Organic Chemistry I
CHEM 201
1st Term 2011-2012
Instructor:
Assoc. Prof. Khaled S. Abdel Halim
k.abdulhalem@uoh.edu.sa
1
Chapter III:
Structure and nomenclature of saturated hydrocarbons
In this chapter, we shall study the following topics:
• Saturated & Unsaturated Organic Compounds.
• Isomerizations.
•Nomenclature of organic hydrocarbon .
•Chemical properties of Alkanes.
•Hydrocarbon Resources.
2
Organic compounds can be classified according to the type of chemical bond
into saturated (sigma bond)and unsaturated hydrocarbons (Pi bond).
Also, it could be classified into aliphatic and aromatic compounds!
Saturated
hydrocarbons
provide the
general carbon
skeletons of
organic
compounds
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Isomerization
Organic compounds have the same molecular formula and different structures are called
Isomers. As the number of C atoms increases, the number of isomers increases
astronomically.
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Isomers or Not?
The order of attachment of the atoms is the factor that determines if two structural
formulas represent isomers or the same compounds!
A ring or Unsaturation?
Compounds with general formula CnH2n contains one double bond equivalent
either one double bond or one ring
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Nomenclature of organic compounds
(i) Common Name (trivial/nickname)
(ii) IUPAC Name ( International Union of Pure & Applied Chemistry)
Alkanes
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7
Branched side chains
Propane
Butane
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Multiple branches
• The prefixes are placed alphabetically, each with its number indicating the position of
attachment to the parent . Substitution groups are consolidated in the name (di- , tri-, tetra, penta-, hexa-).
• Functional groups are named as prefixes to the parent name. its position is specified as
low as possible (longest continuous chain…………lowest number).
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Functional group nomenclature:
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Hydroxyl group
receives the
lowest possible
prefix number.
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Aldehyde and carboxylic groups must be at the beginning, take number 1
always. No number is used in its name to indicate the position.
Benzene is a parent except when attached to alkane with functional group or to
alkane chain of seven or more carbon atoms (considered substitution with
prefix – phenyl.
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Examples
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Homework (Ch.III)
3.5, 3.6, 3.9, 3.13, 3.16, 3.17, 3.22, 3.23, 3.25, 3.28
Write a report on :
Hydrocarbon Resources
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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
1
Chapter IV:
Stereochemistry
In this chapter, we shall study the following topics:
•Aspects of stereochemistry.
•Geometric Isomerism.
• Rules of nomenclature.
• Cyclic compounds.
2
Stereochemistry :
is the study of organic molecules in space which means that how
atoms are arranged in space relative to each other.
There are two different types of isomerism in organic compounds:
•Structural isomers: differ in order of attachment of atoms ;
(CH3)2CHCH3 and CH3CH2CH2CH3
•Stereoisomerism: differ in arrangement of atoms in space such as
geometric isomerism resulting from rigidity in molecules and occurs in
alkenes and cyclic compounds forming groups being cis (same size)
or trans (opposite size), cis- trans isomers.
3
Requirement of geometric isomerism in alkenes: each C atom involved
in the pi bond have two different groups attached to it such as H, Cl, CH3 and Cl.
Same compounds, (trans)
Structure isomers –
position of double bond
geometric isomers
4
Nomenclature of geometric isomers by (E) and (Z) system:
If the higher priority atoms or groups are on the same side ……….(Z-Zusamman, together )
If the higher priority atoms or groups are on the opposite side ………(E- Entgegen, across)
Name the following compounds by E and Z system :
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How about this compound?!
Sequence rules for order of
(Cahn- Ingold- Prelog system)
(1)
(2)
(3)
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(4)
7
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Geometric isomerism in cyclic compounds
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Homework “Ch.4”
4.2, 4.3, 4.4, 4.5, 4.6, 4.24, 4.25, 4.27,
Write a report on : “ The importance of stereochemistry for organic compounds
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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
1
Chapter V:
Mechanism of organic reaction
In this chapter, we shall study the following topics:
• Alkyl Halides.
• Substitution Reactions.
• Elimination Reactions.
2
Importance and types of organohalogen compounds
* Organohalogen compounds are used as solvents, insecticides and some as intermediates
for the synthesis of other organic compounds. Some organohalogen compounds are exist
naturally such as Thyroxine (component of hormone thyroglobulin). Choramphenicol
(Chloromycetin) is antibiotic against typhoid fever.
Three common types of organohalogen compounds;
R-X
Ar-X
3
Classification of alky halides:
Alkyl halides can be classified into 4 types; methyl, primary, secondary and tertiary
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5
Organic reaction mechanism
• The detailed description of how a reaction occurs is called a reaction mechanism. The
reaction mechanism depends on many factors and a large amount of experimental data
should be performed to support the reaction mechanism.
• Molecules must be first collide to undergo a chemical reaction . The colliding molecules
must contain enough potential energy for bond breakage.
• There are two common reaction mechanism in organic chemistry, Substitution and
Elimination reactions.
Substitution Reactions
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Substitution versus Elimination Reactions
• C-X bond is polar with partial positive charge on the halogenated carbon of the alkyl
halide. This carbon is susceptible to attach by an anion and the result is substitution
reaction (reaction in which atom, ion, or group is substituted for another. The halide (X)
in this case is called leaving group. Practically, Cl, Br, and I are good enough leaving groups
in substitution reactions (F - is a stronger base than other halid ions and is not a good
leaving group). The species that attacks the alkyl halide (OH-, CH3O- ) is called Nucleophile
(Nu-: nucleus lover – Lewis base). The substitution reaction in that case is named
Nucleophilic substitutions, [SN2 and/or SN1].
• Elimination reaction can occur when alky halide is treated with strong base. Organic
molecule loses atoms or ions from its structure and alkene is formed. Elmination
reactions are also called dehydrogenation reactions (H and X are lost from alkyl halide).
•Hydroxide ion or alkoxide (RO-) can react as Nu in substitution reactions or as a base in
elimination reaction. The reaction type mainly depends on different factors as structure
and strength of alky halide, solvent, temperature,…etc).
•Generally, methyl and primary halides tend to yield substitution products while teriary
alky halides yields elimination products. Secondary alkyl halides are intermediate and
give both!
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Nucleophilicity & Basicity
•All bases can act as nucleophiles and all nucleophiles can act as bases.
• Basicity is a measure of reagent’s abaility to accept a proton in acid- base reaction.
•Nuceophilicity is a measure of a reagent’s abaility to cause a substitution reaction.
•A reagent like OH- (strong base and good nucleophile ) can act both as a nucleophile and
as a base in one reaction vessel (competing substitution and elimination reactions).
Elimination Reaction
Substitution Reaction
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SN2 Reaction mechanism
* SN2 is a type of reaction mechanism means Substitution –Nucleophilic reaction
bimolecular.
•Bimolecular means that the transition state involves two particles (Nu:- and RX).
* Any methyl or primary alkyl halide undergoes SN2 when reacts with any relatively
strong nucleophile such as OH - , RO-, CN- .
The potential energy required to reach the transition state forms an energy barrier(point of
maximum energy). For a colliding alkyl halide and nucleophile to reach the transition state,
they need a certain minimum amount of energy (activation energy, Ea). The difference
between the average potential energy of reactants and products is the change in enthalpy
∆H of the reaction.
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SN2 Reaction Kinetics
The rate of chemical reaction is a measure of how fast the reaction proceeds, i.e., how
fast reactants are consumed and products are formed. Reaction kinetics study and
measure the reaction rate. The rate of SN2 reactions depends on many parameters such as
temperature, solvent, concentration of the reactants and structures of the reactants.
The rate of SN2 reaction is proportional to the concentration of both reactants (alkyl halide
& Nu), so SN2 is follow second order kinetics.
Under the same conditions, the reaction with lower Ea has a faster rate, this is because of
greater number of molecules will have enough energy to react when less energy is required.
Relative rates of reaction are related to the energies of the transition states, i.e., the
reaction with lower-energy transition state is the faster reaction.
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Structure & steric hindrance effects on the rate of SN2 reactions
The structure of alky halide is greatly affects the rate
of SN2 reactions . Methyl halide shows the fastest rate,
followed by primary alkyl halides and then secondary
alkyl halides. Tertiary alkyl halides do not undergo SN2
reactions . This phenomena can be explained in steric
hinder effect. As the number of alkyl groups attached
to the head of carbon increases, the transition state
becomes increasingly crowded with atoms, repulsions
between groups increase, the energy of the system is
high, so the rate will be decreased.
Steric hindrance effect
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The SN1 Reaction
•Such type of substitution reaction is a special type for tertiary alkyl halides which do not
undergo SN2 reactions because of steric hindrance effect. So, t-alkyl halides when treated
with weak base such as H2O or ethyl alcohol , substitution products are formed (beside
minor products of elimination product). SN1 reaction is called solvolysis reaction from
solvent.
•The mechanism of SN1 reaction is very complicated but for simplicity we will consider
that SN1 reaction is ionic reaction, stepwise mechanism, include unimolecular transition
state and first order rate .
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Rate of SN1 reaction
The rate of SN1 reaction is determined by how fast RX can ionize
and form R+ (Carbocations). The ionization step (step 1) is called
the rate determining step (slowest step).
The rate of SN1 reaction depends on the relative stability of carbocations, that’s why talkyl halides undergo SN1 reactions while methyl, 1o and 2o are not. A tertiary alkyl
hailde yields a carbocation that is more stabilized than the carbocations from methyl,
primary and secondary halides.
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Why carbocation of t-alkyl halide is the most stable one?!
This is because of inductive effect and hyper conjugation!
Inductive effect of alkyl group means the nature of alkyl group as electron donor
group increase the electron density over the electropositive center.
Hyper conjugation theory proposed the partial overlap of SP3-S orbital with the empty
p orbital of the positively charged carbon. In t- butyl cation, nine C-H bonds can help
disperse the charge of positive carbon.
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Rearrangements of carbocations
(special SN1 reaction mechanism for secondary alkyl halide):
Unexpected product?!
When secondary alkyl halide undergo SN1 reaction, the secondary carbocation is formed
with higher energy (less stable) and thus rearrangement of groups is occurred (1,2
methide shift ) to form more stable tertiary carbocation. The net results of the reaction is
the formation of two products (minor expected product & major unexpected products).
1,2 shifts can be happened in different carbocations to increase its stability!
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Elimination reaction mechanism
Reaction mechanism of E1
Like SN1 reactions, E1 is first order and unimolecular and first step is the rate determining
step (slow step). The carbocation tends to lose a proton to the base to become stable. SN1
and E1 are competing reactions and both are relatively unimportant.
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Reaction mechanism of E2
E2 reactions is carried out by heating alkyl halide with strong base like SN2 (OH-, Na+,
CH3CH2O-). E2 reaction is bimolecular occurs in one step as SN2. The E2 reaction is similar
with E1 reactions in that t-alkyl halides undergo reaction fastest than primary alkyl halides.
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Evidence of E2 reaction mechanism
To confirm the previous mechanism of E2 reaction, kinetic isotope experiment is used.
The bond between C-D is very strong (D is deuterium, isotope of H), so the second
reaction is very slow which confirm that E2 mechanism is occurred by lose of D from
methyl group attached to the head carbon of alkyl halide
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Beta elimination reactions
E1 & E2 reactions are types of β elimination reactions because beta H is lost
when alkene is formed.
Saytzeff rule:
The question now; which alkene is formed and why?!
The alkene with greatest
number of alkyl groups on
doubly bonded carbon atoms
predominates in the product
mixtures.
The stability comes from the double bond
character (alkyl group e donor groups)
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Trans alkene is more stable than cis alkene because of less steric
hindrance in trans alkene
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Hofmann Products:
In some cases, the major product of E2 reactions is the less substituted (less stable
alkene), a rule that opposite to Saytzeff rule. In that case the reaction is called Hofmann
reaction. The Hofmann product is formed due to steric hindrance effect which caused by
one of the following factors:
1- Increase the size of attaching base
2- Bulkiness of groups surrounding the leaving group
3- Increase the size of leaving group itself.
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Factors governing substitution and elimination reactions
Substitution & Elimination reactions are competing reactions, all products can be formed
in one reaction flask!
Is it possible to control the reaction to give only one product?! Yes, by controlling the
reaction conditions such as:
1. The structure of alkyl halids
2. The nature of Nu or base
3. The nature of solvent
4. The concentration of Nu or base
5. The temperature.
Strong base/strong Nu
CN- OH- OR-
weak base/weak Nu
H2O ROH ClBr- 26
The polarity of the solvent is greatly affects the reaction. The polarity of liquids is
measured by dielectric constant. A highly polar solvent has a large dielectric
constant. Very polar solvent such as water encourages SN1 reaction by helping
stabilize the carbocations through solvation.
The concentration of Nu or base can control the SN2 and E2 reactions while the
high concentration of Nu or base has no effect on SN1 and E1.
Increasing the temperature of the reaction will increase the rate of substitution
and elimination reactions because it increase the value of Ea. So, increasing
temperature usually leads to a great increase in elimination products where it
needs higher activation energy than substitution reactions.
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Synthesis of organic compounds from alkyl halides
SN2 and E2 are useful for synthesizing organic compounds from alkyl halides as it give
only one product while SN1 and E1 are not where it give mixtures of products.
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Homework (Ch. 5)
•
•
•
•
•
•
•
•
•
•
•
•
5.3
5.7
5.9
5.10
5.15
5.19
5.21
5.22
5.31
5.34
5.36
5.47
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