Sunday, December 21, 2014

Chemistry Concepts Recall U to Z



U

Uranium,



V

Valence, Valence bond approach. VSEPR Model

W

X

Y

Z

Zerothlaw of thermodynamics

Chemistry Concepts Recall P to T



P

Phosphorous,

Q


R

Radium, Reaction, Reactant, Roasting,

S

Sulphur

T

Titanium, Tritration, Tungsten,

Chemistry Concepts Recall K to O



k

Kossel - Lewis approach to bonding

L

Liquid state

M

Magnesium, Manganese,Mercury, Molybdenum,

N

Nickel,


O

Organic Chemistry, Oxidation, Oxygen,

Chemistry Concepts Recall F to J




F

Fission of a covalent bond, Fundamental particles, Fusion,

G

Gaseous state, Gibbs energy, Graham's law of diffusion or effusion,

H

Hydrogcarbons, Hydrogen

I

Ionization potential

J


IIT JEE Revision - Electrophilic Addition Reactions of Alkenes with X2, HX, HOX and H2O (X=halogen)

Reaction mechanism

Reaction takes place in two steps.

Step 1. Attacking molecule gets partially polarised and as it becomes closer to the pi bond of the double bond, the electron cloud of the pi bond repels the electron cloud of the attacking molecule further. As a result, the nearer end of the attacking molecule acquires partial positive charge. At the same time, electromeric effect comes into operation in the double bond and the pi electron pair shifts to one of the carbon atoms making it negatively charged. Thus the partially positively charged atom of the attacking molecule attacks the negatively charged carbon of the alkenes and a new bond is formed. This leaves the other carbon atom with positive charge and also the other atom of the attacking molecule with negative charge.

Step 2: The negatively charged atom of the attacking molecule reacts with positively charged carbon of the alkene to complete the formation of addition product.



Electrophilic Addition Reactions of Alkenes with X2, HX, HOX and H2O (X=halogen)


Halogens (particularly chlorine and bromine) react with alkenes in the presence of an inert solvent (e.g. CCl4) to form dihalogen derivatives:

The reaction with flourine is explosive whereas iodine reacts very slowly.

Alkenes react with halogen acids (HCL, HBr, or HI) to form alkyl halides.

In these reactions one part of the molecule attaches itself to one carbon atom of the double bond whereas the other part to the second carbon atom of the double bond.

However,if the alkene is unsymmetrical, then two products are possible depending upon the carbon atom to which the halogen atom is attached.

Markovnikov rule: during the addition across unsymmetrical multiple bond, the negative part of the attacking reagent joins with the carbon atom which carries smaller number of hydrogen atoms while the positive part goes to the carbon atom with more hydrogen atoms.

Due to fact that the reaction proceeds according to Markow(v)nikov's explanation, addition of HBr to Propene gives 2-Bromopropene as the major product up to 90%.

Exception to Markovnikov rule - Kharasch effect - Peroxide effect: During the addition of HBr to an unsymmetrical alkene in the presence of organic peroxids (e.g., benzoyl peroxide), Br atom will join to the carbon carrying more hydrogen atoms while H atom will go to the other carbon atom.

Only HBr shows peroxide effect. HF, HCl and HI do not exhibit peroxide effect.

Alkenes react with hypohalous acids (HOX) or halogen Cl2 or Br2 in the presence of H2O to give halohydrins. In this reactin, markonikov;s rule is followed and halogen is the positive aprtg and OH is the negative part.

Water adds to alkenes in the presence of mineral acids (catalytic hydration of alkenes). Addition occurs in accordance with Markownikov's rule and we get alcohols from this addition.

Ethene gives ethanol
Propene gives Propan-2-ol as the major product.

21 December - Chemistry Knowledge History




John Mayow baptized 1641 (birth date uncertain): discovered that air contained two gases, one of which ("spiritus nitro-aerous") supported life and combustion.

Hermann Joseph Muller born 1890: theory of genes; mutation by X-rays; Nobel Prize (medicine), 1946.




December - Chemistry Knowledge History

John Mayow's Scientific Work

Mayow published at Oxford in 1668 two tracts, on respiration and rickets,


Accepting Boyle's experiments and theory that air is necessary for combustion, Mayow showed that fire is supported not by the air as a whole but by a more active and subtle part of it. This part he called "spiritus igneo-aereus," or sometimes "nitro-aereus", In combustion the nitro-aereae  supplied by the air combined with the material burnt.  Mayow observed  that antimony, strongly heated with a burning glass, undergoes an increase of weight  and he attributed it  to nothing else but these particles.

Mayow argued that the same particles are consumed in respiration, because he found that when a small animal and a lighted candle were placed in a closed vessel full of air the candle first went out and soon afterwards the animal died. However, if there was no candle present the animal lived twice as long. He concluded that this constituent of the air is absolutely necessary for life, and supposed that the lungs separate it from the atmosphere and pass it into the blood. Mayow also came out with the idea that muscles work or contract due to combination of nitro aereus  with other combustible (salino-sulphureous) particles in the body; hence the heart, being a muscle, ceases to beat when respiration is stopped. Heat in animals is due to the union of nitro-aerial particles, breathed in from the air, with the combustible particles in the blood, and it occurs in muscles during violent exertions.

In effect, therefore, Mayow gave a remarkably correct anatomical description of the mechanism of respiration and argued for the existence of oxygen, under the guise of his "spiritus nitro-aereus," as a separate entity distinct from the general mass of the air. Mayow perceived the part "spiritus nitro-aereus" plays in combustion and in increasing the weight of the calces (oxides) of metals as compared with metals themselves. Mayow described inspiration a mechanism for introducing oxygen into the body, where it is consumed for the production of heat and muscular activity. He even vaguely conceived of expiration as an excretory process. Using bell-jars over water Mayow showed that the active substance - nitro-aereus that we today call oxygen constitutes about a fifth part of the air.

http://en.wikipedia.org/wiki/John_Mayow


Mutation of genes

Genes mutate due to thermal agitations. One gene may mutate but others around may remain stable.
Therefore high energy radiation can produe gene mutations.

Read Muller's Nobel Lecture on Mutation of Genes
http://www.nobelprize.org/nobel_prizes/medicine/laureates/1946/muller-lecture.html


December Month Chemistry Knowledge History

Saturday, December 20, 2014

22 December - Chemistry Knowledge History

Vladimir Markovnikov born 1838: synthesis of cyclobutane and cyclopentane derivatives; Markovnikov's rule for additions to alkenes.
William Lloyd Evans born 1870: chemistry of carbohydrates;
John Clarke Slater born 1900: orbital approaches to quantum chemistry (Slater-type orbitals, Slater determinant); tetrahedral carbon compounds.
Arie Jan Haagen-Smit born 1900: nature and source of smog; smog abatement.




Markovnikov's rule for additions to alkenes.

When  the alkene which is unsymmetrical reacts with halogen acid,  two products are possible depending upon the carbon atom to which the halogen atom is attached.

Markovnikov rule: during the addition across unsymmetrical multiple bond, the negative part of the attacking reagent joins with the carbon atom which carries smaller number of hydrogen atoms while the positive part goes to the carbon atom with more hydrogen atoms.

Due to fact that the reaction proceeds according to Markow(v)nikov's explanation, addition of HBr to Propene gives 2-Bromopropene as the major product up to 90%.

Exception to Markovnikov rule - Kharasch effect - Peroxide effect: During the addition of HBr to an unsymmetrical alkene in the presence of organic peroxids (e.g., benzoyl peroxide), Br atom will join to the carbon carrying more hydrogen atoms while H atom will go to the other carbon atom.

Propene + HBR in the presence of Benzoyl peroxide gives 1-Bromopropane.