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Formation of glycols with Syn Addition. Osmium tetroxide. Syn addition. also. KMnO 4. Anti glycols. Using a peracid, RCO 3 H, to form an epoxide which is opened by aq. acid. epoxide. The protonated epoxide is analagous to the cyclic bromonium ion. Peracid: for example, perbenzoic acid.

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Formation of glycols with Syn Addition

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Formation of glycols with syn addition l.jpg

Formation of glycols with SynAddition

Osmium tetroxide

Syn addition

also

KMnO4


Anti glycols l.jpg

Anti glycols

Using a peracid, RCO3H, to form an epoxide which is opened by aq. acid.

epoxide

The protonated epoxide is analagous to the cyclic bromonium ion.

Peracid: for example, perbenzoic acid


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An example

Are these unique?

Diastereomers, separable (in theory) by distillation, each optically active


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Ozonolysis

Reaction can be used to break larger molecule down into smaller parts for easy identification.


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Ozonolysis Example

For example, suppose an unknown compound had the formula C8H12 and upon ozonolysis yielded only 3-oxobutanal. What is the structure of the unknown?

The hydrogen deficiency is 18-12 = 6. 6/2 = 3 pi bonds or rings.

The original compound has 8 carbons and the ozonolysis product has only 4

Conclude: Unknown  two 3-oxobutanal.

Unknown

C8H12

ozonolysys

Simply remove the new oxygens and join to make double bonds.

But there is a second possibility.


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Another Example

d

Hydrogen Deficiency = 8. Four pi bonds/rings.

a

c

Unknown has no oxygens. Ozonolysis product has four. Each double bond produces two carbonyl groups. Expect unknown to have 2 pi bonds and two rings.

b

To construct unknown cross out the oxygens and then connect. But there are many ways the connections can be made.

Look for a structure that obeys the isoprene rule.


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Mechanism

Consider the resonance structures of ozone.

Electrophile capability.

Nucleophile capability.

These two, charged at each end, are the useful ones to think about.


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Mechanism - 2


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Mechanism - 3


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Mechanism - 4


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Hydrogenation

No regioselectivity

Syn addition


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Heats of Hydrogenation

Consider the cis vs trans heats of hydrogenation in more detail…


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Heats of Hydrogenation - 2

The trans alkene has a lower heat of hydrogenation.

  • Conclusion:

  • Trans alkenes with lower heats of hydrogenation are more stable than cis.

  • We saw same kind of reasoning when we talked about heats of combustion of isomeric alkanes to give CO2 and H2O


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Heats of Hydrogenation

Increasing substitution

Reduced heat of Hydrogenation

By same reasoning higher degree of substitution provide lower heat of hydrogenation and are, therefore, more stable.


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Acid Catalyzed Polymerization

Principle: Reactive pi electrons (Lewis base) can react with Lewis acid. Recall

Which now reacts with a Lewis base, such as halide ion to complete addition of HX yielding 2-halopropane

Variation: there are other Lewis bases available. THE ALKENE.

The new carbocation now reacts with a Lewis base such as halide ion to yield halide ion to yield 2-halo-4-methyl pentane (dimerization) but could react with another propene to yield higher polymers.


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Examples of Synthetic Planning

Give a synthesis of 2-hexanol from any alkene.

Planning:

Alkene is a hydrocarbon, thus we have to introduce the OH group

How is OH group introduced (into an alkene): hydration

  • What are hydration reactions and what are their characteristics:

  • Mercuration/Reduction: Markovnikov

  • Hydroboration/Oxidation: Anti-Markovnikov and syn addition


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What alkene to use? Must involve C2 in double bond.

Which reaction to use with which alkene?

Markovnikov rule can be applied here. CH vs CH2.

Want Markovnikov!

Use Mercuration/Reduction!!!

Markovnkov Rule cannot be used here. Both are CH.

Do not have control over regioselectivity.

Do not use this alkene.

For yourself : how would you make 1 hexanol, and 3-hexanol?


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Another synthetic example…

How would you prepare meso 2,3 dibromobutane from an alkene?

Analysis:

Alkene must be 2-butene. But wait that could be either cis or trans!

We want meso. Have to worry about stereochemistry

Know bromine addition to an alkene is anti addition (cyclic bromonium ion)


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This did not work, gave us the wrong stereochemistry!

This worked! How about starting with the cis?


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Addition Reaction General Rule…

Characterize Reactant as cis or trans, C or T

Characterize Reaction as syn or anti, S or A

Characterize Product as meso or racemic mixture, M or R

Relationship

Characteristics can be changed in pairs and C A R will remain true.

Want meso instead?? Have to use trans. Two changed!!


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Alkynes


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Structure

sp hybridization


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Acidity of Terminal Alkynes

Stronger base

Weaker base

Other strong bases that will ionize the terminal alkyne:

Not KOH


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Important Synthetic Method: Dehydrohalogenation

1. Dehydrohalogenation…

An alkyl halide can eliminate a hydrogen halide molecule, HX, to produce a pi bond.

Recall that HX can be added to a double bond to make an alkyl halide.

HX can also be removed by strong base, called dehydrohalogenation.

Preparation of alkene

Strong base

RCH=CHR+ HX RCHXCH2R

Or rewriting

base

RCHBrCH2R RCH=CHR

Also, if we start with a vinyl halide and a very strong base (vinyl halides are not very reactive).

NaH

RCH=CHBr RCCH


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Synthetic planning (Retrosynthesis)

Work Backwards…..

Trace the reactions sequence from the desired product back to ultimate reactants.

Starting reactant

Target molecule.

Overall Sequence converts alkene alkyne

But typical of synthetic problems side reaction occurs to some extent and must be taken into account.

C


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More Sythesis: Nucleophilic Substitution

Use the acidity of a terminal alkyne to create a nucleophile which then initiates a substitution reaction.

Note that we still have an acidic hydrogen and, thus, can react with another alkyl group in this way to make RCCR’

Alkyl halides can be obtained from alcohols


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Reactions: alkyne with halogen

RCCR + Br2 RBrC=CBrR

No regioselectivity with Br2.

Stereoselective for trans addition.


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Reactions: Addition of HX

The expected reaction sequence occurs, formation of the more stable carbocation.

Markovnikov orientation for both additions.

Now for the mechanism….


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Mechanism

The expected reaction sequence occurs, formation of the more stable carbocation.


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Addition of the second mole, another example of resonance.


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Reactions: Acid catalyzed Hydration (Markovnikov).

Markovnikov addition, followed by tautomerism to yield, usually, a carbonyl compound.


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Reactions: Anti Markovnikov Hydration of Alkynes, Regioselectivity

Step 2

Step 1

Similar to formation of an anti-Markovnikov alcohol from an alkene

Step 1, Internal Alkyne: addition to the alkyne with little or no regioselectivity issue.

Alternatively Asymmetric, terminal, alkyne if you want to have strong regioselectivity then use a borane with stronger selectivity for more open site of attack.

Less exposed site.

Aldehyde not ketone.

More exposed site.

sia2BH


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Tautomerism, enol  carbonyl

Step 2, Reaction of the alkenyl borane with H2O2, NaOH would yield an enol. Enols are unstable and rearrange (tautomerize) to yield either an aldehyde or ketone.

Overall…

internal alkyne   ketone (possibly a mixture, next slide)

Terminal alkyne   aldehyde


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Examples

Used to insure regioselectivity.

As before, for a terminal alkyne.

But for a non-terminal alkyne frequently will get two different ketones

Get mixture of alkenyl boranes due to low regioselectivity.


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Reduction, Alkyne  Alkene

1. Catalytic Hydrogenation

If you use catalysts which are also effective for alkene hydrogenation you will get alkane.

You can use a reduced activity catalyst (Lindlar), Pd and Pb, which stops at the alkene. You obtain a cisalkene.

Syn addition


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Reduction - 2

2. Treatment of alkenyl borane with a carboxylic acid to yield cis alkene.

Instead of H2O2 / NaOH

Alkenyl borane

3. Reduction by sodium or lithium in liquid ammonia to yield the trans alkene.


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Plan a Synthetic SequenceRetrosynthesis

YES!

Synthesize butan-1-ol from ethyne. Work backward from the target molecule.

A big alkyne can be formed via nucleophilic substitution. This is the chance to make the C-C bond we need.

Is read as “comes from”.

Major problem: make big from small. Be alert for when the “disconnect” can be done.

Catalytic Lindlar reduction

  • BH3

  • H2O2, NaOH

Convert ethyne to anion and react with EtBr.

Do a “disconnect” here.

Target molecule

Catalytic reduction Lindlar

Addition of HBr.

Now, fill in the “forward reaction” details

Can we get an alkyne from smaller molecules?

Not yet! So how can we get it?

How about joining molecules to get an alkene? Not yet!! So how can we get an alkene?

Ask yourself! Do we know how to join any two molecules together to yield an alcohol?


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