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Chapter 4 & 5
Alkenes and Alkynes
*Skip section 4.4, 5.5, 5.7
I.
Molecular formula (3.3, 4.1)
Alkane CnH2n+2 = a saturated hydrocarbon
E.g
C-C-C-C-C-C
E.g
C H
or
C H
add a ring:
hexane
add π bond:
*2 missing H’s = a site/degree of unsaturation (DU)
Determine degree of unsaturated (DU) from the molecular formula and structural formula


One degree of unsaturation is equivalent to 1 ring or 1 double bond (1 π bond).
Two degrees of unsaturation is equivalent to 2 double bonds, 1 ring and 1 double bond, 2 rings, or
1 triple bond (2 π bonds).
E.g C9H16
II.
Determine formula
Alkene structure (4.1)
CHM 201-Dang1
3-D sketch
3-D sketch
Cis-trans isomerism
•
•
Because of restricted rotation about a carbon-carbon double bond, an alkene with
two different groups on each carbon of the double bond shows cis-trans isomerism.
Aka as geometric isomers
Nomenclature for Alkenes and Alkynes (4.2, 4.5, 5.7)
To name an alkene;
 The parent name is that of the longest chain that contains the C=C.
 Number the chain from the end that gives the lower numbers to the carbons of the C=C.
 Locate the C=C by the number of its first carbon.
 Use the ending -ene to show the presence of the C=C
 Branched-chain alkenes are named in a manner similar to alkanes in which substituted groups are
located and named

Don’t forget to include the configuration where stereochemistry is appropriate (cis, trans, E, Z)

A compound with two double bonds is called –diene (or triene, tetraen, etc..)

If the structure contains a triple bond, it is named as an “#-alkyne.”
CHM 201-Dang2
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*Greater atomic number => higher priority
If two groups are the same  consider the next atom and so on …
Alkene and Alkyne Physical Properties (B&P 4.3)
Nonpolar molecules have same trends in boiling points and solubility “like dissolves like”
Which has a higher boiling point? 1-hexene or 1-pentyne
Alkene stability (B&P 5.6) and conjugated dienes
**If more than one π bonds present, then conjugated π systems are more stable than conjugated
CHM 201-Dang4
More alkyl substituents bonded to the sp2 carbon of alkene, the greater its stability
E.g Which is more stable ?
What can we investigate from a chemical reaction?
A. Thermodynamics
i.
Compare E (stability) of starting materials (SM) and products ΔH < 0, ΔG <0
ii.
Predict direction of equilibrium (forward or reverse)
ΔG = ΔH - TΔS
CHM 201-Dang5
*Energy vs. Progress of Rxn Diagrams
ΔH >0, ΔG >0
E is consumed
Reactants favored
Reactants lower E, more stable
Reverse rxn is spontaneous
ΔH < 0, ΔG <0
Products are favored
Products lower E, more stable
Forward rxn is spontaneous
Kinetics:
-rate of reaction
A + B  C + D
Rate = k [A]x [B]y
k = rate constant @ given temperature
E.g
CH3—Cl
+ -OH  CH3—OH
+ Cl-
If [-OH] is doubled, rate doubles
If [CH3Cl] is doubled, rate doubles
Rate law = k [-OH][CH3Cl]
CHM 201-Dang6
The rate is “first order” with respect to each reactant
(exponent = 1) and “second-order” overall (sum of exponents)
Reaction Rate Variable (4.9)
-to achieve TS, need a collision with enough Energy
1.
Increasing temperature, increasing rate
2.
Increasing concentration, increasing rate
3.
decreasing activation Energy (Ea), increasing rate
Rate Determining Step
-Step with highest T.S Energy
- has highest Ea + is slowest
Reactive Intermediates
-
High energy species (unstable)
-
Produced then consumed in reaction (doesn’t appear in net rxn)
-
Different from transition state (T.S)
Energy vs. Progress of reaction (Reaction Coordinate) (4.9)
Diagram for Two-Step Mechanism
CHM 201-Dang7
E.g
CHM 201-Dang8