4.1 & 4.2- Stereoisomerism and Aromaticity

Define structural isomerism

Compounds that have the same molecular formula but differ in their arrangement of atoms

What are the three different types of structural isomerism?

Chain

Position

Functional group

What is chain isomerism?

When isomers have different branches or are straight chain

Position isomerism?

Same molecular formula functional group in different position.

Functional group isomerism?

same molecular formula but different functional group

Define stereoisomerism

Compounds that have the same shortened structural formula but have a different arrangement of atoms in space

What are the two types of stereoisomerism?

E/Z and optical

What does E/Z isomerism occur in?

Compounds that have a C to C double bond as there is restricted rotation around the double bond

Both carbon atoms each have two different groups attached to them

When it is E and when is it Z?

E when the groups with the highest atomic number on each carbon are on opposite sides

Z when the groups with the highest atomic number on each carbon are on the same side

How do molecules have optical isomers?

When their mirror images are non-superimposable

What is a chiral centre?

A carbon atom with four different groups attached to it

What can a chiral centre not have?

A double bond attached to it

Know how to draw non-superimposable images

What are the properties of optical isomers?

1. Their physical properties are identical except for the direction of rotation of plane-polarised light, as an enantiomer may rotate the plane-polarised light to the right or left

2. They react at different rates with optically active reagents

Are their chemical properties the same?

Yes

What are enantiomers?

A pair of optical isomers

What is an equimolar mixture/ racemic mixture?

Optically inactive as there is no overall rotation of plane polarised light

What do arenes have?

One or more rings of carbon atoms in which the bonding electrons are delocalised

What is one example?

Benzene C6H6

What is its structural formula?

What is the Kekule structure?

Why is the delocalised structure more accurate?

1. Bond length

2. Resistance to reaction

3. Enthalpy change of hydrogenation

Explain bond length

Single and double bonds do not have the same length. If kekule, the double bonds would be shorter than the single, but all of the bonds in benzene have a length intermediate between single and double

Resistance to reaction

If Kekule is correct, benzene should readily decolourise bromine. Does not readily decolourise bromine and does not readily undergo addition reactions

Enthalpy change of hydrogenation

Less exothermic than expected, as delocalisation leads to a more stable structure

What is the difference between the expected enthalpy change of hydrogenation and the actual change called?

The delocalisation energy

So what is the structure of benzene?

-6 electrons

-6 singularly occupied p orbitals

-Delocalised pi system is above and below the plane of the ring

-3 sigma bonds around each carbon atom

-P orbitals overlap side by side

-Bond angle 120 degrees and planar molecule

-C6H6

How are the delocalised bonds formed?

Two adjacent p orbitals overlap to form a pi bond. In benzene, 6 p orbitals overlap to form a delocalised system of pi bonds that extends over all 6 carbon atoms

Each carbon atom forms how many sigma bonds?

3- 2 to neighbouring carbon atoms and one to a H atom

What does benzene burn with oxygen with?

A smoky flame due to its high carbon content

What type of reaction does benzene take place in?

Electrophilic substitution

Why?

The delocalised electrons are an electron dense area, so are susceptible to attacks by electrophiles

What are the 3 examples?

1. Nitration of benzene

2. Halogenation of benzene

3. Friedel-Crafts alkylation

What are the conditions and reagents required for nitration?

50:50 mixture of concentrated nitric acid and concentrated sulphuric acid

Under reflux at 50 degrees C

Why can the temperature not exceed this?

Because otherwise further substitution may occur

What is formed?

Nitrobenzene

What is the catalyst?

The concentrated sulphuric acid

How is the electrophile formed?

HNO3 + 2H2SO4 -> H3O+ + +NO2 + HSO4-

What is the mechanism of the substitution?

The HSO4 transfers its electrons for on top of the O to the H in the second step to form H2SO4

What do you need to remember to do?

Put the plus sign next to the N in +NO2

Move the horseshoe around depending on the Carbon atom that the electrophile is attaching to

What does the +NO2 act as and why?

The electrophile because it accepts a pair of electrons

Define electrophile

An electron pair acceptor

What happens in the second step?

The carbocation intermediate has 4 pi electrons, delocalised over 5 carbons. H+ is eliminated to complete the reaction.

What are the conditions for halogenation?

Reflux with halogen in the presence of a catalyst

Eg, for chlorination- AlCl3, FeCl3

For bromination- AlBr3, FeBr3

Mechanism for chlorination of benzene

The compound IBr reacts with benzene. Would iodobenzene or chlorobenzene be the main product of the reaction?

What is the equation for friedel-crafts reactions?

C6H6 + CH3Cl -> C6H5CH3 + HCl

What is the hydrogen atom replaced by?

An alkyl group

Why is this important?

Because it increases the carbon atoms in a compound by forming C-C bonds

How is the electrophile formed?

AlCl3 + CH3Cl -> AlCl4- + +CH3

What is the mechanism for the substitution?

How is the catalyst reformed?

AlCl4- + H+ -> AlCl3 + HCl

Compare the reactivity of alkenes with arenes

Alkenes are more reactive as the double (pi) bond is a region of high localised electron density, which can induce a dipole in a neighbouring molecule, causing electrophilic addition to occur

The electron density of the pi bonds in benzene is delocalised, so does not have a region of high electron density so cannot induce a dipole

What does the shorter and stronger bond between carbon and chlorine in chlorobenzene result from?

The pair of electrons in a p orbital on chlorine overlapping with the ring pi system of electrons.

What does the resulting bond need?

More energy to break

What does this mean?

Extreme conditions are needed to produce phenol from chlorobenzene

What test could be used to distinguish between chlorobenzene and chlorobutane?

Reflux with NaOH

Cool and acidify with nitric acid

Add silver nitrate

A white precipitate will be formed with the halogenoalkane only

The C-Cl bond in the halogenoalkane is weaker and will react with OH- (nucleophilic sub)

C-Cl bond in chlorobenzene is stronger so will not react

Electrons in p orbital as Cl overlaps with delocalised pi system

Making C-Cl bond shorter and stronger