nucleophilic substitution sn1 sn2 nucleophile halogenoalkane in organic chemistry

Nucleophilic Substitution, SN1, SN2, Nucleophile, HalogenoalkaneIn Organic Chemistry

IB Chemistry, Nucleophilic Substitution, SN1, SN2, Halogenoalkanes, Nucleophile in Organic Chemistry

Halogenoalkanes - Hydrocarbons with

• Halogen attached (F, CI, Br, I)

• (1°, 2°, 3°) halogenoalkane - number of alkyl gps attach to carbon bonded to halogen

• Polar bonds due to high EN of halogen

Nucleophilic Substitution -

• Nucleophile (non bonding electron) attack the partial positive charge carbon (nucleus)

• Chloride (halogen) - leaving group and substituted by nucleophile

• Nucleophile replace/substitute the halogen

Mechanism of Nucleophilic Substitution (SN1 and SN2 )

SN1 - Substitution Nucleophilic Unimolecular

• SN1 - 2 steps, unimolecular ( first order)

• 1st step - slow/rds, Carbocation formation

• 2nd step - fast, Nucleophilic attack carbocation

• Rate = k [substrate], First order overall

• Rate depend on conc substrate NOT conc nucleophile

• Nature of the nucleophile doesn’t affect the rate

Click HERE for more info

Stable Intermediate Carbocation

• SN1 - produce intermediate carbocation

• Carbocation - positive charged on carbon

• Carbocation formation - sp2 hybrid (Trigonal Planar)

• Nucleophile attack from both sides

• Racemic Mixture

SN2 - Substitution Nucleophilic Bimolecular

• 1 step mechanism, Bimolecular collision

• Rate = k[substrate][nucleophile], Second order overall

• Rate depend on conc of substrate and nucleophile

• Bond making/breaking occur together result in trigonal bipyramidal shape

• Inverted configuration (backside attack by Nucleophile)

Tertiary Halogenoalkane - SN1 Primary Halogenoalkane - SN2

SN1 and SN2 due to Steric Hindrance and Inductive effect

Tertiary - 3 alkyl gp - SN1 • Tertiary - High Inductive/Steric hindrance

• High Inductive effect - 3 alkyl gp donates/push e towards carbocation

• Stabilizes positive charge on it

• High Steric Hindrance - 3 alkyl gp hinder/blocks Nu from attacking, prevent SN2

Primary - 1 alkyl gp - SN2• Primary - Low Steric Hindrance/Inductive effect

• Low Steric Hindrance - allows Nu to attack from one side - SN2 possible

• Low Inductive effect - 1 alkyl gp, result in less stable carbocation - prevent SN1

IB examples for SN1 and SN2 reactions

SN1 reaction, Hydrolysis of 2 -Bromo 2- Methylpropane with warm aq dil NaOH

(CH3)3CBr + OH- ------> (CH3)3COH + Br-

Tertiary - SN1 • 2 steps mechanism

• 1st step, slow/rds, Heterolysis (breaking C-Br bond) forming carbocation

• 2nd step, fast, nucleophile OH- reacts with carbocation

Click HERE to view

SN2 reaction, Hydrolysis of Bromoethane with warm aq NaOH

CH3CH2Br + OH → CH3CH2OH + Br

• Primary - SN2

• One step mechanism

• Bond making/breaking simultaneous in transition state

Click HERE to view

Factor affecting rate of Nucleophilic Substitution (SN1 / SN2)1. Nature of Halogen

• Bond length increase, Bond strength decrease from CI to I, easier for nucleophile to attack by SN2

2. Nature of Halogenoalkane

• Tertiary (SN1) faster than Primary (SN2)Formation Carbocation (SN1) faster than formation of transition state (SN2)

3. Nature of Nucleophile

• Negatively charged more reactive than neutral nucleophile

Click HERE to view

Substitution with Nucleophile (Ammonia and potassium cyanide)

• NH3 /CN- acts as nucleophile

• SN2 - form amine and nitrile

• Substitute X with NH3 -amine

• Substitute X with CN- nitrile

Click HERE to view

CH3CH2Br + NH3 ---> CH3CH2NH2 + HBr• NH3 as nucleophile

• SN2 pathway, one step

• Product - Alkyl amine

CH3CH2Br + CN- ----> CH3CH2CN + Br-

• CN- as a nucleophile

• Product - Alkyl nitrile , addition 1 carbon • Reduction of nitrile with H2/catalyst nickel - produce amine

Bromoethane with aq ethanolic ammonia/ potassium cyanide

Products act as Nucleophile, producing 1°, 2°, 3° Amines

KeynotesRxn 1 : Product C2H5NH2 acts as nucleophile for reaction 2Rxn 2 : Product C2H5NH2 reacts with C2H5Br , producing (C2H5)2NHRxn 3: Product (C2H5)2NH acts as nucleophile reacts with C2H5Br producing (C2H5)3N

Reaction Pathway with SN2 substitution (OH-, NH3, CN-)

CH3CH2Br + OH- CH3CH2OH + Br- CH3CH2Br + NH3 CH3CH2NH2 + HBrCH3CH2Br + CN- ---- > CH3CH2CN + H2/Ni CH3CH2CH2NH2

(2 carbon) ( 3 carbon ) ( 3 carbon )

Video, SN2 reaction with NH3 and KCN

More Video on SN1 and SN2

Click HERE to view

Thanks to all pictures and video contributors for the above post

Acknowledgements

Thanks to source of pictures and video used in this presentation

Thanks to Creative Commons for excellent contribution on licenseshttp://creativecommons.org/licenses/

Prepared by Lawrence Kok

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