Structure and Reactivity | Organic Chemistry 1

Structure and reactivity in organic chemistry are studied in this chapter: Bronsted acids/bases, acid strength, Lewis acids/bases, nucleophiles and electrophiles, chemical functional groups, types of isomers, types of arrows, heterolytic and homolytic cleavage

Bronsted Acids / Bases

Bronsted acid:

A Bronsted acid is a proton donor and therefore contains a hydrogen atom. A loss of proton from the acid forms its conjugate base. Common examples of Bronsted acids are HCl, H2SO4, H3O+, acetic acid (CH3COOH), p-toluenesulfonic acid (TsOH)

Bronsted base:

A Bronsted base is a proton acceptor. It must be able to form a bond to a proton by donating an available electron pair. A gain of proton by a base forms its conjugate acid. Common examples of Bronsted bases are HO-, RO-, H2N-, R2N​​​​​​​-, H-


Acid Strength

A strong acid readily donates a proton, forming a weak conjugate base. The more readily an acid donates a proton, the smaller its pKa, the stronger it is considered. An acid can be deprotonated by the conjugate base of any acid having a higher pKa

Base stability:

The more stable a base, the weaker the base and the stronger the conjugate acid. The effects which affect the stability of a base are:

  • Resonance effects: the more forms a chemical compound has, the more stable it is
  • Hybridization effect: lone pairs of electrons are stabilized by the s character of the orbital (25 % in sp3, 33 % in sp2, and 50% in sp). Therefore, the increase in basicity correlates with hybridization as follows: sp < sp2 < sp3
  • Inductive effect: bases are stabilized by electron-withdrawing inductive effect (-I effect)

How to compare the acidity of 2 acids:

  1. Draw the conjugate bases

  2. Determine which conjugate base is more stable

  3. Determine which acid is the strongest: the more stable the conjugate base, the more acidic the acid


Compare the acidity of the 2 following acids:


1. Draw the conjugate bases:


2. Compare their stability:

A stabilized by resonance effect

3. Determine the strongest acid:


Lewis Acids / Bases

Lewis acid:

A Lewis acid is an electron pair acceptor. It must have an unfilled valence shell, a partial positive charge or a proton. Common examples of Lewis acids are BF3, AlCl3, +CR3, H+

Lewis base:

A Lewis base is an electron pair donor and therefore contains a lone pair or a π bond. It donates this electron pair to any electron deficient compound. Some common Lewis bases are -OH, -OR, NH3, NR3-NH2, H2O


Nucleophile vs. Electrophile


An electron-rich species which can form a covalent bond by donating a pair of electrons to an electron-poor atom. A nucleophile can be neutral or negatively charged and is usually symbolized by Nu-
Lewis bases are nucleophiles. Lone pairs and π bonds are nucleophilic sites



An electron-poor species which can form a covalent bond by accepting a pair of electrons from a nucleophile. An electrophile can be neutral or positively charged and is usually symbolized by E+
Lewis acids are electrophiles. Electronegative heteroatoms like N, O, or halogens X create electrophilic carbon atoms

Functional Groups

Functional group: 

An atom or group of atoms with characteristic chemical and physical properties. It is the reactive part of the molecule


Carbonyl group (compound with a C=O bond):

Types of Isomers

Constitutional isomers:

Chemical species with the same molecular formula but which differ in the way atoms are connected to each other. They are also called structural isomers



Isomers with the same molecular formula, the same connections but which differ in the way atoms are oriented in space (different 3D geometry). Hashed-wedged line structures are used to depict the 3D arrangement

cis-isomer: isomer with 2 substituents on the same side of the ring or double bond
trans-isomer: isomer with 2 substituents on opposite sides of the ring or double bond

Types of Arrows

Reaction arrow: drawn between the starting materials and products in an equation

Double reaction arrows: drawn between the starting materials and products in an equilibrium equation

Double-headed arrow: drawn between resonance structures

Full-headed curved arrow: shows movement of an electron pair

Half-headed curved arrow: shows movement of a single electron

Heterolytic vs. Homolytic Cleavage

Heterolytic cleavage:

Bond breakage in which the bond electron pair is unevenly divided between products. A normal double-barbed arrow shows the movement of the pair of electrons. Heterolytic bond cleavage of a neutral molecule results in the formation of a cation and an anion


Homolytic cleavage:

Bond breakage in which the bond electron pair is evenly divided between products. A single-barbed arrow shows the movement of a single electron. Homolytic bond cleavage results in the formation of radicals