Organic Chemistry Definitions A-Z

An equimolar (1:1) mixture of a Lewis acid anhydrous ZnCl₂ and concentrated HCl- Lucas Reagent, is used to identify and classify unknown alcohol (R-OH) as primary (1^o), secondary (2^o) or tertiary (3^o). The Lucas test is based on the speed at which corresponding insoluble alkyl chlorides (R-Cl) are formed post reaction. The formation of alkyl chlorides solution appears as cloudiness/emulsion, and it is correlated with the reactivity of the alcohol, 3^o alcohol being the most and 1^o being the least.

The substances that on mixing with a suitable dispersion medium readily form a colloidal sol are called lyophilic (solvent loving) and the sols thus formed are known as the lyophilic sols.

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.’

The mass number is the total count of the number of protons and neutrons present in the nucleus of an atom, denoted as A.

For example, ¹²C has 6 protons and 6 neutrons; its mass number is 12.

A mass number is always a whole number. The mass number is also known as the atomic mass number or nucleon number.

The melting point of a substance is the temperature at which a  solid, ordered crystalline substance is in equilibrium with a more random liquid state, thereby undergoing a phase change.

Since the components of the solid substances are bound by strong intermolecular attractive forces, it also reflects the energy needed to weaken these attractive forces and move freely in the liquid state. 

The meta directors are a class of atoms or a group of atoms that, when attached to an aromatic ring, render it with the ability to direct an incoming electrophile to its meta (third or fifth) position in an electrophile aromatic substitution reaction.

A metallic solid is made of many metal atoms composed of kernels and electrons, the positive kernels held in arrays while the negative electrons float around them, at the same time, attracted to each other due to their opposite electrostatic nature and engaged in bonding known as the metallic bond.

As a reactive species, a methylene refers to an electron-deficient carbene, represented as :CH₂. 

Carbenes are carbon-containing electrophiles that contain only 6 valence electrons instead of the standard eight required for stability. It has an unshared electron pair and two bonds, making it a divalent species, highly unstable, having a fleeting existence. Though classified as electrophiles, methylene species have no formal charge and are neutral.

A molecular formula expresses in positive whole numbers the total count of each atom in a molecule. 

For example, in the molecular formula of H₂O, the total count of Hydrogen atoms is 2, and that of Oxygen atoms is 1. Another example of a complex molecule, such as glucose, expressed in a molecular formula as C₆H₁₂O₆, has a total of 6 carbon and oxygen atoms, and 12 hydrogen atoms.

A molecular formula is always a multiple of the empirical formula, and their relationship is expressed as,

Molecular mass is the sum of the masses of all the atoms in the molecule, denoted as m.

The mass of the atoms is measured in the average atomic mass unit, a weighted average of all the element's naturally occurring isotopes.

The sum is represented in amu (μ) or Da unit.