categories of organic categories of organic reactionsreactions

There are so many types of organic There are so many types of organic reactions. We’re going to focus on just a reactions. We’re going to focus on just a few.few.

Addition ReactionsAddition Reactions Substitution ReactionsSubstitution Reactions Elimination ReactionsElimination Reactions Oxidation – Reduction Reaction Oxidation – Reduction Reaction

Bond Breaking:Bond Breaking: Polar and Non-polar Polar and Non-polar

Bond Forming:Bond Forming: Polar and Non-polar Polar and Non-polar

I- Addition ReactionsI- Addition ReactionsIn alkenes and alkynesIn alkenes and alkynes• In In additionaddition reactions, reactions,

reactants are added to reactants are added to the carbon atoms in the carbon atoms in the double or triple the double or triple bond.bond.

• The double or triple The double or triple bond is easily broken, bond is easily broken, since it is highly since it is highly reactive.reactive.

In hydrogenationIn hydrogenation,, • Hydrogen atoms add to the carbon atoms of a double bond or triple Hydrogen atoms add to the carbon atoms of a double bond or triple

bond.bond.• converts unsaturated molecule to saturated converts unsaturated molecule to saturated • alkene or alkyne + Halkene or alkyne + H22 → alkane → alkane• A catalyst such as Pt or Ni is used to speed up the reactionA catalyst such as Pt or Ni is used to speed up the reaction.. Unsaturated vegetable oils reduced to produce saturated fats used in Unsaturated vegetable oils reduced to produce saturated fats used in

margarine and cooking productsmargarine and cooking products

1- 1- HydrogenationHydrogenation(addition of Hydrogen)(addition of Hydrogen)

HC CH + 2H2

NiHC CH

H H

H H

H2C CH2

H HPt

H2H2C CH2 +

2- 2- HydrationHydration(addition of water H(addition of water H22O)O)

• an acid Han acid H++ catalyst is required. catalyst is required.• water (HOH) adds to a double bond.water (HOH) adds to a double bond.• an H atom bonds to one C in the double bond.an H atom bonds to one C in the double bond.• an OH bonds to the other C.an OH bonds to the other C. HH OHOH HH++ │ │ │ │CHCH33─CH=CH─CH─CH=CH─CH33 + + HH──OHOH CHCH33─CH─CH─CH─CH─CH─CH33

HydrationHydration

When hydration occurs with a double bond that has When hydration occurs with a double bond that has anan

unequal number of H atomsunequal number of H atoms,,• the H atom bonds to the C in the double bond the H atom bonds to the C in the double bond

with the more H.with the more H.• the OH bonds to the C in the double bond with the OH bonds to the C in the double bond with

the fewer H atoms.the fewer H atoms. OHOH HH

HH++ │ │ │ │CHCH33─CH=CH─CH=CH22 + + HH──OHOH CHCH33─CH─CH─CH─CH22

3- 3- Halogenation Halogenation

In Halogenation,In Halogenation, halogen atoms add to the carbon atoms halogen atoms add to the carbon atoms

of double bond or triple bond.of double bond or triple bond. Addition of BrAddition of Br22 and Cl and Cl22 to alkenes to yield 1,2- to alkenes to yield 1,2-

dihalidesdihalides

A colour test for c=c BondA colour test for c=c Bond

C C + Br2 C C

Br

Br

HalogenationHalogenation

4-4- Hydrohalogenation HydrohalogenationIn hydrohalogenation,In hydrohalogenation, the atoms of a hydrogen halide add the atoms of a hydrogen halide add

to the carbon atoms of a double bond to the carbon atoms of a double bond or triple bond.or triple bond.

CH3CH=CH2 HBr CH3CH-CH2

Br HCH3CH-CH2

H Br

1-Bromopropane (not observed)

2-BromopropanePropene++

MarkovnikovMarkovnikov’’s Rules Rule In the addition of HX, HIn the addition of HX, H22O, or ROH to an alkene, H O, or ROH to an alkene, H

adds to the carbon of the double bond having the adds to the carbon of the double bond having the greater number of hydrogens.greater number of hydrogens.

II- Substitution Reactions Are 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 Needs ultraviolet lightultraviolet light to initiate the to initiate the reaction.reaction.

Nucleophilic Substitution Nucleophilic Substitution ReactionsReactions

Each step involves the substitution of one nucleophile Each step involves the substitution of one nucleophile (chloride ion, Cl(chloride ion, Cl--, or hydroxide ion, HO, or hydroxide ion, HO--) by another) by another

Nucleophile: a molecule or ion that donates a pair of electrons to another molecule or ion to form a new covalent bond

To be a good leaving group the substituent must be able to leave as a relatively stable, weakly basic molecule or ion

In alkyl halides the leaving group is the halogen substituent-it leaves as a halide ion

Because halide ions are relatively stable and very weak bases, they are good leaving groups

Substitution 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 SThe SNN22 ReactionReactionSSNN2 reaction2 reaction SubstitutionSubstitution,, Nucleophilic Nucleophilic,, Bimolecular. Bimolecular. Essential features of the SEssential features of the SNN22 Reaction Reaction

– Takes place in a single step without Takes place in a single step without intermediatesintermediates

– Incoming nucleophile reacts with the alkyl Incoming nucleophile reacts with the alkyl halide or (the halide or (the substratesubstrate) from a direction ) from a direction opposite the group that is displaced (the opposite the group that is displaced (the leaving group)leaving group)

The SThe SNN22 ReactionReaction The mechanism of the SThe mechanism of the SNN22 reaction reaction

when (when (SS)-2-bromobutane reacts with )-2-bromobutane reacts with ––

OH to give (OH to give (RR)-butan-2-ol)-butan-2-ol The reaction takes place in a single stepThe reaction takes place in a single step Incoming nucleophile approaches from Incoming nucleophile approaches from

a direction 180a direction 180ºº away from the leaving away from the leaving halide ion, thereby inverting the halide ion, thereby inverting the stereochemistry at carbon stereochemistry at carbon

Substitution, Nucleophilic, Bimolecular Substitution, Nucleophilic, Bimolecular –– S SNN22

C X

Nuc : C XNuc CNuc + X

transition state

CarbocationsCarbocations Carbocation:Carbocation: a species in a species in

which a carbon atom has which a carbon atom has only six electrons in its only six electrons in its valence shell and bears valence shell and bears positive chargepositive charge

Carbocations areCarbocations are– classified as 1°, 2°, or 3° classified as 1°, 2°, or 3°

depending on the number depending on the number of carbons bonded to the of carbons bonded to the carbon bearing the carbon bearing the positive charge.positive charge.

– electrophiles; that is, they electrophiles; that is, they are electron-loving. are electron-loving.

– Lewis acids.Lewis acids.– Order of stability:Order of stability:3° › 2° › 1°› methyl.3° › 2° › 1°› methyl.

The SThe SNN1 Reaction1 Reaction Unimolecular nucleophilic substitution Unimolecular nucleophilic substitution

reactionreaction Mechanism of the Mechanism of the

SSNN1 reaction of 2-1 reaction of 2-bromo-2-bromo-2-methylpropane methylpropane with Hwith H22O involves O involves three stepsthree steps– The first step The first step ––

spontaneous, spontaneous, unimolecular unimolecular dissociation of dissociation of the alkyl the alkyl bromide to yield bromide to yield a carbocation a carbocation –– is rate-limitingis rate-limiting

SSNN1 mechanism 1 mechanism 11stst step is rate determining step is rate determining

III-III- Elimination Reactions Elimination Reactions Elimination: removal of atoms or group of atoms from Elimination: removal of atoms or group of atoms from

adjacent carbon to form a carbon-carbon double bondadjacent carbon to form a carbon-carbon double bond These are the opposite of additionsThese are the opposite of additions This used to prepare alkenesThis used to prepare alkenes An important elimination reaction is An important elimination reaction is

DehydrohalogenationDehydrohalogenation

Dehydrohalogenation (-HX)Dehydrohalogenation (-HX)

strongbase

X = Cl, Br, I

+ " "C C

X

H XC C

H

CombustionCombustion

A reaction in which a compound A reaction in which a compound (often carbon) reacts with oxygen (often carbon) reacts with oxygen C + OC + O22 COCO22

CHCH4 4 + 2O+ 2O22 COCO2 2 + 2H+ 2H22OOCC33HH88 + 5O + 5O22 3CO3CO22 + 4H + 4H22OOCC66HH1212OO66 + 6O + 6O22 6CO 6CO22 + 6H + 6H22OO

IV- Oxidation/ReductionIV- Oxidation/Reduction Oxidation:Oxidation: the loss of electronsthe loss of electrons

– alternatively, the loss of H, the gain of alternatively, the loss of H, the gain of O, or bothO, or both

Reduction:Reduction: the gain of electronsthe gain of electrons– alternatively, the gain of H, the loss of alternatively, the gain of H, the loss of

O, or bothO, or both

Oxidation and ReductionOxidation and Reduction3 hydrocarbon oxidation levels3 hydrocarbon oxidation levels

CH3CH3 CH2=CH2 HC CH[O] [O]

oxidation # -3 -2 -1of carbon

Oxidation Oxidation -- ReductionReduction

Oxidation of AlcoholsOxidation of Alcohols– Oxidation of Primary Alcohols to Oxidation of Primary Alcohols to

AldehydesAldehydes A primary alcohol can be oxidized to an A primary alcohol can be oxidized to an

aldehyde or a carboxylic acidaldehyde or a carboxylic acid– The oxidation is difficult to stop at the aldehyde The oxidation is difficult to stop at the aldehyde

stage and usually proceeds to the carboxylic acidstage and usually proceeds to the carboxylic acid

– Oxidation of Secondary Alcohols to Oxidation of Secondary Alcohols to KetonesKetones Oxidation of a secondary alcohol stops at Oxidation of a secondary alcohol stops at

the ketonethe ketone

Oxidation-ReductionOxidation-ReductionReactions involve electron Reactions involve electron

transferstransfers

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

OH

OH

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

OH Br

يــأيــها النبي على يصلــون ومالئكتــه الله يــأيــها إن النبي على يصلــون ومالئكتــه الله إنتسليمـــا وسلمـــوا عليه صلــوا آمنوا تسليمـــا الذيـن وسلمـــوا عليه صلــوا آمنوا الذيـن