Organic Chemistry I
CHEM 201
1st Term 2011-2012
Instructor:
Assoc. Prof. Khaled S. Abdel Halim
k.abdulhalem@uoh.edu.sa
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Chapter VI:
Free Radical Reactions
In this chapter, we shall study the following topics:
• What is the meaning of radical reaction.
• Kinetics of radical reactions.
• Free radical Initiators and Inhibitors.
• Polymers.
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What is meant by radical reactions?!
*Free radical reactions are used to prepare industrially organohalogen compounds. Radical
reactions have biological and practical importance.
* Many polymerization reactions are free radical reactions.
Radical is an atom or group of atoms that has one or more unpaired electrons (incomplete
octet), its highly reactive, highly energy, unstable, short- lived, nonisolable intermediate.
The chlorination of methane is a classical example of a free radical reactions.
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Mechanism of free radical reactions
Free radical reaction is a stepwise reaction including the following steps:
* Initiation
* Propagation
* Termination
(1) Homolytic cleavage
Chain reaction
(2) Propagation
Radical reactions are often characterized by many products because the reaction is not a
selective.
Write equations for the chlorination of : ethane methane
cyclohexane
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Relative reactivities of halogens
Free radical chlorination of methane is exothermic reaction (heat releasing).
The relative reactivity of halogen toward alkane is vary from Iodine to fluorine. F2
undergoes explosive reactions with hydrocarbons, I2 is non reactive toward alkanes. Cl2 &
Br2 are the most useful radical halogenating agents. This is because of lower activation
energy of the rate determining step.
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Kinetics of free radical reactions
The kinetics of radical reactions are quite complex because the radical reaction is a cyclic
process (chain reaction). But there are some evidences point to hydrogen abstraction step
as the rate determining step (slow step). The question become which hydrogen is
abstracted?
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The previous reactivity sequence arises from the stabilities of the transition states leading
to the radical. It’s just like that of carbocation stability (increase as we proceed from
methyl to tertiary because of increasing the stability……..(more stable=less energy).
The possibility of resonance structure will
enhance the radical reactivity because it increase
the stability of carbocation intermediate as in
the case of allylic and benzylic structures
Unlike carbocations, radical
rearrangements are not common.
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Selective radical halogenations: Br2 or Cl2 is the best halogen for radical reactions?
The high selectivity of Br2 arises from its less reactivity than Cl2
The hydrogen abstraction step in chlorination reaction is exothermic with low value
activation of transition state close to the reactants. While the same step in bromination
reaction is endothermic with high value activation energy of transition state close to
products.
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Benzylic and allylic halogenation
The same rules can be applied to benzylic
and alkene (allyic position)
Different experimental conditions should be applied to prevent reaction with double bond in
alliylic or benzylic compounds such as low concentration of halogen or high temperature
reaction or using specific reagent such as NBS (N-bromosuccunimide) which provide Br
radical at very low concentration.
At high concentration of Br2,
addition reaction is occurred.
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Other radical reactions:
There are some organic reactions rather than halogenation reactions occur by
radical mechanism such as:
Pyrolysis
&
autoxidation
Pyrolysis is the thermal decomposition of organic compounds in the absence of oxygen.
Pyrolysis is used industrially for the cracking of high molecular weight compunds into
lower molecular weight compounds, such as pyrolysis of wood into methanol, cracking of
petroleum,… etc.
Autoxidation is also a type of radical reactions including initiation, propagation and termination steps,
such as autoxidation of fat and oil to carboxylic acids, autoxidation of aldehyde to carboxylic acids.
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Free radical Initiators & Inhibitors
Free radical Initiator is anything that can initiate a radical reaction such as light,
free radical catalysts (peroxides, RO-OR).
Free radical inhibitor is anything that inhibits or trap or stop the radical reaction,
such as antioxidant & preservative substances, for example phenols compounds
form stable & unreactive radical because of resonance structures.
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Polymers
Polymer is a term used to describe large molecules consisting of repeating structural units,
or monomers, connected by covalent chemical bonds. Polymers could be organic and
inorganic materials. Polymers might be synthetic (artificial) or natural components.
Polymers include protiens, muscle fibers, enzymes, starch, cellulose, rubber, nylon, polyeste
clothes, polyacrylic, tires, melamine, Teflon, epoxy, toothbrushes, plastics… the technology of
macromolecules has become a giant in the world of industry!
Polymers are classified into 3 general categories: Elastomers, Fibers & Plastics.
Many polymers can be prepared by radical reaction through addition mechanism because
they are formed by the addition of monomers (one part) to each other without the loss of
atoms or groups.
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Mechanism of radical polymerization
There are two ways in which styrene molecules could join together to form
polystyrene : (1) head- to- tail,
or
(2) head-to- head and tail-to-tail
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Polystyrene and polyvinyl chloride (PVC) are examples for head-to- tail polymerization
Sometimes and to achieve desired properties, more than one monomer can be used for
production of polymer (copolymer such as Saran (used in kitchen wrap)
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Homework
Study problems : Pages 270-272
Give answer for questions;
6.14, 6.15, 6.16, 6.19, 6.20, 6.21, 6.22, 6.26, 6.27
Write a report on “ Technology of Polymerization”
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Organic Chemistry I
CHEM 201
1st Term 2011-2012
Instructor:
Assoc. Prof. Khaled S. Abdel Halim
k.abdulhalem@uoh.edu.sa
Chapter VII:
Alcohols
In this chapter, we shall study the following topics:
• Nomenclature & Classification of alcohols.
• Basicity & Acidity of alcohols.
• Synthesis of alcohols.
• Reactions of alcohols.
Nomenclature of alcohols (R-OH):
Alcohols are organic compounds containing hydroxyl group (-OH) bonded to SP3 hybridized
carbon. The word alcohol is coming from Arabic word “alkuhl”. Alcohols have many
applications as solvents, reagents, bactericidal reagent, antifreezing substance,…etc.
Alcohols have IUPAC name and common name (trivial name) as most of organic compounds.
Classification of alcohols
Alcohols like alkyl halides can be classified into methyl, primary, secondary , or tertiary as
well as allylic or benzylic
When a hydroxyl group is directly bonded to a carbon of
aromatic ring , the compound is phenol which is a class of
organic compounds different from alcohols.
Properties of alcohols: Acidity & Basicity
Alcohol & water are similar in structure & properties. Alcohols act as base when reacted with
strong acids and act as acid when react with bases. It forms oxonium ion (protonated
alcohol) with HCl and forms alkoxide ion when reacts with NaH, KH, Na or K.
Alcohol can undergo ionization like H2O but to a lesser extent than water because the
lower dielectric constant of alcohol.
Preparation of alcohols
1. Fermentation of carbohydrates:
2. Nucleophilic substitution (SN2):
3. Reduction of carbonyl compounds:
4. Hydration of alkenes:
Grignard Reactions
Grignard reagent (R-Mg-X) is one of the most organic reagent formed from the reaction of
Mg metal with organohalogen compounds in presence of ether as a solvent. All types of
alkyl halides can form Grignard reagent.
The importance of Grignard reagent is coming from partial negative charge on carbon atom bonded to
Mg, so it acts as strong base and the alkyl portion can act as a nucleophile. So, it reacts with carbonyl
group and attack the double bond and form compounds with higher number of carbon atoms. Grignard
reactions is an excellent route to prepare alcohols with complex carbon skeletons.
Grignard reagent can act as base when react with acidic hydrogen such as H 2O , alcohol,
amines, carboxylic acid or terminal alkynes to form hydrocarbons and metal salt. So, these
compounds should be excluded from Grignard reactions unless hydrocarbon is the desired
product.
Lithium reagents:
Lithium reagents (RLi ) are closely related to Grignard reagent.
Substitution reactions of alcohol
Alcohols undergo substitution reactions only in acidic medium while it gives no reaction in
alkaline or neutral medium, this is because of OH- group is strong base (poor leaving group).
The rate of substitution is mainly depends on the type of hydrogen halides and type of alcohol
as well. Also, the mechanism of substitution (SN1 or SN2) depends on the type of alcohol.
The predominant alkyl halide formed in the reaction of 3-methyl -2- butanol with HBr is a
product of rearrangment. Give the structure of the product and show how it is formed?.
Elimination reactions of alcohols
Alcohols like alkyl halides, form alkenes by elimination reaction because water is lost (dehydration
reactions). The reaction is occurred in presence of acid and heat. The mechanism is E2 or E1 according
to the type of alcohol. Rearrangment for carbocations is possible in E1 mechanism to form stable
alkenes. Saytzeff rule is also applied.
Saytzeff rule
In aromatic alcohols, the resonance structure of benzene ring affects the product.
Study problems
7.12, 7.13
Esterfication reactions
Alcohols react with carboxylic
acids to form esters.
Inorganic esters of alcohols are also formed from alcohols and mineral acids (HCl, HNO3,
H2SO4) or acid halides of mineral acids (SOCl2). Exampels of inorganic esters are nitrate
esters, phosphate esters, sulfate esters, sulfonate esters, tosylate esters.
Oxidation of alcohols
When organic molecule gains oxygen or loses hydrogen, it is oxidized. While when it gains hydrogen or
loses oxygen it reduced. In all cases, oxidation of organic molecule is associated by increasing the
oxidation state (oxidation number) of carbon atom.
This not an oxidation-reduction reactions
as oxidation state of carbon unchanged
Alcohols can be oxidized into Aldehyde, ketones, or carboxylic acids and these reactions are
used in the laboratory, industry and biologically systems as well.
Ethanol is biologically oxidized to acetaldehyde (and then to carboxylic acid) in liver with
the aid of enzyme (alcohol dehydrogenase). The acetate ion is then undergoses
esterification with the thiol coenzyme A (HSC0A).
CH3CH2OH
There are many oxidizing agents used for oxidation of alcohols such as alkaline potassium permangante (KMnO4 + OH-),
hot conc. Nitric acid, chromic acid, sod. Dichromate, CrO3 complexed with pyridine or with pyridine HCl , PCC.
Homework
Study problems : Pages 310- 312
Give answer for questions;
7.22, 7.26, 7.29, 7.32, 7.34, 7.39, 7.40
Organic Chemistry I
CHEM 201
1st Term 2011-2012
Instructor:
Assoc. Prof. Khaled S. Abdel Halim
k.abdulhalem@uoh.edu.sa
Chapter VIII:
Alkenes & Alkynes
In this chapter, we shall study the following topics:
• Synthesis of alkenes & alkynes.
• Electrophonic addition reactions.
• Catalytic hydrogenation reactions.
• Oxidation reactions.
• Uses of alkenes & alkynes.
Structure & Nomenclature of alkenes & alkynes
Alkene is hydrocarbon with one double bond, sp2- hybridized carbon with bond angle 120 o . The old name of alkenes is
Olefins (olefiant gas, oil forming gas). Alkyne is hydrocarbon with one triple bond , sp- hybridized carbon with 180o
bond angle. Alkyne is more polar than alkene (degree of unsaturation is 2). Alkynes are weaker acids than water and
stronger acid than ammonia.
Most of open chain hydrocarbons like
alkanes, alkenes and alkynes can be
represented in line formulas:
Synthesis of alkenes & alkynes:
Alkenes can be prepared by elimination reactions
of alcohols (in strong acid) or alkyl halides.
Alkynes can also be prepared by elimination
reactions of alkyl halides under stronger
conditions.
Complex alkynes can be prepared from simpler ones by SN2 reaction or from Grignard synthesis :
Addition Reactions:
The common types of addition reactions of alkenes
occurred with H2, X2 and HX. In these reactions, sp2
hybridized carbon is re-hybridized to sp3 and the exposed
pi electrons are attractive to electrophiles (E+) such as H+ ,
electrophilic attack to form carbocation which is then
attacked by a nucleophile to yield product.
Electrophilic Addition of hydrogen halides
For unsymmetrical alkenes, there are two possible
products unlike symmetrical alkenes. In such case,
Markovnikov’s rule should be applied “ In addition
of HX (HCl, HI) to unsymmetrical alkenes, the H+ of
HX goes to carbon with the greatest number of
hydrogen”.
The reason for Markovnikov’s
rule comes from the stability of
the intermediate carbocations
3o>2o> 1o
Anti-Markovinkov addition of HBr
The addition of HBr to alkenes gives product opposite to Markovenkov rule. This is can be
attributed to the difference in mechanism of reaction. When HBr reacts with alkene in
presence of initiator, the reaction proceeds through free radical mechanism.
Addition of halogen (Br2 & Cl2)to alkenes and alkynes
The reaction mechanism is proceeds through formation of carbocation bridge intermediate.
Mixed dihalide products are observed when using two different types of halogens (ions) such
as Br2 in NaCl or Br2 and H2O
Catalytic addition hydrogenation
Hydrogenation reactions are exothermic and needs high
activation energy, so catalyst should be used to decrease the
value of activation energy. Finely divided metal or metal oxide
or inert carrier can be used as catalysts.
Hydrogenation
(Hardening ) of oil is
the most common
example for catalytic
addition of H2
Other addition reactions on alkenes and alkynes
Alkenes and alkynes can reacts by addition mechanism using reagents rather than X2, HX or
H2. These reagents such as sulfuric acid, water, mercuric acetate, Borane (hydroboration),
carbene (R2C: methylene)
Anti-Markovnikov addition
Oxidation of alkenes
Alkenes can be oxidized to variety of products depending on the structure of alkenes and the
reagent used. Generally, two types of oxidation can be exist namely oxidation with out
cleavage of sigma bond and the second is oxidation with cleavage of sigma bond
Homework
Study problems : Pages 464- 466
Give answer for questions;
10.26, 10.27, 10.29, 10.30, 10.33, 10.37
Write a report on : “Industrial hydrogenation of fat & oil”