aromatic compounds

Question 1

Aliphatic

Answer 1

Straight or branched chain organic substances

Question 2

Aromatic/ Arene

Answer 2

Includes one or more ring of 6 carbon atoms with delocalised bonding

Question 3

Benzene features and formula

Answer 3

C6H6

Colourless, sweet smelling, flammable

Carcinogen

Hexagonal ring = ARENE

H-C-C bond angle is 120

Question 4

2 structures showing benzene structure

Answer 4

Kekule model

Delocalised model

Question 5

Kekule model

Answer 5

Suggesting that the strucutre was based upon the 6 membered ring of carbon atoms joined by single and double bonds

ALTERNATING

Question 6

Evidence to disprove the kekulé model

Answer 6

1. lack of reactivity of benzene
2. The lengths of carbonc arbon bonds in benzene
3. Hydrogen enthalpies

Question 7

Lack of reactivity in benzene- disproving kekulé model

Answer 7

If benzene contained C=C bonds, it should decolarie bromine in an electrophilic addition reaction

HOWEVER, doesnt happen

Question 8

Lengths of carbon carbon bonds in benzene disproving the kekulé model

Answer 8

Using X ray diffraction , you can measure bond lengths

Found that all the bonds are 0.139nm in length ( in between single0.153 and double0.134)

Question 9

Hydrogen enthalpies - disproving kekulé model

Answer 9

If benzene had the kekuléstucture , it would be expected to have a hydration 3 time that of cyclohexene ( one double bond)

-120 * 3 BUT only -280kJ

Question 10

What is the delocalized model - BENZENE

Answer 10

Each carbon uses 3 of its available electrons in the bonding to 2 carbon and 1 hydrogen

Each carbon atom has 1 electron in a p orbital at right angles to the plane of the bonded carbon and hydrogen

Adjacent P orbitals overlap sideways in both directions, above and below plane, forming ring of electron density

System of pie bonds

DELOCALISED electrons

Question 11

Monosubsituted benzene

Answer 11

Benzene ring is considered parent chain

ALkyl group group, halogen and nitro groups = PREFIX

Question 12

Substituent benzene

Answer 12

When benzene is attached to an alkyl group with a functional group or 7 + carbons

PHENYL is used for prefix ( for benzene

Question 13

Exceptions to benzene naming rules?

Answer 13

Benzonic acid ( benzene + COOH)

Phenyl amine (+Nh2)

Benzaldehyde( +CHO)

Question 14

More than one substituent group ? benzene naming

Answer 14

Number the ring using smallest numbers possible

ALPHABETIC order

Question 15

Reactivity of benzene

Answer 15

Undergoes substitution reactions in which a hydrogen atom on the benzene ring is replaced by another atom or group of atoms

ELECTROPHILES - electrophilic substituion

Question 16

Nitration of Benzene

Answer 16

Reacts with nitric acid slowly
Nitrobenzene formed

Reaction is catalyzed by sulfruic acid and heated to 50 degrees to obtain a good rate of reaction

One of hydoens is repplaced by no2

Question 17

What happens if nitration of benzene exceeds 50 degrees

Answer 17

May lead to production of DINITROBENZENE

Question 18

NITRATION of benzene mechanism

Answer 18

1- No 2+ ion is produced by the reaction of concentrated nitric acid with conc sulphric acid

2- Electrophile accepts a pair of electrons from the benzene ring to form a dative covalent bond. The organic intermediate fromed is unstable, breaks downt o form nitrobenzene and H+ ion

3- H + reacts with H2So4- to regenerate catalyst

Question 19

Nitration of benzene reaction - no2+ produced equation

Answer 19

HNO3 + H2SO4——– No2+ + H2so4- + H20

Question 20

Halogenation of benzene

Answer 20

halogens dont react unless halogen carrier catalyst is present

E.g. Fecl3, FeBr3, Al,Cl3

Generated in situ ( in reaction vessel) from metal and halogen

Question 21

Bromination of benzene

Answer 21

1. the electrophile is bronomium ion ( br+) which is generated when the halogen carrier catalyst ( FeBr3) reacts with bromine
2. Bronomium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond

Organic intermediate e is unstable and breaks down to form bromobenzene and a H + ion

3. Hydrogen combines with Fe Br 4- ion = FebR3

Question 22

Alkylation reactions / FRIEDAL CRAFTS

Answer 22

reflux
Substitution of a hydrogen atom in the benzene ring by alkyl group

Haloklane in the presence of AlCl3 - acting as a hydrogen carrier catalyst, generating the electrophile

Alkylation increases the number of carbon atoms in compound by forming carbon carbon bonds

Question 23

Acylation reactions

Answer 23

reflux
Benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst
Aromatic ketone is formed
phenyl ketones

Question 24

Reactivity of alkenes vs arenes

Answer 24

Alkenes are more reactive

React with bromine, decoloraising

Question 25

Why doesn’t bromine reaxt with benzene

Answer 25

Doesnt react unless carrier catalyst is present

Has delocalized pi electrons spread above and beow plane of the carbon atoms in the ring structure

Electron desntiy around any 2 carbon atoms in teh benzene ring is less than that in a C=C double bond in an alkene

When a non polar molecule such as bromine appraoches hte benzene ring, there insufficient pie electron density around any 2 carbona toms to polarise thebromine molecule

Question 26

What are phenols

Answer 26

Type of chemical containing a hydroxyl and an aromatic ring DIRECTLY bonded

If its not bonded exactly ti te OH its considered an alcohol

Question 27

Phenol

Answer 27

C6H5OH

A type of phenol

Question 28

Phenol reactivity compared to benzene

Answer 28

More reactive than benzene

The OH group means its more likely to undergo electrophilic substitution

One of th lone pairs of electrons from the oxygen p orbital of the OH group begin donated to teh pie system of the phenol electron density of the benzene ring is increased

INcreased electron density attracts electrophiles more strongly than with benzene

Question 29

Unsubsiuted benzene ring- positin of substrituion

Answer 29

Can be ANYWHERE

All of teh carbon atoms are same

Question 30

Substtiuted ring - position of substiution

Answer 30

The functional group can change the electorn densoty at certain carbon atoms

Makes them more or elss likely to react

Question 31

Electron donating groups

Answer 31

e.g. -OH and - NH2

Electrons in orbitals that overlap with teh delocalised ring and increase its electron density

Increase electron density at carbons 2-, 4- and 6- so electrophiles are most liekly ot react at these positions

Question 32

Electron withdrawing groups

Answer 32

E.g. No2

No obritals that can overlap with delocalized ring and is electronegative

Withdraws electon density from the ring

Especially at 2,4, and 6 carbons, so electrophiels unlikely to reat here = directed to 3,5

Question 33

Phenol + bromine water

3Br2

Answer 33

decolourise
The OH is electron donating
No hydrogen carrier catalyst

3Hbr produced as by product

Question 34

Reactions with phenol

Answer 34

SUBSTITUTION
With dilute nitric acid
With bromine water
With bases and sodium

Question 35

Phenol with dilute nitric acid

Answer 35

2 isomers of nitrophenol and water

Easier than benzene

COncentrated nitric acid and conc sulfuric acid catalyst

Question 36

Phenol with bases and sodium

Answer 36

Form salts

Sodium hydroide solution

Sodium hyroxide and water produced

Doesn’t react with sodium carbonate - not a strong enoguh acid