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Ch 7- Alkenes and Alkynes I
Division of Material
• Alkenes and Alkynes are very versatile molecules in
Organic Chemistry
• As a result, there is a lot of information we need for
them.
• To make it manageable, the information is divided
into two chapters.
• In this chapter, we will focus on their general
properties and how to make them
• In chapter 8, we will focus on the reactions they
participate in.
E/Z nomenclature system
• The E/Z system is just like the cis/trans system
for double bonds, only the E/Z system is much
more versatile!
• Example:
• For the E/Z system, you assign priorities to the
two groups bonded to each carbon of the
double bond, just like we did with R/S
E/Z nomenclature system
• If the two higher priorities are on the same
side, it is designated Z
• If the two higher priorities are on the opposite
side, it is designated E
Relative stabilities of Alkenes
• Heats of reactions, or more specifically, Heats
of Hydrogenation, can be used to identify the
stability of various alkenes
• Overall Stabilities:
Synthesis of Alkenes via Elimination
Reactions
• Eliminations are most important means of
making alkenes
• We will look at two types:
– Dehydrohalogenation of alkyl halides
– Dehydration of Alcohols
Dehydrohalogenation of Alkyl Halides
• It is best to promote E2
– Use 2o and 3o alkyl halides
– Use bulky base with 1o
– Use high concentration/Strong base
– Use alkoxides
– Use high temps
Zaitsev’s Rule
• So far, we have seen examples where only one
double bond product, or equivalent double
bond products, were possible
• Examples:
Zaitsev’s Rule
• In some cases, more than one product is
possible
• Example:
• When a small base, such as ethoxide or
hydroxide, is used, the most stable alkene will
form
Zaitsev’s Rule
• The most stable alkene will be the more
substituted alkene, as we saw with the heats of
hydrogenation
• Whenever an elimination occurs to give the most
highly substituted alkene, it is said to follow
Zaitsev’s Rule.
• The reasoning:
– T.S. lower energy
– Happens faster
Kinetic Control
• In general, the preferential formation of one
product because the free energy of activation
is lower than another product, therefore, the
rate of its formation is faster, is called Kinetic
Control of Product Formation.
Hofmann’s Rule
• If a bulky base is used, such a potassium
t-butoxide, the formation of the less
substituted alkene is favored
• This is due to the steric bulk of the base and
its access to the beta hydrogen be eliminated
• Example
• When an elimination yields the less
substituted alkene, it is said to follow
Hofmann’s Rule
Stereochemistry of the E2 reaction
• Experimental evidence shows that the five
atoms in the T.S. of an E2 reaction must lie in
the same plane.
• There are two ways this can happen:
– Anti-coplanar (preferred)
– Syn-Coplanar (only occurs with certain rigid
molecules)
Stereochemistry of the E2 reaction
• Part of the evidence comes from cyclic
molecules.
• Consider 1-chlorocyclohexane in the chair:
• Neither an axial-equatorial nor an equatorialequatorial orientation allows formation of an
anti-coplanar T.S.
Stereochemistry of the E2 reaction
• Consider the different behavior of two
diastereomers of 2-methyl-1-isopropyl-4methylcyclohexane:
Acid Catalyzed Dehydration of Alcohols
• Heating most alcohols with a strong Acid causes the
alcohol to lose the equivalent of a molecule of
water (dehydrate) and form an alkene
• Generic Reaction:
• This reaction is an elimination and is favored at high
temps
• The most commonly used acids in labs are Sulfuric
Acid and Phosphoric Acid
Important Characteristics of
Dehydration Reactions
1) The temperature and concentration of acid
required depends on structure of alcohol
-Primary most difficult to dehydrate
-Secondary uses milder conditions
-Tertiary extremely easy to dehydrate
2) Some primary and secondary alcohols also
undergo rearrangements of their carbon
skeletons during dehydration
Mechanisms for Dehydration of 2o and
3o Alcohols
• Secondary and Tertiary alcohols dehydrate via
E1 mechanism
• The slow step, RDS, is the second step, the
formation of the carbocation
• This explains the order of reactivity with the
tertiary alcohol reacting easiest, due to the
tertiary carbocation being the most stable.
Mechanism for Dehydration of 1o
Alcohols
• Because primary carbocations are so unstable,
primary alcohols dehydrate via E2, not E1
• Example:
Rearrangements
• Understanding the stability of carbocations, we can
now explain the rearrangement seen in some
dehydrations.
• Example:
– The rearrangment occurs in the second step to form a
more stable carbocation.
– 1,2 shift- when atoms or groups rearrange to an
adjacent carbon.
Rearrangements
• Rearrangements will always occur when an alkyl
group or a hydrogen can shift to form a more
stable carbocation!!
• 1,2-methyl shift
• 1,2-hydride shift
• Remember, these shifts occur to increase stability
so other forms of stability must be considered as
well!
Rearrangements
• Only the carbocation rearranges, so
dehydration of primary alcohols can not have
rearrangements since they are E2 and not
carbocation is formed!
• However, as we will see in Ch 8, the alkene
product can react with the acid by using its pi
electrons to abstract a proton from an acid
forming a carbocation.
Rearrangements
• This is technically not a rearrangement!
• It is three consecutive reactions!
• A dehydration followed by an addition
followed by another elimination!!
Synthesis of Alkynes by Elimination
• Alkynes can be made from alkenes via
compounds called Vicinal Dihalides.
• Vicinal Dihalide- a hydrocarbon with halogens
on adjacent carbons.
• Vicinal dihalides can be made by the addition
of halides to alkenes.
– ex
Synthesis of Alkynes by Elimination
• Vicinal dihalides can then undergo two
consecutive eliminations to form alkynes.
• If the alkyne formed is a terminal alkyne, then
we will have to use 3 equivalents of base and
follow with acid to get the product because
the acetylenic hydrogen is acidic!
Synthesis of Alkynes by Elimination
• Geminal Dihalides can also be converted to
alkynes via a double dehydrohalogenation
• Geminal Dihalide- a hydrocarbon with two
halogens on the same carbon
• Geminal dihalides are made from the reaction
of ketones with phosphorus pentachloride
• Ex.
Acidity of Terminal Alkynes
• Remember that the Hydrogen on a terminal
alkyne is acidic, but not as acidic as alcohols
and water.
• Relative Acidity:
• Strength of the conjugate base is the opposite:
Alkylation of Alkynide Ions, revisited
• Earlier, we saw that the alkynide ion is created
when a terminal alkyne is mixed with NaNH2 in
ammonia
• We also saw that the alkynide ion can react with a
methyl halide or a primary alkyl halide with no
branching at the beta carbon
• We should now recognize this as an Sn2 reaction
with the alkynide ion as the nucleophile and the
alkyl halide as the substrate
Alkylation of Alkynide Ions, revisited
• When the alkyl halide is a secondary or
tertiary, the alkynide acts as a base instead
and the reaction becomes an E2
Hydrogenation of Alkenes
• Alkenes will react with hydrogen gas in the
presence of a variety of metal catalyst to form
alkanes.
• In this reaction, you are adding one hydrogen
to each carbon of the double bond
• Reactions where the substrate is soluable and
the catalyst is not are called Heterogeneous
Catalysis
Hydrogenation of Alkenes
• Reactions where both the catalyst and the substrate
are soluble are called Homogeneous catalysis
• Rhodium and Ruthenium complexes with a various
phosphorus ligands are used in homogeneous
catalysis
• As we have seen, these types of reactions are called
Hydrogenation and are an example of an addition
reaction.
Mechanism for Catalytic
Hydrogenation
• The mechanism is quite different for this
reaction because it takes place on the surface of
the metal
• This reaction is also an example of syn-addition
• Syn-addition- an addition that places parts of
the adding reagent on the same side of the
reactant
Anti-addition
• The opposite of syn-addition is an antiaddition in which the parts are added to
opposite sides of the reactants
• Ex.
• In chapter 8, we will see a number of
examples of syn and anti-additions
Hydrogenation of Alkynes
• Typical conditions reduce alkynes to alkanes.
• There are special reagents that will reduce
alkynes only to the alkenes.
1) The P-2 catalyst is formed when Nickel
acetate is combined with Sodium Borohydride
Hydrogenation of Alkynes
• The P-2 catalyst allows the syn-addition of 1
molecule of hydrogen to an alkyne which
results in the cis-alkene.
Hydrogenation of Alkynes
2) Another special catalyst, called Lindlar’s
catalyst, also performs syn-addition of only 1
molecule of hydrogen to alkynes
-Lindlar’s catalyst is formed with Paladium
deposited on Calcium carbonate in quinoline
Hydrogenation of Alkynes
3) Anti-addition of hydrogen to an alkyne is
performed in the presence of lithium or sodium
metal in ammonia. The hydrogen gas is not a
reactant and the product is the trans alkene.
7.15 Introduction to Organic Synthesis
• We went over this earlier, deals with why we
do it, retrosynthesis, etc.
• Read over this section to review
End of Chapter material
• Good summaries/reviews:
– p 329 Preparation of alkenes and alkynes
– p 335 Summary and review tools
– p 336 Synthetic Connections and concept map