Presentation on theme: "O RGANIC R EACTIONS Dr. M. Abd-Elhakeem Faculty of Biotechnology Organic Chemistry Chapter 3."— Presentation transcript:
O RGANIC R EACTIONS Dr. M. Abd-Elhakeem Faculty of Biotechnology Organic Chemistry Chapter 3
A DDITION REACTIONS Addition reactions are organic reactions where two or more molecules combine to form a larger one. Addition reactions are limited to chemical compounds that have multiply bonded atoms, such as molecules with carbon-carbon double bonds, i.e., alkenes, or with triple bonds, i.e., alkynes. Also included are molecules containing carbon - hetero double bonds like those with carbonyl (C=O) groups or those with imine (C=N) groups.organic reactiondouble bondsalkenestriple bondsalkyneshetero carbonylimine
There are three main types addition reactions Electrophilic additionElectrophilic addition (addition of positive part of a regent to an electron rich part of a molecule) Nucleophilic additionNucleophilic addition (addition of negative part of a regent to an electron poor part of a molecule) Free radical additionFree radical addition: the symmetrical reagent divided into symmetrical radicals instead of charged ions
Addition on Carbon – Carbon double bond
When an unsymmetrical reagent (like HBr) adds to unsymmetrical alkene (like CH 3 CH=CH 2 ) the positive part of the reagent becomes attached to the double bonded carbon atom having the greatest number of hydrogen atoms Markovinkove's rule:
H + Br-
The addition reagent may be HCl, HCN, HOCl that react with alkene by the same mechanism.
Anti-Markovinkove's rule: The presence of electron withdrawing substituent in the alkene molecule or presence of peroxide as a catalyst yield product of anti Markovinkove's rule. i.e. hydrogen is added to the carbon atom that has the lower number of hydrogen
The symmetrical reagent will be added by the free radical mechanism Alkynes make this reaction to give disubstituted alkene
Nucleophilic addition on carbonyl group
+ - H+H+ CN - cyanohydrine
E LIMINATION REACTIONS opposite of addition - one reactant splits into two products. An important class of elimination reactions are those involving alkyl halides, reacting with a base to form an alkene alkyl halides basealkene
The second reaction is not considered elimination reaction but substitution reaction Note that the difference in the solvent of the reagent change the product completely
Another example of elimination reaction is the dehydration of alcohols where one alcohol molecule loss a molecule of water to form alkene
Mechanism a- protonation of alcohol b- Elimination of water molecule to yield carbonium ion
c- Carbonium ion lose a proton to give the alkene The ease of the reaction is 3 o ry > 2 o ry > 1 o ry Easiness of the reaction means needing of lower energy, increasing of reaction rate, obtaining higher yield.
In case of alcohol excess the product is ether by elimination of one water molecule from each two molecules of alcohols
R EARRANGEMENT REACTIONS are broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another atom in the same molecule.