Category : Most asked questions

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.

    Examples:

  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.

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Nomenclature of anions

Mr Khemistry

Nomenclature of anions

One of the common questions that pops up from time to time is how do i write the formula of sodium nitrite? And what’s the difference between sodium nitrite and sodium nitrate? Or sodium nitride? This has to do with the nomenclature of anions.

The convention for naming anions is as follows:

ide ending is for a monoatomic anion of an element. e.g. hydride, fluoride, oxide, sulfide, nitride, phosphide etc.

ite ending is for polyatomic anions containing oxygen, oxyanions. -ite is for the one with less oxygen atoms. eg, nitrite, sulfite

ate ending is for polyatomic anions containing more oxygen atoms. eg nitrate, sulfate

If there’s a series of 4 oxyanions, hypo prefix refers to the anion with less oxygen and per- prefix refers to the one with more oxygen. e.g. hypochlorite, perchlorate.

Hopefully now you have a clearer understanding of the nomenclature of anions. For H2 Chemistry, this is not super crucial but it’s good to know. Usually questions will give you the formula of the compound, not just its name.

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How to solve Organic Synthesis Question

Mr Khemistry

Organic Synthesis Question

Organic synthesis question

In any synthesis questions, we first need to note the number of carbon atoms, is there an increase or decrease in the carbon chain?

In the above question, yes the carbon chain increased from 8 to 9 carbons. This informs us that somewhere in the intermediate steps, a step-up reaction involving nitrile group will be needed. 

Then we take a look at the changes in the functional groups. We are starting with alkene and ending with alkene and carboxylic acid group.

To insert a nitrile group, we need to first add halogenoalkane functional group as a precursor. This is a common way to introduce the CN group via nucleophilic substitution in the H2 Chemistry Syllabus.

Next we need to consider how to keep the C=C group in the target molecule. Inevitably, when we add the halogen to the C=C, the alkene functional group will disappear. So how do we reform the C=C after adding the halogen? 

There are 3 options,

  1. HCl
  2. Cl-Cl in tetrachloromethane
  3. Cl-Cl in aq

First option is out as it will convert the functional group to halogenoalkane, which leaves us no way to reform the alkene through elimination.

Second option is also out as both Cl will be substituted by CN group.

Option 3 is our answer as Cl will be added to the the terminal carbon atom and OH will be added to the more substituted carbon atom (Markovnikov Rule) Choosing the correct reagents and condition for the first step is crucial to solving the question as it will set the course of synthesis.

Sequencing the Synthesis Route

After which we can just substitute the terminal Cl with CN and hydrolyse it to COOH. The alkene group can also be reformed via elimination of water.

Organic Synthesis Question

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Tuition and Chemistry Crash Course

Mr Khemistry

Frequently asked questions

Q: When is it too late to go for chemistry tuition?

A: There’s no deadline that says it’s too late to go for tuition. However, a good gauge would be to look at your test scores. If you consistently failed your tests and give yourself excuses, the time will come when you will have to face the music, i.e. get retained or even asked to leave the college. To be fair, there is a big jump from O level to A level standard. But if you are failing, then you are probably not coping with the content at JC level, get help as soon as possible.

Common excuses from students range from “I never study” to “i’m busy with xxx” and even “the test is too hard, everyone is also failing“. Truth is, if you really didn’t study for the test, how would you know whether you understood the content? The JC curriculum is packed and to say you’re too busy to study shows you have bad time management. Lastly, the same people who set your tests are going to set your promos…

Remember this, you are not just competing with your classmates or even your schoolmates. You’re competing with the whole Singapore cohort and you can be sure that others are going to do something about their grades.

Private or Group?

Q: Should i go for private or group chemistry tuition?

A: Firstly, you need to ask yourself are you a motivated person? Secondly, are you totally lost in Chemistry? Thirdly, do you have a very tight schedule? If you are a motivated person who can revise yourself, go for group. If you are totally lost and needs lots of help late in the year (after June), go private. Same for students who have very tight schedules, they will need to hire a private tutor to accommodate their timing.

Chemistry crash course

Q: Are crash courses any good?

A: Again, we need to assess where you are. If you are scoring below 30{8445fa0408f68c331c03e03f10d6a7bff33fb7168cc52e9b2191ddf5ec3671a6} for your tests, you need good consistent revision. Due to the time constraints for crash courses, most crash courses wouldn’t be able to let students attempt the questions. It will be a good refresher for the main concepts that’s all. Here at Khemistry, we try to couple additional private lessons where we guide you along as you attempt the questions. This will complement the crash course refresher.

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H2 Chemistry Tuition

Mr Khemistry

H2 Chemistry Tuition

Q: Why isn’t there group tuition for H1 Chemistry like for H2 Chemistry Tuition?

A: Because there isn’t enough demand to form a group, so usually we try to give 1-on-1 private tuition. However, if you are able to bring along 2 more friends, we will open up a new slot for H1 group tuition. Do note that for classes with only 2 students will be prorated accordingly.

 

Q: What makes you an expert in H2 Chemistry Tuition?

A: Our principal tutor, Mr Eric Kua, taught H2 Chemistry at JC level for more than 10 years. Thus he is well versed in the syllabus and the common mistakes made by students. Mr Kua has also taught students from most of the Junior Colleges in Singapore, including Eunoia JC.

Some students have asked if it’s possible to have an exclusive class for their college. The answer is yes, provided there are at least 4 students to start the class. 

 

Q: Why should I get Chemistry tuition?

A: Chemistry is the central science, all combination in science stream for JC includes chemistry. And chemistry is the only science that is compulsory for entry into medicine, requirement is that students have a “good” pass in chemistry (See appended image below from NUS website)

NUS Entry requirement for medicine

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Qualities of a good tuition teacher

Mr Khemistry

Qualities of a good tuition teacher:

  • Talks in a way you understand and at a pace you can keep up with
  • Points out common mistakes during lessons
  • Knows the current syllabus well
  • Charges reasonable rates
  • Patient

How you can look out for qualities of a Good Tuition Teacher:

Look for ex-MOE teachers, they are qualified and experienced. Even better is those with many years of experience as they would know common mistakes made by students. They would also be able to better handle group/class lessons due to their experience.

Some parents would ask why not look for the cheapest rate? The answer might surprise you…

Trained teachers would not cheapen their services as they are professionals just like lawyers and doctors. On the other hand, they would also not charge exorbitant rates as that would disadvantaged students who come from not so well-to-do families. 

However, non-MOE trained tuition teachers who aren’t familiar with the syllabus and common mistakes might charge lower rates. 

Let’s use an analogy: If you are climbing a mountain, would you choose a good guide who just communicates well or would you choose an experienced guide who have guided hundreds of climbers before? The answer is pretty obvious.

Word of caution:

There are some tuition centres that charge lower rates as they simply group all the students from different levels and streams together.

We don’t think that is a good idea as students are in fact overpaying just for consultation. Here at Khemistry, we offer classes that caters for  H2/IP students specifically so that every student learns from lessons pitched to their level. Why H2 classes only? We answer that here.

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What does the units “mol/dm3” refer to in standard enthalpy change of reaction?

Mr Khemistry

Enthalpy Change of Reaction

Some of you may be thinking the enthalpy change of reaction just a general definition for any reaction but the units actually don’t refer to any specific reactants or products.

It refers to the molar quantities of all the reactants and products. For eg,

2H2 (g) + 2O2 (g) → 2H2O (l)

It means 2 moles of hydrogen reacting with 2 moles of oxygen to form 2 moles of water.

Thus, if you are to find the enthalpy change of reaction for:

H2 (g) + O2 (g) → H2O  (l)

The ΔHrxn will be half the value of the first equation.

If you are sharp, you might notice that this equation is also applicable for enthalpy change of combustion for hydrogen, enthalpy change of formation for water.

This means that some enthalpy definitions overlap and there’s nothing wrong with that 🙂

This enthalpy change of reaction is useful for equations that doesn’t neatly fall into common enthalpy definitions.

So it is important to state the appropriate equation for the particular ΔHrxn quoted!

Do remember for Energetics, it’s very important to include state symbols in your equations as different physical states has different energy levels.

More commonly asked questions here.

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Strength of Hydrogen Bond

Mr Khemistry

Strength of Hydrogen Bond

Qn: Which of these compounds has the strongest hydrogen bond, HF, H2O or NH3? How do we estimate the strength of hydrogen bond?

Ans: HF. Due to the largest electronegativity difference between H and F. Followed by H2O and then NH3.

Qn: But then why does water have the highest boiling point among them? Doesn’t this mean that water has the strongest hydrogen bond?

Ans: No. This only means water has the most extensive hydrogen bonds as it forms two hydrogen bond per molecule. HF forms only one hydrogen bond per molecule as it only has one hydrogen atom. NH3 forms only one hydrogen bond per molecule as N has only one lone pair per molecule.

So we need to differentiate between the strength of the individual hydrogen bonds and the overall extensiveness of the hydrogen bonds formed 🙂

More common questions here

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Common mistake in drawing hydrogen bond

Mr Khemistry

Common mistakes in drawing hydrogen bond

One of the most common mistakes is to write the dipole across the hydrogen bond, e.g.

A dipole exists as a pair, partial positive and partial negative. It also depicts the uneven electron density distribution in a covalent bond thus it first forms in a single molecule where
the hydrogen atom is bonded to a very electronegative element such as F/O/N. As the hydrogen is almost “stripped” bare of its electron, it is strongly attracted to the lone pair on a highly electronegative F/O/N atom. This electrostatic attraction is known as a hydrogen bond. e.g.

Another type of hydrogen bonding within the same molecule is known as Intramolecular hydrogen bonding. Which type is overcome during boiling?

More here

Come sign up for our weekly group tuition to find out! 🙂

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Why do we need to acidify after heating with NaOH(aq) in test for halides?

Mr Khemistry

Halides test

Ans: The purpose of acidifying is to neutralise the excess OH- ions that will form a precipitate with Ag+ ions.

This is an often overlooked step in the halides test.

Another common question is why does the precipitate take varying time to appear? This has to do with the type of halide we have.

More common questions here.

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