ib chemistry on nucleophilic substitution, sn1, sn2 and factors affecting rate of hydrolysis of...

Nucleophilic Substitution

Factors affecting Rate of Nucleophilic Substitution

Nature of Nucleophile • SN2 mechanism (only) • Nucleophile – electron pair donor •anion more reactive > neutral species due to electron charge density(negative charged) • order of reactivity/better nucleophile • CN - > OH- > NH3 > H2O

H H │ │

CH3 - C – Br + OH- > CH3 –C –Br + H2O

│ │ H H

Nature of Halogen • SN1 and SN2 mechanism • polarity of bond decrease ↓ • bond strength decreases ↓ • halogen leaves easily • rate hydrolysis fastest iodo > bromo > chloro compounds

H H H

│ │ │ CH3- C –I > CH3 - C – Br > CH3 – C - CI │ │ │ H H H

CH3 CH3 CH3

│ │ │ CH3 - C – I > CH3 –C –Br > CH3 –C –CI │ │ │ CH3 CH3 CH3

SN2 (1o) mechanism I - > Br - > CI - SN2 (1o) mechanism OH - > H2O

CH3 CH3

│ │ CH3 - C – Br + OH- = CH3 –C –Br + H2O │ │ CH3 CH3

SN1 (3o) mechanism No effect!

Faster ↑

No effect

Nature of Halogenoalkane • SN1 > SN2 mechanism • 3o > 2o > 1o

• 3o – by SN1 - carbocation - faster • 1o - by SN2 – transition state - slower

SN1 (3o) mechanism I - > Br - > CI -

CH3 CH3 H

│ │ │ CH3 - C – Br > CH3 –C –Br > CH3 –C –Br │ │ │ CH3 H H

3o > 2o > 1o

SN1 (3o) > SN2 (1o)

Formation of 2 methylpropan-2-ol

Tertiary carbocation are more stable due to inductive effect • Three alkyl gp stabilises the carbocation by inductive effect – pushing electrons to carbocation (reducing positive charge) making it more stable

Hydrolysis of 2-Bromo-2-Methylpropane (3o) by SN1 mechanism

CH3 │

CH3 - C – Br

│

CH3

Carbocation formation (Intermediate) Nucleophile OH- attacking the carbocation

Heterolytic fission - Carbocation and Br- formation

(CH3)3CBr → (CH3)3C+ + Br- 1st step (slow)

(CH3)3C+ + OH- → (CH3)3COH 2nd step (fast)

CH3 CH3

│ │ CH3 - C – Br + OH- CH3 –C –OH + Br - │ │ CH3 CH3

3o Halogenoalkane by SN1


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http://www.youtube.com/watch?v=JmcVgE2WKBE&feature=plcp

Hydrolysis of Bromoethane (1o) by SN2 mechanism

H

│

OH- + CH3 – C –Br

│

H

Bond Breaking and Making at transition state Bromine substituted with OH- group

H H

│ │ CH3 - C – Br + OH- CH3 – C –OH + Br - │ │

H H

Single step mechanism – Bond breaking and Bond making in transition state • Involve collision between 2 molecules • no bulky alkyl gp, less steric effect – • allows nucleophile to attack the electron deficient carbon from the opposite site

Nucleophile colliding with bromoethane

CH3CH2Br + OH- → CH3CH2OH + Br- Single step

1o Halogenoalkane by SN2

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http://www.youtube.com/watch?v=h5xvaP6bIZI&feature=related

Reaction of Halogenoalkanes

Reactivity for halogenoalkanes • Carbon bonded to halogen group – F, CI, Br, I

• High electronegativity on halogen group

• High reactivity – due to polarity of C+- CI -, C+-Br -

• Nucleophile – species with lone pair electron – donate to carbon center

•Reaction for Halogenoalkanes • Substitution reaction

Types of halogenoalkane Primary 10 – One or NO alkyl gp on C attach to halogen gp

Secondary 2o – Two alkyl gp on C attach to halogen gp

Tertiary 3o – Three alkyl gp on C attach to halogen gp

H │ CH3 - C – Br │ H

H │ H - C – Br │ H

Primary halogenoalkane 10 - SN2

Single Step

Nucleophilic Substitution SN2 • Undergo SN2 mechanism, Bimolecular Nucleophilic Substitution

CH3CH2Br + OH- → CH3CH2OH + Br-

• Single step mechanism – Bond breaking and Bond making in transition state • Involve collision between 2 molecules • Rate is dependent on concentration of CH3CH2Br and OH-

• Molecularity = 2 • Experimentally rate expression = k [CH3CH2Br][OH-]

Transition state

Bond making and bond breaking

Nucleophile OH attack

Br2 leaving group

Single step

OH- + CH3CH2Br → [ HO---CH2(CH3)---Br ] → CH3CH2OH + Br-


SN2

CH3 │ CH3 - C – Br │ CH3

CH3 │ CH3 - C – Br │

H










H │ CH3 - C – Br │ H

H │ H - C – Br │ H


Single Step



• Single step mechanism – Bond breaking and Bond making in transition state • Involve collision between 2 molecules • Rate is dependent on concentration of CH3CH2Br and OH-

• Molecularity = 2 • Experimentally rate expression = k [CH3CH2Br][OH-]

Transition state

Bond making and bond breaking

Nucleophile OH attack

Br2 leaving group

Single step

OH- + CH3CH2Br → [ HO---CH2(CH3)---Br ] → CH3CH2OH + Br-


SN2


Tertiary halogenoalkane 30 – SN1

Nucleophilic Substitution SN1 • Undergo SN1 mechanism, Unimolecular Nucleophilic Substitution

(CH3)3CBr + OH- → (CH3)3COH + Br-

• Two steps mechanism

1st step – slow step, rate determining step, formation of carbocation by heterolysis

(CH3)3CBr → (CH3)3C+ + Br-

2nd step – fast step, OH- reacting with carbocation forming product

(CH3)3C+ + OH- → (CH3)3COH

• Rate is dependent on concentration of (CH3)3CBr

• Molecularity = 1

• Experimentally rate expression = k [(CH3)3CBr]

CH3 │ CH3 - C – Br │ CH3

(CH3)3CBr → (CH3)3C+ + Br- 1st step (slow)

(CH3)3C+ + OH- → (CH3)3COH 2nd step (fast)






(CH3)3CBr + OH- → (CH3)3COH + Br-

SN1






Secondary 2o –Two alkyl gp on C attach to halogen gp


Secondary halogenoalkane 20 -SN1 and SN2


CH3CH(CH3)Br + OH- → CH3CH(CH3)OH + Br-

• Single step mechanism – Bond breaking and Bond making in transition state

• Involve collision of 2 molecules

• Rate is dependent on concentration of CH3CH(CH3)Br and OH-


• Experimentally rate expression = k [CH3CH(CH3)Br][OH-]

CH3 │ CH3 - C – Br │

H

Nucleophilic Substitution SN1 • Undergo SN1 mechanism, Unimolecular Nucleophilic Substitution

CH3CH(CH3)Br + OH- → CH3CH(CH3)OH + Br-

• Two steps mechanism

1st step – slow step, rate determining step, formation of carbocation by heterolysis

CH3CH(CH3)Br → CH3CH(CH3)+ + Br-

2nd step – fast step, OH- reacting with carbocation forming product

CH3CH(CH3)+ + OH- → CH3CH(CH3)OH

• Rate is dependent on concentration of CH3CH(CH3)Br


• Experimentally rate expression = k [CH3CH(CH3)Br]






AND

CH3CH(CH3)Br + OH- -> CH3CH(CH3)OH + Br-

SN1

SN2

Questions on Nucleophilic Substitution

Which Rate of hydrolysis is faster ?

H H │ │

CH3 - C – Br + OH- CH3 –C –OH + Br -

│ │ H H

H H │ │

CH3 - C – Br + H2O CH3 –C –OH + Br -

│ │ H H

CH3 CH3

│ │ CH3 - C – Br + OH- CH3 –C –OH + Br-

│ │ CH3 CH3

CH3 CH3

│ │ CH3 - C – Br + H2O CH3 –C –OH + Br-

│ │ CH3 CH3

CH3 – CH2 – I + OH- CH3 –CH2 –OH + I -

CH3 – CH2 –Br + OH- CH3 –CH2 –OH + Br -

CH3 CH3

│ │ CH3 - C – Br + OH- CH3 –C –OH + Br-

│ │ H H

CH3 – CH2 –I + OH- CH3 –CH2 –OH + I -

CH3 CH3

│ │ CH3 - C – Br + OH- CH3 –C –OH + Br-

│ │ CH3 CH3

CH3 –CH2 –Br + OH- CH3 –CH2 –OH + Br -

or

or

or

or

or

√

√

√

√

A

A

A

A

A

B

B

B

B

B

Nature of Nucleophile, OH- > H2O

Nature of Halogen , I - better leaving gp > Br -

Nature of Halogen , I - better leaving gp > Br -

Nature of Halogenoalkane, 3o > 1o

Rate the same 3o Halogenoalkane NOT affected by nucleophile

Rate the same

H H │ │ CH3 CH2- C- Br + OH- CH3 CH2-C –OH + Br - │ │

H H

CH3 CH3 │ │ CH3 C-Br + OH- CH3 C-OH │ │

CH3 CH3

CH3 CH3 │ │ CH3 C- Br + OH- CH3 C- OH │ │

H H

Questions on Nucleophilic Substitution


Tertiary halogenoalkane 30 - SN1

Secondary halogenoalkane 20 - SN2 and SN1

Single step mechanism - Bond breaking + Bond making in transition state

Two step mechanism – Formation of carbocation

OH

OH

OH

OH

Single step mechanism - Bond breaking + Bond making in transition state

Two step mechanism - Formation of carbocation

OH OH

OH OH

SN1

SN2

SN1

SN2

SN2

SN1

ib chemistry on nucleophilic substitution, sn1, sn2 and factors affecting rate of hydrolysis of...

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ch3 c br ch3 c br

br ch3

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br ch3ch2oh