Wednesday, May 1, 2013

HYDROGEN BONDING

The electrostatic force of attraction between partially positively charged hydrogen in a polar molecule and an electronegative atom is called hydrogen bond.
It is represented by dotted line as shown below.
H--------X
where X is an electronegative atom.
Characteristics of Hydrogen bond 
1) The nature of H-bonding is purely electrostatic.
2) The hydrogen atom must be covalently bonded to a highly electronegative atom in order to acquire sufficient positive charge to participate in hydrogen bonding.
3) The electronegative atom should possess high charge density so as to attract positively charged hydrogen atom effectively. Thus H-bonding is observed with smaller and highly electronegative atoms such as F, O and N.
4) The hydrogen bond is stronger than van der Waal's forces of attraction. However it is weaker than the covalent bond and ionic bond.
5) The strength of H- bond is directly proportional to the electronegativity of electronegative atom. Thus the strengths of hydrogen bonds with different electronegative atoms varies as follows:
H------F > H------O > H-------N

Read more at: HYDROGEN BONDING

Tuesday, April 30, 2013

ETHYL CHLORIDE | ALCOHOLIC KOH | ELIMINATION | DEHYDROHALOGENATION


Why elimination occurs instead of nucleophilic substitution when ethyl chloride reacts with alcoholic potassium hydroxide?
Explanation:
Ethyl chloride can undergo both nucleophilic substitution as well as elimination reactions with strong alkali like KOH.
When it undergoes nucleophilic substitution, ethyl alcohol is formed as major product.
More at:
ETHYL CHLORIDE | ALCOHOLIC KOH | ELIMINATION | DEHYDROHALOGENATION

Sunday, November 11, 2012

200 solved problems in advanced organic synthesis for CSIR NET & GATE exams

As promised, the “Solved Problems in Advanced Organic Synthesis” book for CSIR NET & GATE exams is updated with 200 problems and is available freely to those who have purchased it previously. However, the rate is now revised to Rs. 875/-. Please download your updated version from the link I have provided at the time of purchase. Please mail me for the new password.
Further updates are NOT free. I have changed the policy. You need to participate in the discussions and help me to get further updates. Mail me for further details.

Click here to check the sample copy of Solved Problems in Advanced Organic Synthesis at Google docs

Wednesday, March 23, 2011

Basic introduction to concepts in organic chemistry at adichemistry

Let us start with the question "What is Organic chemistry?".
The simple answer is: It is the chemistry of carbon containing compounds, which are otherwise known as organic compounds.
So it is pretty easy to recognize that we should start our journey of organic chemistry by exploring the chemical nature of carbon.
So the next question is: What is carbon?
    * Carbon is an element with atomic number (Z) = 6.
    * Its ground state electronic configuration can be represented as: 1s22s22p2 (or) 1s22s22px12py12pz0

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Friday, March 18, 2011

Wurtz reaction at adichemistry


* In Wurtz reaction, two alkyl halide molecules are coupled in presence of sodium metal in anhydrous ether or tetrahydrofuran to form a new carbon carbon bond and thus by giving a symmetrical alkane.
Dry Ether
R-X  +  2Na  +  X-R ----------------->  R-R + 2NaX  
Where X = halogen
* The Wurtz reaction must be performed under anhydrous conditions because the alkyl free radical formed (see the mechanism) during the reaction is strongly basic and can abstract proton from water.
* In case of alkyl and aryl fluorides as well as aryl chlorides, tetrahydrofuran is used as solvent instead of ether. 
* The Wurtz reaction is limited to synthesis of symmetrical alkanes with even number of carbon atoms only. The number of carbons in the alkane is double that of alkyl halide (n ---> 2n type reaction)
* If dissimilar alky halides are used, a mixture of alkanes is formed. It is usually difficult to separate the mixture and hence wurtz reaction not a suitable method to synthesize unsymmetrical alkanes.
E.g. The Wurtz reaction between R-X and R'-X yields not only R-R' but also R-R and R'-R'. This mixture cannot be separated easily.
* Methane cannot be prepared by this method.
* A modification of this reaction involving alkyl and aryl halides is called Wurtz-Fittig reaction. If only aryl halides are subjected to coupling, the reaction is called as Fittig reaction.