Organic ChemistryAGUNG RAHMADANIDEPARTMENT OF PHARMACYUNIVERSITY OF MULAWARMANSAMARINDA2014Types of Organic Reactions

categories of organic reactions There are so many types of organic reactions. Were going to focus on just a few.Addition ReactionsSubstitution ReactionsElimination ReactionsOxidation Reduction Reaction

I- Addition ReactionsIn alkenes and alkynesIn addition reactions, reactants are added to the carbon atoms in the double or triple bond.The double or triple bond is easily broken, since it is highly reactive.

In hydrogenation, Hydrogen atoms add to the carbon atoms of a double bond or triple bond.converts unsaturated molecule to saturated alkene or alkyne + H2 alkaneA catalyst such as Pt or Ni is used to speed up the reaction.Unsaturated vegetable oils reduced to produce saturated fats used in margarine and cooking products

1- Hydrogenation(addition of Hydrogen)

2- Hydration(addition of water H2O)an acid H+ catalyst is required.water (HOH) adds to a double bond.an H atom bonds to one C in the double bond.an OH bonds to the other C. H OH H+ CH3CH=CHCH3 + HOH CH3CHCHCH3

HydrationWhen hydration occurs with a double bond that has anunequal number of H atoms,the H atom bonds to the C in the double bond with the more H.the OH bonds to the C in the double bond with the fewer H atoms. OH H H+ CH3CH=CH2 + HOH CH3CHCH2

3- Halogenation In Halogenation, halogen atoms add to the carbon atoms of double bond or triple bond. Addition of Br2 and Cl2 to alkenes to yield 1,2-dihalides

A colour test for c=c Bond

Halogenation

4- HydrohalogenationIn hydrohalogenation, the atoms of a hydrogen halide add to the carbon atoms of a double bond or triple bond.

Markovnikovs Rule In the addition of HX, H2O, or ROH to an alkene, H adds to the carbon of the double bond having the greater number of hydrogens.

II- Substitution ReactionsAre characteristic reaction of saturated compounds such as alkanes.In a substitution reaction, one atom, ion or group is replaced (substituted) by anotherCH4 + Cl2 CH3Cl + HClCH3Cl + Cl2 CH2Cl2 + HClCH2Cl2 + Cl2 CHCl3 + HClCHCl3+ Cl2 CCl4 + HCl

Needs ultraviolet light to initiate the reaction.

Nucleophilic Substitution ReactionsEach step involves the substitution of one nucleophile (chloride ion, Cl-, or hydroxide ion, HO-) by anotherNucleophile: a molecule or ion that donates a pair of electrons to another molecule or ion to form a new covalent bondTo be a good leaving group the substituent must be able to leave as a relatively stable, weakly basic molecule or ionIn alkyl halides the leaving group is the halogen substituent-it leaves as a halide ionBecause halide ions are relatively stable and very weak bases, they are good leaving groupsSubstitution of alkyl halides with nuclophiles can occur by an SN2 or SN1 path, a fundamental difference between them is the timing of bond breaking and bond forming steps

The SN2 ReactionSN2 reactionSubstitution, Nucleophilic, Bimolecular.Essential features of the SN2 Reaction Takes place in a single step without intermediatesIncoming nucleophile reacts with the alkyl halide or (the substrate) from a direction opposite the group that is displaced (the leaving group)

The SN2 Reaction The mechanism of the SN2 reaction when (S)-2-bromobutane reacts with OH to give (R)-butan-2-olThe reaction takes place in a single stepIncoming nucleophile approaches from a direction 180 away from the leaving halide ion, thereby inverting the stereochemistry at carbon

Substitution, Nucleophilic, Bimolecular SN2

CarbocationsCarbocation: a species in which a carbon atom has only six electrons in its valence shell and bears positive chargeCarbocations areclassified as 1, 2, or 3 depending on the number of carbons bonded to the carbon bearing the positive charge.electrophiles; that is, they are electron-loving. Lewis acids.Order of stability:3 2 1 methyl.

The SN1 Reaction Unimolecular nucleophilic substitution reactionMechanism of the SN1 reaction of 2-bromo-2-methylpropane with H2O involves three stepsThe first step spontaneous, unimolecular dissociation of the alkyl bromide to yield a carbocation is rate-limiting

SN1 mechanism 1st step is rate determining

III- Elimination ReactionsElimination: removal of atoms or group of atoms from adjacent carbon to form a carbon-carbon double bondThese are the opposite of additionsThis used to prepare alkenesAn important elimination reaction is Dehydrohalogenation

Dehydrohalogenation (-HX)

CombustionA reaction in which a compound (often carbon) reacts with oxygen C + O2CO2CH4 + 2O2CO2 + 2H2OC3H8 + 5O23CO2 + 4H2OC6H12O6 + 6O2 6CO2 + 6H2O

IV- Oxidation/ReductionOxidation: the loss of electronsalternatively, the loss of H, the gain of O, or bothReduction: the gain of electronsalternatively, the gain of H, the loss of O, or both

Oxidation and Reduction3 hydrocarbon oxidation levels

Oxidation - Reduction

Oxidation of AlcoholsOxidation of Primary Alcohols to AldehydesA primary alcohol can be oxidized to an aldehyde or a carboxylic acidThe oxidation is difficult to stop at the aldehyde stage and usually proceeds to the carboxylic acid

Oxidation of Secondary Alcohols to KetonesOxidation of a secondary alcohol stops at the ketone

Oxidation-ReductionReactions involve electron transfers

Oxidation of biomolecules often occurs as dehydrogenation, electron acceptor which are usually coenzymes are needed for such reactions to occur.

Practice Exercises:Classify the following rxns as substitution, elimination, or addition.

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