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Updated Mar 18, 2026
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Understanding Organic Chemistry - AQA High School Guide
G.k
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Basic Concepts and Formulae
Ever wondered why petrol burns differently than cooking oil? It's all about hydrocarbons - compounds made only of carbon and hydrogen atoms. These molecules form the backbone of organic chemistry and everything from fuels to plastics.
Saturated hydrocarbons contain only single bonds between carbons, making them quite stable. Unsaturated hydrocarbons have double bonds , which makes them more reactive and useful for making other chemicals.
You'll encounter several types of formulae in your exams. The molecular formula shows exactly how many of each atom you have (like C₄H₁₀), whilst the empirical formula gives the simplest ratio. Structural formulae show how atoms connect, and displayed formulae reveal every single bond - essential for understanding reactions.
Top Tip: When drawing organic molecules, remember each carbon must have exactly 4 bonds. If you can't see 4 bonds to other atoms, the missing ones are to hydrogen atoms!
Homologous Series and Functional Groups
Think of homologous series as chemical families - groups of compounds that behave similarly because they share the same functional group. Just like family members share traits, these compounds share chemical properties whilst differing by CH₂ units.
Alkanes (like butane) are your basic hydrocarbons with only single bonds. Alkenes contain double bonds, making them perfect for creating plastics. Alcohols have an -OH group, which is why they mix with water unlike pure hydrocarbons.
Each family has its own naming pattern. Alkanes end in "-ane", alkenes in "-ene", and alcohols in "-ol". Aldehydes end in "-al", ketones in "-one", and carboxylic acids in "-oic acid". Learning these patterns makes naming compounds much easier.
Remember: The functional group determines how a molecule behaves chemically - it's like the molecule's personality!
Naming Rules and Priority System
Naming organic compounds follows a clear hierarchy, just like deciding who gets served first in a queue. Carboxylic acids have the highest priority, followed by aldehydes, ketones, alcohols, alkenes, and finally halogenoalkanes.
Start by finding the longest carbon chain and use the appropriate stem . The highest priority functional group gets the suffix and the lowest possible number on the carbon chain.
Branch chains and lower priority groups become prefixes. For example, in 3,5-dimethylheptane, you've got a 7-carbon chain with methyl groups on carbons 3 and 5. Multiple identical groups get prefixes like "di-", "tri-", or "tetra-".
Exam Strategy: Always identify the longest carbon chain first, then find the highest priority functional group - this determines your base name and numbering system.
Alkenes and Haloalkanes
Alkenes are unsaturated hydrocarbons with the general formula CₙH₂ₙ, and that double bond makes all the difference. They're much more reactive than alkanes and can form polymers like polythene. When naming, the number indicates where the double bond starts .
Multiple double bonds get special treatment - "diene" for two, "triene" for three. Sometimes you'll see E or Z prefixes, which describe the arrangement of groups around the double bond (stereoisomers).
Haloalkanes are simply alkanes with halogen atoms (F, Cl, Br, I) replacing some hydrogens. Name them using prefixes: fluoro-, chloro-, bromo-, or iodo-. For example, 2-bromobutane has a bromine on the second carbon of a 4-carbon chain.
Key Point: When multiple functional groups are present, list prefixes in alphabetical order, ignoring any "di-" or "tri-" prefixes.
Alcohols and Aldehydes
Alcohols contain the -OH functional group, which makes them polar and able to form hydrogen bonds. This explains why ethanol mixes with water but hexane doesn't. Simple alcohols end in "-ol" , whilst multiple -OH groups create diols or triols.
When alcohols appear alongside higher priority groups, they become "hydroxy-" prefixes instead of taking the suffix. For instance, 2-hydroxypropanoic acid has both alcohol and carboxylic acid groups, but the acid takes priority.
Aldehydes always have their C=O group on carbon 1, so you don't need to specify its position - it's ethanal, not ethan-1-al. This carbonyl group makes aldehydes quite reactive and gives them distinctive smells (vanilla contains vanillin, an aldehyde).
Memory Trick: ALdehydes are ALways on carbon 1 - the carbonyl carbon must be at the end of the chain.
Ketones, Carboxylic Acids, and Reaction Mechanisms
Ketones have their C=O group somewhere in the middle of the carbon chain, unlike aldehydes. For chains with 5 or more carbons, you need to specify the position . Acetone (propanone) is the most common ketone - it's nail polish remover.
Carboxylic acids always sit at the end of carbon chains, so numbering always starts from the acid group. These compounds are weak acids (like ethanoic acid in vinegar) and can form salts with metals or bases.
Homolytic fission introduces reaction mechanisms - when bonds break, each atom keeps one electron, forming free radicals. These highly reactive species have unpaired electrons (shown with a dot) and are crucial in many organic reactions, including combustion and polymerisation.
Mechanism Tip: Use single-headed arrows (→) to show single electron movement in free radical reactions - this is different from the double-headed arrows used in other mechanisms.
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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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Understanding Organic Chemistry - AQA High School Guide
G.k
@g_k111
Organic chemistry might seem overwhelming at first, but it's actually just about understanding how carbon and hydrogen atoms connect to form different molecules. Once you grasp the basic naming rules and functional groups, you'll be able to identify and draw... Show more
Basic Concepts and Formulae
Ever wondered why petrol burns differently than cooking oil? It's all about hydrocarbons - compounds made only of carbon and hydrogen atoms. These molecules form the backbone of organic chemistry and everything from fuels to plastics.
Saturated hydrocarbons contain only single bonds between carbons, making them quite stable. Unsaturated hydrocarbons have double bonds , which makes them more reactive and useful for making other chemicals.
You'll encounter several types of formulae in your exams. The molecular formula shows exactly how many of each atom you have (like C₄H₁₀), whilst the empirical formula gives the simplest ratio. Structural formulae show how atoms connect, and displayed formulae reveal every single bond - essential for understanding reactions.
Top Tip: When drawing organic molecules, remember each carbon must have exactly 4 bonds. If you can't see 4 bonds to other atoms, the missing ones are to hydrogen atoms!
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Homologous Series and Functional Groups
Think of homologous series as chemical families - groups of compounds that behave similarly because they share the same functional group. Just like family members share traits, these compounds share chemical properties whilst differing by CH₂ units.
Alkanes (like butane) are your basic hydrocarbons with only single bonds. Alkenes contain double bonds, making them perfect for creating plastics. Alcohols have an -OH group, which is why they mix with water unlike pure hydrocarbons.
Each family has its own naming pattern. Alkanes end in "-ane", alkenes in "-ene", and alcohols in "-ol". Aldehydes end in "-al", ketones in "-one", and carboxylic acids in "-oic acid". Learning these patterns makes naming compounds much easier.
Remember: The functional group determines how a molecule behaves chemically - it's like the molecule's personality!
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Naming Rules and Priority System
Naming organic compounds follows a clear hierarchy, just like deciding who gets served first in a queue. Carboxylic acids have the highest priority, followed by aldehydes, ketones, alcohols, alkenes, and finally halogenoalkanes.
Start by finding the longest carbon chain and use the appropriate stem . The highest priority functional group gets the suffix and the lowest possible number on the carbon chain.
Branch chains and lower priority groups become prefixes. For example, in 3,5-dimethylheptane, you've got a 7-carbon chain with methyl groups on carbons 3 and 5. Multiple identical groups get prefixes like "di-", "tri-", or "tetra-".
Exam Strategy: Always identify the longest carbon chain first, then find the highest priority functional group - this determines your base name and numbering system.
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Alkenes and Haloalkanes
Alkenes are unsaturated hydrocarbons with the general formula CₙH₂ₙ, and that double bond makes all the difference. They're much more reactive than alkanes and can form polymers like polythene. When naming, the number indicates where the double bond starts .
Multiple double bonds get special treatment - "diene" for two, "triene" for three. Sometimes you'll see E or Z prefixes, which describe the arrangement of groups around the double bond (stereoisomers).
Haloalkanes are simply alkanes with halogen atoms (F, Cl, Br, I) replacing some hydrogens. Name them using prefixes: fluoro-, chloro-, bromo-, or iodo-. For example, 2-bromobutane has a bromine on the second carbon of a 4-carbon chain.
Key Point: When multiple functional groups are present, list prefixes in alphabetical order, ignoring any "di-" or "tri-" prefixes.
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Alcohols and Aldehydes
Alcohols contain the -OH functional group, which makes them polar and able to form hydrogen bonds. This explains why ethanol mixes with water but hexane doesn't. Simple alcohols end in "-ol" , whilst multiple -OH groups create diols or triols.
When alcohols appear alongside higher priority groups, they become "hydroxy-" prefixes instead of taking the suffix. For instance, 2-hydroxypropanoic acid has both alcohol and carboxylic acid groups, but the acid takes priority.
Aldehydes always have their C=O group on carbon 1, so you don't need to specify its position - it's ethanal, not ethan-1-al. This carbonyl group makes aldehydes quite reactive and gives them distinctive smells (vanilla contains vanillin, an aldehyde).
Memory Trick: ALdehydes are ALways on carbon 1 - the carbonyl carbon must be at the end of the chain.
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Ketones, Carboxylic Acids, and Reaction Mechanisms
Ketones have their C=O group somewhere in the middle of the carbon chain, unlike aldehydes. For chains with 5 or more carbons, you need to specify the position . Acetone (propanone) is the most common ketone - it's nail polish remover.
Carboxylic acids always sit at the end of carbon chains, so numbering always starts from the acid group. These compounds are weak acids (like ethanoic acid in vinegar) and can form salts with metals or bases.
Homolytic fission introduces reaction mechanisms - when bonds break, each atom keeps one electron, forming free radicals. These highly reactive species have unpaired electrons (shown with a dot) and are crucial in many organic reactions, including combustion and polymerisation.
Mechanism Tip: Use single-headed arrows (→) to show single electron movement in free radical reactions - this is different from the double-headed arrows used in other mechanisms.
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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
Android user
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
iOS user
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 Knowunity AI. 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
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
Android user
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
iOS user
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 Knowunity AI. 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