nucleophilic substitution sn1 sn2 nucleophile halogenoalkane in organic chemistry
TRANSCRIPT
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
Check out more video tutorials from my site and hope you enjoy this tutorialhttp://lawrencekok.blogspot.com