Nucleophilic substitution sn1 sn2 nucleophile halogenoalkane in organic chemistry
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Transcript of Nucleophilic substitution sn1 sn2 nucleophile halogenoalkane in organic chemistry
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Nucleophilic Substitution, SN1, SN2, Nucleophile, HalogenoalkaneIn Organic Chemistry
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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
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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
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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
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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
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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)
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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
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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
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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
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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
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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
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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
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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 )
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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
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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
Check out more video tutorials from my site and hope you enjoy this tutorialhttp://lawrencekok.blogspot.com