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Unit 5. Alkenes and Alkynes. Alkenes. Nomenclature Physical Properties Synthesis Reactions. Alkenes. Contain at least one carbon-carbon double bond (C=C). Also called olefins. Are unsaturated. (Alkanes are saturated.). Properties of C=C Bonds. sigma ( σ ) bonds. pi ( π ) bond.

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unit 5

Unit 5

Alkenes and Alkynes

alkenes

Alkenes

Nomenclature

Physical Properties

Synthesis

Reactions

alkenes1
Alkenes
  • Contain at least one carbon-carbon double bond (C=C).
  • Also called olefins.
  • Are unsaturated. (Alkanes are saturated.)
properties of c c bonds
Properties of C=C Bonds

sigma (σ) bonds

pi (π) bond

  • C=C consists of a σ bond and a π bond.
  • C=C is shorter than a C-C bond.
properties of c c bonds1
Properties of C=C Bonds
  • The pi bond forces the six atoms involved to be planar.
  • The e- density of the pi bond is above and below the plane.
alkenes2
Alkenes
  • σ bond dissociation energy = 347 kJ/mol
  • π bond dissociation energy = 264 kJ/mol
  • The pi bond is relatively easy to break, which makes C=C a functional group.
  • The pi bond blocks nucleophilic attack.
elements of unsaturation
Elements of Unsaturation
  • Alkenes contain C=C.
  • Alkynes contain C≡C.
  • Nonaromatic rings contain two fewer H’s than the corresponding straight chain compound.
  • Compounds containing one or more of the above contain elements of unsaturation.
elements of unsaturation1
Elements of Unsaturation
  • An element of unsaturation corresponds to two fewer H atoms than in a saturated formula, CnH(2n+2).

CH3CH2CH2CH2CH3

pentane, C5H12

CH3CH=CHCH2CH3

2-pentene, C5H10

1 E.U.

cyclopentane, C5H10

1 E.U.

elements of unsaturation2
Elements of Unsaturation
  • If the compound contains
    • a halogen, count the halogen as an H.
    • oxygen, ignore it when counting.
    • nitrogen, count as ½ C.
  • E.U. = ½ (2*(number of C + ½ number of N) + 2 - (number of H + X))
  • Knowing the elements of unsaturation in a compound helps you find a structure from a molecular formula, something you may find useful in NMR.
elements of unsaturation3
Elements of Unsaturation
    • Find the E.U. of C6H3NBrCl.
  • E.U. = ½ (2*(number of C + ½ number of N) + 2 - (number of H + X))
  • E.U. = ½ (2*(6 + ½) + 2 - 5)
  • E.U. = ½ (2*6.5 + 2 - 5)
  • E.U. = ½ (15-5)
  • E.U. = ½ (10)
  • E.U. = 5
elements of unsaturation4
Elements of Unsaturation
  • Determine the number of elements of unsaturation and propose a structure given the following formula and IR spectrum.
    • C3H4Cl2
    • E.U. = ½ (2*3 + 2 - (4+2))

= ½ (8-6)

= 1 E.U.

elements of unsaturation5
Elements of Unsaturation
  • C3H4Cl2

1 E. U.

SDBSWeb: http://riodb01.ibase.aist.go.jp/sdbs (National Institute of Advanced Science and Technology, 4/7/2009)

elements of unsaturation6
Elements of Unsaturation
  • Determine the number of elements of unsaturation and propose a structure given the following formula and IR spectrum.
    • C4H4O2
    • E.U. = ½ (2*4 + 2 - 4)

= ½ (10-4)

= 3 E.U.

elements of unsaturation7
Elements of Unsaturation
  • C4H4O2

3 E. U.

SDBSWeb: http://riodb01.ibase.aist.go.jp/sdbs (National Institute of Advanced Science and Technology, 4/7/2009)

elements of unsaturation8
Elements of Unsaturation
  • Determine the number of elements of unsaturation and propose a structure given the following formula and IR spectrum.
    • C4H4O2
    • E.U. = ½ (2*4 + 2 - 4)

= ½ (10-4)

= 3 E.U.

elements of unsaturation9
Elements of Unsaturation
  • C4H4O2

3 E. U.

SDBSWeb: http://riodb01.ibase.aist.go.jp/sdbs (National Institute of Advanced Science and Technology, 4/7/2009)

nomenclature of alkenes
Nomenclature of Alkenes
  • Apply the same rules you learned for the alkanes.
  • Use the root name of the longest chain containing the double bond, but change -ane to -ene.
  • Number the carbon atoms from the end closer to the double bond.
nomenclature of alkenes1
Nomenclature of Alkenes
  • When the chain contains more than three C atoms, use a number to give the location of the double bond.

CH3CH=CH2

propene

common: propylene

CH3CH=CHCH2CH3

2-pentene

pent-2-ene

trans-6-chloro-5-methylhex-2-ene

nomenclature of alkenes2
Nomenclature of Alkenes
  • Cycloalkenes are assumed to have the double bond in the number 1 position.

3-chlorocyclopentene

nomenclature of alkenes3
Nomenclature of Alkenes
  • The double bond is always between carbon 1 and carbon 2.
  • Number such that you get to the substituent sooner.

3-bromo-4-methylcyclopentene

nomenclature of alkenes4
Nomenclature of Alkenes
  • A compound with two C=C’s is a diene; a compound with three C=C’s is a triene, etc.

CH3CH=CHCH=CHCH=CH2

1,3,5-heptatriene

hepta-1,3,5-triene

2-chlorocyclopentadiene

nomenclature of alkenes5
Nomenclature of Alkenes
  • Alkenes as substituents are called alkenyl groups.

=CH2

methylene group

methenyl

-CH=CH2

vinyl group

ethenyl

-CH2-CH=CH2

allyl group

2-propenyl

phenyl and nitro groups
Phenyl and nitro groups
  • The benzene ring as a substituent is called a phenyl group.
  • Nitro groups are common substituents on benzene ring.

-NO2

nitro group

phenyl group

nomenclature of alkenes6
Nomenclature of Alkenes
  • Alkenes as substituents are often named using common names.

4-methylenecyclopentene

H2C=CHCl

vinyl chloride

H2C=CH-CH2Cl

allyl chloride

nomenclature of alkenes7
Nomenclature of Alkenes
  • cis-trans (geometric) isomers
    • Must have an identical group on each C of the double bond, but not two identical groups on one C of the double bond.
    • cis: identical groups are on the same side of the double bond.
    • trans: identical groups are on opposite sides of the double bond.
nomenclature of alkenes8
Nomenclature of Alkenes

cis-2,3-dichlorobut-2-ene

cis-2,3-dichloro-2-butene

trans-2,3-dichlorobut-2-ene

trans-2,3-dichloro-2-butene

1,1-dichloro-2-methylpropene

nomenclature of alkenes9
Nomenclature of Alkenes
  • trans cycloalkenes are unstable unless the ring has at least 8 C atoms.
    • All cycloalkenes are assumed to be cis unless they are specifically named trans.

trans-cyclodecene

cis-cyclodecene

nomenclature of alkenes10
Nomenclature of Alkenes
  • E-Z nomenclature
    • Can be used instead of cis-trans, but must be used when cis-trans doesn’t apply.
    • E-Z is patterned after the Cahn-Ingold-Prelog convention for asymmetric C atoms.
    • Assign priorities to the two groups on each C of the double bond.
      • If the higher priority groups are on the same side of C=C, the isomer is “Z”.
      • If the higher priority groups are on opposite sides of C=C, the isomer is “E”.
nomenclature of alkenes11
Nomenclature of Alkenes

cis-2,3-dichloro-2-butene

(Z)-2,3-dichloro-2-butene

(E)-2-bromo-1-chloropropene

2-ethyl-1-pentene

(Z)-2-bromo-1-chloropropene

nomenclature of alkenes12
Nomenclature of Alkenes
  • E-Z nomenclature
    • Designate the isomer present for each double bond.

(2Z,4E,6Z)-3-chloroocta-2,4,6-triene

(2Z,4E,6Z)-3-chloro-2,4,6-octatriene

uses of alkenes
Uses of Alkenes
  • Intermediates in the synthesis of
    • polymers (e.g., polyethylene, polypropylene, polystyrene)
    • drugs
    • pesticides
  • Ethylene is used to make
    • ethanol
    • acetic acid
    • vinyl chloride
    • ethylene glycol
stability of alkenes
Stability of Alkenes
  • Information comes from heats of hydrogenation.
    • The more exothermic the heat of hydrogenation, the less stable the bond.
stability of alkenes1
Stability of Alkenes
  • The most stable double bonds are those with the most alkyl groups attached (Zaitsev’s rule).
      • alkyl groups are e--donating
      • serves to separate bulky alkyl groups
  • trans isomers are generally more stable than cis.
stability of alkenes2
Stability of Alkenes
  • Cyclopropeneand cyclobutene have extra ring strain due to the C=C.
  • Larger rings can accommodate the C=C without loss of stability.
stability of alkenes3
Stability of Alkenes
  • Bredt’s rule: A bridged bicyclic compound cannot have a C=C at a bridgehead position unless one of the rings contains at least eight C atoms.
physical properties of alkenes
Physical Properties of Alkenes
  • Boiling points and densities are similar to those of the corresponding alkanes.
  • Although classified nonpolar, alkenes are slightly more polar than alkanes.
    • Pi electrons are more polarizable.
    • Vinylic bonds are slightly polar.