Level 3A

slide71

+

+

Q.32

NaNO3(aq)

New Way Chemistry for Hong Kong A-Level 3A

slide72

NO3

New Way Chemistry for Hong Kong A-Level 3A

slide73

(c) Addition of H – X

X = Br, Cl, OSO3H, OH, etc.

Mechanism required

Acid-catalyzed hydration

New Way Chemistry for Hong Kong A-Level 3A

slide74

Addition of Hydrogen Bromide

  • A molecule of HBr adds to the C=C double bond of an alkene
  • Give a bromoalkane

New Way Chemistry for Hong Kong A-Level 3A

slide75

fast

Reaction Mechanism: Electrophilic Addition Reactions of Hydrogen Bromide to Alkenes

rate-determining step

sp2 hybridized carbonium ion

Br is a nucleophile

New Way Chemistry for Hong Kong A-Level 3A

slide76

racemic mixture

50%

*

*

50%

50%

50%

fast

sp2 hybridized trigonal planar

If the resulting C is chiral

New Way Chemistry for Hong Kong A-Level 3A

slide77

Q.33

New Way Chemistry for Hong Kong A-Level 3A

slide78

Q.33

(a) one  bond and one  bond are broken

(b) two  bonds are formed

(c) Heat evolved during bond formation >Heat required during bond breaking

Addition reactions are usually exothermic

view movie

New Way Chemistry for Hong Kong A-Level 3A

slide79

Regioselectivity of Hydrogen Halide Addition: Markovnikov’s Rule

CH2=CH2 & CH3CH=CHCH3 are symmetrical alkenes.

CH3CH=CH2 is an asymmetrical alkene.

New Way Chemistry for Hong Kong A-Level 3A

slide80

Regioselectivity of Hydrogen Halide Addition: Markovnikov’s Rule

  • A hydrogen halide can add to an asymmetrical alkene in either of the two ways
  • The reaction proceeds to give a major product preferentially
  •  the reaction is said to exhibit “regioselectivity”

New Way Chemistry for Hong Kong A-Level 3A

slide81

Regioselectivity of Hydrogen Halide Addition: Markovnikov’s Rule

the addition of HBr to ethene produces bromoethane as the only product

New Way Chemistry for Hong Kong A-Level 3A

slide82

Regioselectivity of Hydrogen Halide Addition: Markovnikov’s Rule

  • When but-2-ene reacts with HBr
  •  2-bromobutane is formed as the only product

New Way Chemistry for Hong Kong A-Level 3A

slide83

Regioselectivity of Hydrogen Halide Addition: Markovnikov’s Rule

  • When propene reacts with HBr
  •  the major product is 2-bromopropane
  •  the minor product is 1-bromopropane

New Way Chemistry for Hong Kong A-Level 3A

slide84

Regioselectivity of Hydrogen Halide Addition: Markovnikov’s Rule

H is given to the rich

New Way Chemistry for Hong Kong A-Level 3A

slide85

Regioselectivity of Hydrogen Halide Addition: Markovnikov’s Rule

Markovnikov’s rule states that in the addition of HX to an asymmetrical alkene, the hydrogen atom adds to the carbon atom of the carbon-carbon double bond that already has the greater number of hydrogen atoms

New Way Chemistry for Hong Kong A-Level 3A

slide86

Regioselectivity of Hydrogen Halide Addition: Markovnikov’s Rule

  • The products formed according to this rule are known as Markovnikov products

New Way Chemistry for Hong Kong A-Level 3A

slide87

Stability of Carbocation and Mechanistic Explanation of the Markovnikov’s Rule

  • Carbocations are a chemical species that contains a positively charged carbon
  • Very unstable
  • Exist transiently during the reaction
  • Classified as primary, secondary or tertiary
  •  according to the number of alkyl groups that are directly attached to the positively charged carbon

New Way Chemistry for Hong Kong A-Level 3A

slide88

Stability of Carbocation and Mechanistic Explanation of the Markovnikov’s Rule

The more stable the carbocation

 the more stable the transition state

the lower the activation energy

 the faster its formation

New Way Chemistry for Hong Kong A-Level 3A

slide89

Stability of Carbocation and Mechanistic Explanation of the Markovnikov’s Rule

  • The stability of the carbocations increases in the order:

New Way Chemistry for Hong Kong A-Level 3A

slide90

Alkyl groups stabilize the positively charged carbocation by positive inductive effect

New Way Chemistry for Hong Kong A-Level 3A

slide91

A greater number of alkyl groups

  •  release more electrons to the positively charged carbon
  •  increase the stability of the carbocation

New Way Chemistry for Hong Kong A-Level 3A

slide92

Stability of Carbocation and Mechanistic Explanation of the Markovnikov’s Rule

  • Consider the addition of HBr to propene:

New Way Chemistry for Hong Kong A-Level 3A

slide93

Stability of Carbocation and Mechanistic Explanation of the Markovnikov’s Rule

  • The hydrobromination of propene involves two competing reactions:

New Way Chemistry for Hong Kong A-Level 3A

slide94

Since the formation of carbocation is the rate-determining step,

the overall reaction is faster if it involves the formation of a more stable carbocation.

New Way Chemistry for Hong Kong A-Level 3A

slide95

Q.34(a)

X = Cl, Br, or I

New Way Chemistry for Hong Kong A-Level 3A

slide96

+



Q.34(b)

heat

cold

New Way Chemistry for Hong Kong A-Level 3A

slide97

On heating, alkyl hydrogensulphates form alkenes and sulphuric acid

New Way Chemistry for Hong Kong A-Level 3A

slide98

conc. H2SO4 / cold

no reaction

addition

Alkane

Separation of a mixture containing an alkane and an alkene.

Alkane / Alkene

insoluble in conc. H2SO4

dissolved in conc. H2SO4

Separated by separating funnel

New Way Chemistry for Hong Kong A-Level 3A

slide99

conc. H2SO4 / cold

no reaction

addition

Alkane

heat

Alkene

Separation of a mixture containing an alkane and an alkene.

Alkane / Alkene

insoluble in conc. H2SO4

dissolved in conc. H2SO4

New Way Chemistry for Hong Kong A-Level 3A

slide100

Alkyl hydrogensulphates can be easily hydrolyzed to alcohols by heating with water

conc. H2SO4 + H2O  dilute H2SO4

acid-catalyzed hydration

New Way Chemistry for Hong Kong A-Level 3A

slide101

(c) Addition of H – X

X = Br, Cl, OSO3H, OH, etc.

Mechanism required

Acid-catalyzed hydration

New Way Chemistry for Hong Kong A-Level 3A

slide102

+



Acid-catalyzed hydration

New Way Chemistry for Hong Kong A-Level 3A

slide103

Acid-catalyzed hydration

The acid catalyst is regenerated

+ H3O+

New Way Chemistry for Hong Kong A-Level 3A

slide104

3

2

Q.34(d)

New Way Chemistry for Hong Kong A-Level 3A

slide105

+



I – Cl

3

1

Q.34(c)

electron-donating

EN : C = I = 2.5

New Way Chemistry for Hong Kong A-Level 3A

slide106

H – Cl

More destabilized by negative inductive effect

Q.34(e)

New Way Chemistry for Hong Kong A-Level 3A

slide107

H – Cl

Q.34(f)

The resonance effect more than compensates the negative inductive effect of Cl

New Way Chemistry for Hong Kong A-Level 3A

slide108

H – Br

>

2

1

Q.34(g)

New Way Chemistry for Hong Kong A-Level 3A

slide109

The +ve charge is shared by the benzene ring by resonance effect.

Stabilized by resonance effect as well as inductive effect(2)

New Way Chemistry for Hong Kong A-Level 3A

slide110

benzylic carbocation

More stable than 3 carbocation

New Way Chemistry for Hong Kong A-Level 3A

slide111

excess H – F

Q.34(h)

New Way Chemistry for Hong Kong A-Level 3A

slide112

2

1

Allylic carbocation is stabilized by resonance effect.

Stabilized by resonance effect as well as inductive effect

Stability : -

Benzylic > allylic > 3 > 2 > 1 > CH3+

New Way Chemistry for Hong Kong A-Level 3A

slide113

H – F

less destabilized by –ve I-effect of F

New Way Chemistry for Hong Kong A-Level 3A

slide114

Effect of substituents on the reactivity of AdE

1. Electron-donating groups increase the reactivity by

(a)  the electron density of C=C bond, thus making it more susceptible to electrophilic attack.

(b) Stabilizing the carbocation intermediate/T.S. by +ve I-effect and/or resonance effect, thus lowering the Ea for the rate-determining step.

New Way Chemistry for Hong Kong A-Level 3A

slide115

Effect of substituents on the reactivity of AdE

2. Resonance effect > inductive effect

3. Electron-withdrawing groups lower the reactivity by working in the opposite ways.

New Way Chemistry for Hong Kong A-Level 3A

slide116

Q.35

New Way Chemistry for Hong Kong A-Level 3A

slide117

Oxidation of alkenes

(a) Combustion

More sooty and luminous than that of corresponding alkanes due to higher carbon contents

New Way Chemistry for Hong Kong A-Level 3A

slide118

KMnO4, H+ or OH

cold

KMnO4, H+ or OH

heat

(b) Reaction with KMnO4

Used as a test for alkenes

carbonyl products

New Way Chemistry for Hong Kong A-Level 3A

slide119

KMnO4, H+ or OH

heat

KMnO4, H+ or OH

heat

If all R groups are alkyl groups,

ketones will be the final products.

New Way Chemistry for Hong Kong A-Level 3A

slide120

KMnO4, H+ or OH

No reaction

heat

New Way Chemistry for Hong Kong A-Level 3A

slide121

KMnO4, H+ or OH

KMnO4, H+ or OH

heat

heat

If either R1 or R2 is H / either R3 or R4 is H,

further oxidation of aldehydes to carboxylic acid will occur.

New Way Chemistry for Hong Kong A-Level 3A

slide122

KMnO4, H+ or OH

heat

aldehyde

carboxylic acid

New Way Chemistry for Hong Kong A-Level 3A

slide123

KMnO4, H+ or OH

KMnO4, H+ or OH

heat

heat

If both R1 & R2 are H / both R3 & R4 are H,

further oxidation to first methanoic acid and then CO2will occur.

New Way Chemistry for Hong Kong A-Level 3A

slide124

methanoic acid

KMnO4, H+ or OH

2CO2 + 2H2O

heat

New Way Chemistry for Hong Kong A-Level 3A

slide125

(c) Ozonolysis

Further oxidation of aldehyde to carboxylic acid by H2O2 is inhibited using Zn dust and CH3COOH

New Way Chemistry for Hong Kong A-Level 3A

slide126

1. O3

2. Zn dust / H2O

(c) Ozonolysis

1. O3

2. Zn dust / H2O

New Way Chemistry for Hong Kong A-Level 3A

slide127

Oxidative cleavage can be used to locate C=C bond in an unknown sample

New Way Chemistry for Hong Kong A-Level 3A

slide128

Q.36

(3Z)-2-methylpenta-1,3-diene

(3E)-2-methylpenta-1,3-diene

New Way Chemistry for Hong Kong A-Level 3A

slide129

Q.36

(3E)-3-methylpenta-1,3-diene

(3Z)-3-methylpenta-1,3-diene

New Way Chemistry for Hong Kong A-Level 3A

slide130

Q.37

New Way Chemistry for Hong Kong A-Level 3A

slide131

Q.37

New Way Chemistry for Hong Kong A-Level 3A

slide132

Q.37

New Way Chemistry for Hong Kong A-Level 3A

slide133

The END

New Way Chemistry for Hong Kong A-Level 3A

slide134

28.4 Preparation of Alkenes (SB p.173)

Example 28-4

Classify the following alcohols as primary, secondary or tertiary alcohols.

(a) CH3CHOHCH2CH3

(b) CH3CH2CH2OH

(c) (CH3)2COHCH2CH2CH3

Answer

(a) It is a secondary alcohol.

(b) It is a primary alcohol.

(c) It is a tertiary alcohol.

Back

New Way Chemistry for Hong Kong A-Level 3A

slide135

28.4 Preparation of Alkenes (SB p.173)

Back

Check Point 28-4

Classify the following haloalkanes as primary, secondary or tertiary haloalkanes.

(a) (c)

(b)

(a) A secondary haloalkane

(b) A primary haloalkane

(c) A tertiary haloalkane

Answer

New Way Chemistry for Hong Kong A-Level 3A

slide136

The more stable C5H11+ carbocation is the tertiary carbocation as shown below:

28.5 Reactions of Alkenes (SB p.177)

Check Point 28-5A

Of the isomeric C5H11+ carbocations, which one is the most stable?

Answer

Back

New Way Chemistry for Hong Kong A-Level 3A

slide137

28.5 Reactions of Alkenes (SB p.179)

Back

Let's Think 1

Both alkanes and alkenes undergo halogenation. The

halogenation of alkanes is a free radical substitution

reaction while the reaction of alkenes with halogens is

an electrophilic addition reaction. Can you tell two differences between the products formed by the two

different types of halogenation?

Answer

Alkenes give dihalogenated products while alkanes usually give polysubstituted products. Another difference is the position of the attachment of the halogen atom. For alkenes, the halogen atom is fixed to the carbon atom of the carbon=carbon double bond. In the substitution reaction of alkanes, the position of the halogen atom varies.

New Way Chemistry for Hong Kong A-Level 3A

slide138

28.5 Reactions of Alkenes (SB p.183)

Check Point 28-5B

(a) What chemical tests would you use to distinguish between two unlabelled bottles containing hexane and hex-1-ene respectively?

Answer

(a) We can perform either one of the following tests:

Hex-1-ene can decolourize bromine water or chlorine water in the dark while hexane cannot.

Hex-1-ene can decolourize acidified potassium manganate(VII) solution while hexane cannot.

New Way Chemistry for Hong Kong A-Level 3A

slide139

28.5 Reactions of Alkenes (SB p.183)

Check Point 28-5B

  • What is the major product of each of the following reactions?
  • (i)
  • (ii)

Answer

New Way Chemistry for Hong Kong A-Level 3A

slide140

28.5 Reactions of Alkenes (SB p.183)

  • (i)
  • (ii)

Check Point 28-5B

New Way Chemistry for Hong Kong A-Level 3A

slide141

(c) Give the products for the following reactions:

(i) CH3CH = CH2 + H2

(ii) CH3CH = CHCH3

(iii) CH3CH = CHCH3 + Br2

Ni

conc. H2SO4

28.5 Reactions of Alkenes (SB p.183)

Check Point 28-5B

Answer

New Way Chemistry for Hong Kong A-Level 3A

slide142

(c) (i) CH3CH2CH3

  • (ii)
  • (iii)

28.5 Reactions of Alkenes (SB p.183)

Check Point 28-5B

Back

New Way Chemistry for Hong Kong A-Level 3A

slide143

28.5 Reactions of Alkenes (SB p.184)

Check Point 28-5C

  • Arrange the following carbocations in increasing order of stability. Explain your answer briefly.

Answer

New Way Chemistry for Hong Kong A-Level 3A

slide144

The increasing order of the stability of carbocations is:

  • Tertiary carbocations are the most stable because the three alkyl groups release electrons to the positive carbon atom and thereby disperse its charge. Primary carbocations are the least stable as there is only one alkyl group releasing electrons to the positive carbon atom.

28.5 Reactions of Alkenes (SB p.184)

Check Point 28-5C

New Way Chemistry for Hong Kong A-Level 3A

slide145

28.5 Reactions of Alkenes (SB p.184)

Check Point 28-5C

  • (b) Based on your answer in (a), arrange the following molecules in the order of increasing rates of reaction with hydrogen chloride.

Answer

New Way Chemistry for Hong Kong A-Level 3A

slide146

(b) The reaction of these compounds with hydrogen chloride involves the formation of carbocations. Therefore, the order of reaction rates follows the order of the ease of the formation of carbocations, i.e. the stability of carbocations:

Therefore, the rates of reactions of the three compounds with hydrogen chloride increase in the order:

28.5 Reactions of Alkenes (SB p.184)

Back

Check Point 28-5C

New Way Chemistry for Hong Kong A-Level 3A