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AQA A Level Chemistry > Armomatic Chemistry > Flashcards

Armomatic Chemistry Flashcards

1
Q

What is benzenes skeletal structure and formula ?

A

•C6H6
•A hexagon with a circle in the middle

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2
Q

What is another name for arenes ?

A

Aromatic compounds, as they were first found in sweet smelling dyes

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3
Q

What is the most common type of reaction for benzene ?

A

Substitution reactions in which one H is substituted for another functional group

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4
Q

What is the shape of benzene ?

A

Flat, regular hexagon. Bond angle = 120 degrees.

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5
Q

What is the bond length between C atoms in benzene ?

A

It’s between the length of C-C and C=C.

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6
Q

What happens to the 4th electron in the p subshell of each C atom in benzene ?

A

It delocalises to form rings of electron density above and below the hexagonal structure

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7
Q

What effect on the stability of benzene does its areas of electronegativity have?

A

It makes benzene more unstable, even though it is unsaturated

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8
Q

Describe the structure of cyclohexa-1,3,5-ene

A

A 6 carbon hexagonal molecule with double bonds on every other carbon

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9
Q

What is the thermochemical evidence that benzene is more stable than cyclohexa-1,3,5-ene?

A

•Hydrogenation of cyclohexene = -120kJmol-1
•Hydrogenation of cyclohexa-1,3-ene =-240kJmol-1
•Hydrogenation of cyclohexa-1,3,5-ene = -360kJmol-1
•Benzene hydrogenation is -208kJmol-1 so is more stable

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10
Q

Apart from thermochemical data, why is cyclohexa-1,3,5-triene not a suitable model for benzene ?

A

•Benzene is symmetrical whilst C135 is not
•C135 readily undergoes addition reactions whilst benzene does not
•Both form different isomers when halogenated.

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11
Q

What is the appearance of benzene at 298k 100kPa ?

A

Colourless liquid

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12
Q

Why does benzene have a relatively high melting point ?

A

There is close packing of the flat hexagonal molecules in the solid phase.

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13
Q

Is benzene soluble in water ?

A

No, it is non polar

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14
Q

Why is benzene dangerous ?

A

It is carcinogenic

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15
Q

How do you name molecules containing a benzene ring ?

A
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16
Q

Why is benzene attacked by electrophiles?

A

There is a high electron density above and below the ring

17
Q

What is delocalisation energy and what is the effect of this on benzenes reactions?

A

•The large amount of energy required to break the aromatic ring apart
•Results in the ring typically staying intact during reactions.

18
Q

What is seen when benzene is combusted ? Why ?

A

•Smokey flames due to soot from incomplete combustion
•This is due to a high carbon:hydrogen ratio.

19
Q

Draw / describe a general electrophilic substitution mechanism of benzene using El+ to represent an electrophile

A

*Arrow from cirlce of benzene ring to El+
*Benzene now has a half ring and El as well as H on the same carbon
*Arrow from C-H to delocalised ring + charge.

20
Q

What ion (name and formula) is used to nitrate benzene ?

A

*NO2+ (+charge is on the nitrogen)
*It is called a nitronium ion or nitryl cation.

21
Q

How is the NO2+ ion generated ? (conditions and equation)

A

*Concentrated H2SO4 and concentrated HNO3
*H2SO4 + HNO3 -> H2NO3+ + HSO4-
*H2NO3+ -> H2O + NO2
*Overall: H2SO4 + HNO3 -> HSO4- + NO2+ + H2O.

22
Q

How is the H2SO4 catalyst regenerated in the nitration of benzene ?

A

HSO4- + H+ -> H2SO4 (H+ is lost from the substitution of benzene)

23
Q

Draw / describe a mechanism and write an overall equation for the nitration of benzene.

A

*C6H6 + HNO3 ->C6H5NO2 + H2O
*Arrow from inner circle of benzene ring to N of NO2+
*Benzene now has both NO2 and H bonded to the same carbon, and a semi circle with +
*Arrow from C-H to + of inner circle.

24
Q

What are the uses of nitrated arenes?

A

*Production of explosives e.g. TNT as it releases lots of heat and gas on explosion
*To make aromatic amines that are used for industrial dyes.

25
Q

How do substituents with a positive inductive effect (e.g. alkyl groups) affect further substitution ?

A

*They release electrons into the delocalised electron ring, increasing the electron density and making further substitution reactions more likely/fast
*Direct substitutions to the 2, 4, 6 positions.

26
Q

How do substituents with a negative inductive effect (e.g. NH2) affect further substitution ?

A

*They remove electrons from the delocalised electron ring, decreasing the electron density and make further substitution less likely/fast
*Direct substituents to 3,5 positions.

27
Q

What type of catalyst is used for a Friedel-Crafts reaction ?

A

A halogen carrier e.g. AlCl3

28
Q

Write an equation to form an electrophile that could be used to acylate benzene, starting with AlCl3 and RCOCl.

A

*AlCl3 + RCOCl -> AlCl4- + RCO+ (+ on C)
*RCO+ can attack benzene.

29
Q

What is happening when AlCl4- is formed in terms of electrons ?

A

Chlorine atoms lone pair of electrons is forming a coordinate bond to Al

30
Q

How is the AlCl3 catalyst reformed?

A

AlCl4- + H+ -> HCl + AlCl3 (H+ is lost from the substitution of benzene)

31
Q

How could you use a Friedel-Crafts mechanism to add a methyl group to a benzene ring ?

A

Use a haloalkane and AlCl3 to create an electrophile that can attack benzene

32
Q

Draw / describe the mechanism for the acylation of benzene from RCO+

A

*Arrow from inner circle of benzene to C+ of COR+
*Benzene now has both COR and H bonded to the same carbon, and a semi circle with + charge
*Arrow from C-H to + of inner circle.

33
Q

If you are considering cyclic compounds, what might happen if two double bonds are next to each other?

A

C=C bonds are close in proximity, so electrons in pi cloud can partially delocalise and move between both C=C p orbitals

34
Q

What effect would electrons in p orbitals moving between two C=C bonds have on the stability of the molecule and its enthalpy of hydrogenation ?

A

Makes the molecule more stable; makes enthalpy of hydrogenation more positive

AQA A Level Chemistry (26 decks)

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