aliphatic: organic compound, hydrogen and carbon joined in straight or branched chain aromatic: organic compound with benzene ring Benzene (C6H6): colorless liquid, boils at 80° - can be extracted by fractional distillation but it’s carcinogenic myths / expectations: assumed to be unsaturated but reactions proved otherwise expected C=C and C-C in cyclic / ring structure Problems with expected structure: 1. testing for unsaturation C=C double bond; undergo addition rxn and decolorise bromine water but benzene doesn’t 2. isomers 3. x-ray diffraction: C-C bonds are equal in bond length 4. enthalpy of hydrogenation: should be 3x value of cyclohexane benzene much lower in reality 5. infrared spectra: adsorption due to C=C stretch in alkenes benzene has different lower peaks actual structure 6 C-C sigma bonds in ring structure each C has unpaired e in p-orbital p-orbitals form pi bond 3e above atom & 3e below 1. test for unsaturation: no C=C 2. isomerism: 3 isomers 3. x ray diffraction: all C-C are identical 4. enthalpy of hydrogenation: spread stability Benzene reactions: - doesn’t undergo addition rxns bc no C=C - hydrogenation heated under pressure, nickel catalyst = cyclohexane - combustion burns in oxygen = CO2 & water incomplete combustion; ratio of benzene smokey - bromination heated under reflux, halogen carrier catalyst can be iron filings: react with benzene -> iron III bromide = bromobenzene, hydrogen bromide - nitration replacement of hydrogen with NO2 warmed, nitric acid, sulfuric acid (catalyst) trisubstitution at higher temps = nitrobenzene & water Alkylation & Acylation - catalyst: AlCl3 or halogen carrier - lack of water; avoid unwanted rxn Alkylation replace hydrogen in benzene with alkyl group -> alkylbenzene and hydrogen chloride Acylation replace hydrogen in benzene with acyl -> keytone & hydrogen chloride Sulfonation replacement of H in benzene with SO3H warmed with fuming sulfuric acid dissolve sulfur VI oxide in sulfuric acid = benzenesulfonic acid & water Electrophilic substitution benzene ring: electron rich delocalised e attract electrophile electrophile attracts 2e; covalent bond ~> intermediate with positive charge intermediate unstable: H leaves as H+, 2e restore pi bond Phenol: hydroxyl group attached to benzene - contain benzene ; can undergo substitution bromination of phenol: - happen at room temp - no need for catalyst - decolorises bromine water -> forms organic prod; white ppt 2,4,6 tribromophenol why phenol reacts more readily than benzene ? 1. OH group: lone pair of e in oxygen pz orbital lone pair interacts with delocalised pi bond increases electron density; activation; more reactive towards electrophile bromine molecules become polarised, Br-Br bond breaks -> Br+ attacks ring Phenol uses: - antiseptics - TCP chlorination of phenol (used as antiseptic)
For the first question part (a), why can't we answer that the proposed structure can form isomers so there must be more than just one structure found therefore, the delocalized structure is the correct one as it has no isomers?
hi miss just wanted to thank you for saving my a level chemistry, your videos are really helpful. I just wanted to ask you when are you going to do topic 14 15 16 and 17 thanks
By hydrogenating the molecule, you are making it more stable (i.e. lower in energy). If the benzene is already quite stable, the amount of energy needed to reduce the stability further is lower compared to the expected value.
@@aleemaasghar379 Apologies, I have not been able to get them done due to my own teaching commitments but I hope to get them completed in my midterm break in February. Thank you for your patience!
31:50 Just a correction here: With UV light, we'll have an addition reaction and so the product would be hexabromocyclohexane and not hexabromobenzene. Thank you for your helpful videos!
17:09 if the kekule’s structure was proved to be wrong why is it still used to represent benzene? Will i lose marks if i draw it with the ring of delocalized electrons or should i stick to the kekule’s structure vice versa. Bit confusing tbh. ☹️
aliphatic: organic compound, hydrogen and carbon joined in straight or branched chain
aromatic: organic compound with benzene ring
Benzene (C6H6): colorless liquid, boils at 80°
- can be extracted by fractional distillation but it’s carcinogenic
myths / expectations:
assumed to be unsaturated but reactions proved otherwise
expected C=C and C-C in cyclic / ring structure
Problems with expected structure:
1. testing for unsaturation
C=C double bond; undergo addition rxn and decolorise bromine water
but benzene doesn’t
2. isomers
3. x-ray diffraction: C-C bonds are equal in bond length
4. enthalpy of hydrogenation: should be 3x value of cyclohexane
benzene much lower in reality
5. infrared spectra: adsorption due to C=C stretch in alkenes
benzene has different lower peaks
actual structure
6 C-C sigma bonds in ring structure
each C has unpaired e in p-orbital
p-orbitals form pi bond
3e above atom & 3e below
1. test for unsaturation: no C=C
2. isomerism: 3 isomers
3. x ray diffraction: all C-C are identical
4. enthalpy of hydrogenation: spread stability
Benzene reactions:
- doesn’t undergo addition rxns bc no C=C
- hydrogenation
heated under pressure, nickel catalyst
= cyclohexane
- combustion
burns in oxygen
= CO2 & water
incomplete combustion; ratio of benzene
smokey
- bromination
heated under reflux, halogen carrier catalyst
can be iron filings: react with benzene -> iron III bromide
= bromobenzene, hydrogen bromide
- nitration
replacement of hydrogen with NO2
warmed, nitric acid, sulfuric acid (catalyst)
trisubstitution at higher temps
= nitrobenzene & water
Alkylation & Acylation
- catalyst: AlCl3 or halogen carrier
- lack of water; avoid unwanted rxn
Alkylation
replace hydrogen in benzene with alkyl group
-> alkylbenzene and hydrogen chloride
Acylation
replace hydrogen in benzene with acyl
-> keytone & hydrogen chloride
Sulfonation
replacement of H in benzene with SO3H
warmed with fuming sulfuric acid
dissolve sulfur VI oxide in sulfuric acid
= benzenesulfonic acid & water
Electrophilic substitution
benzene ring: electron rich
delocalised e attract electrophile
electrophile attracts 2e; covalent bond
~> intermediate with positive charge
intermediate unstable: H leaves as H+, 2e restore pi bond
Phenol: hydroxyl group attached to benzene
- contain benzene ; can undergo substitution
bromination of phenol:
- happen at room temp
- no need for catalyst
- decolorises bromine water
-> forms organic prod; white ppt
2,4,6 tribromophenol
why phenol reacts more readily than benzene ?
1. OH group: lone pair of e in oxygen pz orbital
lone pair interacts with delocalised pi bond
increases electron density; activation; more reactive towards electrophile
bromine molecules become polarised,
Br-Br bond breaks -> Br+ attacks ring
Phenol uses:
- antiseptics
- TCP chlorination of phenol (used as antiseptic)
May God bless you for this.
bless you. don't suppose you have notes like this for all the videos? 🤣
Hi, just wanted to say thank you very much for the videos. They helped me so much :)
I am so glad you are finding them useful!
Thanks a ton!
my savior
For the first question part (a), why can't we answer that the proposed structure can form isomers so there must be more than just one structure found therefore, the delocalized structure is the correct one as it has no isomers?
Hi miss thanks for the video! Is it possible if you could give me the links to the presentation?
hi miss just wanted to thank you for saving my a level chemistry, your videos are really helpful. I just wanted to ask you when are you going to do topic 14 15 16 and 17 thanks
Topic 14A is now available. 14B and 15 will be available in the next 2 weeks. Thank you for your patience!
THANK YOUS ❤
Is heating under pressure the same as heating under reflux?
9:41 Why will the increase in the stability of a structure reduce the enthalpy of halogenation? Thanks for the video :)
By hydrogenating the molecule, you are making it more stable (i.e. lower in energy). If the benzene is already quite stable, the amount of energy needed to reduce the stability further is lower compared to the expected value.
@@MissNatalieChemistry Yes I got you.Thanks so much:)
where is topic 16 and 17 for unit 5
These topics will be available this week. I have been swamped with school work for my own students but I have some free time on my mid term break now.
Hi miss at 14:41 for the Alkylation part is the product Hydrochloric acid or Hydrogen chloride?
It's hydrogen chloride gas as it would be misty fumes. Hydrochloric acid is when the hydrogen chloride is dissolved in water.
@@MissNatalieChemistry I understood it miss thank you so much for your reply :)
hi miss. your videos are very helpful. helping me a lot.
may i ask when will you upload chapter 16 and 17. is it possible to upload it before 15 jan?
I plan to try and upload these videos in January but I cannot guarantee before 15th January. I will try my best though.
@@MissNatalieChemistry Hi. it is feb already. i am anxiously waiting for chapter 16 and 17. plz tell us when you will be uploading it.
@@aleemaasghar379 Apologies, I have not been able to get them done due to my own teaching commitments but I hope to get them completed in my midterm break in February. Thank you for your patience!
@@aleemaasghar379 Apologies for the delay, I have been swamped with school work but I have free time lately so they should be available this week.
31:50 Just a correction here: With UV light, we'll have an addition reaction and so the product would be hexabromocyclohexane and not hexabromobenzene. Thank you for your helpful videos!
why tho? isn't electrophilic addition for C=C double bonds? and UV leads to free radical substitution in alkanes? so I don't quite get this.
Miss when are you going to do topic 13,14and 15
Topic 13 and 14A is now available. 14B and 15 will be available in the next 2 weeks. Thank you for your patience!
17:09 if the kekule’s structure was proved to be wrong why is it still used to represent benzene? Will i lose marks if i draw it with the ring of delocalized electrons or should i stick to the kekule’s structure vice versa. Bit confusing tbh. ☹️
Kekules structure is no longer used to represent benzene. You'd have to draw the ring when drawing benzene.
@@sarahnaz1544 Shiiiii but…. Did i ask?
@@sporadicpenguin3911 uhm yeah you did lol
@@sarahnaz1544 ohhh shiii my bad but thanks
@@sporadicpenguin3911 a heck of a lame individual you are.