•Learning objectives--bond making and breaking process-Attacking species.•Electrophile and nucleophile•FREE RADICLES AND THEIR REACTIONS•Types of reactions-Addition-Substitution-Elimination reactions-hydrolysis

Organic Reactions

- We described hydrocarbons and looked at their structural isomers

- We reviewed how to name hydrocarbons and compounds containing functional groups

We’re now going to focus on a several kinds ofOrganic reactions because without its knowledge we cant understand organic chemistry.

Organic Reaction Types

There are in fact so many types of organic rxn’sIt would be impossible to review them all.

Therefore we’re going to focus on just a few:

- Substitution - Elimination- Addition -Hydrolysis- Polymerization (Condensation & Addition)

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Steps in Mechanisms• We classify the types of steps in a sequence• A step involves either the formation or breaking of a

covalent bond• Steps can occur in individually or in combination with

other steps

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Types of Steps in Reaction Mechanisms

• Bond formation or breakage can be symmetrical or unsymetrical

• Symmetrical- homolytic• Unsymmetrical- heterolytic

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Indicating Steps in Mechanisms• Curved arrows indicate breaking

and forming of bonds• Arrowheads with a “half” head

(“fish-hook”) indicate homolytic and homogenic steps (called ‘radical processes’)

• Arrowheads with a complete head indicate heterolytic and heterogenic steps (called ‘polar processes’)

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Radical Reactions• Not as common as polar reactions• Radicals react to complete electron octet of valence

shell– A radical can break a bond in another molecule and

abstract a partner with an electron, giving substitution in the original molecule

– A radical can add to an alkene to give a new radical, causing an addition reaction

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• Three types of steps– Initiation – homolytic formation of two reactive species with unpaired

electrons• Example – formation of Cl atoms form Cl2 and light

– Propagation – reaction with molecule to generate radical• Example - reaction of chlorine atom with methane to give HCl and

CH3.

– Termination – combination of two radicals to form a stable product: CH3. +

CH3. CH3CH3

Steps in Radical Substitution

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Generalized Polar Reactions• An electrophile, an electron-poor species, combines

with a nucleophile, an electron-rich species• An electrophile (electron poor) is an electron loving

species• A nucleophile is a nucleus (positive charge) loving

species.• The combination is indicate with a curved arrow from

nucleophile to electrophile

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An Example of a Polar Reaction: Addition of HBr to Ethylene

• HBr adds to the part of C-C double bond• The bond is electron-rich, allowing it to function as a

nucleophile• H-Br is electron deficient at the H since Br is much more

electronegative, making HBr an electrophile

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Mechanism of Addition of HBr to Ethylene

• HBr electrophile is attacked by electrons of ethylene (nucleophile) to form a carbocation intermediate and bromide ion

• Bromide adds to the positive center of the carbocation, which is an electrophile, forming a C-Br bond

• The result is that ethylene and HBr combine to form bromoethane

• All polar reactions occur by combination of an electron-rich site of a nucleophile and an electron-deficient site of an electrophile

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Using Curved Arrows in Polar Reaction Mechanisms

• Curved arrows are a way to keep track of changes in bonding in polar reaction

• The arrows track “electron movement” • Electrons always move in pairs• Charges change during the reaction• One curved arrow corresponds to one step in a reaction

mechanism• The arrow goes from the nucleophilic reaction site to the

electrophilic reaction site

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Rules for Using Curved Arrows• The nucleophilic site can be neutral or negatively

charged

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• The electrophilic site can be neutral or positively charged

• Don’t exceed the octet rule (or duet)

Substitution Reactions

• Any reaction in which one atom is replaced by another• Used to place a halogen onto an alkane• The products always are a halocarbon and the acid of

the halogen (ex: hydrobromic acid)• Needs ultraviolet light to initiate the reaction

– Provides the high energy needed to form the excited state

CC

H

H

H

HH

H

+ Cl2hv

CC

H

H

H

Cl

H

H+ HCl

Substitution Rxns

What is the products formed in the following rxn?

CH3CH3 + Br2 sunlight

CH3 CH2Br + HBr

(Why sunlight?)

Elimination Rxns

• Any reaction in which atoms are eliminated from another molecule

• This can be done by

– Elimination of H2

– Elimination of HX– Elimination of H20

Elimination Reactions

-Loss of H2- This process is often referred to as Dehydrogenation

H H H-C-C-H H2C=CH2 + H2 H H Heat, catalyst

This type of rxn takes place in industry in what is know as a catalytic cracking unit

Elimination Rxns

-Loss of HX-Alkyl halides can also undergo elimination. This is as known as dehydrohalogenation

H H H-C-C-H H2C=CH2 + HX H X Base (ex KOH)

The base extract a proton (H+) and X- leaves

Elimination Rxns

-Loss of H2O-Alcohols can undergo elimination via the loss of water. This is known as dehydration

H H H-C-C-H H2C=CH2 + H2O H OH Acid, heat

a) The acid protonates the –OH group, water leaves Positive carbon remains behindb) An adjacent proton (H+) leaves next leaving the electron pair to form the double bond

Addition• Takes place with unsaturated compounds which are

usually more reactive that saturated compounds

– Takes place with both Double and Triple bonds– Two atoms are added across the electron rich bond

• What can be added?– X2

– H2

– H2O– HX

Addition

• Addition of halogen– Normally occurs dissolved in a solvent such as CCl4– Alkenes form dihaloalkanes– Alkynes produce dihaloalkenes or tetrahaloalkanes

CC

H

H

H

H

+ Cl2 CC

H

H

H

H

ClCl

1,2-dichloroethane

Addition• Addition of Hydrogen

– Catalysts normally used such at Pt, Pd or Ni

• Known as Hydrogenation– Alkene becomes an alkane– Alkyne becomes and alkene or alkane

H2C=CH2 + H2 Heat, catalyst

H HH-C-C-H H H

Addition• Addition of Water

– Occurs in the presence of acid (H+)

• Known as Hydration– Alkene becomes an alcohol– Alkynes do not produce alcohols this way! (produces ketone or aldehyde)

Addition• Addition of Hydrogen Halides (HX)

– HX = HCl, HBr, HI (Not HF)– Alkene becomes an alkyl Halide– Alkynes form Monohalo alkenes or dihaloalkanes with the halogens on the same carbon

H2C=CH2 + HX H HH-C-C-H H X

HC=CH + HX H-C-C-H H X

H XH-C-C-H H X

+ HX

Esterification• Alcohol + Organic Acid = Water + Ester• Used to make perfumes, scents and flavors• Combination reaction which involves dehydration.• The alcohol becomes the alkyl group and the acid

becomes -oate

Methyl propanoate

Aspirin – Made by Esterification

H-O-C-CH3 O

+ HO C=O

OH

HO C=O

O-C-CH3 O

Salicylic Acid

Acetyl Salicylic Acid(Common Name)“Aspirin”

(An alcohol and acid)

Acetic acid

Saponification/HYDROLYSIS• Another very old chemical reaction practiced by man

• The hydrolysis of the ester bonds (back to acid + alcohol) in triglycerides using an aqueous sol’n of a strong base to form carboxylate salts and glycerol

• Hydrolysis of fats by a strong base (KOH or NaOH)– Products are soap and glycerol (a triol)

OCH2-0-C-(CH2)14CH3| OCH2-O-C-(CH2)14CH3| OCH2-0-C-(CH2)14CH3

+ 3KOH

OCH2-0H K+ -O-C-(CH2)14CH3| OCH2-OH K+ -O-C-(CH2)14CH3| OCH2-0H K+ -O-C-(CH2)14CH3

A TRIGYCERIDE GLYCEROL 3 SOAP MOLECULES

+

PolymerizationThere are two methods we’ll quickly look at forForming Polymers.

- Addition polymerization

- Condensation polymerization

Home workWhat are bond making and breaking processes.Discuss different types of reactions.Discuss free radical substitution reactions.Five examples of nucleophile and electrophiles.