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CHEM1021 Chemistry 1B Lecture 29

Organic Chemistry •Cover - Heading location (aligned with ”SYDNEY”)

Dr Siobhán Wills - Dalton Building 130, School of Chemistry - [email protected]

- This lecture: Section 16.7, p1002 in Blackmans

Slides based on lectures by Dr A. Fahrenbach and Dr Thomas Name MurphyName

Alkene compounds – Stereo-outcomes H2(g)

+

PtO2

H2(g) Pd/C

Br2(l)

2

+

Alkene compounds – Stereo-outcomes

Enantiomers

≡

Chair flip

3

≡

Rotate molecule 180° around vertical

Aromatic compounds - Learning Objectives Threshold T8.4 Identify that benzene is nucleophilic, by considering its molecular structure and distributions of electron density. T8.5 Demonstrate an understanding pf the concept of resonance stabilisation. T8.6 Predict the product of an electrophilic aromatic substitution reaction of benzene, given the reagents and conditions. Mastery M8.4 Draw the mechanism for electrophilic aromatic substitution reactions using 'curly arrow' notation indicating correctly electron movement and breaking/formation of bonds. M8.5 Draw an energy vs. reaction coordinate diagram for electrophilic aromatic substitution reactions.

4

5

Benzene Benzene was first isolated by Michael Faraday in 1825 and its molecular formula was determined to be C6H6 But the structure of benzene was confusing for a long time Degrees of unsaturation The degree of unsaturation indicates there could be a combination of double bonds and rings in the structure…

6

Benzene - Experimental observations Double bonds mean addition reactions.

XY H H

C

Y

C

H H

X C

H H

Benzene does not react this way!

C6H6 + Br2

C6H6Br2 No reaction!

C

H H

Benzene - Experimental observations Multiple bonds mean addition reactions.

XY H H

C

Y

C

H H

But with slightly different conditions…

C6

+

C6H5Br + Br2

Fe(s) powder catalyst Fe(s) powder catalyst

X C

H H

C

H H

C6H C6H4Br2 + HBr There are three C6H4Br2 structural isomers

This is substitution not addition - alkenes don’t do this

Benzene - Experimental observations Benzene mass Spectrum: m/z = 78 (M+) [consistent with C6H6] 1H

NMR spectrum All 6 hydrogen atoms identical

PPM

9.0

8.0

7.0

3.0

2.0

1.0

Typical alkene 1H shifts 13C

NMR spectrum All 6 carbon atoms identical

PPM

180.0

160.0

140.0

120

100.0

Typical alkene 13C shifts

80.0

60.0

40.0

20.0

Benzene - structure determination The structure of benzene must account for the following observations: • All 6 carbon atoms are equivalent • All 6 hydrogen atoms are equivalent • Why benzene doesn’t react like an alkene • Why THREE structural isomers form when benzene is substituted with Br2 and Fe(s) powder.

Benzene - structure determination In 1865-66 Kekulé proposed the first decent approximation of the structure for benzene.

In this structure all C and H atoms are equivalent! H

H

H

H H

H

(C6H5)

H

Br2 Fe(s) powder catalyst

H

H

Br H

+ HBr

H

(C6H5Br)

But…

Benzene - structure determination When C6H5Br is substituted with bromine there should only be three isomers, but Kekulé’s structure predicts 4 H

H

Br

H

H

Br H

H

Br2 Fe(s) powder catalyst

These two structures are not the same. But Kekulé had a solution…

Br

H Br

H

H

Br

H

H

H

H

H

H

H

Br

H

Br H

Br

H

Br H

H

Benzene - structure determination Kekulé’s structure was effectively a molecule with 3 alkene groups Hydrogenation is exothermic. Hydrogenation of an alkene releases about 118 kJ/mol, therefore:

Benzene - structure determination Kekulé’s structure was effectively a molecule with 3 alkene groups Hydrogenation is exothermic. Hydrogenation of an alkene releases about 118 kJ/mol, therefore:

Benzene - structure determination Kekulé’s structure was effectively a molecule with 3 alkene groups Hydrogenation is exothermic. Hydrogenation of an alkene releases about 118 kJ/mol, therefore: Kekulé’s benzene (“1,3,5-Cyclohexatriene)

Benzene - structure determination Kekulé’s structure was effectively a molecule with 3 alkene groups Hydrogenation is exothermic. Hydrogenation of an alkene releases about 118 kJ/mol, therefore: Kekulé’s benzene (“1,3,5-Cyclohexatriene)

Benzene is much more stable than expected!

Why?

Benzene - structure determination Kekulé was almost right.

Benzene does not sway between two structures, it is a hybrid (a mixture) of two structures We can see how this works by looking at the orbitals in benzene.

Benzene - bonding The C atoms in benzene are sp2 hybridised, so they have sp2 σbonds The C atoms also have unhybridised p-orbitals that overlap sideways

A single π-orbital made by the conjugation of all 6 p-orbitals

An analogy for resonance

A unicorn is a mixed-hybrid of a horse and a rhino Just like the π-orbital in benzene is made by the mixing and hybridisation of p-orbitals

Benzene Because of resonance - Benzene is a regular hexagon 1.09 Å 120°

- Benzene is planar (flat)

1.39 Å

- All C-C bonds are the same (with a length between that of a single bond and a double bond)

- All C-H bonds are the same - All bond angles are 120°

Aromatic bonding Resonance means that benzene doesn‘t consist of alternating single and double bonds Benzene has a different type of bonding – ‘aromatic’ bonding Aromatic bonding allows benzene to spread the p-orbital electrons out in the π-orbital. This is the source of the resonance energy of benzene. The resonance energy gives the molecule stability. Aromatic bonding explains why benzene doesn’t behave like an alkene!

Aromatic bonding Benzene is just the simplest aromatic molecule There are many aromatic molecules, but they all have conjugated p-orbitals that overlap continuously to make a ring-shaped πorbital

Imidazole Pyrene

Anthracene Annulene

Aromatic compounds in Nature

Guanine

Cytosine

Aromatic π-orbitals

The bases that make up the nucleotides in DNA are aromatic molecules

Aromatic compounds in Nature Chlorophyll

The porphyrin ring in chlorophyll that absorbs sunlight for photosynthesis is an aromatic structure.

Benzene - structure determination

Benzene - structure determination Try Q. 1 & 2

26

Benzene - The π-orbital in benzene makes the ring of carbon atoms electron rich. - This high electron density makes benzene nucleophilic. - Because benzene is an electron rich and nucleophile, it will react with electrophiles. Aromatic π-orbital

High electron density

Low electron density Adapted from: Vollhardt and Schore (2014), Organic Chemistry, Structure and Function, Macmillan International Higher Education

- Benzene undergoes substitution reactions and not addition reactions so that it keeps its resonance energy (Aromaticity is a stabilising effect for molecules).

Electrophilic aromatic substitution The generic electrophilic aromatic substitution reaction

One H atom gets substituted by the electrophile

H

EH

H +

EY

-

HY H

H

Electrophile

H

When we look at electrophilic aromatic substitution, we want to know: 1. Why does substitution occur instead of addition, as occurs with alkenes  to understand this we look at the mechanism for electrophilic aromatic substitution 2. What electrophiles and reaction conditions are used for electrophilic aromatic substitution reactions.

Electrophilic aromatic substitution Recall the mechanism for electrophilic addition to alkenes.

X

X

Y H H

C

C

H H

Y

Step 1

C

H H

The first step in electrophilic aromatic substitution is the same as the first step for addition to alkenes

+ C

H H

 Y

Step 2

X C

H

C

H

Step 2 is where electrophilic aromatic substitution is different to addition to alkenes, and we will see why

H H

Electrophilic aromatic substitution The energy diagram for electrophilic aromatic substitution will look something like this: E +

EY

Ea

This first step always has the highest activation energy (Ea) - it is the Rate determining step (RDS) E

G A two step mechanism with a carbocation intermediate, similar to the one we saw for addition to alkenes.

Electrophilic aromatic substitution In step 1 of electrophilic aromatic substitution, the electrophile accepts an electron pair from the π-system on the benzene ring

E +

EY

-

This is the carbocation intermediate This cation still has some resonance energy, like benzene

Electrophilic aromatic substitution The carbocation formed is stabilised by resonance E

The resonance stabilisation means that the cation charge gets spread out around the benzene ring

E

E

+

The Wheland intermediate

E

This cation (Wheland) intermediate still has about 2/3 of the resonance energy that benzene has.

Electrophilic aromatic substitution What makes step 2 different to the addition to alkenes? Consider the result if addition did happen EH

H

H

H H Y

E

H

H

Both of these addition products have lost all the resonance energy that Benzene had.

H +

E

Y



A nucleophile donates an electron pair to bond with Wheland cation intermediate

They have also lost the resonance energy the Wheland cation intermediate had. Y The products want to keep their resonance energy – so something different happens

Electrophilic aromatic substitution In step 2 of electrophilic aromatic substitution the H atom where the electrophile is bonded to benzene is removed. E

E

+

The loss of H+ makes the C atom sp2 hybridised again

π-orbital restored in product

This means the ring of overlapping p-orbitals that make the π-system is restored and the product gets back the resonance energy benzene had

Electrophilic aromatic substitution Overall mechanism:

Substituted product

EY

E

E

Step 1 slow

+

Step 2 fast

Wheland (carbocation) intermediate

What electrophiles (E) can we use?

Electrophilic aromatic substitution There are 4 main electrophilic aromatic substitution reactions to remember:

1.

Nitration

2.

Halogenation

3.

Friedel-Crafts Alkylation

4.

Friedel-Crafts Acylation

Make sure to remember the reagents and products for each reaction

Electrophilic aromatic substitution 1. Nitration NO2

Conc. HNO3 Conc. H2SO4 +

H2SO4 turns HNO3 into NO2 = the electrophile HNO3 + 2H2SO4

+



NO2 + 2HSO4 + H3O

+

Electrophilic aromatic substitution 1. Nitration NO2

Conc. HNO3 Conc. H2SO4 +

NO2

NO2

NO2

Step 1 slow

Step 2 +

fast

Electrophilic aromatic substitution 2. Halogenation X2/MX3

X

X2 = either Cl2 or Br2 MX3 = is a metal halide used to turn the halogen (X2) into a good electrophile use FeBr3 or AlCl3

Electrophilic aromatic substitution 2. Halogenation X

X2/MX3

MX3 are Lewis acids – They have vacant orbitals that accept electron pairs from Cl2/Br2 to make a complex with an electrophilic halogen atom. +

-

BrBrFeBr3 Electrophiles

+

-

ClClAlCl3

Electrophilic aromatic substitution 2. Halogenation X

X2/MX3 +

-

BrBrFeBr3

Step 1 slow

Br

Br

Step 2 +

The mechanism is exactly the same for

fast +

-

ClClAlCl3

Electrophilic aromatic substitution 3. Friedel-Crafts Alkylation R

R-Cl/AlCl3

R-Cl = any alkyl chloride ; e.g. 1-chloropropane

AlCl3 is used to turn the alkyl halide into a good electrophile (just like it did in halogenation)

Electrophilic aromatic substitution 3. Friedel-Crafts Alkylation R

R-Cl/AlCl3

R

Cl

+

-

RClAlCl3 Electrophile

Electrophilic aromatic substitution 3. Friedel-Crafts Alkylation R

R-Cl/AlCl3 +

-

RClAlCl3

Step 1 slow

R

R

Step 2 +

fast

The mechanism is exactly the same for any alkyl chloride

Electrophilic aromatic substitution 3. Friedel-Crafts Alkylation R-Cl/AlCl3

R

Wait! What about stereochemistry? If the alkyl chloride is a 2°or 3° alkyl chloride, then the products of the Friedel-Crafts alkylation will be chiral! That means R/S isomers are possible!

Electrophilic aromatic substitution 3. Friedel-Crafts Alkylation R-Cl/AlCl3

R

Wait! What about stereochemistry? 2°or 3° alkyl chlorides can make stable carbocations! E.g. R-Cl/AlCl3 Cl (S)-2-chlorobutane

This carbocation can react with benzene just like NO2 and it is planar so the product will be a racemate 50% R and 50% S

Electrophilic aromatic substitution 4. Friedel-Crafts Acylation X

RCOCl/AlCl3 RCOCl = any acyl chloride (“acid chloride”) ; e.g.

Acetyl chloride

AlCl3 is used to turn the acyl chloride into a good electrophile (just like it did in halogenation and Friedel-Crafts alkylation)

Electrophilic aromatic substitution 4. Friedel-Crafts Acylation X

RCOCl/AlCl3

O R

C

O

Cl

+

-

R  C  ClAlCl3 Electrophile

Electrophilic aromatic substitution 4. Friedel-Crafts Acylation X

RCOCl/AlCl3

O

O

-

R  CClAlCl3

CR

CR

Step 1 slow



+

 

 

O

+

Step 2 fast

The mechanism is exactly the same for any acyl chloride

Practice problems What reagents were used to make the following transformations

Conc. HNO3 Conc. H2SO4

NO2

This is nitration!

AlCl3

You need to include the entire acyl group that was added as a reagent

This is an acyl group = Friedel-Crafts acylation

Practice problems What is the product of the following reaction? This is Friedel-Crafts alkyation

AlCl3 This alkyl halide is chiral but can form a tertiary carbocation intermediate! = 2 products, one R and one S

(R)-(3-methylhexan-3-yl)benzene

(S)-(3-methylhexan-3-yl)benzene

Benzene - structure determination Try Q. 3-5

52

References Unicorn sketch: By Pearson Scott Foresman - Archives of Pearson Scott Foresman, donated to the Wikimedia FoundationThis file has been extracted from another file: PSF M-600003.png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=34739504 Hores sketch: By Arsdelicata - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=7905942 Rhino image: in the public domain under creative commons licence: CC0 1.0 Universal (CC0 1.0) Public Domain Dedication Vollhardt and Schore (2014), Organic Chemistry, Structure and Function, Macmillan International Higher Education Tilia leaf: By Krzysztof P. Jasiutowicz - first upload pl.wikipedia 11:26, 10 jul (lipiec) 2004 by Kpjas as pl:Grafika:Lisc lipy.jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=166598 Chloroplast: By Kelvinsong - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=26147364

Aromatic compounds - Learning Objectives Threshold T8.4 Identify that benzene is nucleophilic, by considering its molecular structure and distributions of electron density. T8.5 Demonstrate an understanding pf the concept of resonance stabilisation. T8.6 Predict the product of an electrophilic aromatic substitution reaction of benzene, given the reagents and conditions. Mastery M8.4 Draw the mechanism for electrophilic aromatic substitution reactions using 'curly arrow' notation indicating correctly electron movement and breaking/formation of bonds. M8.5 Draw an energy vs. reaction coordinate diagram for electrophilic aromatic substitution reactions.

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