Alkenes and alkynes

1
Alkenes and alkynes

• The chemistry of unsaturation

2
Alkene structure

• Unsaturated hydrocarbons those with one or more
  double or triple bonds. General formula is CnH2n
  (for one double bond)
• Trigonal planar geometry, sp2 hybridization
• Bond angles close to 120º
• No rotation about CC

3
Alkyne structure

• One or more triple bonds
• Linear geometry
• 180º bond angles
• sp hybridization
• One sigma and two pi bonds
• H-CC-H
• Ethyne (acetylene)

4
Nomenclature

• Position of double bond is given by number of
  first doubly bonded carbon
• 1-butene CH2CHCH2CH3
• 2-butene CH3CHCHCH3
• Longest chain always includes double/triple bonds

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Cis-trans isomerism

• trans-2-butene

cis-2-butene
E-Z notation
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E-Z Notation

• Cahn-Ingold sequence priority priority is given
  to higher MW substituents
• Higher priority substituents on same side Z
  isomer (zusammen, or together)

Z-2-chloro-2-butene
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Nomenclature

• Higher priority substituents on opposite sides
  E isomer (entgegen, against)
• E-2-chloro-2-butene

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Nomenclature

• Cycloalkenes double bonded carbons are numbers
  1 2
• 4-methylcyclohexene
• (A number for the double bond is not needed)

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Nomenclature

• Dienes, trienes and polyenes
• Prefixes di2, tri3, etc. are used to indicate
  the number of double bonds
• Each double bond gets its own location number

3-chloro-1,3-pentadiene
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Nomenclature

• 2-methyl-1,4-cyclohexadiene

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Physical properties

• Physical properties of alkenes are similar to
  those of alkanes
• Alkene natural products Terpenes
• Oligomers of isoprene (2-methyl-1,3-butadiene)

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Terpenes

• Essential oils two isoprene units monoterpenes

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Terpenes

• Sesquiterpenes three isoprenes
• Tetraterpenes (8 isoprenes)
• b-carotene (precursor to vitamin A)

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Reactions of alkenes

• Addition reactions
• Addition of hydrogen halides to form alkyl
  halides
• CH2CH2 HBr ? CH3CH2Br
• Mechanism
• Hydrogen halides are polar
• The positive end is attracted to the electrons in
  the double bond

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Mechanism of hydrohalogenation

• protonation step
• nucleophile double bond 1º (primary)
  carbocation
• electrophile H of HCl
  (unfavorable)

?
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Mechanism of hydrohalogenation
?

• chloroethane
• Carbocation mechanism!

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Details of Carbocation Mechanisms

• motion of 1 e- motion
  of 2 e-
• Order of stability of carbocations 3ºgt2ºgt1º
• Primary

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Details of Carbocation Mechanisms

• Secondary
• Tertiary

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Details of Carbocation Mechanisms

• More substituted carbocations are more stable
  because alkyl groups are slightly electron
  donating, and they stabilize the carbocation by
  diluting the positive charge.
• Most stable carbocation possible will be formed

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Details of Carbocation Mechanisms
Tertiary carbocation is always formed never the
secondary
2-methyl-2-butene two possible sites for
carbocation formation
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Details of Carbocation Mechanisms

• Markovnikovs rule in hydrohalide addition, the
  hydrogen adds to the carbon that already has the
  most hydrogens bonded to it.
• The halogen adds to the carbon that has the most
  carbons attached (the location of the positive
  charge)

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Addition of water

• Acid catalyzed addition of water to form alcohols

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Addition of water

• Proceeds by a carbocation mechanism
• Follows Markovnikovs rule (gives the
  Markovnikov product)
• Mechanism

?
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Addition of water
?
protonated ethanol
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Addition of water
ethanol
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Addition of halogens

• Addition of halogens to form vicinal dihalides
• CH2CH2 Br2 ? BrCH2CH2Br
• Carried out in pure reagent, CCl4 or other inert
  solvent
• Additions to cyclic alkenes always give the trans
  product (anti addition)
• Discoloration of bromine solutions is a test for
  alkenes

27
Addition of hydrogen

• Addition of hydrogen (reduction) to form alkanes
  catalytic hydrogenation
• Powdered metal catalyst is used usually Pd, Pt,
  Ni, Ru
• H2 is used under high pressure

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Polymerization reactions

• Polymers are long chains of identical units
  called monomers
• nCH2CH2 initiator ? (-CH2CH2-)n
• ethane polyethylene

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Mechanism of free radical polymerization

• Free radicals are species with one unpaired
  electron
• Formed by heterolytic bond cleavage
• Initiation

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Mechanism of free radical polymerization

• Propagation
• Termination

31
Mechanism of free radical polymerization
32
Polyethylenes

• Low density polyethylene (LDPE)
• Highly branched, clear, low melting
• Made via radical mechanism
• Used for packaging, trash bags
• High density polyethylene (HDPE)
• Linear, opaque, high melting
• Made via an ionic mechanism
• Used for milk and water jugs, grocery bags,
  squeezable bottles