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How does resonance stability work in organic compounds?

Mr Khemistry

Resonance stability in organic compounds refers to the stabilization that arises from the delocalization of electrons within a molecular system. It occurs in compounds with conjugated systems, where pi electrons can move or be shared over multiple atoms or bonds. This delocalization of electrons leads to increased stability due to the spreading of charge or electron density.

Here’s how resonance stability works:

  1. Conjugated Systems: Resonance stability primarily occurs in compounds with conjugated systems, which involve alternating single and multiple bonds or the presence of lone pairs of electrons adjacent to a pi bond. Examples include benzene rings, allylic systems, and carbonyl compounds. In these systems, the p orbitals overlap to form a continuous network of pi bonds along the molecule. (note that all the carbon atoms have to a unhybridized p orbital to form the continuous overlap)
  2. Electron Delocalization: In compounds with resonance stability, pi electrons are not confined to a single bond or atom but can move freely within the conjugated system. The delocalization of electrons allows them to be shared or spread out over multiple atoms or bonds. This electron delocalization stabilizes the compound by lowering its overall energy. You can think of it as spreading out negative charge of the electrons.
  3. Resonance Structures: Resonance stability is often represented by resonance structures, which are different Lewis structures that depict the various electron distributions within the molecule. Resonance structures are represented using curved arrows to show the movement of electrons. These structures differ only in the placement of pi electrons or lone pairs.


  4. Stabilizing Effects: The delocalization of electrons through resonance has several stabilizing effects on the compound. It helps to distribute the electron density more evenly, reducing the electron-rich or electron-poor regions within the molecule. This stabilization can decrease the reactivity of the compound towards electrophiles or nucleophiles. It also helps to disperse charge or electron density across a larger area, minimizing the repulsion between like charges and enhancing stability.
  5. Aromaticity: Aromatic compounds represent the highest level of resonance stability. They possess a fully conjugated ring system and exhibit extraordinary stability due to the extensive delocalization of pi electrons. Aromatic compounds follow Hückel’s rule, which states that a compound is aromatic if it has a planar, cyclic, and conjugated system with 4n+2 pi electrons (where n is an integer, i.e. for benzene n=1). The aromatic compounds, such as benzene, are exceptionally stable and exhibit unique chemical properties.

It’s important to note that resonance stability does not involve the actual movement of atoms but rather the movement of electron density or charge distribution within the molecule. Resonance structures are theoretical representations that describe different electron distributions, and the actual molecule is a hybrid of these structures.

Resonance stability is a key concept in organic chemistry, as it influences the reactivity, chemical properties, and stability of compounds. Understanding the delocalization of electrons through resonance helps predict and explain the behavior of organic compounds in various chemical reactions.

Here are some additional details and concepts related to resonance stability in organic compounds (extra reading not required by H2 syllabus):

  1. Resonance Energy: Resonance energy, also known as delocalization energy, is the stabilization energy gained by a molecule through resonance. It quantifies the stability increase resulting from electron delocalization. Resonance energy is calculated as the difference in energy between the actual molecule and the most stable contributing resonance structure.
  2. Resonance Contributors: Resonance stability is described by multiple resonance structures or contributors, each representing a different electron distribution. The actual molecule is considered a resonance hybrid, with characteristics from all the resonance contributors. Resonance contributors are not separate entities but rather theoretical representations to describe the electron delocalization.
  3. Resonance and Bond Length: Resonance can affect bond lengths within a molecule. When a bond is involved in resonance, it experiences partial double bond character, resulting in a shorter bond length compared to a typical single bond. Conversely, bonds adjacent to resonance systems (such as in allylic or benzylic positions) can exhibit longer bond lengths due to electron delocalization.
  4. Resonance and Bond Order: In resonance structures, bonds involved in delocalization are considered to have partial double bond character. This concept of fractional bond order arises from the sharing of electrons between adjacent atoms. For example, in benzene, each carbon-carbon bond is considered to have a bond order of 1.5 due to the delocalization of pi electrons.
  5. Electrophilic Aromatic Substitution: Aromatic compounds, with their high resonance stability, undergo electrophilic aromatic substitution reactions. In these reactions, an electrophile attacks the aromatic ring, leading to the substitution of a hydrogen atom. The resonance stabilization of the intermediate carbocation formed during this reaction enhances the reaction rate and facilitates the overall process.
  6. Resonance and Acid-Base Behavior: Resonance can influence the acid-base properties of organic compounds. For example, in carboxylic acids, resonance stabilization of the resulting carboxylate anion contributes to their acidic nature. The delocalization of the negative charge over the oxygen atoms stabilizes the anion, making the dissociation of a proton more favorable.
  7. Resonance and Stability of Radicals: Resonance stability is not limited to charged species; it also applies to radicals (species with unpaired electrons). Radicals can exhibit stability through resonance when the unpaired electron is delocalized over adjacent atoms or functional groups. This delocalization reduces the reactivity of the radical and increases its stability. (see eg above)

Understanding resonance stability is crucial for predicting reactivity, understanding reaction mechanisms, and explaining the stability of organic compounds. By considering the various aspects of resonance, chemists can gain insights into the behavior of molecules and design more efficient synthetic routes or develop new strategies in organic synthesis.

If this has helped you understand resonance better, share this post with your friends who might be struggling to understand the concept!

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Differences between H2 and H3 Chemistry

Mr Khemistry


H3 and H2 subjects in Singapore’s Junior College (JC) curriculum differ in terms of syllabus, content, and level of difficulty in assessment. H3 subjects are considered more challenging than H2 subjects, as they are typically designed for students who have a strong interest and aptitude in a particular subject area. These subjects usually have a more advanced syllabus and require students to have a deeper understanding of the content.

H3 subjects also tend to have more demanding assessment criteria and may include additional coursework or research components. H2 subjects, on the other hand, are considered to be of a lower level of difficulty and are typically taken by students who want to study a subject in more depth, but without the added rigor of the H3 level. These subjects usually have a more standard syllabus and may not require as much independent research or coursework.

Q: Should i take H3 Chemistry?

A:  Deciding whether to take H3 (Higher Level 3) Chemistry requires careful consideration of several factors. Here are some key points to help you make an informed decision:

  1. Interest and Passion: Evaluate your interest and passion for chemistry. Are you genuinely enthusiastic about the subject and enjoy exploring its concepts and applications? If you have a strong interest in chemistry and want to deepen your understanding beyond the standard curriculum, H3 Chemistry might be a suitable choice. It allows you to explore advanced topics and engage in independent research.
  2. Academic Strength: Assess your academic strengths and abilities in chemistry. H3 Chemistry is typically more challenging and rigorous than H2 Chemistry. It requires a solid foundation in the subject and strong analytical and problem-solving skills. If you have consistently performed well in chemistry and are confident in your abilities, you may be well-suited for H3 Chemistry.
  3. Future Academic and Career Goals: Consider your future academic and career aspirations. If you plan to pursue a science-related field that requires a strong background in chemistry, such as chemistry research, chemical engineering, or medicine, taking H3 Chemistry can provide you with an advantage. It will deepen your knowledge and better prepare you for higher-level coursework in college or university.
  4. Time Commitment and Workload: Evaluate your overall workload and schedule. H3 Chemistry demands a significant amount of time and effort due to its advanced content and independent research requirements. Assess whether you are willing and able to dedicate the necessary time for study, research, and completing assignments while maintaining a balance with your other subjects and commitments.
  5. Consult with Teachers or Counselors: Seek advice from your chemistry teacher or school counselor. They can provide valuable insights into your academic abilities, potential, and the specific expectations of H3 Chemistry. They may also be able to guide you on how H3 Chemistry aligns with your academic goals and the potential benefits it could offer.

Remember, H3 Chemistry is an optional and advanced level of study. It is not necessary for everyone, and choosing H2 Chemistry instead does not diminish your abilities or potential in the subject. Consider your interests, strengths, future aspirations, and available resources when making your decision.

Q: Does any University Courses require H3 Chemistry for specific programs?

A:   Here are a few examples of universities that may recommend or require H3 Chemistry for certain science-related programs:

  1. National University of Singapore (NUS): NUS offers various science-related programs, including Chemistry, Chemical Engineering, Pharmacy, and Life Sciences. For some of these programs, H3 Chemistry is recommended or advantageous. For example, the NUS Faculty of Science recommends H3 Chemistry for admission to their Chemistry and Pharmacy programs.
  2. University of Cambridge: The University of Cambridge offers a renowned Natural Sciences program, which allows students to specialize in various scientific disciplines, including Chemistry. While H3 Chemistry is not explicitly required, having a strong background in Chemistry, including H3 Chemistry, can enhance your application.
  3. Imperial College London: Imperial College London, known for its science and engineering programs, may recommend H3 Chemistry for admission to their Chemistry or Chemical Engineering programs. The specific requirements and recommendations can vary, so it’s advised to check their admissions website for the most up-to-date information.
  4. Massachusetts Institute of Technology (MIT): MIT is a prestigious institution known for its strong emphasis on science and engineering. While H3 Chemistry is not a formal requirement, having a solid foundation in Chemistry, including advanced coursework, can strengthen your application to science-related programs.

Apart from the Chemistry and Pharmacy programs at the National University of Singapore (NUS), there are other science-related programs that may recommend or require H3 Chemistry. Here are a few examples:

  1. Chemical Engineering: NUS offers a Bachelor of Engineering (Chemical Engineering) program, which involves the study of chemical processes and their applications in various industries. While H3 Chemistry may not be explicitly required, having a strong background in Chemistry, including H3 Chemistry, can be advantageous for admission to this program.
  2. Life Sciences: NUS offers a Bachelor of Science (Life Sciences) program, which focuses on the study of living organisms, including biology, genetics, and biochemistry. While H3 Chemistry may not be a mandatory requirement, having a good foundation in Chemistry, including H3 Chemistry, can be beneficial for admission to this program.
  3. Environmental Science: NUS offers a Bachelor of Environmental Studies program, which encompasses various aspects of environmental science, including environmental chemistry. While the specific requirements may vary, having a background in Chemistry, including H3 Chemistry, can be advantageous for admission to this program.

Q: Where do i obtain more information regarding admissions criteria for science-related programs at NUS?

A:   To obtain more specific and up-to-date information about admission requirements for science-related programs at the National University of Singapore (NUS), you can visit the official NUS admissions website and explore the relevant program pages. Here are some useful resources:

  1. NUS Admissions Website: The official NUS admissions website provides comprehensive information about undergraduate admissions, including program offerings, admission requirements, application procedures, and contact information. You can access the website at:
  2. NUS Faculty Websites: Each faculty at NUS typically has its own website with detailed information about the programs they offer. By exploring the websites of specific faculties, such as the Faculty of Science or the Faculty of Engineering, you can find program-specific details, admission requirements, and frequently asked questions (FAQs).
  3. Contacting the Admissions Office: If you have specific questions or need clarification regarding admission requirements, it’s advisable to directly contact the NUS admissions office. They can provide personalized guidance and address any inquiries you may have. You can find their contact information on the NUS admissions website or send an email to the designated admissions email address.

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Changes in GCE ‘A’ level Education

Mr Khemistry

Changes in GCE ‘A’ level education



    1. No more Mid-Year examinations
    2. General Paper made compulsory
    3. Project Work becomes a pass/fail subject
    4. University Admission Score will be rebased to 70 from 90

1)     Some thoughts regarding the scraping of mid-year exams – less administrative work to prepare for a major exam, more time to put back into lessons and learning. But, like what many others have already pointed out, without this major milestone check, students will be in for a rude shock when it comes to their End of Year Exams.

“The removal of mid-year examinations will not be replaced (emphasis mine) by more school-based assessments, and JCs and MI must not administer more than one weighted assessment per term, MOE said in a separate press release.”

If this is really the case, students will not have a timely feedback on their progress via exams conducted under time-constraint. There are two things that might happen, one is schools will give more assessments but will not call them ‘weighed assessments’, the second is that students and parents might turn to tuition centres to conduct the assessments instead.

The elephant in the room is still the way students are graded at the GCE ‘A’ level exams, as long as the format of the exam doesn’t change, the way the students/parents prepare for it wouldn’t change much.

2)    GP is taken by the vast majority of students anyway, only 100 students took KI as an alternative in 2022, so this change isn’t that significant.

3)    Making PW a pass/fail subject is a de-emphasis on the subject. The most likely effect this will have is that students will spend much lesser time and focus on their content subjects which contribute to University Admission Score. I personally feel that this is a good move as i see the J1 students spend a disproportionate amount of time researching and on meetings.

4)    The fourth content subject and MTL would only count towards the UAS if it scores better compared to GP. A likely outcome for students who are very poor in language will bank on the fourth content subject to replace GP in their UAS.

On the other hand, students who are strong in either GP or MTL might not need to put so much effort for their fourth content subject. Most arts stream student tend to struggle with the contrasting H1 science subject so it will be good for them to leave it out of their UAS. However, the best bet for science students struggling with contrasting H1 arts subject may simply be to focus on GP.

In any case, having two less subjects contributing to UAS is a good step towards relieving the workload for our ‘A’ level students.

What do you think? 🙂

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Transition Element (Common Last Topic)

Mr Khemistry


Minister Chan just announced on 27th July 2021 that the common last topic will be taken out of the A levels examinations this November.

For those of us who already did the topics, it seemed like time was wasted to teach this topic. But the good thing is students will have less to revise this year.

Rightfully so as the multiple disruptions due to tightening/loosening covid measures surely affected the students’ ability to focus.

With the time now freed up from revising transition elements, we can now focus on the topics that are left.

Focus on the Theme: Creative Writing for PSLE Excellence

For those of you who have not done up a revision schedule, now is the best time to plan it.

IMHO, A levels is not something you can decide to pia overnight and score a distinction for it. Far from it.

Unless you consistently revise and put in the effort to practise, it is almost guaranteed that you will not get a quality grade. (A or B)

Such is the rigour of this national examination.


If you need a crash course or intensive revision classes, feel free to drop me a whatsapp @ 83322534.

Will see what i can do to help! Gambatte!


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Tang Ping

Mr Khemistry

Tang Ping

Read an article about a trending movement from China/Hongkong called “Lying flat”.⁠ ⁠ Reading the articles makes me think, this is the other extreme of the FIRE 🔥 movement. But rather then working really hard and saving almost all of your take-home pay so that you can reach financial independence much earlier and retire early, Lying Flat movement simply adopts a monastic mindset.⁠ ⁠

Don’t get married 🤵🏻👰🏻, don’t have kids, don’t buy a car and don’t buy a flat. Just do the minimal work to get by. Some take it to extremes and don’t even socialize in order to save money.⁠ ⁠ The underlying (no pun intended) worldview is one of killing all desires so that you wouldn’t be used as a money-making pawn by the elites.⁠ ⁠ This mindset most likely is spawned by the country’s widening gap in income 💰 between the rich and poor. ⁠ ⁠


Hope SG will manage our equality gap so that our young will not get disenchanted and give up on life. 😔⁠ ⁠

Ref: ⁠ ⁠⁠ ⁠ ⁠

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Transition Elements

Mr Khemistry

Transition Element

Last year due to Covid 19, we did not cover the common last topic, which was Transition Element.

This year, i was waiting for some announcement from SEAB to see whether TE would be tested in A levels this Nov. Alas, the announcement never came.

In the absence of any official announcements, i guess we would have to cover this common last topic this year.

Hopefully, SEAB wouldn’t change its mind at the last minute and say it will not test TE.

Very likely this trend of Circuit breaker and calibrated re-opening will continue for a good 2-3 years. Will need to plan ahead so that disruptions are minimize as much as possible.

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Mr Khemistry will be moving progressively to online classes over the next few years.

Watch this space!

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Mr Khemistry


Recently had a parent WhatsApp a thank you message to me, it was an unexpected surprise for me as the student was usually very quiet in class.

Such a simple message but it had a profound impact on me.  In this period where everyone is feeling a little drained from working/studying from home, a positive message gives great encouragement. I hope everyone will try to spread a little positivity like the #Steadylah campaign by some celebrities. It really does spread cheer and lift up the spirits of a world crippled by Covid lockdowns.

If you would like to receive such messages, why not start to send a few of these out yourself? If you like to reap an apple, first you got to plant the apple seed! 🙂

Share with me how you spread joy to the people around  you! Tag me on Facebook or Instagram, i will ping you back!

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Reflection and awareness

Mr Khemistry

Reflection and awareness

In this fast paced society, it is imperative that we take time for reflection at the end of the day.

To be aware of what is happening around us and how we are living our lives.

I once heard from Bob Procter, ” If all the good of the universe is pour over us, it wouldn’t make the slightest different UNTIL, we become AWARE of it“. Think about it, until a child becomes aware what money is and what it can buy, they wouldn’t actually be happy even if you write them a cheque for a million dollars.

Our awareness, or consciousness, of opportunities around us…needs to be awaken. Through mentors who have done it before, through reading books, through reflections.

Thinking back when i was younger, reflection isn’t a part of my routines and i didn’t really grow in awareness.

Sometimes when i look at students, there are those who reflect upon what they’ve learnt and grew in knowledge. And then there are those who keep repeating the same mistakes no matter how many times they do the same kind of questions.

Parents are understandably worried when they do not see improvements after a few months of tuition, but we would need to see how the student revise their work and if they do any form of reflection.

Reflecting on what they have done and their thought process, then seeing the answers and the thought process required. How to bridge that gap? There’s a few ways students can bridge this gap.

  1. Hard core repetition. Keep on doing prelim and tys questions. Try to remember all the different types of questions and the answers. Rote learning.
  2. Doing a few questions, analyse and try a few more questions to see if you understand the concepts.

There are countless ways which cambridge can  phrase the questions, using the rote learning method might land you a decent grade but most likely not a distinction. There are around 25-30% of questions which are novel, which means you need to think on the spot. If you can’t think using the concepts that you’ve learnt, probably wouldn’t do exceptionally well for Advanced level exams.

To see how we train students in our weekly classes, drop us an message at!

Look forward to hearing from you! 🙂

Déjà vu

Mr Khemistry

Déjà vu

We are transiting to Home-based Learning again. I told my wife that i have a feeling schools and tuition centres are going to be shut down…imminently.

As it turns out, i was right. The rise in unlinked community cases is ominous. This is a very good time to think about the trade-offs we have to take as a nation.

Some friends around me are very pissed that Government did not handle the situation well and imposing lock-down again because they made the mistake of not restricting foreigners from high risk countries (from South Asia).

Thinking a bit deeper, i find that Government is stuck in between a hard place and a rock.

Restricting workers from South Asia will lead to a crunch in the construction sector….which will impact existing work and work that has been delayed due to the previous lockdown. Read article here. It is not as simple as just continuing to work with the existing workers as some of them return home when their passes expires. Without new workers, construction activities will come to a halt and delays lead to job losses…which impacts locals holding related jobs too.

In any case, even if the government shuts down borders quickly and we managed to avoid the rise in community cases…we’d still have to eventually open up to the same countries as we need the workers. Locals are not likely to take up construction jobs overnight. The whole situation will only improve if global cases starts to drop, otherwise, it’s unlikely we’ll be able to isolate Singapore totally.

Covid has caused an uneven economic situation where certain industries are impacted much harder while some others do relatively well. This will most likely exacerbate the economic inequality and cause tensions within the society. Many people think that they are smarter and able to make better decisions than the government. In my humble opinion, not many truly appreciate the constrains we have. Trickle down effects of a hard closure of borders will only be felt after a few weeks.

I urge everyone to stand united and support each other through these trying times!

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Mr Khemistry


Recently, had a friend do a corporate photoshoot for me. Wasn’t expecting too much as the previous photoshoots sessions i had were un-inspiring.

But i was pleasantly surprised. He brought along the lighting gear and the prime lenses…also had an assistant to help with the set-up.

Very different from the photoshoots i had previously. Not that the previous photographers wasn’t professional, it was just a spirit of excellence that Ken (my photographer friend) put into the shoot.

He captured the class in session without disrupting the lesson, knew exactly which is the better side to take my profile picture, generally gives people the feeling of a true professional. Now i know why he’s so sought after. Below are some of the pictures he took for me that day.

I guess in life, you kind of have to go through those who are amateurs, those who just do the bare minimum, the professional and then you have the ones at the top of their game. Ken belongs to the last group, his passion for photography is matched only by his expertise in utilizing those high tech gears and lenses. The end result is an art form which elevates the mundane to the extraordinary. Thank you bro, for your outstanding service!

Here at Mr Khemistry, we aim to provide the best tuition service for JC H2 Chemistry, coupled with a passion to help students achieve their fullest potential. Our tools are pen and paper, but our real weapon is Teaching with Heart. Come join us if you still struggle with Chemistry concepts, we will show you how easy it is to learn Chemistry! 🙂

By the way, for those who are interested in engaging Ken’s service, please google “Blackbox Photography” or Ken Photography.

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