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Benzene Structure and Models
Ever wondered why benzene doesn't behave like other compounds with supposed double bonds? The answer lies in its unique delocalised structure that gives it extraordinary stability.
Benzene (C₆H₆) is a planar hexagonal molecule where each carbon atom bonds to one hydrogen with 120° bond angles. Two competing models explain its structure: Kekulé's model suggests alternating single and double bonds , whilst the delocalised model shows electrons spread evenly around the ring.
The delocalised model is correct because six p-orbitals (one from each carbon) overlap to form two rings of delocalised electrons - one above and below the ring plane. This delocalisation provides benzene with exceptional stability that makes it behave very differently from typical alkenes.
Key Point: The delocalised electrons make benzene much more stable than expected, which explains why it doesn't react like normal alkenes.
Evidence strongly supports the delocalised model over Kekulé's structure through bond length measurements and reaction patterns.
Evidence for Delocalisation
Three pieces of rock-solid evidence prove that benzene has delocalised electrons rather than alternating single and double bonds.
Bond length evidence shows all C-C bonds in benzene are identical - intermediate between single and double bond lengths. If Kekulé was correct, you'd expect three shorter double bonds and three longer single bonds.
Enthalpy of hydrogenation provides the most convincing proof. Benzene releases only -208 kJ/mol when hydrogenated, but Kekulé's structure would predict -360 kJ/mol. This 152 kJ/mol difference represents the extra stability from delocalisation.
Chemical reactivity also supports delocalisation. Benzene undergoes substitution reactions rather than addition reactions typical of alkenes. It doesn't decolourise bromine water because it lacks the reactive double bonds that Kekulé's model suggests.
Remember: The lower enthalpy of hydrogenation proves benzene is more stable than expected - this stability comes from electron delocalisation.
Understanding the Evidence
When tackling exam questions about benzene structure, focus on explaining why the delocalised model better represents reality than Kekulé's alternating bond structure.
The orbital overlap explanation is straightforward: in Kekulé's model, π electrons are localised between specific carbon pairs, whilst in the delocalised model, π electrons spread evenly around the entire ring. This even distribution creates greater stability.
Enthalpy calculations often appear in exams. Remember that if Kekulé was correct, benzene should release about -357 kJ/mol during hydrogenation . The actual value of -208 kJ/mol shows benzene is much more stable.
Exam Tip: Always mention that identical C-C bond lengths, less exothermic hydrogenation, and substitution (not addition) reactions all support the delocalised model.
The key insight is that electron delocalisation explains all of benzene's unusual properties in one elegant theory.
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Understanding Benzene Structure in Chemistry
Benzene is one of the most important compounds in organic chemistry, and understanding its structure is crucial for A-level success. The fascinating thing about benzene is that its actual structure completely defied chemists' expectations, leading to groundbreaking discoveries about electron... Show more
Benzene Structure and Models
Ever wondered why benzene doesn't behave like other compounds with supposed double bonds? The answer lies in its unique delocalised structure that gives it extraordinary stability.
Benzene (C₆H₆) is a planar hexagonal molecule where each carbon atom bonds to one hydrogen with 120° bond angles. Two competing models explain its structure: Kekulé's model suggests alternating single and double bonds , whilst the delocalised model shows electrons spread evenly around the ring.
The delocalised model is correct because six p-orbitals (one from each carbon) overlap to form two rings of delocalised electrons - one above and below the ring plane. This delocalisation provides benzene with exceptional stability that makes it behave very differently from typical alkenes.
Key Point: The delocalised electrons make benzene much more stable than expected, which explains why it doesn't react like normal alkenes.
Evidence strongly supports the delocalised model over Kekulé's structure through bond length measurements and reaction patterns.
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Evidence for Delocalisation
Three pieces of rock-solid evidence prove that benzene has delocalised electrons rather than alternating single and double bonds.
Bond length evidence shows all C-C bonds in benzene are identical - intermediate between single and double bond lengths. If Kekulé was correct, you'd expect three shorter double bonds and three longer single bonds.
Enthalpy of hydrogenation provides the most convincing proof. Benzene releases only -208 kJ/mol when hydrogenated, but Kekulé's structure would predict -360 kJ/mol. This 152 kJ/mol difference represents the extra stability from delocalisation.
Chemical reactivity also supports delocalisation. Benzene undergoes substitution reactions rather than addition reactions typical of alkenes. It doesn't decolourise bromine water because it lacks the reactive double bonds that Kekulé's model suggests.
Remember: The lower enthalpy of hydrogenation proves benzene is more stable than expected - this stability comes from electron delocalisation.
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Understanding the Evidence
When tackling exam questions about benzene structure, focus on explaining why the delocalised model better represents reality than Kekulé's alternating bond structure.
The orbital overlap explanation is straightforward: in Kekulé's model, π electrons are localised between specific carbon pairs, whilst in the delocalised model, π electrons spread evenly around the entire ring. This even distribution creates greater stability.
Enthalpy calculations often appear in exams. Remember that if Kekulé was correct, benzene should release about -357 kJ/mol during hydrogenation . The actual value of -208 kJ/mol shows benzene is much more stable.
Exam Tip: Always mention that identical C-C bond lengths, less exothermic hydrogenation, and substitution (not addition) reactions all support the delocalised model.
The key insight is that electron delocalisation explains all of benzene's unusual properties in one elegant theory.
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Basil
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This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
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very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
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I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
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THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
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This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
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This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
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The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user