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Organic Chemistry - Drawing Structures, Concepts, and Examples

Ylide

The ylide is an electrically neutral molecule that has a negative carbon with an unshared electron pair (usually a carbanion) adjacent to a positive heteroatom (typically nitrogen, phosphorus, or sulfur), where both the atoms have full octets (eight electrons in their valence shell). 

The bond between the carbanion and positive heteroatom is best described as a polar covalent bond with significant ionic and double-bond character, depending on the ylide type.

 

Difference between Qualitative and Quantitative Analysis in Organic Chemistry

 

Qualitative Analysis

Quantitative analysis

Aim

Identifies what compounds or functional groups are present. Therefore, the type of information obtained is descriptive.

For example, whether the compound changed color, evolved gas, or formed a precipitate. 

 

Measures how much of a compound is present and, therefore, numerically describes its quantity. 

Differences between Valence Bond Theory (VBT) and Molecular Orbital Theory (MOT)

Aspect

Valence Bond Theory (VBT)

Molecular Orbital Theory (MOT)

Basic Concept

Describes chemical bonding as the overlap of atomic orbitals, forming localized bonds.

Describes bonding by combining atomic orbitals into molecular orbitals that are delocalized over the molecule.

Bonding Explanation

Focuses on bonds as being localized between two specific atoms.

The shape of sp hybrid orbital - Why is the lobe unequal?

If you look at the shape of s and p orbitals before hybridization, you will notice that - 

  • The s orbital is spherical.

  • The p orbital is dumbbell-shaped, where the two lobes are proportionate along one axis.

However, when these orbitals of the same atom mix to form hybrid orbitals, the newly formed shape is that of teardrops pointing in opposite directions, with one lobe more prominent than the other.
 

Acid

Commonly, it is accepted that a compound is an Arrhenius acid if it liberates hydrogen ions as H+ in water. In the next step, these H+ ions combine with water molecules to form hydronium ions (H3O+). The two steps can be summarised to say that Arrhenius acids are compounds that form hydronium ions, provided water is the solvent.

 

Structural Formulas

Structural formulas are used to display covalently bonded molecules in the order they are bonded. The atoms may bond using single, double, or triple bonds. Some structural formulas show only the atoms without displaying the complete structure with bond angles and, in some cases, even the bonds.

Octet Rule

Atoms generally form bonding arrangements that give them filled shells of electrons like a noble gas configuration. The stability the atom aims for is that of its nearest noble gas.

For example, Lithium (Z = 3, Electronic Configuration = 1s2, 2s1) of the second row would prefer losing one electron to become Li+ (Z = 3, Electronic Configuration = 1s2) having an electronic arrangement similar to its nearest noble gas Helium (Z = 2, Electronic Configuration = 1s2). 

Organic Chemistry Fundamentals - Topic List | CurlyArrows

Organic Chemistry stands out from other scientific disciplines due to its unique focus on carbon. This versatile element forms larger, more diverse molecules, and its reactions with other elements are crucial for enhancing reactivity, improving properties, introducing structural diversity, altering molecular composition, and more. These ambitious transformations are powered by electrons. Therefore, the study of organic chemistry begins with a deep understanding of its key players-carbon and electrons.

Dihedral Angle

A dihedral angle is obtained when two planes pass through three atoms and two bonds, of which one bond is common to both planes. 

In the below example, plane 1 passes through X-C-C, containing three atoms and two bonds. Similarly, the plane 2 passes through C-C-Y. The separation between the two planes or the angle of intersection is denoted using a dihedral angle (ϴ) in degrees (o).

 

Organic Chemistry

Organic chemistry is a branch of chemistry that studies transformations at three levels- macroscopic (visual changes like rotting of an apple), microscopic (atoms, elements, electrons, involved), and symbolic (representing changes using bond breaking and making, electron transfer arrows). It, therefore, studies structures, composition, properties, formation, and reactions of various compounds, mainly carbon-containing compounds, to fully understand transformations.

 

Why do only covalently bonded molecules show Isomerism?

Around the early 1800s, scientists began to notice the presence of isomerism in all types of compounds; however, at the time, it was referred to using different names.

The term allotrope was used to describe different varieties of an element that existed in different physical forms, such as diamond or graphite for carbon or monoclinic or rhombic for sulfur. Polymorphism was used to explain compounds that occurred in more than one crystalline form, like aragonite and calcite, both crystalline forms of calcium carbonate.

History of Isomerism

Accidental discoveries and insatiable curiosity fuel chemistry. 

One such significant discovery is of isomerism, which also occurred around the period when the faith in vitalism, a belief that organic molecules must only come from living organisms, was dwindling. 

It was a pleasant surprise to learn that urea, a by-product of urine produced by human kidneys, could also be synthesized by heating ammonium cyanate, an inorganic compound.

Differentiating complete structural formulas and condensed structural formulas using examples

Draw complete structural formulas and  condensed structural formulas for

        a) three compounds of formula C3H8O

        b) five compounds of formula C3H6O

The condensed structural formula disguises the molecule's true vastness by revealing only the number of atoms present. What remains hidden from sight are the bond connections, the bond angles, and their three-dimensional arrangements, so what we observe is just the molecule's literal condensed form.

How to determine the hybridization of C in CH4?

If only valence shells participate in bonding reactions, then Carbon should only form two bonds using its two unpaired electrons. However, Carbon forms four bonds, as seen in CH4, CH3X, H2C=O, and O=C=O molecules. This discrepancy between the number of valence electrons of an atom and the number of bonds it forms (valency) is explained by the hybridization concept, an extension of the Valence Bond (VB) Theory.

Bond Order for Covalent Bonds

The very premise of a covalent bond is electron sharing. As two atoms share one electron each to form one covalent bond, they may likely share more than once and form more bonds. This information on the number of connections between two atoms is revealed from the Bond Order. 

So, the Bond Order measures the number of bonds between the two atoms in a molecule. The number can be integers like 1, 2, or 3 for single, double, or triple bonds or non-integers like 0.5, 1.3, 1.5, etc.

Drawing Organic Structures- Lewis Structures

The first time the use of two dots or a colon (:) to represent two electrons acting as connecting links between the two atoms was proposed by G.N. Lewis in 1916.

In his landmark paper, ‘the atom and the molecules,’ he attempted to draw molecular structures using dots to represent the covalent nature of the chemical bonds, now popularly known as the Lewis structures.

So, a molecule of Cl2 was Cl : Cl, two electrons connecting two Cl atoms.

Molecular models for organic structure representation

Molecular models are tools used for classroom or research to visualize better and interpret three-dimensional structures of organic molecules. The toolkit can be made of wood, plastic, metal, or other materials.

These models are digitally represented using computer graphics covered under molecular modelling, a branch of chemistry that uses computational and theoretical methods to model and mimic the molecule's behaviour in chemical and biological environments.

Bond Line or Skeletal Formula

What is a Bond-Line or Skeletal Formula?

Carbon and Hydrogen containing bonds are chemically inert; still, they form the backbone of many organic compounds. They provide a skeletal framework that can contain other heteroatoms or functional groups. 

Such a carbon-hydrogen-containing chain of compounds together with other atoms or groups of atoms are conveniently represented using zig-zag lines, called the bond-line or skeletal formula.

Condensed Structural Formula- How to write

What is a Condensed Structural Formula?

The condensed structural formula provides the shortest way to understand the atoms and their numbers, their connection with respect to each other, and therefore, visualize the compounds' structure without elaborately drawing it, but by writing it in a single line.

So, if the elaborate structure of butanoic acid look like A, the condensed structural formula would look like B.

Valence Shell Electron Pair (VSEPR) theory

In his landmark paper, 'The Atom and the Molecule,' G.N. Lewis attempted to describe linkages between the atoms to understand the nature of covalent bonds.

He used dots to represent an atom’s valence electrons and argued that the atoms share their valence electrons to form one, two, or three bonds until they attain a stable octet electron configuration. An exception is the Hydrogen atom that attains a duplet configuration.  

How to identify a functional group from an IUPAC nomenclature- with three examples

The longest carbon chain is the parent chain containing the principal functional group. 

The principle functional group is always given the highest priority, lowest number (1 before 2, 3, 4...), and assigned as a suffix while writing the nomenclature. Therefore, the numbering of the parent chain always starts from the principal functional group.

What is the priority order in polyfunctional compounds while assigning nomenclature?

IUPAC, an international body engaged in developing standards for naming chemical elements and compounds, has laid several rules known as 'IUPAC rules' to identify a molecule by its parent alkane chain and the functional group. It has assigned various suffixes to identify organic chemistry's commonly seen functional groups.

What are Radical inhibitors and their role in a reaction?

A radical reaction usually happens in three steps- Initiation (creation of free radicals), Propagation (multiplication and growth of radical reaction), and Termination (stopping of the free radical reaction).

Radical inhibitors are chemical species that inhibit radical reactions by halting the chain propagation step.

Examples of such inhibitors are Hydroquinone, BHT (2,6-di-t-butyl-4-methylphenol), etc.

Types of Rearrangement reactions in chemistry

An atom or group of atoms migrate from one position to another; such reactions are rearrangement reactions.

Post rearrangement, the molecule formed has the atoms arranged differently but has the same molecular formula as the parent. Therefore, such rearrangements create structural isomers, compounds that differ in structure but have the same molecular formula.

Related Reading - Structural Isomerism

Benzyl

A benzyl, abbreviated as Bn, refers to the phenylmethyl group (C6H5-CH2-), consisting of a benzene ring attached to a methylene (-CH2-) group.

 

What is a metallic bond and how does it form?

A metallic bond is a chemical bond seen in metals consisting of tightly bound metal atoms of the same type. 

Metals are large atoms that do not firmly hold their outermost valence electrons and easily lose them. Once the electrons are lost, the metal atoms become positively charged, called kernels. The position of these kernels is fixed to avoid repulsions and is part of the solid structure. 

Why are Debye Forces called dipole-induced dipole interaction?

Debye intermolecular forces of attraction occur between polar and nonpolar atoms or molecules.

When a polar molecule is in the vicinity of a nonpolar molecule, the electronegative end of that polar molecule attracts the electron cloud of the adjacent nonpolar molecule, thereby distorting it and inducing a pole separation. So, the polar molecule is said to have induced polarity in the neighboring nonpolar molecule.

Why are Debye forces called the induction effect?

Debye intermolecular forces of attraction occur between polar and nonpolar atoms or molecules. When closely placed with a polar molecule (electronegativity difference between 0.5-1.7), the electron cloud of the nonpolar atom (or molecule) is influenced and distorted, leading to the formation of temporary dipoles. As the polarity is induced (forced) in the otherwise nonpolar atom, the Debye forces are said to show an Induction effect.

Which polar atoms will show the strongest Debye interaction?

Debye interactions require polar molecules to show polar-nonpolar type of intermolecular attractive interactions. Therefore, the polar atoms participating in Debye interactions must have comparatively high electronegativity to show stronger interactions. If the electronegativity difference (0.5-1.7) is greater in a polar molecule, the polarity it induces on the neighboring non-polar atom will also be more significant, leading to stronger Debye interactions.

How do polar nonpolar molecules interact? Explain with examples.

A polar molecule has natural poles. Therefore, when next to a nonpolar molecule, it will try to magnetize it temporarily. 

The negative end of the polar molecule will repel the nonpolar molecules’ moving electron cloud, or the positive end will attract its electrons, which can contribute to significant distortions. Such a forced activity can induce a temporary pole separation in the nonpolar molecule. 

Why are London Dispersion forces called induced dipole interactions?

London Dispersion forces are seen in all the atoms and the molecules; however, it is the exclusive force binding nonpolar molecules. Nonpolar molecules are made of two or more atoms, and the electronegativity between two atoms in a covalent bond is less than 0.5. Therefore, no charges are separated to create poles in nonpolar molecules or atoms. Despite not having poles, nonpolar molecules interact using a weak force of attraction known as the London Forces.

Which molecules show London Dispersion Force?

All atoms and molecules show London Dispersion Force, a primary way atoms and molecules interact. Most of the time, molecules have additional stronger forces than the London Dispersion Force, which is the weakest. Therefore, London Dispersion forces are exclusively seen in the nonpolar molecules since they don't have other competing functional groups leading to other interfering intermolecular interactions.

How do nonpolar molecules interact?

A nonpolar molecule is made of two or more atoms where the electronegativity difference between the atoms is less than 0.5. Therefore, the electron density in the bond is equally distributed, and there is no concentration of electrons on one atom to create a charge separation rendering the molecule essentially without poles or nonpolar. Such nonpolar molecules like H2, CH4, He, etc., will interact using the London dispersion forces without forming any positive or negative pole.

Why are London forces weak?

London dispersion force is the weakest of the three van Der Waal forces of interactions universally found in all the atoms and molecules. It is caused by the constant fluctuations in the electron position, creating a temporary dipole. When other nonpolar molecules are in its vicinity, the instantaneous temporary dipole formed earlier distorts the electron cloud of an adjacent molecule, creating another dipole.

Why are Keesom forces are said to have an orientation effect?

Keesom forces occur in polar molecules where the atoms have a considerable electronegativity difference, resulting in the charge separation and formation of positive and negative poles. The negative pole of one molecule electrostatically interacts with the positive pole of the neighboring molecule. Consequently, all the polar molecules in the medium try to orient their dipoles to be in alignment for such electrostatic interactions.

How do polar molecules interact?

A molecule is said to be polar if the bond electrons between the atoms in a covalent bond are unequally distributed, creating two ends. The electron redistribution is marked by showing the separation of charges as a partial charge above the atom's symbol. Therefore, a polar molecule always has a positive and a negative end resulting in a dipole. The pulling of bond electrons by an atom is attributed to its electronegativity.

What are the requirements of van der Waals force?

The van der Waal forces of interactions are weak intermolecular forces. For the atoms or molecules show these interactions only under certain conditions -

1) Distance- van der Waal forces mainly depend on the distance between the associated molecules. These forces become very weak if the distance between the molecules or atoms is greater. These forces are relatively stronger if the atoms or molecules are grouped closely.

How do Van der Waals forces arise?

Van der Waals forces are weak intermolecular attractive forces that occur in polar and nonpolar atoms or molecules due to the shift in their electron positions.

The electrons shift to form electron-dense and electron-deficient poles. Some poles are permanent due to the nature of the atom in a molecule, while others are induced poles. 

Theories on Covalent Bond Formation

Dalton, in 1804, in his work on ‘Atomic Theory,’ proposed the law of chemical combination to explain how atoms form compounds. According to him, atoms of different elements combine in a simple whole-number ratio to give compounds.

For example, two Hydrogen and one Oxygen combine to form water (H2O), or four Hydrogens and one Carbon combine to form methane (CH4), or one Nitrogen combines with three Hydrogens to give ammonia (NH3).

Which side of an unsymmetrical alkene does the reagent attack and why?

Alkenes are double bonds containing molecules. These double bonds are called pie bonds and are made up of two electrons. Due to the presence of the pie electron cloud, alkenes are electron-rich species and interact with electron-deficient species, the electrophiles. The reaction between alkenes and the electrophiles is an addition reaction.

What is Electronegativity?

Sharing resources is essential to build a harmonious world. When the resources are shared ineffectively conflicts emerges. A similar principle extends to Chemistry. 

Atoms are in a state of harmony when the neighbouring atoms shares electrons. When the sharing stops, Chemical reactions triggers.

The imbalance in sharing electrons influences the molecule’s polarity, reactivity, and physical properties. The disagreement on sharing is brought about by electronegativity. 

Metallic Bond

Pre-requisite: Nuclear charge, Valence electrons

Stovetop cooking routinely requires cast iron or aluminium pans but never a glass pan or a pan made of silicon. That’s because the glass pan will shatter at high temperatures, and the silicon pan will burn away.

What is the Strongest Bond in Chemistry?

A chemical bond strength is a force holding the atoms in a bond, and separating such atoms requires energy input. The bonds are of two types- Intermolecular and Intramolecular bonds.

Intramolecular bonds join the atoms in a molecule, whereas Intermolecular bonds are only responsible for closely associating the molecules. Therefore, Intramolecular bonds require higher energy to break than intermolecular bonds.

 

Markovnikov’s Rule

Russian chemist Vladimir Markovnikov proposed Markovnikov’s rule in 1869 to predict the regiochemistry of addition reactions between unsymmetrical alkenes/ alkynes and hydrogen halides to form alkyl halides.

According to the rule, ‘An alkyl halide is formed in a reaction between an unsymmetrical alkene or alkyne and hydrogen halide (HX) when the negative part of the reagent (X-) attaches to the alkene carbon that has fewer number of hydrogen atoms across the double bond.’

Types of Arrows used in Chemistry

A comprehensive list of 18 types of the arrows frequently encountered in chemistry with examples - Chemical Reaction Arrow, Reversible Reaction Arrow, Equilibrium Reaction Arrow, Double Headed Curly Arrow, Fishhook Curly Arrow, Dashed Arrow, The Crossed or The Broken Arrow, Resonance Arrow, Retrosynthesis Arrow, Rearrangement Reaction arrow, Reflux Reaction Arrow, Orbital Electrons Arrow, Co-ordinate Covalent Bond Arrow, Upward Arrow (Gas Evolution), Downward Arrow (Precipitate), Clockwise and Anti-Clockwise/ Stereochemical Arrow, Wavy Arrow, and Dipole Moment Arrow.

What is Degree/Level of Unsaturation (LU) or Double Bond Equivalent (DBE)? How to find and interpret DBE values?

The DBE calculation uses the general molecular formula to find the presence of unsaturation in a compound. The unsaturation is calculated in levels or degrees. 

The lowest degree of unsaturation (DOU) indicates minimum unsaturation, where there is the least loss of hydrogens to form a pie bond or a cyclic ring, like in cycloalkanes.

 

General Molecular Formula

What are haloalkanes and haloarenes? How can they be classified?

A Haloalkane is the Halogen derivative of an alkane obtained by replacing of one or more hydrogen atoms.

For example, when one Hydrogen of an alkane, Methane, is replaced with a Halogen, for instance, Chlorine, the haloalkane obtained is chloromethane.

                                                               Alkane                 Haloalkane

Difference Between Phenol, Phenyl, Benzene, Benzyl, and Aryl

It is easy to identify the simplest aromatic hydrocarbon Benzene that is made of only two atoms- Carbon and hydrogen. Benzene is a planar, cyclic ring molecule with six sides. It has an alternating double bond that contributes to its extra stability by endowing it with resonance, a phenomenon of electron delocalization.  The molecular formula of Benzene is C6H6 and the structure is-

Identify the Organic Compound with Molecular Formula C8H16O2- Aldehydes, Ketones and Carboxylic Acids

An Organic Compound 'A' molecular formula C8H16O2 was hydrolyzed with dilute H2SO4 to give a carboxylic acid 'B' and an alcohol 'C.' Oxidation of 'C' with chromic acid also produced 'B.' On dehydration 'C' gives 1-but-ene. Write the equations for the reaction involved.

1) The first step is find out the Degree of Unsaturation (the number of double bonds or the presence of a ring) in the compound. 

The formula is, 

How to represent molecules using Lewis dot structures? (With Examples)

The Lewis dot structures are used to show the shared electron pairs between the bonded atoms in the molecule and the lone pairs of electrons if any. The representative structures follow the octet rule wherein the atoms combine by either transfer of electrons (loss or gain) or by sharing of valence electrons in a way that the valence shell of the atoms attains the octet configuration.

The electrons involved are shown as dots. A single bond is made up of two electrons, a double four and a triple bond six. Similarly, an atom can have one, two or three lone pairs of electrons.

What is Markownikoff’s/ Markovnikov’s Rule?

Alkenes undergo addition reactions where the substrate (alkene) and the reagent combine to form an adduct. One pie bond of the alkene is sacrificed to create two new stronger sigma bonds with the reagent in the reaction.

If the alkene is symmetrical with the same number of groups on either end, in that case, the alkene adds to the reagent to form one primary product. However, unsymmetrical alkenes with different groups across the double bond can have more than one way of attachment.

 

Why Anilines and Phenols do not undergo Friedel Craft Acylation Reaction?

Friedel Craft acylation reaction is a type of electrophilic aromatic substitution reaction wherein the hydrogen of the aromatic ring is substituted with the acyl group (R-CO-), and a new Carbon-Carbon bond is formed. Such an acylation reaction requires a Lewis acid catalyst such as AlCl3 and an acid chloride (R-CO-Cl). The byproduct formed is HCl.