Bonding, Nomenclature, Properties

1
5 Alkenes
Bonding, Nomenclature, Properties
Structure Hydrogen Deficiency Nomenclature Physic
al Properties Naturally Occurring Alkenes/Terpenes
2
Unsaturated Hydrocarbons

• 1. Alkene contains a carbon-carbon double bond
  and has the general formula CnH2n

3
Unsaturated Hydrocarbons

• 2. Alkyne contains a carbon-carbon triple bond
  and has the general formula CnH2n-2 (Ch 7)

4
Unsaturated Hydrocarbons

• 3. Arene benzene and its derivatives (Ch 21-22)

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Benzene Phenyl Group

• Benzene and derivatives studied in Chapters 21
  22, but some formulas may contain the phenyl
  group.

Ph-
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Structure of Alkenes

• Double Bond
• 1 ?-bond formed by overlap of 2 sp2 hybrid
  orbitals
• 1 ?-bond formed by overlap of 2 parallel 2p
  orbitals

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Valence Bond view of olefins
or double bonds.
x
109.5o 0.154 nm 88 kcals/mol 25 s
tetrahedral 120o 0.134 nm 146 kcals/mol
33 s trigonal 180o 0.120 nm 200
kcals/mol 50 s linear
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Structure of Alkenes

• No rotation about a CC bond - why?

Rotation requires breaking ? bond 63
kcal/mol
9
Index of Hydrogen Deficiency

• Index of hydrogen deficiency (IHD)
• IDH ? (number of rings number of ? bonds)
• Compare Hs of alkane with Hs in a compound
• CnH2n2 CnHx

2
10
Index of Hydrogen Deficiency

• Other elements present
• F, Cl, Br, I add one H - (Group 7)

9 1
7 1
8 1 - 1
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Index of Hydrogen Deficiency

• Problem isopentyl acetate has a molecular
  formula of C7H14O2. Calculate its IHD
• reference hydrocarbon C7H16
• IHD (16-14)/2 1
• Problem calculate the IHD for niacin, molecular
  formula C6H6N2O
• reference hydrocarbon C6H14
• IHD 14 - (6-2)/2 5

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Nomenclature IUPAC Root name - longest
continuous chain containing the olefin.
alkene
indicates CC in chain
Number of Cs in CC chain
6
5
1
2
4
3
2-ethyl-4-methyl-1-pentene
2-hexene
13
x
Nomenclature IUPAC Root name - longest
continuous chain containing the olefin.
alkene
indicates CC in chain
Number of Cs in chain with an CC
Cyclic
Olefin (functional group) - positions 1,2 Number
around the ring to best accommodate
substituents
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IUPAC names?
(7S)-7-bromo-4-(2-iodoethyl)-6-methyl-3-octene
7-bromo-4-(2-iodoethyl)-6-methyl-3-octene
(S)-6-ethyl-1-fluoro-5,5-dimethyl-1-cyclooctene
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Cis/trans isomerism (recall
cycloalkanes - cis/trans)
opposite side ?-bond
cis-1,2-dichloroethene
trans-1,2-dichloroethene
trans generally more stable than cis - dipoles
and 1,2 interactions
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The Cis,Trans System

• Configuration is determined by the orientation of
  atoms of the main chain

trans-3-hexene
cis-3,4-dimethyl-2-pentene
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5 Alkenes
Bonding, Nomenclature, Properties
Structure Hydrogen Deficiency Nomenclature Physic
al Properties Naturally Occurring Alkenes/Terpenes
18
Common Names used lab lecture not on tests
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Cycloalkenes- 3 to 7 cis olefins
rings not large enough to accommodate trans
double bonds
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E,Z Configuration

• uses priority rules (Chapter 3)
• higher priority groups - same side, Z

E
Z
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E/Z - priorities of groups on ends of CC
(1) Atom assigned a priority, higher atomic
number higher priority. (2) Isotopes -
higher atomic mass, higher priority 1H
lt 2H lt 3H H lt D lt T (3) If priority
the same, go to the next set of atoms
-CH2-H lt -CH2-O-H lt -CH2-Cl (4)
double (triple) bonds replaced by single bonds.
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The E,Z System

• Example name each alkene and specify its
  configuration by the E,Z system

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IUPAC names?
?(trans)-longest chain?
(3Z,7S)-7-bromo-4-(2-chloroethyl)-6-methyl-3-octen
e
(7S)-7-bromo-4-(2-chloroethyl)-6-methyl-3-octene
(S)-6-ethyl-1-fluoro-5,5-dimethylcyclooctene
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If same atoms, priority goes to the next point
of difference - atomic number
9-bromo-5-(2-methylpropyl)-4-nonene
(E)- (4E)- cis-
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cis/trans configuration may be absolute or
relative
e.g. cis-1,3-dimethylcyclopentane
or R,S-1,3-dimethylcyclopentane absolute
E,Z and R,S absolute
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Nomenclature - more than 1 unsaturated group
Diene (or diyne, en-yne) longest chain (ring)
with both groups
,-alkadiene
,-alkadiyne
-alken--yne

number so enes (enyne, ynes) have the lowest
possible number (if the same ene gt yne).
1,4-cycloheptadiene
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Dienes, Trienes, and Polyenes

• alkenes with n double bonds, which can be
  cis,trans isomers, 2n stereoisomers are possible
• example 2,4-heptadiene 22 4 cis,trans (E,Z)

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Dienes - E or Z per olefin with number
1,6-dichloro-2,4-dimethyl-2,6-decadiene
( 2Z, 4S, 6E)-
( 2Z, 4S, 6E)-
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Dienes, Trienes, and Polyenes

• vitamin A, a biologically important, sight/color
• four double bonds w/ cis,trans isomerism
• 24 16 stereoisomers

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Physical Properties

• Alkenes are nonpolar compounds
• attractive forces between molecules are
• dispersion forces
• The physical properties of alkenes are similar to
  those of alkanes

to 6.1