(25) session 25 introduction to organic reactions

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Session 25
Organic Chemistry, UNAM School of Medicine1
Introduction to Organic ReactionsIntroduction to Organic ReactionsIntroduction to Organic ReactionsIntroduction to Organic Reactions
Dr L.H.A. Prins (Ph.D.)
Dept. of Pharmacy
UNAM
Learning Outcomes
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By the end of this session, the student should be able to:
Define the terms reaction mechanism, electrophile,
nucleophile, radicals, carbocation, carbanion & leaving
group
Understand the categories of organic reactions
Interpret & use curly arrows to show mechanisms of organic
reactions
Understand homolytic & heterolytic bond cleavage
Identify nucleophiles & electrophiles in organic reactions
Organic Chemistry, UNAM School of Medicine

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Definitions??Definitions??Definitions??Definitions??
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Reaction mechanism =A complete step-by-step description of
how a reaction occurs
Electrophile =An “electron-lover” or a species that accepts an
electron pair from a nucleophile in a polar bond-forming reaction
Nucleophile =A “nucleus-lover” or a species that donates an
electron pair to an electrophile in a polar bond-forming reaction
Radicals =Atoms, molecules or ions with unpaired electrons that
have valence shells which are not completely filled with electrons
Definitions??Definitions??Definitions??Definitions??
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Carbocation =A “carbon cation” or
a species that contains a + charged C atom
having a total of 6 valence e−
Carbanion =A “carbon anion” in which
C has a lone pair of e- & bears a − charge
having a total of 8 valence e−
Leaving group =A group that is displaced in a substitution
reaction
-

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Categories of organic reactionsCategories of organic reactionsCategories of organic reactionsCategories of organic reactions
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4 important categories of organic reactions:
1. Addition reactions
2. Elimination reactions
3. Substitution reactions
4. Rearrangement reactions
Addition reactionsAddition reactionsAddition reactionsAddition reactions
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Addition reactions occur when two reactants add together to
form a single product
The generalised reaction is:
Specific example of addition reaction: Reaction of an alkene with a
hydrogen halide to yield halo-alkanes (alkyl halides)
A + B C
Reactants Product
CH2=CH2 + HBr CH3CH2Br

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Elimination reactionsElimination reactionsElimination reactionsElimination reactions
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Elimination reactions occur when a single reactant splits up into
two products
(Elimination is the opposite
of addition)
Specific example of elimination reaction: Reaction of a halo-alkane
(alkyl halide) with a base
A B + C
Reactant Products
CH3CH2Br CH2=CH2 + HBr
NaOH
Substitution reactionsSubstitution reactionsSubstitution reactionsSubstitution reactions
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Substitution reactions occur when two reactants exchange
parts to give two new products
Specific example of substitution reaction: Reaction of 2-bromo-2-
methylpropane with water to yield 2-methylpropan-2-ol & hydrogen
bromide
A−B + C−D A−C + B−D
2 Reactants 2 New products

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Rearrangement reactionsRearrangement reactionsRearrangement reactionsRearrangement reactions
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Rearrangement reactions occur when a single reactant
undergoes reorganisation of bonds & atoms to yield an isomeric
product
Specific example of rearrangement reaction:Treatment of 1-butene
with an acid catalyst to yield a structural isomer, 2-butene
A B
Single reactant Isomeric product
Acid catalyst
1-butene 2-butene
How organic reactions occurHow organic reactions occurHow organic reactions occurHow organic reactions occur
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Reactive species:
Nucleophiles (Nu) are e−-rich atoms or species that donate
electrons (“nucleus-loving”)
A nucleophile has a pair of electrons it can share
Some nucleophiles are neutral & some are negatively charged
Examples:

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Reactive species:
Electrophiles (E) are e−-deficient atoms or species that accept
electrons (“electron-loving”)
An electrophile has an atom that can accept a pair of electrons
Most electrophiles are + charged, have an atom that carries a
partial + charge or have an atom that does not have an octet of
electrons.
Examples:
H+, NH4
+, NO+ CH3CH2
+, H−Cl, CH3-Br, etc.
δ+ δ−
δ+ δ−
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Reaction mechanisms:
Reaction mechanisms describe how molecules react with each
other & predict reaction products
“Curly arrows” are used to represent reaction mechanisms
−Show the direction of the movement of an e- pair
∴∴∴∴ Show how e− move as new covalent bonds are formed & existing
covalent bonds are broken
−Drawn from an e−-rich centre such as a lone pair or covalent bond
(tail of arrow), to an e−-deficient centre (head of arrow)

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“Curly arrows”:
−Head of arrow indicates ultimate destination of e− pair, which will
either be:
An electronegative atom that can support a − charge (a leaving group)
An electropositive atom that has a partial or full + charge
−Overall charge is always conserved in the reaction
Example:
δ+ δ−
Overall charge = 0 (+1 −1 = 0)
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Example: (Step 1 of reaction)
Existing ̟ bond has been broken & a new C–H σ bond is formed
2 e− that formerly formed ̟ bond, now form σ bond between C & H
H–Br bond breaks, with Br keeping the bonding e−
Product of 1st step in reaction is a carbocation i.e. a + charged carbon
1 carbon is + charged because the sp2 carbon that did not form the
new bond with H has lost a share in a pair of e−
δ+ δ−

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Example: (Step 2 of reaction)
A lone pair on the − charged bromide (Br) ion forms a bond with the
+ charged C of the carbocation
Both steps of the reaction involve the reaction of an electrophile
with a nucleophile
Overall, the reaction involves addition of HBr to an alkene
Reaction is called an electrophilic addition reaction, because in
1st step of reaction, an electrophile (H+) is added to the alkene
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General rules regarding the use of curly arrows:
1. Always drawn away from
the − charge towards the + charge
2. Drawn to indicate movement of e−
(NOT to indicate movement of an atom)

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General rules regarding the use of curly arrows:
3. Arrow always starts at e− source such as a bond or a lone pair
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Homolytic bond cleavage:
When a bond breaks such that each of the atoms retains one of the
bonding electrons
Denoted by 2 x single barbed curly arrows pointing away from the
bond
These single electron species are known as (free) radicals

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Heterolytic bond cleavage:
When a bond breaks such that both of its electrons stay with one of
the atoms
A − charged species (an anion) & a + charged species (a cation) is
formed during this bond breakage
Anion: Species that retains the electrons from the bond
Cation: Stripped of the electrons from the bond
Quiz: Organic reactions introduction
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1. w

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Quiz: Organic reactions introduction
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2. Use curly arrows to show the movement of electrons in each of the following
reaction steps:
Thank you
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END

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Quiz answers:
1. w
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Quiz answers:
2. w

(25) session 25 introduction to organic reactions

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