Topic 5E

Reactions of alkenes and alkynes

Reaction Mechanisms 52

In an organic reaction:• we break bonds and form bonds, and• these bonds are covalent; electron pairs are involved• A mechanism describes the sequence in which bond breaking and bond formation occurs as well as how the energy

of the system changes during a process• Use arrow notation to depict movement of electrons and energy profile diagrams to depict energy changes during that

process.

Reaction Energy Profiles 53

Exothermic reaction pathway:

Reaction Coordinate

Energy

Transition state

Reactants

Products

Activation energy barrierA

B

C

Reaction Energy Profiles 53

Endothermic reaction pathway:

Reaction Coordinate

Energy

Transition state

Reactants

Products

Activation energy barrierA

B

C

A two-step reaction 54

• Intermediates lie in shallow energy wells• Rate-determining step— step with highest EA

Reaction Coordinate

Energy

Transition state 1

ReactantsProducts

AB

C

Transition state 2

IntermediateDE

Addition reactions summary 55

RCH CH2

RCH CH2

X H

HX RCH CH2

HO HH+/HOH

RCH CH2

X X

X2

RCH CH2

H H

H2

RCH CH2

HO OH

OsO4 or KMnO4

Addition reactions summary 37

RCH CH2

RCH CH2

X H

HX

Addition of Hydrogen Halides

55

• Hydrogen halides react with alkenes

• Alkyl halides are formed

CH2 CH2 H Br CH3 CH2 BrEthyl bromide

Addition of Hydrogen Halides 56

• Two products are possible• Only 2-chloropropane is formed• H always adds to side of double bond with most hydrogens — the

Markovnikov rule

Cl

CH3CHCH3

CH3–CH=CH2 + HCl

CH3CH2CH2ClNot formed

Mechanism 57

Acids are sources of electrophilic H+ H+ is attacked by the electon pair ofbond leaving a carbocation a positive carbon

(Double headed arrows for electron pairs)

Mechanism animation 57

QuickTime™ and aCinepak Codec by SuperMatch decompressor

are needed to see this picture.

Addition of HCl to 2-methylpropene

Ocol, Bcorgchm CD ROMMcMurry 2.0

Mechanism in detail 57

• Two electrons from bond form bond with H+, an electrophile

• This leaves sp2 carbon with five electrons, a carbocation

CH3 H

HH

CH3 H

H

H

H+

vacant pzorbital

sp3

Mechanism in detail 58

• Finally Cl– , a nucleophile,donates a pair of electrons to carbocation forming a C—Cl bond

• Both carbons are sp3 hybridised.• H—Cl has added across the double bond

CH3

H

H

H

HH

H

HCl

H+

Cl –

sp2

sp3

sp3

CH3

Mechanism in detail 58

CH3CH CH2

+CH3CHCH3 (not CH3CH2CH2

+)

H+

secondarycarbocation

primarycarbocation

C CH2

+H+CH3

CH3

CH3

CH3

C CH3

+CH3

CH3

C CH2

H

not

tertiarycarbocation

primarycarbocation

Why?

Classification of carbocations

58

• Three Groups attached to cation centre

CH3

CH3H H

C H CH3 C CH3 CH3 C CH3

+ + +

Primary Secondary Tertiary

• Two Groups attached to cation centre

• One Group attached to cation centre

Stability of carbocations 59

• Alkyl groups push electrons through sigma bonds, they are electron donating• They are positively INDUCTIVE (+I) and charge is stabilised through delocalisation or

dispersion• The more alkyl groups the greater the stability• Inductive effects operate over only one to two bonds

CH3

CH3H H

C H CH3 C CH3 CH3 C CH3

+ + ++ + + + +

+

Primary Secondary Tertiary

RCH2+ << R2CH+ < R3C+

increasing carbocation stability

Mechanism in detail 59

CH3CH CH2

+CH3CHCH3 (not CH3CH2CH2

+)

H+

secondarycarbocation

primarycarbocation

C CH2

+H+CH3

CH3

CH3

CH3

C CH3

+CH3

CH3

C CH2

H

not

tertiarycarbocation

primarycarbocation

Why?RCH2

+ << R2CH+ < R3C+

increasing carbocation stability

Stability of carbocations 59

• More stable carbocation is formed more easily• It is a lower activation energy process

En

ergy

Reaction Coordinate)

CH3CH=CH2 + H +

CH3CH2CH2 not formed

Ea (1°)

CH3CHCH3 observed reactionEa (2°)

Other additions of HX 60

An addition of HBr:

C C

CH3

CH3

CH3

H

CH CH

CH3

CH3

CH3not

+

2carbocation

C CH2CH3

CH3

CH3

3 carbocation

+

H +C

CH2CH3

CH3 Br

CH3Br–

2-bromo-2-methylbutane

Other additions of HX 61

An addition of HI:

C CH2

CH3CH2

CH3CH2

C CH3

CH3CH2

CH3CH2

I

CH3

CH3CH2CCH2CH3I–+H +

3 carbocation (not 1 )

3-iodo-3-methylpentane

An addition of HCl:

CH3CH3

H

CH3

ClCl –+

H+

3 carbocation1-Chloro-1-methylcyclohexane

Summary 61

• Positive part adds to the carbon of the double bond which has the greater number of hydrogens attached to it

• Today this is better stated that addition of an electrophile gives the most stable carbocation

• This is a general rule for addition to alkenes• All alkenes can be expected to react in this manner

Markovnikov's Rule:

Alkene addition reactions 61

RCH CH2

RCH CH2

X H

HX RCH CH2

HO HH+/HOH

Hydration of alkenes 61

CH3CH CH2 CH3CHCH3 H2OH+ OH

2-propanol(an alcohol)

•An example of Markovnikov addition of water

Hydration of alkenes 61

Mechanism:

Alkene addition reactions 63

RCH CH2

RCH CH2

X H

HX RCH CH2

HO HH+/HOH

RCH CH2

X X

X2

Addition of halogen molecules

63

RCH CH2

RCH CH2

X X

X2

CH2 CH2 BrCH2CH2BrBr2

1,2-dibromoethane

CH3CH CH2 + Cl2 CH3CH CH2

Cl Cl

General mechanism 63

CH2

Br Br

H H

H

H

Br

Br

+CH2CH2Br + Br –

H

HH

HBr+

but

then Br –

XCH2

A "bridged bromonium ion" is formed

Addition is trans.

Bromination of bacon fat 63

QuickTime™ and aGraphics decompressor

are needed to see this picture.

Bacon fat contains unsaturated fats which add bromine

Movie from Saunders General Chemistry CD-ROM

Addition of bromine, Br2 63

Addition is trans.

Br

BrH H

trans-1,2-dibromocyclohexane

Br

HBr

HBr2

Diequatorial conformation

Br2 addition to a ring 63

• A "bridged bromonium ion" is formed

• Addition gives the trans product by anti addition

H H

Br

BrH H

trans-1,2-dibromocyclohexane

–Br

Br+

Anti-addition 63

• A "bridged bromonium ion" explains why only trans is formed since only anti addition is possible

• Stepwise addition would give the cis and trans product

Br

H

Br

H

H

Br H

Br

transcis

cis and trans-1,2-dibromo-cyclohexane

BrHH

+

Br–

Br–

H H–Br

Br+

Addition of chlorine 63

• A bridged chloronium ion is formed

• Cl– attacks to give the trans addition product

• The reaction is general for alkenes with halogens

Cl2Cl

Cl–

H

Cl

Cl

H

Addition reactions summary 65

RCH CH2

RCH CH2

X H

HX RCH CH2

HO HH+/HOH

RCH CH2

X X

X2

RCH CH2

HO OH

OsO4 or KMnO4

Cis addition to alkenes 64

• A cyclic osmic ester is first formed• Water converts this to the cis diol

C CR''

R'''

R

R'

O OOs

O O

C C

R''R'''

RR'

O OOs

O O

OsO4

cis-1,2-diol

C C

R''R'''

RR'

HO OH

H2O

OsO2

Cis diol (glycol) formation with osmium tetroxide:

Diol (glycol) formation 65

• A cyclic manganese ester is formed• Water converts this to the cis diol• Brown MnO2 is generated (purple colour of KMnO4 lost)

C CR''

R'''

R

R'

O OMn

O O

C C

R''R'''

RR'

O OMn–

O O

MnO4–

cis-1,2-diol

C C

R''R'''

RR'

HO OH

H2O

MnO2Permanganate reacts similarly:

Diol formation — cyclic alkenes

65

• Syn-addition to cyclic alkenes affords hydroxyl groups on the same face

• Cyclic alkenes afford the cis-1,2-diol

H

OH

H

OH

H

O

H

OMnO2

–

cis-1,2-cyclohexanediol

Addition reactions summary 65

RCH CH2

RCH CH2

X H

HX RCH CH2

HO HH+/HOH

RCH CH2

X X

X2

RCH CH2

H H

H2

RCH CH2

HO OH

OsO4 or KMnO4

Addition of hydrogen, H2 65

• Addition of hydrogen is exothermic by 120kJmol–1

• Addition has a very high activation energy though• With catalysts, addition occurs via a low energy path• Pt, Pd, Ni, Rh and Ru

C CR''

R'''

R

R'

C C

R''

R'''R

R'

H H

+ H2

High EA

Catalytic action 66

• Catalytic pathways may be multistep• Overall activation energy is lower

En

ergy

Progress of reaction

uncatalysed high EA

Catalysed (multistep, low EA each step)

Schematic of catalytic hydrogenation

66

• Hydrogen is absorbed onto the surface

• H—H bond weakened and hydrogens becomeatom-like

H HH H

metalsurface

metalsurface

Schematic of catalytic hydrogenation

66

• Hydrogens react with -bond (stepwise)

• Hydrogens attach on same face (syn addition) to give cis product.

metalsurface

metalsurface

H H

R R

R R R RR R

H H cis-addition

H

R R

R R R RR R

H H

Cis addition to cyclic alkenes

67

• Hydrogens add to one face of the alkene• The result is cis addition

CH3

CH3

CH3

CH3

H

HH2/Pt

cis-1,2-dimethylcyclohexane

Reaction of alkynes

• Very similar to alkenes:– They add halogens twice, X2

– They add hydrogen halides twice, HX– They add hydrogen twice, H2

– They add water with acid (hydration)– Markovnikov's rule applies

• They are slightly acidic and react with strong bases

Addition of bromine

• Two molecules of bromine add successively

H C C HH

C CBr

Br

H+ Br 2

C C

Br

H

H

Br

Br2CH CHBr2

1,1,2,2-tetrabromoethane

+ Br 2

Addition of HBr

• Two molecules of HBr add successively• Markovnikov addition in both steps

CH3C CH CH3C CH2

Br

CH3CCH3

Br

Br

HBr HBr

2,2-dibromopropane

Addition of H2

• Normal catalysis leads to double addition

• Less active catalysts allow syn addition of one molecule

R C C R + H2 C C

R

H

R

H

R CH2 CH2 R'

special catalyst

normal catalyst

Reactions as an acid

• Very strong base required

R C C H R C C +–

NH3–

NH2

Strong base Alkynide ion

Liq. NH3

Alkynide formation

• Anions (electron pairs) in sp hybrid orbitals arecloser to the carbon nucleus

• More stable than anions in sp2 or sp3 orbitals

% s character:

Electrons further from positive nucleus

s sp sp2 sp3

100 253350

Index of hydrogen deficiency

(Double bond equivalents)67

• Hexane C6H14

• Hydrogen deficiency is TWO• 2H is equivalent to either a double bond or a ring

cyclohexane1-pentene

C6H12

Index of hydrogen deficiency

68

• TWO double bond or • ONE ring and ONE double bond or• TWO rings

cyclopentene1-pentyne

C5H12 – C5H8 = 2 x H2

Equivalent to either

H2

C C

CH2

H2

C C

H

H

Combustion analysis 68

• Microanalysis gives the percentages of carbon and hydrogen• These, when divided by the atomic numbers, give the relative

proportion of each element

C: 88.16%

H: 11.84%

Then C = 88.1612.01 = 7.341

H = 11.841.008 = 11.75

Combustion analysis 69

• This the empirical formula• Compare to the weight of the compound• If the same, this is the molecular formula• If not the molecular formula will be a factor of the empirical formula

Divide by lowest number:

CH1.6 x 2 = C2H3.2

x 3 C3H4.8

x 4 C4H6.4

x 5 C5H8 molar mass 68