GENERAL CHEMISTRY II

1
GENERAL CHEMISTRY II

• CHAPTER 5
• Alkenes (Olefins)

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Introduction

• Olefins have at least one double covalent bond
  between 2 carbon atoms (CC).
• Olefins are unsaturated HC because of the
  presenting double bond(s).
• Olefins have 2 types with their formulae
• 1. Linear structural alkenes (CnH2n)
• 2. Ring structural cycloalkenes (CnH2n-2)
• Linear structural alkenes have two types of their
  double covalent bonds (CC)
• 1. Terminal alkenes (CC at the end)
• 2. Internal alkenes (CC in between)

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Introduction

• Double bond in alkenes has 2 types, which are a ?
  bond and a ? bond.
• Each carbon is sp2 hybridized and trigonal
  planar, with bond angles of approximately 1200.

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Introduction

• Bond dissociation energies of the CC bonds in
  ethane (a ? bond only) and ethylene (one ? and
  one ? bond) can be used to estimate the strength
  of the ? component of the double bond.

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Introduction

• Cycloalkenes having fewer than eight carbon atoms
  have a cis geometry. A trans cycloalkene must
  have a carbon chain long enough to connect the
  ends of the double bond without introducing too
  much strain.
• trans-Cyclooctene is the smallest isolable trans
  cycloalkene, but it is considerably less stable
  than cis-cyclooctene, making it one of the few
  alkenes having a higher energy trans isomer.

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Introduction
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Nomenclature

• An olefins parent name is given to the olefin
  based on
• 1. longest chain of C atoms in its molecule.
• 2. location of CC in its molecule.
• Greek numerical prefixes are combined together
  with olefins suffixes during their nominations.
• Methylene is its empirical formula, but only
  exists in methane compound (e.g. methylene
  fluoride (CH2F2)
• Examples
• 1. An alkene that contains only 2 C atoms in
  its molecule is named ethene (eth- represents two
  C atoms and ene represents the alkene itself)
• 2. Another alkene that contains 6 C atoms and
  has double bond between 3rd and 4th C atom in its
  molecule is named 3-hexene.

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Nomenclature

• For isomerized (or branched) olefins, H atom (or
  atoms) attached at any of C atoms in the chain is
  (or are) replaced by alkyl(s) or other
  nonmetallic substituent(s), following rules below

• Identify the parent name. The primary (or main)
  chain must contain the double bond(s).
• Number the primary chain. Start at the end closer
  to the double bond (the double bond determines
  the numbering). If the double bond is equidistant
  from the ends, start at the end closer to the
  first branch (e.g. 3-hexene).
• Number the substituents (already shown in
  previous chapter). Each substituent has its own
  number.
• Write the name. Separate numbers with commas and
  words with hyphens. List the substituents
  alphabetically. Use numerical prefixes if the
  substituents are from the same kind.

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Nomenclature
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Nomenclature
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Nomenclature
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Nomenclature

• Some alkene or alkenyl substituents have common
  names.
• The simplest alkene, CH2CH2, named in the IUPAC
  system as ethene, is often called ethylene.

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Physical Properties

• Easily volatile matter
• Lower boiling and dew points than water
• Lighter (less density) than water
• Insoluble in water but soluble in organic
  solvents
• Miscible in water but immiscible in organic
  solvents
• For C2 to C4 gaseous form
• For C5 to C14 liquid form
• For C16 and above solid form
• Combustible when ignited with any kind of flame

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Physical Properties

• Most alkenes exhibit only weak van der Waals
  interactions, so their physical properties are
  similar to alkanes of comparable molecular
  weight.
• The CC single bond between an alkyl group and
  one of the double bond carbons of an alkene is
  slightly polar because the sp3 hybridized alkyl
  carbon donates electron density to the sp2
  hybridized alkenyl carbon.

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Physical Properties

• A consequence of this dipole is that cis and
  trans isomeric alkenes often have somewhat
  different physical properties.
• cis-2-Butene has a higher boiling point (4 0C)
  than trans-2-butene (1 0C).
• In the cis isomer, the two Csp3Csp2 bond dipoles
  reinforce each other, yielding a small net
  molecular dipole. In the trans isomer, the two
  bond dipoles cancel.

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Chemical Properties

• Contains only C and H
• React with air to produce CO2 and H2O.
• React instantly with other chemicals, which one
  of 2 bonds in CC is broken and placed with 2
  chemical substituents to produce saturated
  compounds.
• Examples
• C2H4 decolorizes Br2 producing C2H4Br2.
• C3H6 reacts with water producing propanols.
• C3H4 (1-cyclopropene (olefin)) combines with H2
  producing C3H6 (cyclopropane (paraffin)).
• Carcinogenous substances (can cause cancers)

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Chemical Properties
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Chemical Properties
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Synthesis of Olefins

• Dehydrogenation (removal or elimination of H
  atoms) of paraffins, example C2H6 removes H2
  producing C2H4.
• Dehydration (removal of water molecule) of
  alcohols, example C2H5OH removes H2O producing
  C2H4.
• Hydrogenation (addition of 2 H atoms) of alkynes,
  example C2H2 accepts H2 producing C2H4.
• Other methods of synthesizing olefins

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Synthesis of Olefins

• Recall that alkenes can be prepared from alkyl
  halides and alcohols via elimination reactions.

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Synthesis of Olefins

• Also recall that these elimination reactions are
  stereoselective and regioselective, so the most
  stable alkene is usually formed as the major
  product.

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Reaction and Mechanism

• Reactions of Olefins
• Combustion
• C2H4(g) 3O2(g) ? 2CO2(g) 2H2O(l)
• Cracking (Hydrogenation)
• C2H4(g) H2(g) ? C2H6(g)
• Addition
• C2H4(g) HF(g) ? C2H5F(g)
• C2H4(g) H2O(l) ? C2H5OH(l)
• C2H4(g) Br2(l) ? C2H4Br2(l)
• C2H4(g) H2O2(l) ? C2H4(OH)2(l)

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Reaction and Mechanism
Addition of Alkenes

• The characteristic reaction of alkenes is
  additionthe ? bond is broken and two new ? bonds
  are formed.
• Alkenes are electron rich, with the electron
  density of the ? bond concentrated above and
  below the plane of the molecule.
• Because alkenes are electron rich, simple alkenes
  do not react with nucleophiles or bases, reagents
  that are themselves electron rich. Alkenes react
  with electrophiles.

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Reaction and Mechanism
Addition of Alkenes

• Because the carbon atoms of a double bond are
  both trigonal planar, the elements of X and Y can
  be added to them from the same side or from
  opposite sides.
• Syn addition takes place when both X and Y are
  added from the same side.
• Anti addition takes place when X and Y are added
  from the opposite sides.

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Reaction and Mechanism
Addition of Alkenes
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Reaction and Mechanism
Hydrohalogenation Electrophilic Addition of HX

• Two bonds are broken in this reactionthe weak ?
  bond of the alkene and the HX bondand two new ?
  bonds are formedone to H and one to X.
• Recall that the HX bond is polarized, with a
  partial positive charge on H. Because the
  electrophilic H end of HX is attracted to the
  electron-rich double bond, these reactions are
  called electrophilic additions.

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Reaction and Mechanism
Hydrohalogenation Electrophilic Addition of HX

• Addition reactions are exothermic because the two
  ? bonds formed in the product are stronger than
  the ? and ? bonds broken in the reactants. For
  example, ?H0 for the addition of HBr to ethylene
  is 14 kcal/mol, as illustrated below.

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Reaction and Mechanism
Hydrohalogenation Electrophilic Addition of HX

• The mechanism of electrophilic addition consists
  of two successive Lewis acid-base reactions. In
  step 1, the alkene is the Lewis base that donates
  an electron pair to HBr, the Lewis acid, while
  in step 2, Br is the Lewis base that donates an
  electron pair to the carbocation, the Lewis acid.

• In Step 1, the ? bond attacks H atom of HBr, thus
  forming a new C-H bond while breaking H-Br bond.
  Because the remaining C atom of original double
  bond is left with only 6 electrons, a carbocation
  intermediate is formed. This step is
  rate-determining because two bonds are broken but
  only one bond is formed.
• In Step 2, nucleophilic attack of Br- on the
  carbocation forms the new C-Br bond.

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Reaction and Mechanism
Hydrohalogenation Markovnikovs Rule

• With an unsymmetrical alkene, HX can add to the
  double bond to give two constitutional isomers,
  but only one is actually formed
• This is a specific example of a general trend
  called Markovnikovs rule.
• Markovnikovs rule states that in the addition of
  HX to an unsymmetrical alkene, the H atom bonds
  to the less substituted carbon atomthat is, the
  carbon that has the greater number of H atoms to
  begin with.

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Reaction and Mechanism
Hydrohalogenation Markovnikovs Rule

• The basis of Markovnikovs rule is the formation
  of a carbocation in the rate-determining step of
  the mechanism.
• In the addition of HX to an unsymmetrical alkene,
  the H atom is added to the less substituted
  carbon to form the more stable, more substituted
  carbocation.

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Reaction and Mechanism
Hydrohalogenation Summary
32
Reaction and Mechanism
Hydration Electrophilic Addition of Water

• Hydration is the addition of water to an alkene
  to form an alcohol.

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Reaction and Mechanism
Hydration Electrophilic Addition of Water

• In Step 1, the ? bond attacks H3O, thus forming
  a new C-H bond while breaking H-O bond. Because
  the remaining C atom of original double bond is
  left with only 6 electrons, a carbocation
  intermediate is formed. This step is
  rate-determining because two bonds are broken but
  only one bond is formed.
• In Step 2, nucleophilic attack of H2O on the
  carbocation forms the new C-O bond.
• In Step 3, removal of a proton with a base (H2O)
  forms a neutral alcohol. Because the acid used in
  Step 1 is regenerated in this step, hydration is
  acid-catalyzed.

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Reaction and Mechanism
Hydration Electrophilic Addition of Water

• Alcohols add to alkenes, forming ethers by the
  same mechanism. For example, addition of CH3OH to
  2-methylpropene, forms tert-butyl methyl ether
  (MTBE), a high octane fuel additive.
• Note that there are three consequences to the
  formation of carbocation intermediates
• Markovnikovs rule holds.
• Addition of H and OH occurs in both syn and anti
  fashion.
• Carbocation rearrangements can occur.

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Reaction and Mechanism
Halogenation Addition of Halogens

• Halogenation is the addition of X2 (X Cl or Br)
  to an alkene to form a vicinal dihalide.

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Reaction and Mechanism
Halogenation Addition of Halogens

• Halogens add to ? bonds because halogens are
  polarizable.
• The electron rich double bond induces a dipole in
  an approaching halogen molecule, making one
  halogen atom electron deficient and the other
  electron rich.
• The electrophilic halogen atom is then attracted
  to the nucleophilic double bond, making addition
  possible.

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Reaction and Mechanism
Halogenation Addition of Halogens

• In Step 1, four bonds are broken or formed the
  electron pair in ? bond and a lone pair of a
  halogen atom are used to form two new C-X bonds.
  The X-X bond is also cleaved heterolytically,
  forming X-. This step is rate-determining.
• The three-membered ring containing a positively
  charged halogen atom is called a bridged halonium
  ion. This strained three-membered ring is highly
  unstable, making it amenable for opening of the
  ring in Step 2.
• In Step 2, nucleophilic attack of X- opens the
  ring of halonium ion, forming a new C-X bond, and
  relieving the strain in the three-membered ring.
• Carbocations are unstable because they have only
  six electrons around carbon. Halonium ions are
  unstable because of ring strain.

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Reaction and Mechanism
Halohydrin Formation

• Treatment of an alkene with a halogen X2 and H2O
  forms a halohydrin by addition of the elements of
  X and OH to the double bond.

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Reaction and Mechanism
Halohydrin Formation

• In Step 1, four bonds are broken or formed the
  electron pair in ? bond and a lone pair of a
  halogen atom are used to form two new C-X bonds
  in the bridged halonium ion. The X-X bond is also
  cleaved heterolytically. This step is
  rate-determining.
• In Step 2, nucleophilic attack of H2O opens the
  ring of halonium ion, forming a new C-X bond.
  Subsequent loss of a proton forms the neutral
  halohydrin.

• Even though X is formed in step 1 of the
  mechanism, its concentration is small compared to
  H2O (often the solvent), so H2O and not X is the
  nucleophile.

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Reaction and Mechanism
Halohydrin Formation

• Although the combination of Br2 and H2O
  effectively forms bromohydrins from alkenes,
  other reagents can also be used.
• Bromohydrins are also formed with
  N-bromosuccinimide (NBS) in aqueous DMSO
  (CH3)2SO.
• In H2O, NBS decomposes to form Br2, which then
  goes on to form a bromohydrin by the same
  reaction mechanism.

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Reaction and Mechanism
Halohydrin Formation
Because the bridged halonium ion is opened by
backside attack of H2O, addition of X and OH
occurs in an anti fashion and trans products are
formed.
With unsymmetrical alkenes, the preferred product
has the electrophile X bonded to the less
substituted carbon, and the nucleophile (H2O)
binds to the more substituted carbon.
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Reaction and Mechanism
Halohydrin Formation
As in the acid catalyzed ring opening of
epoxides, nucleophilic attack occurs at the more
substituted carbon end of the bridged halonium
ion because that carbon is better able to
accommodate the partial positive charge in the
transition state.
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Reaction and Mechanism
Halohydrin Formation
Halohydrins have been used in the synthesis of
many naturally occurring compounds. Key steps in
the synthesis of estrone, a female sex hormone,
are illustrated below.
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Alkenes in Organic Synthesis

• Suppose we wish to synthesize 1,2-dibromocyclohexa
  ne from cyclohexanol.
• To solve this problem we must
• Work backwards of the product by asking What
  type of reactions introduce the functional groups
  in the product?
• Work forwards from the starting material by
  asking What type of reactions does the starting
  material undergo?

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Alkenes in Organic Synthesis

• Working backwards from the product to determine
  the starting material from which it is made is
  called retrosynthetic analysis.
• Stepwise of retrosynthetic analysis in this case
  is as follows
• For working backwards, 1,2-dibromocyclohexane can
  be prepared by adding Br2 to cyclohexene.
• For working forwards, cyclohexanol can be
  acid-catalysis dehydrated to form cyclohexene.
• In this case, cyclohexene is called a synthetic
  intermediate, because it is the product of one
  step and a starting material of another.
• So it is concluded that the conversion from
  cyclohexanol to 1,2-dibromocyclohexane requires
  2-step sequential synthesis and this synthesis is
  complete, and alkene (cycloalkene) plays as the
  central role of this synthesis.

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Alkenes in Organic Synthesis
Work backwards
Work forwards
Combination of both work wards