Wednesday, July 20, 2011

Hydrohalogenation - Electrophilic Addition of HX


During this reaction,

  • 2 bonds are broken, pi bond and H-X bond
  • 2 bonds are formed, C-X and C-H
  • In H-X and H has a small positive charge X is negatively charged.
  • H is attracted to electron rich double bond.
  • Therefore Electrophilic addition occurs.
  • Addition reactions are exothermic.
  • 2 sigma bonds formed are stronger than the the sigma and pi bond broken.

Mechanism 
consists of 2 successive Lewis acid-base reactions. ( 2 step mechanism)
[1] Alkene is the Lewis base. H-X is the Lewis acid.
     The Activation energy is higher than that of [2].
     Therefore [1] is slower than [2].
     2 Bonds break only one is formed.
     Therefore ΔH01 is (+)
     Results with the formation of a carbocation. 
     Rate determining step.
[2] X- is the Lewis base. Carbocation is the Lewis acid.
     The Activation energy is lower than that of [1].
     Therefore [2] is faster than [1].
     Only bond making occurs,one bond is formed.
     Therefore ΔH01 is (-)
  • Markovnikov's Rule  - With an unsymmetric alkene H atom bonds to the less substituted carbon.
  • Hammond postulate  - Structure of the transition state of an endothermic reaction resembles the products while that of an exothermic reaction resembles reactants.


Stereo Chemistry
A new stereogenic center is formed. New enantiomers are formed in equal amounts making a racemic mixture.This is due to the fact that the nucleophilic attack of the X- occurs from either side of the trigonal planar carbocation and have equal probability. 
Therefore syn and anti addition can occur.  

Extra reading




Alkenes



In alkenes ,
  • the pi bond is weaker than the sigma bond.
  • trans alkenes are generally more stable than cis alkene.
  • stability in creases as the number of R groups connected to the double bond increases.
Have Van der Waals interactions.
Therefore melting point(mp) s and boiling point(bp)s are low.
Mp and bp increases as the number of C atoms increases. This is due to the increasing surface area.
C-C bond between an alkyl bond and one of the double bond carbons is slightly polar because sp3 hybridized carbon donates electron density to the sp2 hybridized carbon.

Reactions
Characteristic reaction of Alkenes is Addition.
Pi bond is broken and two sigma bonds are formed.
Because alkenes are electron rich they react with Electrophiles.
Simple alkenes do not react with nucleophiles or electron rich reagents.
Because the C atoms of the double bond are trigonal planar addition can occur from either from the same side(syn) or both sides(anti).

Important Addition reactions of Alkenes
  1. Hydrohalogenation
  2. Hydration
  3. Halogenation
  4. Halohydrin formation
  5. Hydrocarbonation ( Oxidation)

Thursday, July 7, 2011

Nucleophiles and Bases


Similarities and Differences

Nucleophiles and bases are structurally similar. They both have either a lone pair or a pi bond. They differ from what they attack. bases attack protons while
nucleophiles attack other electron deficient atoms,
usually carbons.
Basicity is a measure of how readily an atom donates its electron pair to a proton.
It is characterized by an equilibrium constant in acid-base reactions
thus a thermodynamic property.
Nucleophilicity is a measure of how readily an atom donates its electron pair to another atom,
characterized by the rate constant
thus a kinetic property.

Nucleophilicity// basicity in 3 instances

1.) From two nucleophiles with same nucleophilic atom, the stronger base is the stronger nucleophile
eg :- OH- is stronger nucleophile than CH3COO- because H2O is more basic than CH3COOH ( pKa H2O > pKa CH3COOH )
2.) Negatively charged nucleophiles are stronger than its conjugated acid
eg :- OH- is a stronger nucleophile than H2O
3) In a row of the periodic table nucleophilicity and basicity increases from right to left.

Steric Hindrance

But when steric hindrance is involved nucleophilicity does not // basicity. More the steric hindrance is lesser the nucleophilicity is. eg :- (CH3)3O- has more steric hindrance than CH3Ch2O-.Out of both stronger base is (CH3)3O- while stronger nucleophile is CH3Ch2O-. This is because while a proton can move near the stericaly hindered (CH3)3O- the CH3 groups repel other larger atoms and groups.

Polar Protic Solvents
In polar protic solvents anions form strong inter molecular hydrogen bonds. Smaller nucleophiles are strongly solvated because smaller the nucleophile more concentrated the charge is.Therefore smaller nucleophiles are weaker nucleophilesin protic solvents such as water and ROH.
RS- is stronger nucleophile than RO-

Polar Aprotic Solvents

In polar aprotic solvents such as acetone,acetonitrile,... the anions form dipole - dipole inter molecular interactions and are incapable of forming hydrogen interactions. Cations form ion - dipole interactions. Because dipole -dipole interactions are not as strong as hydrogen interactions the anions are not very well solveted.. Therefore the anions in polar aprotic solvents are highly reactive as both nucleophiles and bases. Thus smaller the size of the atom stronger the nucleophilicity.

Common Nucleophiles
Negatively Charged
O : OH- ,OR-,CH3COO-
N : H3-
C : CN- , CHC-
X- : Cl-, Br-, I-
S : HS- , RS-

Neutral
O : H2O ,ROH
N : NH3 ,RNH2
S : H2S ,RSH

Wednesday, July 6, 2011