THE CHEMISTRY OF ARENESA guide for A level students

2008KNOCKHARDY PUBLISHING SPECIFICATIONS

KNOCKHARDY PUBLISHING

ARENESINTRODUCTIONThis Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards. Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available. Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...

www.knockhardy.org.uk/sci.htm Navigation is achieved by... either or clicking on the grey arrows at the foot of each page using the left and right arrow keys on the keyboard

ARENESCONTENTS Prior knowledge Structure of benzene Thermodynamic stability Delocalisation Electrophilic substitution Nitration Chlorination Friedel-Crafts reactions Further substitution

ARENESBefore you start it would be helpful to

know the functional groups found in organic chemistry know the arrangement of bonds around carbon atoms recall and explain electrophilic addition reactions of alkenes

STRUCTURE OF BENZENEPrimary analysis revealed benzene had...an a a

empirical formula of CH molecular mass of 78 molecular formula of C6H6

and and

STRUCTURE OF BENZENEPrimary analysis revealed benzene had...an a a

empirical formula of CH molecular mass of 78 molecular formula of C6H6suggested that benzene was...

and

Kekul

PLANAR CYCLIC and HAD ALTERNATING DOUBLE AND SINGLE BONDS

STRUCTURE OF BENZENEHOWEVER...

it did not readily undergo electrophilic addition - no true C=C bond only one 1,2 disubstituted product existed all six CC bond lengths were similar; C=C bonds are shorter than C-C the ring was thermodynamically more stable than expected

STRUCTURE OF BENZENEHOWEVER...

it did not readily undergo electrophilic addition - no true C=C bond only one 1,2 disubstituted product existed all six CC bond lengths were similar; C=C bonds are shorter than C-C the ring was thermodynamically more stable than expected

To explain the above, it was suggested that the structure oscillated between the two Kekul forms but was represented by neither of them. It was a RESONANCE HYBRID.

THERMODYNAMIC EVIDENCE FOR STABILITYWhen unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.

THERMODYNAMIC EVIDENCE FOR STABILITYWhen unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.

C6H10(l) + H2(g) > C6H12(l)

2

3

- 120 kJ mol-1

THERMODYNAMIC EVIDENCE FOR STABILITYWhen unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.

C6H10(l) + H2(g) > C6H12(l)Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane

Theoretical - 360 kJ mol-1(3 x -120)

C6H6(l) + 3H2(g) > C6H12(l)2 3

- 120 kJ mol-1

THERMODYNAMIC EVIDENCE FOR STABILITYWhen unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.

C6H10(l) + H2(g) > C6H12(l)Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane

Theoretical - 360 kJ mol-1(3 x -120)

C6H6(l) + 3H2(g) > C6H12(l)2 3

Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale

- 120 kJ

mol-1

Experimental - 208 kJ mol-1

THERMODYNAMIC EVIDENCE FOR STABILITYWhen unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.

C6H10(l) + H2(g) > C6H12(l)Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane

Theoretical - 360 kJ mol-1(3 x -120)

MORE STABLE THAN EXPECTED by 152 kJ mol-1

C6H6(l) + 3H2(g) > C6H12(l)2 3

Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale

- 120 kJ

mol-1

Experimental - 208 kJ mol-1

It is 152kJ per mole more stable than expected. This value is known as the RESONANCE ENERGY.

THERMODYNAMIC EVIDENCE FOR STABILITYWhen unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.

C6H10(l) + H2(g) > C6H12(l)Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane

Theoretical - 360 kJ mol-1(3 x -120)

MORE STABLE THAN EXPECTED by 152 kJ mol-1

C6H6(l) + 3H2(g) > C6H12(l)2 3

Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale

- 120 kJ

mol-1

Experimental - 208 kJ mol-1

It is 152kJ per mole more stable than expected. This value is known as the RESONANCE ENERGY.

HYBRIDISATION OF ORBITALS - REVISION2p

The electronic configuration of a carbon atom is 1s22s22p2

2 2s

1

1s

HYBRIDISATION OF ORBITALS - REVISION2p

The electronic configuration of a carbon atom is 1s22s22p2

2 2s

1

1s

If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s22s12p3

2p 2 2s

1

1s

The process is favourable because of the arrangement of electrons; four unpaired and with less repulsion is more stable

HYBRIDISATION OF ORBITALS - REVISION

The four orbitals (an s and three ps) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent.

2s22p2

2s12p3HYBRIDISE

4 x sp3

sp3HYBRIDISATION

HYBRIDISATION OF ORBITALS - REVISION

Alternatively, only three orbitals (an s and two ps) combine or HYBRIDISE to give three new orbitals. All three orbitals are equivalent. The remaining 2p orbital is unchanged. 2s22p2 2s12p3HYBRIDISE

3 x sp2

2p

sp2HYBRIDISATION

STRUCTURE OF ALKENES - REVISION

In ALKANES, the four sp3 orbitals repel each other into a tetrahedral arrangement.

In ALKENES, the three sp2 orbitals repel each other into a planar arrangement and the 2p orbital lies at right angles to them

STRUCTURE OF ALKENES - REVISION

Covalent bonds are formed by overlap of orbitals.

An sp2 orbital from each carbon overlaps to form a single C-C bond.

The resulting bond is called a SIGMA ( ) bond.

STRUCTURE OF ALKENES - REVISION

The two 2p orbitals also overlap. This forms a second bond; it is known as a PI ( ) bond. For maximum overlap and hence the strongest bond, the 2p orbitals are in line. This gives rise to the planar arrangement around C=C bonds.

ORBITAL OVERLAP IN ETHENE - REVIEW

two sp2 orbitals overlap to form a sigma bond between the two carbon atoms

two 2p orbitals overlap to form a pi bond between the two carbon atoms

s orbitals in hydrogen overlap with the sp2 orbitals in carbon to form C-H bonds

the resulting shape is planar with bond angles of 120

STRUCTURE OF BENZENE - DELOCALISATIONThe theory suggested that instead of three localised (in one position) double bonds, the six p (T) electrons making up those bonds were delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It also gave a planar structure.

6 single bonds

STRUCTURE OF BENZENE - DELOCALISATIONThe theory suggested that instead of three localised (in one position) double bonds, the six p (T) electrons making up those bonds were delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It also gave a planar structure.

6 single bonds

one way to overlap adjacent p orbitals

STRUCTURE OF BENZENE - DELOCALISATIONThe theory suggested that instead of three localised (in one position) double bonds, the six p (T) electrons making up those bonds were delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It also gave a planar structure.

6 single bonds

one way to overlap adjacent p orbitals

another possibility

STRUCTURE OF BENZENE - DELOCALISATIONThe theory suggested that instead of three localised (in one position) double bonds, the six p