Organic Reactions PDF

ORGANIC REACTIONS  Writing Equations for Organic Reactions Equations for organic reactions are usually drawn with a single reaction arrow () between the starting material and product. The reagent, the chemical substance with which an organic compound reacts, is sometimes dra...

ORGANIC REACTIONS  Writing Equations for Organic Reactions Equations for organic reactions are usually drawn with a single reaction arrow () between the starting material and product. The reagent, the chemical substance with which an organic compound reacts, is sometimes drawn on the left side of the equation with the other reactants. At other times, the reagent is drawn above the arrow itself.  ORGANIC REACTIONS Although the solvent is often omitted from the equation, most organic reactions take place in liquid solvent. The solvent and temperature of the reaction may be added above or below the arrow. The symbols “h” and “” are used for reactions that require light and heat respectively.  ORGANIC REACTIONS  Different ways of writing organic reactions   ORGANIC REACTIONS  When two sequential reactions are carried out without drawing any intermediate compound, the steps are usually numbered above or below the reaction arrow. This convention signifies that the first step occurs before the second step, and the reagents are added in sequence, not at the same time.  KINDS OF ORGANIC REACTIONS Addition reaction – two or more molecules combine to form a larger one.  compounds that have multiple bonds,  carbon-carbon double bonds C=C (alkenes),  with triple bonds C≡C (alkynes).  Molecules containing carbon—hetero double bonds like  carbonyl (C=O) groups, or amine (C=N) groups  undergo addition as they too have double bond character.  ADDITION REACTION  Addition is a reaction in which elements are added to the starting material  Addition of Halogens to Alkenes ▪ Bromine and chlorine add to alkenes to give 1,2-dihaldes, an industrially important process ▪ F2 is too reactive and I2 does not add ▪ Cl2 ( Chlorination) reacts as Cl+ Cl- ▪ Br2 is similar ▪ addition of Cl2 (Chlorination)  ADDITION REACTION Two reactants combine together to make one product. - Addition of Br2 - Bromination  ADDITION REACTION  alkene gives the products of alkyl halide  Addition of HX X–halogen element Cl, Br, and F  hydrohalogenation  ADDITION REACTION  alkene gives the products of alkyl halide  Addition of HX  In the addition of HX to alkene, the H attaches to the carbon with fewer alkyl substituents and the X attaches to the carbon with more alkyl substituents.  ADDITION REACTION  alkene gives the products of alkyl halide  Addition of HBr  Addition of HX to Alkenes: Markovnikov’s Rule ▪ Markovnikov observed in the 19th century that in the addition of HX to an alkene, the H attaches to the carbon with more H’s and X attaches to the other end (to the one with more alkyl substituents). This is Markovnikov’s rule Addition using Markovnikov’s Rule  ADDITION REACTION In an addition reaction, new groups X and Y are added to the starting material. A  bond is broken and two  bonds are formed.  hydration  alkene + water ---- alcohol ADDITION OF H2O TO ALKENES: MARKOVNIKOV’S RULE  Addition reactions to Alkynes ▪ Addition of H2 using chemically deactivated palladium on calcium carbonate as a catalyst (the Lindlar catalyst) produces a cis alkene ▪ The two hydrogens add syn (on the same side of the triple bond)  Addition of Bromine to alkynes ▪ Initial addition gives trans intermediate ▪ Product with excess reagent is tetrahalide  Elimination reactions - one molecule splits into two  In most organic elimination reactions, hydrogen are lost to form the double bond:.  Elimination is a reaction in which elements of the starting material are “lost” and a  bond is formed.  Addition and elimination reactions are exactly opposite. A  bond is formed in elimination reactions, whereas a  bond is broken in addition reactions.  In an elimination reaction, two groups X and Y are removed from a starting material. Two  bonds are broken, and a  bond is formed between adjacent atoms. The most common examples of elimination occur when X = H and Y is a heteroatom more electronegative than carbon.  ELIMINATION REACTIONS  with acid catalyzed reaction of an alcohol to yield water and an alkene  dehydration – removal of water yield alkene and water with acid catalyst  dehydration of ethanol with acid catalyst yield ethene  ethanol -- ethene  ELIMINATION REACTIONS  dehydration of cyclohexanol with acid catalyst yield cyclohexene  cyclohexanol -- cyclohexene  ELIMINATION REACTIONS H Br H H C C H + NaOH C=C + H2O H H H H H + NaBr  ELIMINATION REACTIONS  dehydrogenation of 2-methylpropane --->  product 2-methylpropene Rearrangement reactions – a molecule undergoes changes in the way its atoms are connected.  Rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomers of the original molecule. ( isomers are groups of atoms with the same molecular formula but with different arrangement.)  REARRANGEMENT REACTION  Often a substituent moves from one atom to another atom in the same molecule. In the example below the substituent R moves from carbon atom 1 to carbon atom 2: REARRANGEMENT REACTION  Substitution reaction – A functional group in a particular chemical compound is replaced by another group.  parts from two molecules exchange  SUBSTITUTION REACTION A substitution is a reaction in which an atom or a group of atoms is replaced by another atom or group of atoms. In a general substitution, Y replaces Z on a carbon atom.   SUBSTITUTION REACTION  Substitution reactions involve  bonds: one  bond breaks and another forms at the same carbon atom.  The most common examples of substitution occur when Z is a hydrogen or a heteroatom that is more electronegative than carbon. SUBSTITUTION REACTION  SUBSTITUTION REACTION  Bromination-of-benzene SUBSTITUTION REACTION SUBSTITUTION REACTIONS OF BENZENE AND ITS DERIVATIVES  SUBSTITUTION REACTIONS In a substitution reaction, a hydrogen atom on a benzene ring is replaced by an atom or group of atoms. Type of substitution H on benzene replaced by  Halogenation Chlorine or bromine atom (Chlorination or Bromination)  Nitration Nitro group (—NO2)  Sulfonation —SO3H group  HALOGENATION In the halogenation of benzene  an H atom of benzene is replaced by a chlorine or bromine atom  a catalyst such as FeCl3 is needed in chlorination  a catalyst such as FeBr3 is needed in bromination  NITRATION In the nitration of benzene,  an H atom of benzene is replaced by a nitro (–NO2) group from HNO3  an acid catalyst such as H2SO4 is needed  SULFONATION In the sulfonation of benzene,  an H atom on benzene is replaced by a —SO3H group from SO3  an acid catalyst such as H2SO4 is needed SUMMARY TABLE: EFFECT OF SUBSTITUENTS IN AROMATIC SUBSTITUTION  Ortho- And Para- Directing Groups 2- and 4- directing groups: –OH, NH2 OH 2nd substitution at 2- or 4- positions 1 6 2 nitration of phenol OH OH OH 5 3 6 1 2 NO2 4 + 5 3 4 4 NO2  Meta- Directing Groups OH 3- directing groups: –NO2 1 6 2 chlorination of nitrobenzene meta – directing group NO2 NO2 5 3 1 4 6 2 5 3 Cl 4 NITRATION OF BENZALDEHYDE O O2 N O CH HNO3 H2SO4 CH 75-84%  META DIRECTING O Cl CCl Cl2 O FeCl3 CCl 62% BROMINATION OF NITROBENZENE Br NO2 Br2 Fe NO2 60-75%