Organic Chemistry I Phan Minh Giang

Chapter 3
UNSATURATED HYDROCARBONS
ALKENES AND ALKYNES
Organic Chemistry

Alkenes

IUPAC nomenclature of alkenes

Molecular structures and Reactivity
Organic Chemistry

Natural Alkenes and Polyenes

Organic Chemistry

IUPAC Nomenclature
Organic Chemistry

IUPAC Nomenclature
Organic Chemistry

Relative Stability: Heats of Hydrogenation

Organic Chemistry

Relative Stability of Alkenes

Hyperconjugation and Steric effects account for the relative stabilities

of alkenes.
Organic Chemistry

Relative Stability of Alkenes

Organic Chemistry

Relative Stability of Alkenes

Steric effects
Organic Chemistry

Structure and Reactivity of Alkenes

Organic Chemistry

Structure and Reactivity of Alkenes

Organic Chemistry

Structure and Reactivity of Alkenes

3
Organic Chemistry

Polarity and Dipole Moment of Alkenes

Organic Chemistry

Structure and Reactivity of Alkenes

Polar reactions: ionic mechanism and electrophiles as reagents.

Electrophilic addition mechanism

Nu:
Rate-limiting step of the
electrophilic addition
Organic Chemistry

Chemical Reactions of Alkenes

Chemical properties
1) Electrophilic addition to alkenes
Mechanism of electrophilic addition (AE)
Regioselective reactions
Markovnikov’s rule
2) Catalytic hydrogenation
3) Oxidation
Syn-dihydroxylation
Oxidative cleavage
4) Stereochemistry of addition and oxidation reactions
Stereospecific reactions
Organic Chemistry

Important Terms and Concepts

Electrophilic addition
Electrophile
p bond as nucleophile
Carbocation stability
Markovnikov’s rule
Markovnikov addition products
Anti-Markovnikov addition products
Regioselectivity and Regioselective reactions
Stereoselectivity vs. Stereospecificity
Stereospecific reactions
Syn-addition
Anti-addition
Oxidative cleavage
Organic Chemistry

Reactions of Alkenes

• A reaction mechanism is written as a series of chronologically

ordered steps, with each step describing the motion of one or
more electrons. When the steps are added together, the result
is the net reaction.
• Polar reactions in which two electrons move together when
bonds are formed or broken. When electrons move in pairs,
bond breaks in a heterolytic manner.
• Radical reactions involve the movement of a lone electron. In
radical processes, bond breaking and forming reactions are
called homolytic.
Organic Chemistry

Structure and Reactivity of Alkenes

Organic Chemistry

Reactions of Alkenes

Reaction type: Polar reaction

Mechanism type: Electrophilic addition (AE)

1) Hydrohalogenation: addition of hydrogen halide (HX = Cl, Br, I)

2) Halogenation: addition of halogen (X2 = Cl, Br, I)
3) Halohydrin formation: addition of X2/H2O
4) Hydration: addition of water (H2O)
5) Hydroboration-Oxidation: addition of BH3.THF

Reaction type: Radical reaction

Mechanism type: Radical addition
1) Hydrohalogenation: addition of hydrogen bromide (HBr) in the
presence of peroxides (RO-OR)
Organic Chemistry

Reactions of Alkenes

H Br H OH Br
HBr H2O, H+ Br2

CH2Cl2 Br

H Cl H OH Cl
HCl H2O, H+ H
Cl2
H H
H H H CH2Cl2 Cl

H H I
HBr H2O, H+ I2

I OH CH2Cl2 I

Br OH Br
HBr H2O, H+ Br2

H CH2Cl2 Br
H
Organic Chemistry

Reactions of Alkenes
Organic Chemistry I

Reactions of Alkenes

Reaction type: Reduction-Oxidation reaction

1) Catalytic hydrogenation
saturation of double bond
metal-catalyzed addition of H2
2) Mild oxidation
formation of cis-1,2-diols
- with osmium tetroxide (OsO4)
- with cold potassium permanganate (KMnO4)
3) Oxidative cleavage of carbon-carbon double bond
formation of carbonyl compounds
- with ozone (O3)
- with hot KMnO4
Organic Chemistry I

Reactions of Alkenes
Organic Chemistry I

Reactions of Alkenes
Organic Chemistry

Electrophilic Addition to Alkenes

Electrophilic: ionic mechanism initiated by an electrophile.

Addition: reaction type is an addition.
Organic Chemistry I

Reactions of Alkenes: Summary

Organic Chemistry

Reactions of Alkenes: Important Terms

Regioselectivity: Regioselective reactions

Organic Chemistry

Reactions of Alkenes
Stereochemistry: Stereospecific reactions
Organic Chemistry I

Reactions of Alkenes

Regioselective reaction (Regioselectivity)

Stereospecific reaction (Stereospecificity)

Organic Chemistry

Hydrohalogenation: Addition of HX
Regiochemistry: Markovnikov
Stereochemistry: not specific

ether

halogen acid
HCl, HBr, HI

Vladimir Markovnikov 1869

Organic Chemistry

Mechanism of Hydrohalogenation

rate-limiting step

General rule
Organic Chemistry

Mechanistic Explanation of Markovnikov’s Rule

hyperconjugation and inductive effect

sp2-hybridized
Organic Chemistry

Energy Diagram of Hydrohalogenation

Step1: carbocation formation
rate-limiting step
step1 step 2 Step 2: nucleophilic
slow
attack to the carbocation
fast
pathway 1: more
energetically
favored

starting
material
product
Organic Chemistry

Non-stereoselective Reactions

Cyclic compounds as models for stereochemical investigation.

stereochemistry: not specific

(Carbocations are of planar structure and achiral).
Organic Chemistry

Carbocation Rearrangement

1,2-hydride and alkyl shifts

Organic Chemistry

Anti-Markovnikov Hydrohalogenation

General: RO-OR
(organic peroxides)

regiochemistry: anti-Markovnikov
stereochemistry: not specific
Organic Chemistry

Mechanism of anti-Markovnikov Hydrohalogenation

Guide: Stability of the intermediates

in the rate-limiting step

Polar reaction
Ionic mechanism Radical reaction
Radical mechanism
Organic Chemistry

Acid-catalyzed Hydration of Alkenes

Regiochemistry: Markovnikov
Stereochemistry: not specific
Organic Chemistry

Mechanism of Acid-catalyzed Hydration

1) Reaction of the
alkene double bond
with H+ yields a
carbocation
intermediate.
2) The carbocation then
reacts with water as
nucleophile to yield a
protonated alcohol
(ROH2+) product.
3) Loss of H+ from the
protonated alcohol
gives the neutral
alcohol and
regenerates the acid
catalyst.
Organic Chemistry

Acid-catalyzed Hydration
Organic Chemistry

Carbocation rearrangements
Organic Chemistry

Oxymercuration-Demercuration
Organic Chemistry I

Oxymercuration-Demercuration

BACK DONATION

Một phương pháp êm dịu hơn (Sự oxy thủy ngân hóa - khử hóa) để thực hiện sự cộng
nước Markonikov là sử dụng một muối thủy ngân trong một hỗn hợp của tetrahydrofuran
(THF) và nước để xúc tác cho phản ứng. Chất này sẽ cộng ở nhiệt độ phòng để tạo thành
một sản phẩm oxy thủy ngân và thủy ngân được trao đổi cho H trong một bước thứ hai sử
dụng natri bo hydride. Sự cộng này hiếm khi bị làm phức tạp bằng sự chuyển vị vì
chất trung gian carbocation không tham gia vào sự oxy thủy ngân hóa.
Organic Chemistry

Anti-Markovnikov Addition of Water

Hydroboration-Oxidation
Organic Chemistry

Mechanism of Hydroboration-Oxidation

regiochemistry: anti-Markovnikov
stereochemistry: syn-addition
Organic Chemistry

Halogenation: Addition of Halogens

Stereospecific reaction
Stereochemistry: anti-addition Inert solvents: CH2Cl2, CCl4
Organic Chemistry

Halogenation
Organic Chemistry

Mechanism of Halogenation

Donor-Acceptor
Organic Chemistry

Mechanism of Halogenation
1) Alkene p electrons attack Br2, pushing out Br- and leaving a bromo
carbocation.
2) The neighboring bromine atom uses a lone pair of electrons to overlap
the vacant p orbital, forming a cyclic bromonium ion.
3) Br- ion attacks the bromonium from the top (unshielded) face to give
trans-1,2-dibromocyclopentane.
Organic Chemistry

Halohydrin Formation

Regioselectivity:
Markovnikov
stereochemistry:
anti-addition
Organic Chemistry

Mechanism of Halohydrin Formation

Organic Chemistry

Halohydrin Formation
Organic Chemistry

Electrophilic Addition Reactions

Organic Chemistry

Stereospecific Halogenation

Stereospecificity is determined by stereochemistry of the starting materials

and stereochemistry of the reactions.
Organic Chemistry

Stereospecific Electrophilic Addition Reactions

Give the structure of the major organic product in each of the following
addition reactions
Organic Chemistry

Reduction or Oxidation Reactions

Organic Chemistry

Catalytic Hydrogenation of Alkenes

Organic Chemistry

Catalytic Hydrogenation of Alkenes

1) The adsorption of
hydrogen is essentially a
chemical reaction: unpaired
electrons on the surface of
the metal pair with the
electrons of hydrogen and
bind the hydrogen to the
surface.
2) The collision of an alkene
with the surface bearing 3) A stepwise transfer of hydrogen atoms takes
adsorbed hydrogen causes place, and this produces an alkane before the
adsorption of the alkene. prganic molecule leaves the catalyst surface.
Organic Chemistry

syn-1,2-Dihydroxylation

Hydroxylation of an alkene – the addition of an –OH group to each of the

alkene carbons. The reaction occurs with syn stereochemistry and yields a
1,2-dialcohol, or diol, product (also called a glycol). Since oxygen adds to
the alkene during the reaction, we call this an oxidation.

1) OsO4
2) Na2SO3 (NaHSO3)/H2O
C C C C
(or KMnO4, OH)
OH OH
syn addition
Organic Chemistry

syn-1,2-Dihydroxylation
Organic Chemistry

syn-1,2-Dihydroxylation
Organic Chemistry

Stereochemistry of syn-1,2-Dihydroxylation
Organic Chemistry

trans-1,2-Dihydroxylation
Organic Chemistry

trans-1,2-Dihydroxylation
Organic Chemistry

Stereospecific syn-Dihydroxylation

H3C H
1) OsO4
(Fischer Projection)
2) NaHSO3/H2O
H CH2CH3

H3C CH2CH3
KMnO4, OH
(Fischer Projection)
H H
Organic Chemistry

Oxidative Cleavage with Hot KMnO4

Organic Chemistry

Oxidative Cleavage with Hot KMnO4

Organic Chemistry

Oxidative Cleavage by using Ozonolysis

1) O3
2) Zn/H2O
or Zn/CH3CO2H
or (CH3)2SH
to carboxylic acid
if use 2) H2O2

R O R R'
R'
Zn/H2O O + O + Zn(OH)2
R H R
O O H
Organic Chemistry

Oxidative Cleavage: Ozonolysis

Mechanism
Organic Chemistry

Oxidative Cleavage: Ozone vs. KMnO4

Organic Chemistry

Concept Map of Alkene Chemistry