Chapter 25 key concepts

Question 1

Structure of Benzene

Answer 1

Planar hexagonal cyclic hydrocarbon with a delocalised system of light bonds

Question 2

When to use prefix Phenyl

Answer 2

When benzene attached to:
alkyl group has more than 7 carbons

Alkyl group attached to a functional group
*here benzene is the substituent group

Question 3

Evidence to disprove kekules model

Answer 3

Lack of reactivity of benzene

• nah e sub
• nah decolourise bromine

Lengths of the C-C bonds in benzene

• x ray diffraction measures bond length
• single bond is 0.153nm double bond 0.134nm. Benzene was 0.139

Hydrogenation enthalpies

• benzene would be called cyclohexa 1,3,5 triene
• With enthalpy of hydrogenation 3 x that of cyclohexane which is 120kjmol-1 actual value was much lower, not 360 but 208kjmol-1

Question 4

Properties and history of benzene

Answer 4

• discovered in 1825 by Michael Faraday who isolated it as an oily residue from a gas used for street lighting
• colourless
• sweet smelling
• highly flammable liquid
• carcinogen

Question 5

Delocalised model

Answer 5

• benzene is planar cyclic hexagonal hydrocarbon
• each C bonds to 2 C 1 H with one e in a P orbital at right angle to the plane of bonded C and H
• ring of e den forms when p orb e overlap sideways in both directions above and below plane of C making a system of pin bonds that spreads over all 6C in the ring
• the 6 e are delocalised

Question 6

Priority groups

Answer 6

CA.             S. oic acid p.carboxy
Ester.   S.oate p. Alkoxycarbonyl
Amide   s.amide p.amido
Nitrile.   S. nitrile p.cyano
Aldehyde   s.al   p.formyl
Ketone. S.one p.oxo
Alcohol. S.ol P.hydroxy
Amine.  S.amine P.amino
Alkene  s.ene p. Alkenyl
Alkyne. S.yne p. Alkynyl
Alkane   S.ane. P. Alkyl
Ether s.-  p. Alkoxy
Alkyl halide s.- p. Bromo chloro iodo fluoro

Question 7

Uses of nitrobenzene

Answer 7

Important starting material in the prep of

• Dyes
• Pharmaceuticals
• Pesticides
• paracetamol (a common painkiller to alleviate headaches and symptoms of fever)

Question 8

Uses of phenylethanone

Answer 8

In the perfume industry 
-smells of honeysuckle 
Jasmine
Strawberry 
Almond 
Cherries

Question 9

Alkene vs Arene

Answer 9

Alkene

• decolourise bromine in ELECTROPHILLIC ADDITION
• Pi bonds contain LOCALISED ELECTRONS above and below the place of the TWO CARBON ATOMS IN THE DOUBLE BOND making area of high e den
• able to polarise a bromine molecule

Benzene

• only decolourises bromine IF a halogen carrier is present
• DELOCALISED PI ELECTRONS
• electrons spread above and below the PLANE OF THE CARBON ATOMS IN THE RING
• the e den around any 2 C in benzene is less than that in alkene CC double bond
• doesn’t polarise bromine molecules as INSUFFICIENT E DEN AROUND ANY 2 C

Question 10

Phenol vs benzene

Answer 10

Phenol

• nitrated without concentrated H2SO4
• doesn’t need Halo carrier for bromination cuz e den in phenol ring is enuff to polarise bromine molecules
• bromine and nitric acid react more readily with phenol than benzene
• increased reactivity because lp of Electrons from the Oxygen P orbital of the OH group is donated to phenols pi system increasing e den
• increasing e den attracts e philes
• phenols aromatic ring is more susceptible to e phillic attack

Benzene

• needs a halogen carrier for bromination
• not enuf e den around and 2 given carbons to polarise a bromine molecule
• needs concentrated H2SO4 and 50degrees to react be nitrated

Question 11

Differentiate between phenol and alkene

Answer 11

Add bromine water, both decolorise bromine water but only phenol makes white ppt

Or use pH indicator. Phenol is slightly Acid and alkene is neutral

Question 12

Why bromine reacts more with phenol than benzene?

Answer 12

Ox LP is delocalised into the ring

Increasing electron density of the ring

Now able to polarise non polar molecules such as bromine

All e are delocalised throughout benzene structure so benzene can’t polarise bromine